Rosetta finds magnetic field-free bubble at comet

This article is mirrored from the main ESA web portal

ESA’s Rosetta spacecraft has revealed a surprisingly large region around its host comet devoid of any magnetic field.


Comet Halley, by ESA's Giotto, from a distance of about 2000 km. Giotto flew by the comet during the night between 13 and 14 March 1986. Credit: ESA/MPAe Lindau

When ESA’s Giotto flew past Comet Halley three decades ago, it found a vast magnetic-free region extending more than 4000 km from the nucleus. This was the first observation of something that scientists had until then only thought about but had never seen.

Interplanetary space is pervaded by the solar wind, a flow of electrically charged particles streaming from the Sun and carrying its magnetic field across the Solar System. But a comet pouring lots of gas into space obstructs the solar wind.

At the interface between the solar wind and the coma of gas around the active comet, particle collisions as well as sunlight can knock out electrons from the molecules in the coma, which are ionised and picked up by the solar wind. This process slows the solar wind, diverting its flow around the comet and preventing it from directly impacting the nucleus.

Along with the solar wind, its magnetic field is unable to penetrate the environment around the comet, creating a region devoid of magnetic field called a diamagnetic cavity.


Artist's impression of the diamagnetic cavity at Comet 67P/Churyumov-Gerasimenko. Credit: ESA–C.Carreau

Prior to Rosetta arriving at Comet 67P/Churyumov-Gerasimenko, scientists had hoped to observe such a magnetic field-free region in the environment of this comet. The spacecraft carries a magnetometer as part of the Rosetta Plasma Consortium suite of sensors (RPC-MAG), whose measurements were already used to demonstrate that the comet nucleus is not magnetised.

However, since Rosetta’s comet is much less active than Comet Halley, the scientists predicted that a diamagnetic cavity could form only in the months around perihelion – the closest point to the Sun on the comet’s orbit – but that it would extend only 50–100 km from the nucleus.

During 2015, the increased amounts of dust dragged into space by the outflowing gas became a significant problem for navigation close to the comet. To keep Rosetta safe, trajectories were chosen such that by the end of July 2015, a few weeks before perihelion, it was some 170 km away from the nucleus. As a result, scientists considered that detecting signs of the magnetic field-free bubble would be impossible.


The decrease in magnetic field strength measured by Rosetta’s RPC-MAG instrument at Comet 67P/C-G on 26 July 2015 at a distance of about 170 km from the comet. Credit: ESA/Rosetta/RPC/IGEP/IC

“We had almost given up on Rosetta finding the diamagnetic cavity, so we were astonished when we eventually found it,” says Charlotte Götz of the Institute for Geophysics and extraterrestrial Physics in Braunschweig, Germany.

Charlotte is the lead author of a new study, published in the journal Astronomy and Astrophysics, presenting the detection of a diamagnetic cavity obtained by RPC-MAG on 26 July. The paper describes one of the most spectacular measurements from almost 700 detections of regions with no magnetic field made by Rosetta at the comet since June 2015.

“We were able to detect the cavity, and on many occasions, because it is much bigger and dynamic than we had expected,” adds Charlotte.

To investigate why the magnetic field-free cavity is so much bigger than predicted, Charlotte and her colleagues looked at measurements performed around the same time by other instruments, such as Rosetta's scientific camera, OSIRIS, and the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA, to verify whether any anomalous changes in the comet's activity could be pushing the cavity away from the nucleus.

While one of the cavity detections, on 29 July, occurred in conjunction with a strong outburst of gas and dust recorded by other instruments on Rosetta, this seems to be an isolated case. Almost all of the other observations of magnetic field-free regions, including the one recorded on 26 July, were not accompanied by any appreciable increase of outgassing.


OSIRIS narrow-angle camera images of Comet 67P/C-G, obtained on 26 July 2015 from a distance of about 170 km. The image at left was taken at 14:43 GMT, the middle image at 15:13 GMT, and the final image at 16:54 GMT. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“To account for such a big cavity in the simulations, we would need the outgassing rate to be 10 times higher than was measured at the comet by ROSINA,” says co-author Karl-Heinz Glassmeier from Technische Universität Braunschweig, Germany, principal investigator of RPC-MAG.

The most likely explanation seems to lie, instead, in the dynamical nature of the cavity boundary.

Boundaries between plasma regions with different properties are often unstable, and small oscillations can arise in the pile-up region of the solar wind, where it encounters the magnetic field-free region, on the Sun-facing side of the comet. If these oscillations propagate and get amplified along the boundary, in the direction opposite the Sun, they could easily cause the cavity to grow in size.

Such a moving instability would also explain why the measurements of magnetic field-free regions are sporadic and mainly span several minutes, with the 26 July one lasting 25 minutes and the longest one, recorded in November, about 40 minutes. The short duration of the detections is not a result of Rosetta crossing the cavity – the spacecraft moves much too slowly with respect to the comet – but of the magnetic field-free regions repeatedly passing through the spacecraft.

“What we are seeing is not the main part of the cavity but the smaller pockets at the cavity boundary, which are occasionally pushed farther away from the nucleus by the waves propagating along the boundary,” adds Charlotte.

Scientists are now busy analysing all the magnetic field-free events recorded by Rosetta, to learn more about the properties of the plasma in the comet environment and its interaction with the solar wind. After perihelion, as the comet moved away from the Sun and its outgassing and dust production rate declined, the spacecraft was able to move closer to the nucleus, and the magnetometer continued detecting magnetic field-free regions for several months, until the latest detection in February 2016.

“Three decades ago, Giotto’s detection was a great success, because it was the first confirmation of the existence of a diamagnetic cavity at a comet,” says Matt Taylor, Rosetta Project Scientist at ESA.

“But that was only one measurement, while now we have seen the cavity at Rosetta’s comet come and go hundreds of times over many months. This is why Rosetta is there, living with the comet and studying it up close.”


First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko,” by C. Götz et al. is published in the journal Astronomy & Astrophysics.

The results will be presented at the 50th ESLAB Symposium “From Giotto to Rosetta”, held 14–18 March in Leiden, the Netherlands.



  • ianw16 says:

    Excellent article. Just to add to "This was the first observation of something that scientists had until then only thought about but had never seen"; there were the AMPTE artificial comet experiments back in 1984-5, which also showed the magnetic field pile-up region, and a diamagnetic cavity. There are a number of papers on those experiments, some, amazingly, still paywalled. This one gives a decent overview and report of one of them:
    This was inspired by the predictions and modelling of Biermann et al, who published a paper on this in 1967: (available on adsabs, but I've given up linking to them; it never works!)

  • Giorgio Bonito says:

    Just wondering... is it possible that boundary's instability is related to the irregular shape of the comet and its rotation?

    • ianw16 says:

      Highly unlikely, I'd say. At the distances from the comet that Rosetta was at, any anisotropic gas release due to the nucleus shape, or areas of higher outgassing (which they dismiss) would be 'evened out'. That is; although it's tempting to think of the gas following the same collimated paths as the visible dust jets, that isn't the case. In the jets what we are seeing is sunlight reflected from the dust. The dust tends to remain reasonably collimated for some distance.
      The gas, on the other hand, is invisible. This will actually spread out in what I can only describe as a mushroom shape. Far more hemispherical. After ~300 km (5-10 mins), it will be quite isotropic, wherever it has come from.

  • Dave says:

    Nice article, but only one recorded instance when the cavity coincided with a strong outburst of gas and dust, looking forward to more detail in the future

    • ianw16 says:

      ".....but only one recorded instance when the cavity coincided with a strong outburst of gas and dust....."

      Which was explained in the article and the paper: "The most likely explanation seems to lie, instead, in the dynamical nature of the cavity boundary.
      Boundaries between plasma regions with different properties are often unstable, and small oscillations can arise in the pile-up region of the solar wind, where it encounters the magnetic field-free region, on the Sun-facing side of the comet. If these oscillations propagate and get amplified along the boundary, in the direction opposite the Sun, they could easily cause the cavity to grow in size."

      The only other observations they have been able to make of these phenomena have been the flyby mission to Halley, and a few similar short lived observations from the AMPTE experiments (sans comet) in 84-85.

      So with over 700 observations to now work with, I am sure the plasma physicists will be able to add to their store of knowledge on a process that Biermann first proposed nearly 50 years ago.

      Yaaay science!

      • Dave says:

        Hi Ian

        're Which was explained in the paper'

        But is wasn't!

        After starting with 'But a comet pouring lots of gas into space obstructs the solar wind.

        The explanation then goes on to say that the model, to account for such a big cavity would need the out gassing rate to be 10 times bigger.
        This is fair enough, but we have seen more than 1 very large eruption, albeit we see the dust not the gas. Of course the instruments may not of been pointing in the right direction or be in the right place to detect other occurrences of the comet eruptions.

        the rest of the analysis is less certain.

        ie 'the most likely explanation seems to lie, instead, in dynamical nature of the cavity boundary'

        'moving instability would also explain why measurement of magnetic fields are sporadic'

        All this is fine and looks like the observations so far point to a different model than dust & gas from the comet obstructing the solar wind., this is also fine,, but its hardly as settled as you state and the further information to come will be very interesting in further understanding the process.


        • ianw16 says:

          Yes, but the point is, we KNOW why the cavity forms, and we KNOW why the IMF piles up, and we KNOW the processes that occur due to it. As I posted above, this was first modelled and predicted by Biermann in 1967, and then confirmed by the AMPTE experiments and the Halley flyby.
          The intimate details of the oscillations have yet to be fully presented, as this is based on the just the early observations, but it will add greatly to our knowledge of the fine detail.
          The FACT that a cavity forms, and has a magnetic field pile up region ahead of it is well documented. And from the AMPTE experiments, we know that the presence of a comet was not necessary to see those things.
          Unfortunately, the Halley and AMPTE findings were restricted to simply flying through these regions.
          Now we can see this finer detail, which will no doubt lead to increased knowledge and an ability to refine the models that are currently used.

      • Kamal says:

        IanW, Dave: I think Dave is right with the caution, we are in deep waters, but I can also see IanW's excitement at getting a first oar at them. One interesting question (which your quote from the paper brings to light) is the connection to solar angle. Dust gets pushed back by radiation pressure, I do not know if there is any such simply-stated mechanisms applying to ions. The hypothesis that oscillations get amplified in an anti-solar direction needs observational sustenance. Could there be some kind of hysteresis which only the most powerful dust jets like the one on 29 July 2015 are able to break through?

        • ianw16 says:

          See my reply to Dave, above. It is not necessarily common knowledge that this process, i.e. diamagnetic cavity, magnetic pile up region, has been seen before. I believe Biermann suggested the experiment back in '61. A very good year, as it happens!
          We know why this happens, and we've seen it before. We don't even need a comet in the middle of all this gas to see it happen.
          See some of the AMPTE papers I've linked to. If not, a simple search on Google Scholar, or normal Google will show what you need to see.
          In other words; no comet required.

  • Kamal says:

    Claudia: Along with some work presented earlier, this is really heading towards an explanation of the ion tail. Do these or other observations show the rise in its energy due to mass loading?

    • Charlotte says:

      Hi Kamal, the ion tail is indeed a very interesting place to look at the magnetic field. Unfortunately, RPC-MAG can only measure the field where Rosetta is located, and due to the trajectory we have not had any direct observations of the magnetic field or the plasma in the tail.

      • ianw16 says:

        Hi Charlotte,
        The interaction between Siding Spring and Mars must have been of huge interest. I read the paper, and it seems that the draped magnetic field had a fairly large effect on Mars' rather weak magnetosphere.

        I believe a trip out to the tail is planned, so maybe, a little late, you'll get some data. Maybe a little too late 🙁

    • Kamal says:

      Charlotte: The magnetic activity very close to the nucleus must somehow link to the beginning of the ion tail, just as we see for the dust activity and the dust tail. The causal chain may be quite complicated and perhaps beyond the range of activities on Rosetta, but it looks like a promising start has been made on the problem. Also looking forward to reading about what was detected during the 1500 km excursion post-perihelion, which tried to detect the bow shock. Even a null result might be worth knowing.

  • Paul Everitt says:

    I would love to see DETAILED HARD SCIENCE generated by the lander. If the lander portion of the mission failed, say so. Otherwise, can you PLEASE point me to the link to the hard science achieved by the lander?

  • originalJohn says:

    Well, quite astounding that the headline to this post should be celebrating the "finding" of a diamagnetic cavity, and a larger one than expected, surrounding this comet nucleus. The investigators seem little interested in the magnetic field itself, which they know is present because their magnetometer measurements are showing the time dependent variation in magnetic field strength at the points of measurement, with the dip in field strength lasting for a maximum of 40 minutes in a single measurement over 700 measurements spread over many months. No justification for calling it a cavity without a full 3 dimensional map of the whole magnetic field of the nucleus. No justification either for calling it diamagnetic. Diamagnetism is a property of a material which causes it to repel an externally applied magnetic field. If this happened and the solar wind carried the external field it would be possible in theory for a cavity to form. But to do this either the nucleus would have to be diamagnetic ( I don't think they would say it is superconducting) or the inner coma would have to be diamagnetic and it seems they have no evidence of that. It would require a 3D magnetic field map of the whole of the inner coma from the nucleus out and as far as I am aware they do not have that, since they have little or no measurements of anything within tens of kilometres of the nucleus.

    No, it seems here that what we have is another case of extreme modelism, as with the original Giotto Halley case which is referred to. Everything said about a diamagnetic cavity measured by Giotto was based on two magnetometer readings, a whole two, one going into the coma and one going out, everything else, including the assertion that a diamagnetic cavity thousands of kilometres in extent existed was derived from interpretation of those two readings using a preconceived model of the interaction of the solar wind with the coma, a huge assumption.
    Let us be clear that to call it a cavity you would have to be certain from the data that it was a 3 dimensional shape fully separated from the surrounding environment by a continuous boundary. Giotto had no evidence of that. They assumed it.

    Similarly in this case it appears that they are relying on a model. They have no magnetometer readings close to the nucleus and probably few if any where the boundary of their cavity is supposed to be. They have 700 scattered readings within the coma where a fluctuating magnetic field has been measured and they have said that this indicates zero magnetic field within the claimed cavity region. It does not of course. It represents a fluctuating magnetic field, and no consideration at all has been given to other possible explanations for this fluctuation. The whole assumption is that diamagnetism has caused a total deflection of the solar wind away from the nucleus, but without emphasising the fact of for periods of up to 40 minutes at a time with the field being re established in between fluctuations, presumably from a fluctuating loss of diamagnetism. So to talk in terms of a magnetic field free bubble at the comet is completely misleading as it was too with Giotto.

    Giotto had some excuse. It was a flyby. Rosetta on the other hand has been with this comet for over a year and has had plenty of opportunity to map the full magnetic field of the comet with reasonable resolution. The discussions and descriptions about magnetic fields however remain vague with no clear distinction made between magnetic fields associated with the Sun and the magnetic field of the comet. The Sun we know has its own magnetic field, a dipolar field centered on the body of the Sun. Then there is a completely separate Sun related field which is never mentioned but which is highly influential and that is the radial magnetic field which surrounds each filament of the solar wind current. Finally there is the magnetic field of the comet itself. We know it exists because this paper reports measurements of its fluctuation but we have little idea of its extent or variation in its intensity, particularly close to the nucleus.

    Finally with the reasonable possibility that the comet coma is a plasma and the certain knowledge that the solar wind is a plasma no consideration at all has been given to the recognised boundary effects that are known to occur between plasmas of different properties, particularly the extreme accelerations that can occur to ions crossing these boundaries. Easy enough to check. Find the boundary then measure the proton and electron energies on either side of it. How interesting too it would be to see a map of the magnetic field surrounding the nucleus and its interaction with the solar wind. To spend more than a year with the comet without gathering this data and still having to rely on models could be seen as a major missed opportunity.
    Ok, I have assumed that what they have measured is the fluctuating strength of the comet's magnetic field and they might say they are talking about the Sun's magnetic field ( we have no idea of its strength at hundreds of millions of kilometres. Would be interesting to know) or the magnetic field of the solar wind filaments. Who knows. Do the investigators know. Have they characterised these fields. Can they distinguish them. It seems not because they only ever refer to a magnetic field, without attributing it.

    • ianw16 says:

      Hopefully Martin can stop laughing long enough to reply to this post!
      And for the umpteenth time, the nucleus has no magnetic field of its own. It is induced by the pile up of the IMF as the solar wind is slowed and then stopped, and forced to flow around the comet.
      As I've said before, you would (and indeed, did) see this whether or not there was a comet within the gas.

      • Martin says:

        At the moment the 50th ESLAB meeting is taking place in Leiden, the Netherlands, titled "From Giotto to Rosetta". Next week I will be in Braunschweig, I might write a detailed answer together with Charlotte then.
        But I would suggest that originaljohn reads Aflvén's 1957 paper on the creation of cometary tails.

        • originalJohn says:

          Martin, in that paper Hannes Alfven was theorising, before the dawn of the space age, about the structure of the comet coma and its interaction with the solar wind. Many of the ideas expressed, particularly relating to magneto hydrodynamics and more particularly to the concept of a "frozen in" magnetic field he withdrew from later on in his career. Since then huge amounts of data which Alfven had no knowledge of have been collected by numerous satellites and probes from the US, Russia, Japan and Europe. The understanding of the interplanetarty environment is now much different and much clearer. It is well established that the solar wind is a plasma, a moving stream of charged particles, an electric current. It is now accepted that the comet coma is a plasma, has the properties of a plasma regardless of how much neutral matter it might contain. The coma is therefore an electrically conducting region and by definition exhibits its own variable magnetic field. The frozen in concept is obsolete and redundant.

          You have no need now to rely on Alfven's theorising Martin. You are there accompanying a comet with a craft loaded with instrumentation. You are in an almost infinitely better position than Alfven ever was. You already know far more than he did. How you interpret it is another matter. You don't believe if Alfven were here now his interpretation would concur with yours do you? He eventually understood well the properties of plasma double layers, the ubiquitous boundaries between different plasma regions. He would have understood too the importance of those properties in the interaction between the coma and the solar wind and the nucleus, bounded by is own double layer, and any impinging ions. He would probably have been interested in those interactions to the exclusion of almost everything else. The ion analysers and magnetometers would have been hot with priority use and the orbiter would have been grazing the surface of the nucleus at every opportunity.

          • ianw16 says:

            Alfven certainly had problems with the 'frozen-in' concept as applied in some circumstances, and I can find papers, or references therein, to those going back to at least 1971. However, he was also well aware of Biermann's 1967 paper that proposed the diamagnetic cavity, or contact discontinuity, as it was called. It was he, after all, who had helped to shape his thinking with the previously linked 1957 paper.

            However, I can find no evidence of Alfven coming out in print to criticise this paper, nor alter the model. In fact, in 1973, along with Asoka Mendis, he published a paper in Nature in which they attempt to model the induced magnetic field:
            From the abstract: "The presence of magnetic fields in the plasma tails of comets is suggested by the rayed structures and helical features observed in them. According to the present picture of the solar wind-comet interaction (see, for example, ref. 2) it is, however, not at all clear how the interplanetary magnetic field could mix with the cometary plasma in the tail because the solar wind and the associated interplanetary field is separated from the cometary plasma by a contact surface (i.e. the cavity boundary; my addition)." Ref. 2 is a link to a 1971 paper by Biermann in which, amongst other things, he restates his model from the 1967 paper with some amendments:

            I can find nothing further by Alfven on comets, but Mendis continued to write a number of papers where his views can only be described as 'mainstream', given the level of knowledge about comets available at the time. After the discovery of a diamagnetic cavity at Halley, he seems to have no problem with it, and models where it should have been at Grigg-Skjellerup, visited by Giotto in 1992:

            In the last paragraph of their 1973 paper, Mendis & Alfven thank W. I. Axford for "a useful discussion". Axford was also what you would call a 'mainstream' thinker when it came to comets. and mentions the "likely existence of a Venus-type magnetosheath", and references Biermann, in the introduction to this paper:
            Mendis was a co-author of that paper.

            So if Alfven was having doubts about his 'frozen-in' concept at least as early as 1971, he certainly had the time and opportunity to voice them with regard to cometary modelling at the time of the 1973 Nature paper. Instead, by citing Biermann, he essentially gives an implicit acceptance of that model.
            I realise that he died in 1995, but he surely would have said something if he thought the plethora of papers on Halley were horribly wrong in their conclusions. Or indeed, a large number that preceded that discovery, many of which would have cited Biermann, or similar models based on Biermann's.
            The fact that he was obviously on good terms with Mendis and Axford, and would have been well aware of the ideas prevalent in cometary modelling, yet chose to say nothing, implies to me that he was perfectly fine with it. Which is just as well, as they were shown to be essentially correct by AMPTE in '84-5, Halley in '86 and now 67P.

          • Martin says:

            "Many of the ideas expressed, particularly relating to magneto hydrodynamics and more particularly to the concept of a "frozen in" magnetic field he withdrew from later on in his career. "

            No, Alfven did not withdrew this, he was just worried about that fact that this new tool of "frozen in" field was used by the community without thinking about the necessities for the field to be "frozen in". Indeed these days we know much more about space plasmas, and lo-and-behold for most interactions that we are looking at the field can be considered to be "frozen in". It took a lot of effort, e.g. by A.T.Y Lui et al. to find breaking of the frozen in condition (see

            "It is well established that the solar wind is a plasma, a moving stream of charged particles, an electric current."
            Again, no. The solar wind is a moving magnetoplasma, with equal amounts of ions and electrons, so it is NOT a current. The solar wind is quasi-neutral, which means that charge fluctuations are limited to the size of the Debye length. Thus equal amounts of electrons and ions are flowing out, and thus the total current one could calculate here is: number-electrons * electron charge + sum(number-ion-species * ion-species charge) = 0. Apparently this seems to be a difficult concept for electric universe proponents. But yes, there can be currents embedde in the solar wind, e.g. the heliospheric current sheet, which is created by the oppositely directed magnetic fields in the two hemispheres of the heliophere (similar to the current sheet in the Earth's magnetotail). This current, however, is perpendicular to the magnetic field (parker spiral).

            "You don't believe if Alfven were here now his interpretation would concur with yours do you?"
            I am sure that Alfven would agree with our current papers. As you say we know much more now that he ever did, and we could show him our data, and he would see that for the interactions that we look at (if you need to keep coming up with frozen-in) this assumption is well founded, because for a plasma the conductivity is high (not infinite, yes we know that) and thus the magnetic diffusion time is very long, much much longer than the time of the interactions that we are looking at, and thus the magnetic field is not diffusing significantly over the interaction time, and therefore assuming frozen-in conditions is okay. And now and then it is not okay as shown in the paper by Lui et al. (there are more papers but Tony is a good colleague and friend of mine).

            "He eventually understood well the properties of plasma double layers, the ubiquitous boundaries between different plasma regions."
            Yes, DLs *could* be created, I know these structures well, because I did my PhD on double layers. But just having a boundary between two different plasmas does not mean that there will be a DL, I guess I should have written that more clearly in the wiki on DLs.

            "He would have understood too the importance of those properties in the interaction between the coma and the solar wind and the nucleus, bounded by is own double layer, and any impinging ions."
            And prithee, what exactly are you saying here? Properties of the DL? Or properties of the interaction? What impinging ions? You just put words on a string here and basically are saying nothing. If you think we should look at something, then you should clearly state what you think we should look at and not some generalities. Apart from a DL, what you claim above is exactly what I have done in my latest publication (

            "The ion analysers and magnetometers would have been hot with priority use and the orbiter would have been grazing the surface of the nucleus at every opportunity."
            The RPC instruments are always switched on, most of the mission in burst mode, so we get enormous amounts of data of the plasma physics taking place near the comet. "grazing the surface of the comet" would not be a good idea, for one from a flight dynamics point of view and also because the global interaction of the solar wind with the outgassing comet would even be studied better with long flybys, far-close-far, so we get a cut through the coma. Unfortunately, RPC is not the only instrument on the spacecraft, so compromises will have to be made. The RPC data is excellent for the study of all kinds of plasma physical phenomena and we are working hard on it.

            However, if you think you can do better, then the data are part by part uploaded to ESA's Planetary Science Archive ( I am sure we will be seeing your paper appear soon here on the blog.

          • ianw16 says:

            Actually, I did find a bit more from Alfven on comets. They were in works of a more general nature. Some of the highlights, which are contrary to the electric comet idea, are as follows:

            >"Lai proposed...........This view might seem to be contraindicated by the fact that roughly matching amounts of oxygen are deduced from observed OH emission. Most workers would take this as a support for the concept of water molecules as the source for both dissociated hydrogen and hydroxyl radicals."
            From: Arrhenius, Alfven, Fitzgerald; Asteroid and Comet Exploration, 1973

            >"As Venus, like the comets, has no appreciable intrinsic magnetic field, the solar wind interaction with her is likely to be essentially the same."
            And then goes on to describe a model by Gringauz that uses the 'frozen-in' concept.

            >"The described current system gives a magnetic field which can be depicted as solar wind field lines wrapped up by the obstacle which it meets."

            >"In the present epoch magnetohydrodynamic effects are not important for the motion of the big celestial bodies (planets, satellites and also asteroids) but are still influencing the evolution of comets and, of course, the state of the magnetospheres (including the heliosphere) and ionospheres."

            All from: Alfven, Cosmic Plasma, 1981.

            And then, from 1987, there is this table, in which it is obvious that he considers the 'frozen-in' concept to be legitimate at comets:

            He doesn't consider it legitimate in the tail of the comet, but Martin would know far more than I about how that is modelled. Perfectly fine for the coma, though.

            EU proponents might help themselves if they read what Alfven ACTUALLY said, rather than cherry picking parts of it, and repeating it as a mantra every time inconvenient evidence shows up. Took me less than a day to find that.

      • ianw16 says:

        Further to that, from the paper linked to in this article:
        "This pile-up then abruptly stops at the contact surface where the magnetic field strength drops to zero ***because the comet does not have a magnetic field of its own***
        (Auster et al. 2015).
        My emphasis.

        Auster et al refers to:
        ""Based on magnetic field measurements during the *descent* and subsequent multiple touchdown of the Rosetta lander Philae on the comet 67P/Churyumov-Gerasimenko, we show that **no global magnetic field** was detected within the limitations of analysis."

        Again, my emphasis.

        • originalJohn says:

          "Frozen in " is not a helpful description with respect to magnetic fields in plasmas Martin whether Alfven initiated it or not, and who really knows what he would think of it now, with present day knowledge. For a start it is a term applicable to solids hence the transition from liquid to solid is called freezing. But a fixed field associated with a plasma makes little sense. A plasma is an array of ions and in an electric field ions in motion. The ions have no magnetic field. They have a static charge. When they are in motion in an electric field they establish magnetic fields associated with each current carrying plasma filament. Without the motion there is no field. The frozen in concept is therefore a description of the consisent flow of ions in the plasma. But if the flow stops there is no magnetic field. It is not frozen in because there is nothing to generate the magnetic field without current flow.

          To continue this contradiction you deny that the solar wind is a moving stream of charged particles but is what you describe as a moving magneto plasma, a "magnetised" plasma. So how do the ions of the plasma manifest the magnetism in your view, if not through their motion.

          And you go on to assert that the solar wind has equal amounts of ions and electrons and is not therefore a current. Similar to the ions and electrons flowing in an electrolyte bath which are not a current eh. And you cite the Debye length as a limiting factor in charge fluctuation. Yes because ions and electrons further apart from each other than the Debye length are screened and cannot neutralise each other. But this does not stop you presenting a relationship that shows that the electron charge and the positive ion charge neutralise each other resulting in a zero current flow. The calculation is invalid Martin. Neutralisation of screened charges is a difficult concept for anyone, EU proponent of otherwise.

          In addition Martin you are assuming the electrons and positive ion are travelling in the same direction. I don't think you have any evidence of this. It is much more likely that those electrons and ions are flowing in opposite directions in the electric field established between the electron rich interstellar medium and the Sun. Hence the magnetic field of the solar wind.

          Then you talk about the heliospheric current sheet as a characteristic of the solar wind but somehow separate from the ion flow in the solar wind. So you believe the heliospheric current sheet right out to the boundary of the helioshere (for that is where the hypothetical Parker spiral extends to) is caused by the direction of the Sun's dipolar magnetic field being in opposite directions in the northern and southern hemispheres, with the prolific ( 4-700km/sec, 2-10 ions /cm cubed) ion flow of the solar wind making no contribution. Incidentally an ion flow which is known to accelerate with distance from the Sun. I wonder what you believe causes the acceleration.

          With regard to the extract from my comment about double layers that you highlight I was of course referring to the properties of double layers but the double layer is caused by the interaction so no problem there. And the impinging ions would be any ions in either plasma arriving at the boundary formed by the double layer. So in the case of the coma and the solar wind, probably mainly negative ions from the coma and solar wind protons. And yes the words are on a string because that is what a sentence is Martin.

          The properties of the double layer I am insinuating are the extreme accelerations the double layer electric field can impart to ions crossing the boundary, in opposite directions depending on charge. The opposite of a barrier or a bow shock. Of course a barrier effect could be interpreted in ions being reversed in direction within the double layer. For example, at the coma boundary, electrons arriving from outside the coma or positive ions from within the coma.

          You say Martin " if you think we should look at something..." but of course it is not my place to think what you should look at. To a plasma physicist it should be self evident from the theory. It is obvious from the above content of this reply to you that the boundary between the coma and the solar wind deserves attention with respect to the possible presence of a double layer and positive ion and electron energies on either side of it.
          However the more important potential double layer region would be at the boundary between the comet nucleus and the coma, because this would control the movement of ions towards and away from the nucleus surface if it existed. This would be difficult to examine because if such a double layer was there it is likely that it would be only a few metres or even centimetres in extent (thickness). Nevertheless it would be highly significant in the behaviour of the comet.

          As a plasma specialist Martin these double layer questions should be right up your street and any clarification would be a major contribution. From what you have said you should be a somewhat rare exception to this statement by Alfven:
          "Students using astrophysical textbooks remain essentially ignorant of even the existence of plasma concepts, despite the fact that some of them have been known for half a century. The conclusion is that astrophysics is too important to be left in the hands of astrophysicists who have gotten their main knowledge from these textbooks. Earthbound and space telescope data must be treated by scientists who are familiar with laboratory and magnetospheric physics and circuit theory, and of course with modern plasma theory."

          • ianw16 says:

            I suspect Martin has better things to do than carry on this pointless dialogue. However, the irony of a plasma physicist, who studied in Alfvens lab, being lectured to by somebody who believes in electric comets; non-existent, undetectable discharges; impossible and undetectable combustion woo; that Keplerian orbital dynamics means a comet might take less time to travel from perihelion to aphelion, than aphelion to perihelion; and that the solar wind flux can increase by many orders of magnitude between Rosetta and the comet, will not be lost on either him or numerous others. Particularly given that 20 months alongside this comet has still provided zero evidence in favour of this mythology inspired nonsense.

          • Gerald says:

            Originaljohn: " are assuming the electrons and positive ion are travelling in the same direction. I don't think you have any evidence of this. It is much more likely that those electrons and ions are flowing in opposite directions in the electric field established between the electron rich interstellar medium and the Sun."

            Your thinking is wrong, Originaljohn.

    • Martin says:

      "few if any near the boundary"?????
      the magnetic field is measured with a 22 Hz cadence, the boundaries are well resolved.

    • Robin Sherman says:

      I'll take a mathematical model based on experimental data over a "reasonable possibility" based on no evidence whatsoever I think.

    • Harvey says:

      Retry, capta issue.
      originaljohn. You would save yourself a lot of embarrassment if you took the trouble to do a little reading before posting these diatribes.

      A quick search shows 62 papers involving comet and diamagnetic cavity, stretching back to 1983; there will be more, because there are synonyms. So, if I was an onlooker with no knowledge, I have a choice; Believe you, and that the hundreds of qualified people involved in preparing & refereeing this work are talking nonsense; or the reverse; take your pick.

      We know the nucleus has no magnetic field from direct measurement by Philae & Rosetta; that's all that matters in that regard.

      We also know the coma is a plasma; 'reasonable possibility'? - complete certainty from direct measurement & obvious physics, photoionisation (which for some reason you don't believe in, but it provides the plasma you crave!)

      Diamagnetism is indeed a property of a material; and conducting materials - which include plasmas - are very well know to behave as pseudo diamagnetic materials; pseudo because it doesn't apply to time invariant conditions; its an approximation, like most things, in other circumstances. All you had to do was look at wiki on diamagnetism
      "All conductors exhibit an effective diamagnetism when they experience a changing magnetic field. The Lorentz force on electrons causes them to circulate around forming eddy currents. The eddy currents then produce an induced magnetic field opposite the applied field, resisting the conductor's motion."

      There are innumerable documents on the diamagnetic effect in plasmas on the web; 193,000 hits on diamagnetic plus plasma! ; perhaps you could have reviewed a few. (Much of it is magnetic confinement fusion related as well as astrophysical.) Most of course 'vanish in a cloud of vector calculus; pretty quickly, because they have to. If you don't understand vector calculus you are on a hiding to nothing regarding the detail. But you will see that the concept of a plasma being diamagnetic is exceedingly well established in many contexts.

      Superconductors are a complete red herring;; type I superconductors are 'perfect' diamagnets below the critical field; so what.

      Rosetta can't be in two places at once; its path is constrained by flight dynamics, craft safety, thermal & solar power input, & the demands of multiple instruments. We get what we get - a huge amount of fantastic data, but no complete 3D high resolution instantaneous map of the magnetic field! I wish. From that data we construct models consistent with what we have, & refine them when there is more data or a discrepancy arises. If you can provide a *better* model which improves on the current one *and sticks to the laws of physics* do please publish it. I won't hold my breath however.

      With no model we just have a heap of unintelligible data which tells us very little.

      You seem to think a measured magnetic field comes 'labelled with its origin'. It does not; its simply a vector field at that point, which of itself says nothing about why it is there. The magnetometer has three output, Bx, By, Bz - there is no signal saying 'this field comes from the sun' or whatever. In most cases the net field is a complex interaction of several fields & the effects of a mobile plasma - not a crude linear superposition of those fields in isolation.. Any such 'labelling' essentially comes from the very models you despise.
      You cannot simply 'superimpose' 'the sun's field' & 'the comets field'; the comet nucleus has no field; its the interaction of the expanding, moving photoionised plasma with the interplanetary field that produces the net Bx,y,z.

      As we have seen many times, you simply attack things you don't understand. Please sign up for 'Physics 101' somewhere; it would save you a lot of self embarrassment; and deprive the rest of us of a mixed source of irritation & amusement in roughly equal measure.
      (Oh, please include the law of electrostatic induction in your studies, then go back & read your explanation of how the 'discharges' work.)

      • originalJohn says:

        Ok Harvey , point by point. Happy to reply to you as you made some effort to limit personal insinuations and stick to the text of the argument.

        First off I am not the least bit embarrassed and I reiterate everything I said. It is not a diatribe. It is a reasoned, fact based criticism and i know very well what I am talking about.
        It is of course not at all surprising that there is a plethora of papers on comets and diamagnetic cavities. It is the dogma. Once it becomes the consensus it conditions the interpretation of the data. None of them prove that the diamagnetic cavity exists any more than the recent rosetta one we are discussing does. The qualifications, the preparations and the refereeing all take place within the constraints of the dogma.

        We do not know the nucleus has no magnetic field. We know that is not magnetic ie it is not composed of a ferromagnetic or paramagnetic material. There is however, as you know, another highly significant way a magnetic field can be developed. From the flow of an electric current.
        Such a field has indeed been reported by the ESA with this comet, at some distance from the nucleus. I remember you yourself commenting on the weakness of it it in this blog, even though every expectation was that it would be relatively weak as the nucleus is so relatively small. Have you forgotten. Has everybody forgotten. Perhaps we need a statement from Herr Glassmeier.

        "Reasonable possibility" on my part was a sarcasm which you failed to grasp. Of course I know the coma is a plasma, from theoretical considerations. I also know that photoionisation is the only possible source of plasma within the dogma. Embarrassing really because it is an acknowledged weak effect which ESA themselves have stated can typically account for ionisation of only 1 ppm of the gas in the coma. And I also know that there are much more prolific plasma sources, outside the dogma.
        And lets get one thing clear. This is not an emotive issue for me. I don't crave a plasma explanation. I am simply objectively interested in the truth and it amuses me how the dogmatists bend over backwards to avoid it. Futile I am afraid.

        I understand diamagnetism well enough Harvey. i am no "expert" on it but I suspect neither are you. I have absolutely no argument with the possibility of diamagnetism. Thousands of publications on it mean nothing as proof of whether or not a diamagnetic cavity exists around this comet or any other. More or less a waste of time therefore in studying any of those papers. What is simply needed is a presentation of Rosetta data which demonstrates the existence of a diamagnetic cavity or not. The Rosetta investigators have a unique opportunity. They do not have to model it or assume it. Decide on the definition of a cavity, measure the magnetic field strength at numerous points around the nucleus within the expected cavity region ( in whatever way they judge to be the most efficient and productive). If the field strength within the whole of a certain region is uniformly zero that would be the beginnings of an argument. All that has been measured so far is a magnetic field somewhere that for short periods ( of minutes) fluctuates to zero ( or very low strength) and returns to its former level. Is that sufficient evidence of diamagnetism or a cavity for you Harvey. We have no measurements of solar wind current flow near the nucleus but already the spurious assumption that it is zero, repelled by the hoped for diamagnetism.

        Now of course I understand the problems of distinguishing magnetic fields and i also know there are ways of addressing those problems. Collect enough data around the nucleus and perhaps characteristics such as specific field strengths might begin to emerge, to allow the characterisation of regions of the field. Measure the direction of the field and perhaps indications of its source might emerge. Assess from the data the degree of uniformity or the variation of different regions of the field. Combine the field strengths and directional data over a large enough sample and plot them in three dimensions using the wonders of computer technology and magically separate fields and their interactions may even appear. There has been plenty of time on this mission to do this, to make the dots and connect them.

        Finally I don't attack at all. I criticise things I DO understand which are flawed in numerous ways. If there were no flaws I would have no comment, except perhaps praise. I am ok on physics 101 thanks and nothing I have said about the issue we are discussing contradicts physics in any way. We are not anyway discussing physics. We are discussing interpretation of results and whether a particular interpretation is valid, or whether a particular approach to interpretation is valid.

        There IS one course i would have agreed that I need had you suggested it and that would be dogmatism 101. But that is one I gladly forgo.
        I am fine that you are irritated and the fact that you are amused by my attitude is interesting. You will be pleased to know I am as amused by your attitude. Who will have the last laugh I wonder.
        (I am well versed in the theory of electrostatic induction too and it was part of my explanation to you previously about how comets acquire charge around the aphelion region of their orbit, which you did not accept.)

        • Harvey says:

          Probably little point in dragging this out.

          No, I didnt really think you would be embarrassed. That needs understanding of the fact that one is badly wrong - yes, its happened to me 🙂

          You apply, as I’ve previously remarked, no ‘quality control’ to your understanding.
          The rest of us pass exams, submit theses to be grilled by externals, teach students (very harsh critics of poorly understood taught material!), set exams with model answers which must pass internal & external review, submit papers to referees, are reviewed by membership & Fellowship committees of learned societies, & so on, in a continual process.
          Now of course it might all be a self-reinforcing house of cards; after all the scientific world once believed in phlogiston 
          But there is a final test; engineering application.
          The results of my calculations are often reduced
          to a physical device which must work; Consultancy customers are perhaps the strongest test of all.
          Has a customer ever paid you large sums of money for your physics calculations? Have you ever fired up something you designed, mechanical & optical systems, electronics to test if it does what it’s supposed to? Indeed Rosetta itself depends totally on the underlying physics & resultant engineering being correct.

          Your last paragraph is a perfect example. You *think* you are ‘well versed in the theory of electrostatic induction’.
          How do you know you are?
          In fact, by its very nature it is incapable of providing the charge & power source to drive the fictional discharges.
          All it does is move charge around in the body concerned (in a rather different way for insulators & conductors.) It cannot act as a charge/power source to drive some huge external discharge circuit.

          As Matt has kindly told us, *all* (his word, my emphasis) jets are illuminated by scattered light off dust.
          The magnetic fields observed are completely inconsistent with any large scale discharges , diamagnetic cavity or not.
          Discharges never were a viable explanation anyway, but that’s a complete observational killer for the ‘discharges’ no complex models needed. Fatal.
          (As previously, a caveat; I would not rule out small scale local discharges, ‘lightening type’ effects, perhaps driven by triboelectricity for example, like those seen in volcanic clouds (very different conditions); but none have been observed that I know of.)

        • ianw16 says:

          And yet again you conveniently ignore the fact that they measured the magnetic field, and found it to be zero. This was in addition to finding that the nucleus itself is non-magnetised. I'll repeat the links I posted earlier:
          From the paper referenced in this article: "
          "This pile-up then abruptly stops at the contact surface where the magnetic field strength drops to zero ***because the comet does not have a magnetic field of its own***
          (Auster et al. 2015).
          My emphasis.

          Auster et al refers to:

          ""*****Based on magnetic field measurements during the descent***** and subsequent multiple touchdown of the Rosetta lander Philae on the comet 67P/Churyumov-Gerasimenko, we show that ****no global magnetic field**** was detected within the limitations of analysis."

          Again, my emphasis.

          The part where they say "magnetic FIELD measurements, made DURING THE DESCENT, ought to be a clue.

          And you still haven't explained how a diamagnetic cavity and magnetic pile up region formed during the AMPTE experiments. Just gas, no comet.

        • ianw16 says:

          According to you: " We have no measurements of solar wind current flow near the nucleus but already the spurious assumption that it is zero, repelled by the hoped for diamagnetism."

          According to the real scientists making the observations: "Up until early May, a weak signal at solar wind energies
          appeared intermittently when Rosetta was within the outer region. Since early May, the only time a solar wind signal has been observed was during the excursion, well outside the boundary and just prior to the impact of a CME at CG."

          ESLAB abstracts, p. 64-65.

        • Martin says:

          Quote:" We are not anyway discussing physics. We are discussing interpretation of results and whether a particular interpretation is valid, or whether a particular approach to interpretation is valid."

          Well, that made my day. Apparently originaljohn has no idea what it means to analyse and interpret data, if he thinks that that is "not physics". I would love to see him do analysis without physics. His comments about "plotting the data" and (maybe) "magically separate fields and their interactions may even appear", how are we to know if these are "interactions" when we are not doing any physics?
          And by the way, the plasma density (RPC) is indeed in the range of 1 ppm compared to the neutral density (COPS).

          • Harvey says:

            Kind of you to join in.
            I fear you have joined a rather uphill struggle 🙂

            However can I take the opportunity to congratulate the team, a superb mission, I've enjoyed reading the papers so far and look forward to more.

          • originalJohn says:

            No Martin, you made your own day. The interpretation is not the physics. It is the application of the physics. The
            same physics can result in different interpretations. You can say there is evidence of a cavity, I can say there is not. You can say the low readings result from diamagnetism, I can say there are other field distribution possibilities. Get it Martin, same physics, different interpretations.

            So I never implied there was no physics in it. I said we were not arguing about the physics. Of course you need the physics to plot
            the data but there is indeed a possibility that if you plot the data different fields will manifest themselves in the plot. Have you never done anything like that Martin.

            And can I get this clear. You are announcing that the average plasma density throughout the coma is 1 ppm?

  • logan says:

    Hi Claudia. Would find nice if ESA astro/cosmo nauts also wear a patch with smaller flags of all the Community..

  • ianw16 says:

    Totally unrelated to any of the above; however, just before the ESLAB conference closed, it was announced that, for the first time, CO2 ice has been detected on the surface of a comet!

  • originalJohn says:

    Claudia, may I ask, is the Rosetta orbiter carrying any radiation pyrometers?

  • Matt Taylor says:

    Although directed at Claudia, i can answer - The instruments onboard Rosetta are conveniently listed and described here.
    Among them are a number of instruments that are able to determine temperatures, such as VIRTIS and MIRO, by examining e-m radiation emission.

    • originalJohn says:

      Thanks Matt. No dedicated pyrometers then.

      What is the temperature sensitivity of VIRTIS and MIRO.

      • Harvey says:

        The modern definition of 'pyrometer' provided in wiki would in fact include VIRTIS and MIRO, although the word it most commonly used for instruments measuring high temperatures.
        These instruments have of course far wider capability than a simple furnace-type 'pyrometer'.

        There is no simple single number answer to 'temperature sensitivity', it will depend on range, spatial resolution, target temperature, etc. There is also a difference between absolute accuracy and MRTD, minimum resolvable temperature difference and NETD, noise equivalent temperature difference.

        Plenty of data on VIRTIS etc is available, see for example

      • Booth says:


        Still fishing I see!

        Perhaps you missed my recent post regarding VIRTIS temperature maps. As noted near the end of said post, your "basal" jet temperatures have been measured and they are not at all consistent with electrical discharges or combustion chemistry. A measured maximum "surface" temperature of 220 K at Hapi, the most active region on the comet during rendezvous and initial mapping, is much more consistent with sublimation processes. VIRTIS surface temperatures continue to be consistent with sublimation processes to this day!

        However, let's get back to your fishing adventure. VIRTIS is the Visible and Infrared Thermal Imaging Spectrometer. The instrument's name clearly indicates that it is designed to measure temperature. A pyrometer measures thermal infrared radiation. VIRTIS-M measures thermal infrared radiation in the 4000 to 5000 nm range. VIRTIS-M is a pyrometer! Asked and answered!

        When you ask about temperature sensitivity, I assume you understand basic thermodynamics and instrumentation engineering? No? It is clear to me that you have, either never read any of the specification documents I have provided links to, or you do not comprehend the basic science and engineering being discussed.

        Instrument sensitivity is not a trivial matter. For example, the following list is a subset of factors used to define the "sensitivity" of the VIRTIS-M IR channel:
        Spectral resolution : 10 nm
        Spatial resolution : 0.25 mrad
        Radiometric resolution (SNR) : greater than 100
        Relative radiometric calibration : less than 1%
        Mean dark current : less than 2 fA at 90 K

        So, what particular aspect of sensitivity are you looking for? Given that this latest tangent is an obvious fishing expedition, I assume you actually want the instrument's dynamic temperature range. In other words, can VIRTIS measure high temperatures? Simply stated ... YES!

        Again, I refer you to my VIRTIS temperature map post (linked above). If VIRTIS was used to image the Sun, it would be possible to "integrate" the hyperspectral data to recover a temperature of ~5800 K. Not trivial, but doable. You will also recall that VIRTIS was an important instrument on the Venus Express mission. To provide meaningful science in that environment, the scientists and engineers calibrated the instrument at 770 K, well above the VIRA reference. On the off chance that VIRTIS were to record an anomalously high temperature event at 67P, new calibration data could be generated that would provide a reasonable measure of the temperature. Clear?

        No matter how hard you try, originaljohn, there are no temperature anomalies! There are no unexpected temperature spikes! All hyperspectral data falls within the range expected of a cold celestial body, covered by a nonvolatile organic-rich mantle, sublimating volatile ices. The temperature maximums reported by VIRTIS are correct! Interpretation is not required! Basal jet temperatures of 220 K were measured at Hapi. This maximum is consistent with a cold cometary body sublimating volatile ices. The science is correct! There is nothing in the EU/EC model that fits the data being reported by any of the instruments onboard Rosetta!

        Originaljohn, if there were combustion or discharge activities at 67P, VIRTIS-M would be able to record the event. No electrical discharge or combustion temperatures have ever been recorded!!!

        As I close this out, I must sadly confess that I don't know MIRO as well as I should. Thus, I cannot and will not comment on what the instrument is capable of. However, in the next several days, I will take the time to review the MIRO specifications to see what information may be gleaned. I suspect it will not bode well for the EU/EC! Now, rather than asking others to do all the heavy lifting, Sir, you might want to be proactive and do your own research! If you open your eyes, you might actually learn something!

        • originalJohn says:

          Ok Booth, you are right. I did miss your recent post about the VIRTIS temperature maps. I have now read it and I would like to thank you for the time and effort you put into it. Very interesting and educational for me as unlike you I am not a specialist in the field. However if its intention was to answer my simple question about VIRTIS M and the Sun it failed because in your eagerness to educate me you didn't understand the question. Of course I appreciate that any device can be precalibrated to detect different wavelengths and that invisible radiation has no real colour. I understand also that it would be perfectly reasonable to set your detection device up for the range of temperatures you were expecting, to optimise the performance. However we were previously discussing the significance of a white (no colour not spectral white) indication in your set up for the observations of this comet. My question was If with that same set up you were to aim VIRTIS at the Sun what colour would you record. I asked the question because I believe the answer would be white. You on the other hand replied that you could make the Sun whatever colour you like, missing the point.

          And a camera Booth. Your definition appears to be a device that collects and records visible radiation. Well let me assure you Booth that there are devices in crystallography for example, perhaps not an expertise of yours, that collect and record X rays. Such devices are universally known as cameras. So my definition would be a device that collects and records EM radiation of any wavelength, which is why I call VIRTIS M a camera.

          Now, you talk about fishing Booth in a derogatory sense as though I am trying to catch you out. You and your friends. I have no EU agenda as you believe. I have however come to the conclusion over years that the current "official" explanation of comets is a delusion, a hypothesis represented as fact, so I am keen to assess all alternative explanations and make my own judgement about which if any is correct.

          The electrical explanation is highly plausible and the combustion explanation is my own idea derived from a consideration of what is on the surface of the nucleus, what is in the coma, and the appearance of the jets. Also because the commonest chemical way to generate water is by combustion of hydrocarbons in oxygen and we are looking at a body which is a water generator which is coated in hydrocarbons and which has now been found to be surrounded by a cloud of oxygen. Not much missing there Booth. Just the initiation energy, the match to light the flame, and potentially that is the energy of the solar protons. Whereas you prefer to stick with the ice sublimation hypothesis as fact despite the depressing for you lack of evidence that the nucleus contains ice. Despite also the fact that you have been fishing for ice since the start of this mission, and before in fact with the ice screws and harpoons.

          And now you are claiming that you have all the temperature evidence you need to be absolutely certain there is no heat being generated by any reaction at the comet nucleus surface. Really Booth. Are you satisfied. If so you are easily satisfied. You talk now about "basal" jet temperatures, in fact to me "your basal jet temperatures" as though I own the concept, when it is of equal significance to everybody. And what is basal with respect to a jet. Is it a new technical jet term you have coined. I know what basal means but for a gas and dust jet it is vague. Lets assume that you are referring to the lowest plane in the jet with respect to the surface of the nucleus. This is of course the surface of the nucleus, plainly and simply. So you are referring to the surface temperature of the nucleus. i don't know if you have data for the surface temperature within the boundary of the jet ( please tell me if you do) or whether you are talking about near the jet or even not particularly near the jet. But in any event I am sure you would agree that the lowest temperature in a combustion flame for example is at the very lowest point in the flame.
          If we consider a methane burner it is at the point where the gas exits the burner ports just before it ignites. Need I say that that temperature is far from typical of the maximum or even the average flame temperature. In some burner designs you could touch that burner port surface without harm.

          Yet returning to the comet situation that is the information you have and it satisfies you as representing the maximum possible jet temperature. No need for further investigation in your view. Well Booth, sorry to disappoint you but that is not science.

          SImilarly Booth if you place you finger millimetres away from a stable flame you feel little heat compared to the true flame temperature. So the temperature of a combustion flame is extremely localised. And for that reason in ice cold low density surroundings the flame temperature could be easily missed if it was not expected. Measurements made outside the visible jets therefore have no significance, particularly measurements made kilometres out in the coma. The comet is a tiny source compared to the size of the coma and any heat expanding into the coma would soon be diluted. Or would it. Do you have a temperature map of the coma.

          So what is the point of all this. Well quite simply Booth if you don't look for the temperature within the jets you are likely to miss it. That is the situation you are in now, potentially, and unfortunately it satisfies you.

          Finally let me assure you Booth that my eyes are wide open and I have learned a great deal in my time, and continue to do so.

          • ianw16 says:

            Combustion is irrelevant, as it is impossible at a comet. Why look for things that a) would have already shown up, and b) are scientifically impossible?
            For anybody that can show how combustion works at a comet, get to it! A first class return ticket to Stockholm awaits. Otherwise, it remains in the file marked "impossible pseudoscience."
            You don't need to wait for a journal to peer review it; write it up, post it somewhere, and provide a link. I, for one, would be happy to repost it on any number of physics forums, or contact authors who have written papers on combustion. That way you could get some pre-peer review comments, and save the embarrassment of subjecting it to actual peer review.
            Be sure to include all the equations regarding O2 content, partial pressure etc.

  • Harvey says:

    Try without the less than and greater than symbols which seem to cause trouble!


    Behind the 'pyrometer' query lies a discussion from a while back. At times several of us here have felt like King Canute trying to hold back a tide of, bluntly, nonsense; with little more success, but I hope more reason! So your intervention as a source of direct data is very welcome; but I would entirely understand if you can't find time for more.

    Some time ago it was agreed by the proponents of these 'theories' that temperature measurements of the comet and jets would be a clear decider; they predict (no numerical theory!) very high values. So there are three questions I think which would unequivocally 'nail it'. In reality of course many other things do that already; but this is easy to understand and agreed as a decider.

    So if you and the team can be bothered, no hard feelings if not:

    - what, roughly, is the highest temperature *ever observed anywhere on the comet, at any time*, by VIRTIS and MIRO? Obviously I'm not asking them to trawl through all the images - I'm sure the team would know the maximum of the highest scale used so to speak.
    Ie T less than xxxK always, everywhere.

    - 'for the avoidance of doubt' , were the VIRTIS data *ever* saturated in the sense that it simply reported a lower bound to the temperature, T greater than yyyK? If so, some comment on why would be great.
    Obviously not single pixel transient cosmic ray upsets, actual data.

    - more interestingly, has it been possible to get any data on the temperature of the jets themselves. I can see this is potentially instrumentally problematic. When the jets were very active, Rosetta was at a large distance and MIRO may not have had the spatial resolution. VIRTIS on the other hand may have problems if the dust cloud is not optically thick in the 2-5um region, in getting a signal or interpreting it. I appreciate that 'temperature' for the gas phase is not a simple concept (Tr, Tv, Tt...if T is valid at all.) But is there any 'jet temperature' data, is it possible to quote in a simplistic way a maximum seen?

    I sincerely hope this is the last time I bother you. If those results are what the 'conventional' group here expect, but don't convince the 'others', I give up!

  • Harvey says:

    Note. Don't include greater than or less than signs in posts, they mess it up!!!

    • Matt Taylor says:

      quickly, i recall that VIRTIS saw surface temperatures of above 50 C during perihelion, possibly higher, but that these plunged immediately when going into the dark. So not that high and VERY well explained by solar illumination and reflection. I cant recall values off the top of my head at the moment.
      Coma is a more difficult target. Most of what you ask will come from the analysis of the data in the next year or so. The teams are working very hard to operate the instruments at the moment. This year is more complicated than 2014 in terms of science and spacecraft operations. many thanks to all for your interest in Rosetta and science!

      • ianw16 says:

        Thanks for that, Matt. The temperature you recall would be in reasonable accord with those found at Tempel 1 and Hartley 2:

        High temp of ~ 325K at Tempel 1, measured at 1.51 AU, and around 360K at Hartley 2, measured at 1.06 AU.

        If Harvey is reading this, perhaps he might give a short explanation of how relevant ro-vibrational and spin temperatures are, with regards to the sublimated gas. I've seen plenty of these reported for various comets, including this one (I think). Would like to find out how relevant it is before I dig through a shed load of papers, though.

        • Harvey says:

          In many cases you can assign three temperatures to a gas; a translational, rotational and vibrational temperature. In some cases the different vibrational modes can have different temperatures.
          In high pressure environments all of them will be equal, and the distributions Boltzmann.
          But in low pressure environments it can be very different.

          On average it typically only takes one or two collisions for Tt and Tr to exchange energy. It varies a bit with species, but it's always fast. So translation and rotation rapidly equilibrate.
          But the number of collisions for vibrations to equilibrate is far, far higher hundreds, thousands even low tens of thousands of collisions. So Tv can differ wildly from Tt and Tr - in a molecular discharge it's perfectly possible to have the former in the hundreds and the latter in the thousands of K. This depends heavily on species (the Slater Lambert law for example if you want to look it up.) resonance effects can be strong, eg in an N2-CO2 collision.

          MIRO measures Tt of water etc by measuring the Doppler width. That is explicitly Tt. Tr can be derived from the rotational line intensity profiles; I don't think Rosetta has anything with the spectroscopic resolution to do that. Although if MIRO monitors more than one line in the same molecule (I forget if it does) it can get it. Tv can also be derived spectroscopically, and generally requires lower resolution. Again MIRO might if it measures two lines in different vibrational bands of the same gas.

          Should we care?
          If you have a density profile, you know the collision rates. So you could model the way the temperatures vary in the out flowing gas. It would add a level of detail and consistency checking to your model, rather than perhaps being a big gee wiz thing in itself.
          It's possible all three temperatures remain validly a temperature and equal in the whole flow field, and the distinction is completely unimportant! But one would need to be sure that is true before bandying a single 'temperature' about.

          • Harvey says:

            Sorry one further clarification. The collisions not only say equilibrate Tt and Tr, but makes the distribution thermal. Once you have had a few T or R changing collisions the distributions will be validly thermal. So T and R easily thermalise, V can take much longer.

          • Harvey says:

            A final PPS - big subject.

            It's important to recall that whilst there are strict selection rules on total angular momentum (J) change for absorption of a photon, typically delta J=+/- 1 or zero (sometimes zero), there are *NO* selection rules for delta J in a collision. So in, for example, CO2 at 300K, molecules are spread from J=0 to say J=40 or so, relatively few are J greater than 40. So you might think it needs quite a lot of collisions to thermalise that. But its less than you expect, because it can go fro 0 to 30 in one go, it doesnt have to go 'stepwise'.
            (Fo complicated reasons, in C16O2, every other state is missing, because 16O has zero nuclear spin, but lets not get into that 🙂 )

            But I would re stress, it quite possible *none of this matters*, that everything has a perfectly good, single temperature. But that needs to be confirmed.

          • ianw16 says:

            Thanks for the explanation.
            The only paper I could find re 67P, given a quick search, was:
            In that they measure a gas kinetic temperature of 132 K. This paper also has the advantage of showing the nucleus temperature contemporaneous with the gas emission.
            From Halley, there is:
            And also: (usual proviso re adsabs links!)
            These show gas and ion temperatures within the cavity to be very low, from ~ 160 K to an upper limit of ~ 500 K.

  • Harvey says:

    Many thanks Matt.
    It will be interesting to see full results in due course.

    So, can 'discharge theories' finally be interred at the cross roads with a stake through their heart & quanties of garlic strewn around? 🙂

    We dont need the fact that basic physics ruled out 'discharge theories'.
    We have **three**, completely independent sets of direct observations that rule them out; any one would suffice.
    The magnetic fields; the lack of intrinsic emission; and surface temperatures entirely consistent with solar heating. The last was previously agreed as a decider.

    In any rational world that's the end of that.
    I stress, in any *rational* world!

    • originalJohn says:

      Well thank you Harvey for your considered comments in the discussion about temperature and pyrometry. It is a pity however that you continue to feel the need to interject personal digs questioning my rationality and that of others labelled as EU proponents. Clearly we are in actual fact as rational as you are. The judgement you apply is a trite argumentative insult. There are many possible interpretations of any piece of information and all may be fully rational despite contradicting each other. If you don't understand that your rationality could be questioned.
      In fact you when addressing the issue of temperature and encouraged by others went into a detailed analysis and explanation about the concept of temperature. A very rational thing to do. To obscure the issue with irrelevant detail when it is quite obvious we are talking about the general understanding of temperature.

      I would also point out that I have never expressed any particular interest in surface temperature as it is not a good indicator of the temperature of surface reaction, most of the energy being carried away from the surface by the reactants. Yet you (and others) continue to offer measured surface temperature as evidence of the maximum temperature associated with the cometary discharge. This too is rational. Avoiding the real issue and citing irrelevant data as proof.

      It is also clear that you feel the need to be an apologist for the Rosetta team. Is that rational. Perhaps for your personal reasons it is. I feel however that they can look after themselves and your knowledge and experience on the temperature issue would serve better if applied fully critically and objectively.

      I asked the question about pyrometers to get from the horses mouth a clear statement about the temperature measuring capability of the Rosetta instrumentation. It was not intended to instigate a discussion on the semantics of sensitivity or the jargon use of the word. It seems from other replies that temperatures of several thousand degrees K could be measured but it also seems that such temperatures could also be overlooked since in the ViRTIS case a recalibration would be necessary to stretch to that level. The positive intention to search for such temperatures would therefore be required.

      Hopefully that intention exists because nothing offered so far provides conclusive evidence of the maximum temperatures associated with the comet discharge. I would reiterate that the temperature that needs to be measured to prove or disprove any comet discharge mechanism is the temperature within the visible bounds of the discharge, the jets, at points from a few metres to a few kilometres above the surface of the nucleus. A hydrocarbon combustion reaction and/or electrical discharge would indicate temperatures of at least 1000 deg K up to several thousand. Ice sublimation and pressurised vapour discharge a couple of hundred deg K maximum.

      • ianw16 says:

        There are no discharges. End of story. All light is due to the scattering of sunlight from dust. Where would we see these discharges in the electromagnetic spectrum? And why don't we? What temperatures would they be? And why don't we see them? Are they as invisible as the non-existent electric discharge at Tempel 1?
        Sorry, but it is all pure pseudoscience. Not a shred of evidence for any of the nonsense, and a shed load to show it is, in fact, nonsense. Quite in keeping with the scientific credentials of the mythologists who dreamed it up in the first place.

      • Dale Eason says:

        From my understanding of the VIRTIS instrument it would see a hotter object in front of a colder object. So if a jet or gas was hotter than the surface it would see it and measure it. It is also calibrated so that it sees hot temperatures in the range you hope to see.

        So they have measured all the regions around and above the comet and found no temperatures in the range expected of combustion or discharge.

      • Harvey says:

        Ah; see the MIRO paper linked below. At 3.4AU inbound, it unambiguously reports the water water around the comet as typically 165K.
        Since this is from line profiles, it is clearly in the gas phase, and not subject to emissivity issues etc.
        So it seems we are agreed there were no discharges at that time.

      • Gerald says:

        Originaljohn, a pyrometer requires one or two channels to infer/measure temperature. The Rosetta instruments have a resolution hundreds of times better than the requrements of a pyrometer.

        You might require a calibration for higher temperatures, if you want to go down to high accuracies. But (optically dense) features of high temperatures would show up immediately. A presumed hot jet, bright enough to be visible in NavCam images, would be sufficiently dense to produce a typical black body peak in the spectra.
        A calibration, or some refinement would be needed to resolve the temperatures of several jets of different temperatures overlapping each other. But qualitatively you would see it in the spectra. Calibration would be needed to pin the temperature distributions down to high accuracy.
        But this all doesn't make sense without those high-temperature features being present.
        So you get essentially two superposed peaks. One for the scattered light from the Sun, the other by the infrared emissions from the surface. After subtracting the peak from the sunlight, just the "intrinsic" peak of the target remains. Fitting this peak with a black body radiation curve reveals "the" target temperature. These fitting algorithms usually provide a residual, which indicates, whether there is some unconsidered effect.

        Since Matt Taylor said, that the brightness of the jets can be explained "very well" by scattered sunlight (of well-known temperature), this means, that the residuals are very low.
        So there is nothing significant besides sunlight and "intrinsic" infrared emissions showing low "brightness temperature".
        "Brightness temperature" is "the" temperature usually measured in planetology.
        Since 67P's albedo is rather low, this brightness temperature is close to the thermodynamic temperature. For bright patches, the brightness temperature might need some adjustment to get the actual thermodynamic temperature, but not hundreds of Kelvins, maybe up to 10K instead.

  • Harvey says:

    One last try to evade the Capta monster!
    If you have a 'hot' jet in front of a 'cold' comet, the result will depend on whether the jet is 'optically thick' - strongly absorbing - at the wavelengths VIRTIS operates at. If the jet is transmitting, it will have a low emissivity, and could incorrectly appear 'cold'; the analysis needs care.
    MIRO is different. It gets temperatures from line profiles, and these do not depend on the jet absorbing strongly.
    There is no evidence the jets are 'hot', and no physical mechanism to make them so that makes sense. But the interpretation of VIRTIS data to prove that needs care.
    I'm always careful only to claim what the data really proves, not more.

    • Dale Eason says:

      Harvey I assume the investigators are careful. Are your saying that the instruments could have missed the high temperatures that OriginalJohn and/or the EU people are predicting? So their claims are still not disproved?

      • ianw16 says:

        I can't speak for Harvey, but there is ample evidence that jets from comets are not hot. Low surface temperatures in the vicinity of jets, ice entrained within the jets and low dust temperatures within the jets are just some of them.
        This paper covers some of those findings:
        'Water Ice and Dust in the Innermost Coma of Comet
        103P/Hartley 2.' Protopapa, et al.

        There were also contemporaneous surface temperatures made at that comet, which also show low temperatures.
        Add that to the fact that combustion is impossible in a cometary environment, and any electrical discharges would be noticed across the EM spectrum, then I don't think it is something worth bothering with, any more than trying to prove that a reincarnated Elvis is hiding within a spaceship concealed within the comet.

      • Gerald says:

        The claims of the "electric" people have been disproved in many ways, and many times.

      • Harvey says:

        It is in principal true that an optically thin, 'hot' jet seen in front of a 'cold' comet would not be seen by VIRTIS.
        It would be seen by MIRO.
        There should be a hot patch at its base on the surface which would be seen by VIRTIS etc.
        I'm just very careful to be scrupulously careful about what the data do and don't prove.
        *HOWEVER* there is a stick of data which says that the discharge and combustion theories do not hold water; and there is no remotely credible physical theory which supports either.
        So the claim, with regard to jet temperatures only, may not be disproved by VIRTIS data; but is is disproved By a STACK of other data, and by an absence of any remotely credible physics to explain it.

      • Dale Eason says:

        I'm replying to my own post. To say Harvey you don't need to reply because I think today Gerald explained it well. In that the jets were optically dens enough to be seen and to have their temps measured at resolutions appropriate to show that they were pretty cold.


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