What made the comet sing?

This blog post is contributed by Bárbara Ferreira, EGU Media and Communications Manager.

ESA_Rosetta_NavCam_20150131_Mosaic

Comet 67P/Churyumov-Gerasimenko on 31 January 2015, at the time when it was still singing. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Late last year the Rosetta Plasma Consortium (RPC) announced that Comet 67P/Churyumov-Gerasimenko, which ESA’s Rosetta spacecraft has been studying in detail since August 2014, was singing in space. Now, in a paper published in the European Geosciences Union’s open access journal Annales Geophysicae, the RPC team reveals more details about 67P/C-G’s song, including why the comet was singing.

The sounds ‘emitted’ by 67P/C-G are oscillations in the magnetic field around the comet. Its space environment is permeated by the solar wind – a continuous stream of electrically charged gas (called plasma) and magnetic field lines strung along from the Sun – which interacts with the comet’s gas-dust atmosphere. A consequence of this interaction is an induced cometary magnetosphere. In other words, even though the nucleus of 67P/C-G has no magnetic field of its own (as announced at this year’s EGU General Assembly), the comet’s atmosphere or coma is magnetised.

As reported in Annales Geophysicae, the RPC magnetometer on Rosetta started to detect large-amplitude fluctuations in this magnetic field upon arrival of the spacecraft at the comet on 6 August 2014. For four months, until November 2014, the RPC team detected about 3000 cases of wave activity with frequencies of about 40 millihertz.

Sound_comet2

Artist's impression of the 'singing comet' 67P/Churyumov-Gerasimenko. Click on the image to listen to the audio track on Soundcloud. Credit: ESA/Rosetta/NavCam

“This is exciting because it is completely new to us. We did not expect this and we are still working to understand the physics of what is happening,” said RPC principal investigator Karl-Heinz Glassmeier at the time ESA reported the discovery of the ‘singing comet’ waves on the Rosetta blog. Glassmeier is Head of Space Physics and Space Sensorics at the Technische Universität Braunschweig, Germany, and the senior author of the Annales Geophysicae paper.

This observation took the team somewhat by surprise because it is the first detection of waves of this nature at a comet. In previous cometary encounters, such as the International Cometary Explorer and Sakigake spacecraft flybys of comets Giacobini-Zinner and Halley, researchers measured wave activity with frequencies some 10 times lower.

The difference in the 67P/C-G case is that, as Rosetta travelled alongside the comet, the instruments could measure the magnetic field for a long time, and while the comet was still relatively far away from the Sun. The RPC instruments collected the data reported in the new study while the comet was between 400 to 540 million kilometres from the Sun. At this point, the comet’s activity was low: it was not expelling a lot of gas and dust into space, and the induced magnetosphere was just beginning to form.

Since the song 67P/C-G sings at this early stage is very different from the ‘classical sounds’ detected at comets closer to the Sun, the team concluded a new mechanism must generate the 40 millihertz waves. (If you are interested in finding out more about the difference between the two types of cometary sounds, and the processes that generate them, read the more detailed explanation provided by K.-H. Glassmeier below.)

When RPC scientists first uncovered 67P/C-G’s mysterious song, they suspected it had something to do with the comet’s activity – even if low – and the neutral particles it releases into space. Ultraviolet radiation from the Sun causes ionisation of these atoms and molecules, including water molecules. In the plasma environment around the comet’s nucleus, the newborn ions move perpendicularly to the magnetic field, forming what is called a cross-field electric current. It turns out that this current is unstable, and ultimately, it is what makes the comet sing.

“The physical process is somewhat difficult to understand without a deeper understanding of plasma physics, but we can use a simple analogy to have a better idea of what’s going on,” says Glassmeier. “Consider your garden hose. If you start the water flow, there is a chance that the hose starts to oscillate, generating waves. This is about what happens in the plasma. Of course, the flow we have in the cometary situation is not like water, but is a flow of charged particles. But somehow the analogy is suitable.”

The questions left to answer are whether 67P/C-G continues to sing the same song as it gets closer to the Sun, and whether it starts emitting more classical cometary sounds.

Glassmeier says RPC instruments detected the 40 millihertz waves at least until February this year, when Rosetta was about 350 million kilometres away from the Sun. “Around this time, the activity is changing, other features show up, the plasma interaction region becomes much more violent. Singing comet waves are still present, but buried under a variety of other features we are currently trying to understand.”

“Whether we also observe the classical type of cometary waves, like those observed at Halley, is very difficult to judge. We are heavily working on further analysing the dynamics of this region to find out more.”

Based on the paper by: Richter, I., Koenders, C., Auster, H.-U., Frühauff, D., Götz, C., Heinisch, P., Perschke, C., Motschmann, U., Stoll, B., Altwegg, K., Burch, J., Carr, C., Cupido, E., Eriksson, A., Henri, P., Goldstein, R., Lebreton, J.-P., Mokashi, P., Nemeth, Z., Nilsson, H., Rubin, M., Szegö, K., Tsurutani, B. T., Vallat, C., Volwerk, M., and Glassmeier, K.-H.: Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko, Ann. Geophys., 33, 1031-1036, doi:10.5194/angeo-33-1031-2015, 2015.

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A more detailed explanation of the difference between classical cometary waves and the ‘singing comet’ waves, by Karl-Heinz Glassmeier:

Plasma waves play a most important role in coupling newborn ions of cometary origin with the solar wind plasma. As the particle density of the solar wind is rather small, the interplanetary space is almost a vacuum. There are no collisions between solar wind particles and the cometary ions causing the required coupling. Without such a coupling, the cometary ions would move relative to and undisturbed by the solar wind.

However, if the comet-solar wind interaction region is much larger than a typical gyro-radius of a cometary ion, as in the case of Comet Halley, cometary ions constitute so-called ring-beam distributions in the phase space of the solar wind plasma.

Such distributions are heavily unstable and produce the classical cometary waves as observed at Halley and Giacobini-Zinner. These waves, on the other hand, are able to scatter cometary ions as the electric field fluctuations of the waves impact the ion motion.

In this way, the plasma waves couple the solar wind particles and the cometary ions. The waves act as a kind of mediator between solar wind and cometary ions. A more detailed treatment shows that the frequency of the waves as observed in the spacecraft frame of reference is very close to the local gyrofrequency of the cometary ions. [Editor’s note: Recall that, in the presence of a uniform magnetic field, an electrically charged particle moves in a circular motion; the radius of this circle is the gyro or Larmor radius, and the angular frequency is called gyrofrequency.]

The situation we observed at 67P/C-G in the first months after the arrival at the comet is rather different. The interaction region is rather small because of the weak activity of the nucleus. Actually, the size of the interaction region is much less than the gyro-radius of a cometary ion.

I like to call this region the Larmor sphere of the comet. Within the Larmor sphere the newborn ions have not yet been able to perform a complete gyro motion around the solar wind magnetic field. The newborn ions just constitute an electric current perpendicular to both the solar wind flow and magnetic field. This cross-field current is much different from any ring-beam phase space situation observed in case of the large interaction region of Halley or Giacobini-Zinner. In our model, the cross-field current constitutes the very first effect the newborn ions impose onto the ambient plasma. Only later, the newborn ions develop into ring-beam type particle distributions. But in these later stages the plasma volume with the newborn ions have already passed the nucleus, moving downstream.

This is a fundamental difference between the fully developed ring-beam situation at strongly outgassing comets and the cross-field current situation at weakly active comets. In the former case the unstable phase space distribution is moving with the solar wind plasma, while in the latter case the cross-field current is almost fixed with respect to the nucleus.

Comments

62 Comments

  • logan says:

    _ "...the newborn ions move PERPENDICULARLY to the magnetic field, forming what is called a cross-field electric current. It turns out that this current is unstable..."

    Thanks a lot Bárbara, ANGEO and all the Teams 🙂

  • logan says:

    Even more: "...the newborn ions need to be accelerated to constitute a significant current and the waves need to grow."

    "...perpendicular to both the solar wind flow and magnetic field..."

    This model from I. Ritcher et al. sounds well.

    • logan says:

      -This [newborn ion] acceleration should be occurring above surface. Corollary: potential V [and P] increasing above surface. Corollary: magnetic signal becoming significant ABOVE surface.

      This preliminary paper doesn't discard plausibility of low V current slowing sinking into Ducky.

  • Harvey says:

    Its entertaining to note that the ion gyrofrequency is probably more commonly encountered in high fields ~~1T, say in fusion plasmas heated by ICRH (ion cyclotron resonance heating.) There typically frequencies are tens of MHz.
    But here we are in nT fields, so scale down by 10^9 since it scales linearly with B of course, and tens of MHz becomes tens on mHz.

    • Martin says:

      In the magnetometer data we can easily detect ion cyclotron waves if they are present, also in nanoTesla range. At comets they are not so often observed, there the mirror mode waves dominate, but e.g. at Venus or Europa we can see the cyclotron frequencies of protons and of Na, K, Cl respectively.

  • logan says:

    Freed electrons accelerated by several order of magnitude that of ions.

  • andyf says:

    Easily the least illuminating article so far. Ring beam situation? Unstable phase space distribution? Cross-field current distribution? Sorry, but it comes across like word-salad.

    • Harvey says:

      To an extent I take your point; maybe it could have been done better; but having often been in the same situation, I would also defend the shear difficulty of what Glassmeier is trying to do.

      There tends to be an assumption that everything can be condensed into simple, everyday words, but, at least in the small space journalists will give a scientist, and within the attention span of most listeners, it 'ain't necessarily so.'

      Glassmeier has fallen back into the short hand used by physicists when describing equations, which is what he is really thinking about. If we were really discussing this, I'd be visualising equations, and pretty soon we'd be scribbling them on a white board. Phase space, cross product etc we'd use without a moments thought.

      So to the general public a sort of double translation occurs; from the math to the short hand, from the short hand to everyday language. A lot gets 'lost in translation', and as with real language, some things are near impossible to translate without lengthy explanations. For example the concept of a vector cross product is absolutely central here, but it's completely unfamiliar to most readers; there are two kinds of multiplication, one yielding a vector, one a scalar.... What's a scalar.....why......things that take several lectures to undergraduates.

      So yes, maybe he could have done a better job. But what he is trying to communicate is really very complicated, and the guy is a physicist, not a media presenter, give him a break!

  • Lucas says:

    Ouch what happened? Plasma, and not just gas? Electricaly charged with magnetic field? Finely electric effect are starting to reveal, not only kinetic.forces. One more thing doesnt fit - sublimation. Did anyone tested this effect in vacum chamber? Can sublimated (and as we know ionised) gas can be so filamentary? My gues is that you cant reproduce this effect without electricity and magnetic field.

    Or maby I'm wrong?

    • Gerald says:

      Sublimation provides neutral gas from the comet. The solar wind provides/is plasma with entangled magnetic and electric fields (many orders of magnitude weaker than "Electric Universe" presumes). Solar ultraviolet light ionizes neutral gas. The combination of these different environments makes things complex and interesting.

  • Margaret Cho says:

    Is it worth to play the Comet-67P song against aurora?

    Since both aurora & Comet 67P are said to be related to electromagnetic fields, aurora might respond to Comet 67P song in some way...

    Please see my paper, "Aurora vs. Comet 67P" at
    academia.edu search: Ian Saph

    I'd appreciate a response...

    • Gerald says:

      My ad-hoc assessment would be, that a change of the Earth's magnetosphere or a change of the electromagnetic behaviour of the environment of 67P in the suggested way is far beyond our technical possibilities.
      Sending a weak radio signal from Earth towards the comet, or to the magnetosphere of the Earth, via radio dishes, however, would be technically feasible. Usually a scientifically justified reason needs to be provided to get a funding. Such a justification should be part of the proposal; it isn't self-evident to me in this case.
      Sometimes a sufficiently large public interest in such an experiment can be sufficent for an approval, even if there is no scientific justification, but kind of symbolic value instead.

    • Harvey says:

      Capta problems...
      Firstly, the 'sound' at 67P is not a 'sound' in the normal sense at all. They have simply used that as an analogy. It's a complicated oscillation of the plasma and it's magnetic fields, not just a varying density. The 'amplification' was in frequency, not 'strength', to move them into the ear's range; they are far sub-sonic.

      The aurora is fundamentally a property of bodies like the earth having a magnetic field; 67P does not have a magnetic field of its own. So the two are very different things.

      Actually it is possible to modify the earth's ionosphere by heating it with radio waves, and this has been done, link below. But 67P is far too far away for that to be practical, the plasma conditions are very different, and it's not clear why one would want to try.

      https://en.m.wikipedia.org/wiki/Ionospheric_heater

  • logan says:

    _Was strongly sub-estimating ion erosion.

  • Dave says:

    Is there any other information that can be released with some sketches so that we can more easily understand the text.

  • originalJohn says:

    An interesting post Claudia. Call it singing if you wish, the anthropomorphising of a lump of rock, but what it clearly signifies is that this supposedly inert neutral lump of rock is electromagnetically active and is emitting a wave oscillating at 40 millihertz ( a very low frequency, 40 x 10 power minus 3 Hertz).

    Having observed this the investigation then goes of the rails, several times. It starts well enough by reminding us that the nucleus has no intrinsic, internally generated magnetic field ( from previous measurements). It then goes on to further remind us that space is "permeated by the solar wind - a continuous stream of electrically charged gas ( called plasma)". First derailment. It is not a gas. And it is not a gas called plasma. It is plasma, a quite distinct state of matter.
    Then another derailment straight away. The investigation also acknowledges accompanying magnetic field lines "strung along from the Sun" in the solar wind. A schoolboy derailment I am afraid. The reification of magnetic field lines, the implication that lines exist in space. Of course they do not. A field exists for which lines are used as a representational artifact.

    Not impressive but we know what they are getting at. The solar wind is an electric current which like all electric currents manifests an induced magnetic field.

    But then right off the rails and down the embankment. The interaction of the solar electric and magnetic fields with the comets gas and dust atmosphere apparently induces a cometary magnetosphere. So the comet by some magic process at the same time has no magnetosphere and has a magnetosphere. As the investigator says " the comets coma is magnetised" This was a surprise to principal RCP investigator Karl-Heinz Glassmeier who is "still working to understand the physics of what is happening".

    The thing Karl-Heinz has to get straight is that the magnetic field of the solar wind is the consequence of the solar current flow and the magnetic field of the comet coma, its magnetosphere, is a consequence of electric current flow within the coma, but because it is a magnetosphere, current flow originating at the coma. What Karl-Heinz is seeing is the interaction of these two fields.

    The usual effect is invoked (without evidence) of photionisation of coma gas by ultraviolet photons from the Sun as the only possible source of ions, and the created ions are said to establish a cross field current perpendicular to the magnetic field. Confusion here though because it is not clear which magnetic field is being referred to, the Sun's or the comet's. An induced magnetic field is naturally perpendicular to the direction of current flow and vice versa so the " cross field" term is redundant. For the ions to move as a current an electric field must exist and that current flow will induce a magnetic field, a magnetic field distinct from the pre existing solar magnetic field.
    Karl-Heinz needs to acknowledge these two separate current flows, and magnetic fields then things will become more clear to him.

    It would be extremely helpful if measurements could be made of the ion content of the comet jets as a far more likely and abundant source of ions than the weak photoionisation effect. If the comet coma has a magnetosphere it is probable that it is induced by current flow from the surface of the nucleus. This would be easy to prove or disprove. These measurement would simply clarify the situation and avoid the need for unnecessary complications such as the ring beam theory, in an attempt to support the gas sublimation assumption.
    Rather than assuming that the comet jets and the coma are mainly neutral gas let us see some detailed analysis including ion analysis. It is crucial to the whole Rosetta mission.

    The singing anthropomorphised comet is giving us a clue. As Herr Glassmeier observes “Around this time, the activity is changing, other features show up, the plasma interaction region becomes much more violent. "

    • Martin says:

      This is, to quote Pauli, "not even wrong"

    • Gerald says:

      Originaljohn, it seems, you're still suffering of a fundamental confusion of solar wind with an electric current.
      That's two different concepts.
      In the solar wind, positively charged ions and electrons largely move the same direction and transport the same amount of positive and negative charge, such that you don't get an electric current in zero-th approximation.
      Within this roughly electrically neutral medium on a larger scale, there are entangled magnetic and electric fields. But those are small in comparison to the currents you would get when considering the electrons and the positive ions separately.
      That's why actually your way of thinking is derailed on major parts.
      An electric current flow from the surface of the comet would induce a magnetic field. This would have been measured. Instead the magnetic field is low, particularly the locally created magnetic field. Hence your significant electric current flow from the comet's surface is already disproved.
      Ions, e.g. by photoionization, in the presence of electrons (freed by the same photoionization) can move without an electric field, and without forming an electric current.
      Same with ions formed by charge exchange (pairs of ions, one positively, the other negatively charged).
      But a dense plasma of this kind would recombine rapidly to a neutral gas, and dissipate radiation. This radiation would have been measured. Instead only radiation on a reasonable level has been measured.
      You may remember the Alice measurements:
      http://www.esa.int/Our_Activities/Space_Science/Rosetta/Ultraviolet_study_reveals_surprises_in_comet_coma

      Sublimation of ices of volatiles is straightforward.

      • THOMAS says:

        @Gérard
        "Ions, e.g. by photoionization, in the presence of electrons (freed by the same photoionization) can move without an electric field, and without forming an electric current."

        Why then does the post explicitly speak of an electric current? I quote: "In the plasma environment around the comet’s nucleus, the newborn ions move perpendicularly to the magnetic field, forming what is called a cross-field electric current. It turns out that this current is unstable, and ultimately, it is what makes the comet sing."

        The origin of the "newborn ions" is hypothesized to be photo-ionization by UV solar radiation, whereas the true origin is actually clearly the cathode sputtering which has been observed on the nucleus by mission scientists for many months now.

        Whatever, the presence of a magnetic field and a concomitant electric current in the coma is now beyond doubt, and it is futile to continue attempting to deny the fact when mission scientists themselves freely admit it. Do you know better than they do?

        • THOMAS says:

          Sorry my comment was in response to Gerald, not Gérard...

        • Gerald says:

          THOMAS, a copper wire isn't the same as an electric current within this wire.
          Same for the plasma. A plasma is one thing, a current within the plasma something different.
          A moving copper wire is not an electric current, despite containing positively charged atomic nuclei and negatively charged electrons.
          A moving plasma is not an electric current, despite containing positively charged ions and negatively charged electrons.
          Confusing the two concepts leads to subsequent confusion.
          The solar wind is a moving plasma, not an electric current. Electric currents may flow within the moving solar wind, like electric currents may flow within a moving copper wire.

          I have no problem with the post speaking of electric currents within the plasma.
          I do have a problem with fundamentally wrong claims like Originaljohn's "The solar wind is an electric current...".

          Some other parts of Originaljohn's post are so confused, that I can't even tell, whether they are right or wrong.

          • Gerald says:

            ... speaking of Pauli.

          • Harvey says:

            Gerald, An excellent analogy and explanation, which I hope you don't mind if I borrow occasionally in other contexts.

          • Gerald says:

            Thanks for your approval, Harvey! You may use it whenever it helps,

          • originalJohn says:

            No confusion on my part Gerald.

          • originalJohn says:

            The movement of charged particles is an electric current Gerald, as in the solar wind. These charged particles also constitute the plasma. In your copper wire analogy the moving electrons that carry the current are the same structural electrons that bond the copper atoms together. So that part of the copper wire is indeed moving.

            The main difference with a metallic solid is that the positive ions are trapped. In a plasma the positive ions are free.

        • Harvey says:

          We know the neutral density from MIRO etc.
          We know the photoionisation and photo dissociation cross sections from lab measurements.
          We know the solar spectrum from decades of measurements.
          Could you please explain why these will not give rise to photoionisation etc? Do the ions wear sun screen or something?

          Yes, there are indeed a few sputtered ions, at densities many orders of magnitude lower than the photo generated ions, and of different species (which is why they can be distinguished.)

          • Harvey says:

            Let's re phrase that as one of those dreaded equations.
            Molecular density M in say molecules/cm^3, (ex MIRO etc.)
            Photoionisation cross section (spectrum weighted) S in cm^2 (ex the lab)
            Photon density D in per cm^2 per second (ex decades of measurements.)

            So the photoionisation rate R=M*S*D in ions per cm^3 per second.

            Which one is wrong, M, S or D and why?

      • originalJohn says:

        Gerald, a plasma such as the solar wind current is certainly not a neutral medium because of the Debye length constraint. Positive and negative charged particles remain separated. because they are are screened from each other. The neutrality is only numerical and therefore quasi, not actual. Hence the current flow, and the effects associated with it such as magnetic field induction and Lorentz force compression.
        You may not have acknowledged that the comet has a magnetic field but Karl Heinz Glassmeier has. See this "singing comet" post, referring to his publication. Unequivocally he states that the coma is magnetised. It is the comets magnetosphere.

        As in any plasma current, and any electric field with mobile positive ions, the protons and electrons in the solar wind move in opposite directions. There could be some drift of electrons in the proton direction but this is not
        characteristic. Electrons could also be affected by any local superimposed field.

        There is no evidence that the direction of electron motion in the heliosphere is the same as that of the protons. It is assumed, erroneously, in the belief that the motion in the solar wind is only away from the Sun. That is why the wind analogy is misleading.

      • originalJohn says:

        Gerald my understanding of the properties of plasma, such as the solar wind is crystal clear. It cannot be a neutral medium because of the Debye length constraint. Beyond the Debye length any charged particle is screened from all other particles and therefore acts as a separated charge. The neutrality is quasi ie in number only.

        Secondly there is no evidence that electrons and protons in the solar wind move in the same direction, except for regions of local drift . That is an assumption derived from the misleading wind concept As in any electric field the electrons and protons move in opposite directions.

        You note that the magnetic field in the vicinity of the comet is low, therefore there is a magnetic field and there is an electric current flowing from the nucleus, however low. A current is a current.

        It is a simple fact that the ions in the solar wind are in motion and therefore constitute an electric current by definition. The protons also accelerate away from the Sun over interplanetary distances. A force at source could not achieve this. Only an electric field could. Look up the solar current sheet Gerald. It is the flow of charged particles towards and away from the Sun.

    • logan says:

      The point here OriginalJohn is that Ducky is not relevantly magnetized, but Charged,

      • Harvey says:

        It cannot be significantly charged either.
        It's a first year physics problem in electrostatics to get an approximation to the capacitance of 67P.

        It is <1uF
        So if it is charged to a GIGAVOLT, it would store a charge of Q=C*V or 1000C, which would sustain a miserable 1A discharge for just 1000 seconds. from Q=I*T.
        A GV is clearly ludicrous - and you get nothing significant for it. For comparison, an arc welder commonly runs 100A arcs.

        As it is clearly insulating, that charge can't move on the surface either.

        • logan says:

          Hi Harvey. FFind some argumentation a little circular. First about not nucleus discharge occurring. Agree, not by far in the EU range. Second the nucleus should be 'wired' to the solar wind in their coldest years, so uncharged.

          If no discharge occurring, then nucleus should arrive at perihelion quite electrically neutral.

          But then, -and that is pure speculation- the gas envelope is not neutral, but positive.

          So, there is a neutral nucleus facing [without discharging] a positive gas envelope. Active comets could exhibit capacitive behavior.

          • Harvey says:

            Logan
            No, nothing circular about it.
            The plasma around the comet will also be quasi neutral, on a scale greater than the local Debye length. Given the density near the comet that will be quite short now close to 67P, quite likely less than a metre. That means that if you take a sphere or a few metres radius, the number of + and - charges will be closely equal. (It's called quasi neutral, not neutral, because of the way we approximate the equations to allow for fields and currents.)

            Of course for a small enough sphere, eventually you get one ion, one electron, it's not neutral. Make the sphere big enough and they 'average out'. This is routine textbook stuff.

            If 67P is giving rise to a discharge - a current flow in a gas - the current has to come from somewhere. 67P simply cannot store enough charge to run a significant discharge for any length of time without idiotic voltages.
            If it's not stored charge, where does the current come from?! I see no wires connected to a big space power supply connected to 67P!

            The solar wind is many, many orders of magnitude too weak - and both protons and electrons in essentially equal numbers. Our moon is exposed to that 'wind' - but has no coma I've seen.

        • logan says:

          Hi Harvey, Thanks a lot for answering.

          Accepting all of your last arguments for dry Moon and asteroids.

          Learning this Debye length from you long time ago 🙂 Solar plasma from the distance fields neutral.

          But this issue is about atomic particle's kinetics.

          It's about deflection. It's about separating this Solar 'mix'.

          Separating those particles a way, way beyond the Debye length. An interaction of gas with plasma, on a cometary scale.

          Speculation is that bow should be impressing differing kinetics to Solar electrons from Solar protons [We differ just there].

          On shock 67P's coma should contain a relevant deficit of solar electrons to solar protons.

          Gas envelope [which is mainly molecular] should be 'solar protons doped', so charge unbalanced [Even if admitting a totally neutral nucleus].

          http://blogs.esa.int/rosetta/2015/04/14/rosetta-and-philae-find-comet-not-magnetised/#comment-440507

          ....

          On your sketched formulation, How much surface did you estimated for this sintered, highly porous object?

          • Harvey says:

            Logan
            Sorry I don't think I really understand your question.

            Are you asking how the neutral gas and plasma interact?

            The neutral gas, atomic or molecular, is not affected by the magnetic fields.

            But it can be photoionised, a major source of the locally produced ions and electrons. This is described by a 'cross section', which depends on the species, wavelength and process.
            You can also get collisions between neutral species and ions or electrons. The frequency of these reduces rapidly as the density reduces as you move away from the comet into low density regions. But certainly things like 'charge exchange' collisions are of importance. Again, described by a cross section for the process and species of interest, which depends on energy.

            The whole story gets rather complicated I fear!
            Not sure if I've answered, or partly answered your question or not.

            However again, on a scale a good few times the local Debye length, it's neutral. There are no electron or proton deficient layers beyond that scale. Double layers, of much interest to some here, typically occur on a scale of maybe ten Debye lengths or so, and certainly have charge separation. But much beyond that scale, neutral.

          • logan says:

            Hi Harvey, had not seen your last post.

            "...The whole story gets rather complicated I fear"!
            Fully Agree.

            Lots of wanderings and speculations on that top-most interplay of Sun's plasma with the mostly molecular coma.

            If double layer, should be a very special class of them. Simple descriptions written to my level doesn't fit my mental scenarios.

            A few days from now, on the 23th, ROSETTA will go up and take a look at the outer gaseous skin layers of 67P.

            Wishing the best to the Teams! Eagerly awaiting some kind appetizer's data 🙂

          • logan says:

            Would beg to the Teams to allow for testing of a 'pinch' in the geometry of magneto-atmo-sphere; When time comes for ROSETTA to near back.

        • originalJohn says:

          Your 1 microFarad is an assumption Harvey. You have no knowledge of the capacitance of the comet nucleus because you have no knowledge of its material. You also have no knowledge of its conductivity.

          Nevertheless if we assume it is rock and insulating we are clearly not looking for conductivity within the rock structure. We are looking for a surface effect.

          I have explained to you already how an insulating material can conduct at its surface by accumulating free surface electrons which play no part in the structural bond integrity. I have also explained how, initiated by the standard process of electrostatic induction, this could occur on the surface of a comet nucleus.
          Its slow accumulation and slow release of a surface charge is perfectly feasible at fractions of an amp to a few amps and a few hundred volts and remains an excellent hypothesis to account for the observed behaviour.

          It is all however speculative until we have some appropriate data from this mission.

          • Harvey says:

            It is NOT an assumption that it is <1uF
            It is a first year physics problem to do the calculation.
            The capacitance of an isolated object DOES NOT depend on what it is made of.

            (Unless of course we are talking about a classical two plate capacitor with a dielectric in between. That is not the case here; its a classical calculation of an isolated body's capacitance. You do it by calculating the capacitance between concentric spheres, then letting the radius of the outer sphere go to finity.)

            Your 'explanation' using induction is simply completely incorrect. If you place an insulator in an electric field, Charges are slightly displaced from their equilibrium position; they do not accumulate on the surface (but can be placed there from an external source- but that is not 'induction')

            A few amps and a few hundred volts is *wildly* inadequate to support the observed out gassing rates; that's less power than I put into my electric kettle!

            If insulators had these magical mobile surface electrons they would have a conducting surface; we would have no insulators in effect!

            I'm sorry but the reply is simply totally incorrect.

      • logan says:

        As at heliopause, some 'foam-iness' could reside at the outermost coma.

    • ianw16 says:

      "It would be extremely helpful if measurements could be made of the ion content of the comet jets as a far more likely and abundant source of ions than the weak photoionisation effect."

      Like they did at Hartley 2, for instance?
      http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015409.pdf

      It wasn't ionised material, by the way. Just neutral CO2, H2O etc.

      • originalJohn says:

        No ion measurements made at all in the paper you refer to w16.

    • Harvey says:

      The shear arrogance and impoliteness of this is simply breathtaking.

      Prof Dr Glassmeier has published hundreds of peer reviewed papers in this field, and several books. Not only he, but a number of other highly qualified authors put their name to this. He's head of a research group, been involved in numerous space missions, PI on some.

      Yet you think you can 'get (him) straight' on the basics.

      Incredible.

  • Harvey says:

    Out of idle interest, why do I need further evidence of the photoionised plasma when:

    MIRO at 67P and umpteen other instruments at other comets confirm molecular H2O and its density.
    Laboratory measurements give me the photoinisation and photo dissociation cross section.
    We know the solar spectrum rather well.

    From those three, calculation of the resulting plasma density is straightforward.

    • originalJohn says:

      No evidence that the resutling plasma density would be. a consequence of photoionisation.

      Indeed we do know the solar spectrum well and we know that the frequency range of UV photons with sufficient energy to cause photoinisation is a tiny part of it. Probably why ESA previously stated that the expectation is 1ppm of water molecules ionised.

      So a great deal could be deduced from from further data on the coma plasma. If for example the ratio of ions to water was much greater than 1 per million molecules it could fairly safely be assumed that photoionisation was not the primary plasma source.

  • Dave says:

    Claudia - re sketch.

    Thanks very much for responding. The sketch confirms the description in the article. However there are some gaps in the description, it's going to be interesting to see how this unfolds. Thanks again
    Best wishes

  • logan says:

    -Magnetic energy density- [according to fig. 3] modal at exactly 20km an 30km. 'Sharp Layering' at the escaping 'atmosphere'?

    • martin says:

      no, not really, just Rosetta remaining at that distance (20 and 30 km) for prolonged times, therfore the many points at that location.

  • logan says:

    Thanks, Martin. Modal sampling.

  • logan says:

    One of the most beautiful mosaics from H. NAVCAM Team. Truly breath holding!

  • George Nixon says:

    If the activity on comet 67P results only from radiation received from the sun, and the magnet oscillation is activated by the solar wind, all with no input from the GTE (gravitational thermodynamic effect), then the activity during retreat from the sun should close resemble activity observed during approach.

  • Claudia says:

    Just a friendly reminder to all commenters to please keep comments relevant to the topic of the original post and, most importantly, to use respectful language when responding to other comments.
    Any further comments submitted on an unrelated topic and/or deemed aggressive or unnecessarily rude will not be published. Please have a look at the blog house rules here: http://blogs.esa.int/rosetta/2015/01/06/happy-new-year/

    Thanks for your co-operation!

  • George Nixon says:

    Hello Claudia.

    If my post is considered to be not relevant to the topic being discussed, then please accept my apologies because that was not the intention. It is my belief that the ESA Rosetta Project has the potential to be the most important experiment made to-date on behalf of mankind. Due to the fact that both Rosetta and comet 67P are under the influence of gravitation, the potential referred to is the possibility of proving the physical existence of new physics, hence the reference to the presently unproven GTE.

    With regards to relevance to the original article concerning the magnetic oscillations, sublimation of water ice and other frozen substances due to the magnitude of differing types of radiation from the sun, then my comments in my previous post was directed at the argument between some of the above posters. I stated that the departure of comet 67P from the sun should provide information concerning the physical realities with regards to the more obvious solar activity on comet 67P. The truth of that statement is in the fact that the comet will receive close to the same magnitude of radiation from the sun during its departure, as it received during its approach.

    • logan says:

      Hi George. Welcome to the blog. Disciplinary action usually directed to us parishioners. Claudia love us too much as to signal a guilty 🙂

  • George Nixon says:

    Hello to you Logan, and thanks for your welcome.

    To stay relevant to the discussion concerning the physics of the appearance of the comet's oscillating magnetic field, either requires the rational application of known physics leading to an unequivocal conclusion, or if physical explanations are ambiguous. then the possibility of new physics should be considered for the following reason.

    Despite the popular belief regarding gravitation as required by GR, the magnitude of a planet's or a comet's momentum is given by mass times velocity, and due to gravitational acceleration, velocity is constantly changing. Physics requires a force to be acting to change velocity. Because we must apply a force if we wish to change a velocity, then why should there be any difference with regards to Rosetta and comet 67P. It is for that reason I am expecting that the Rosetta Experiment will require new physics to unequivocally provide explanations for the unexpected realities experienced, and to be experienced by comet 67P and measured by Rosetta.

    As a backing for the above statement, The GTE (gravitational thermodynamic effect) activities on comet 67P cannot be attributed to gravitational squeezing as was the excessive volcanic activity on Jupiter's moon Io.

  • George Nixon says:

    With regards to the above post, the immediate reality of the conservation of momentum with regards a constantly increasing or decreasing rate of velocity due to the gravitational effect between the comet and the sun, requires an explanation that conforms with the conservation of energy law. Cavendish experimentally measured the magnitude of gravitational effect between lead spheres by the use of a balance similar to the one designed by John Mitchell, That experiment represented the constant, instantaneous reality of the gravitational effect between those four lead spheres that certainly was not an illusion.

    The immediate reality of the conservation of momentum cannot be logically explained away by reference to gravitational potential increases or decreases. Nature is only concerned with instantaneous pay as you go for present occurrences, and does not accept IOUs to be repaid in the future. The fact that physical changes during the approach are compensated for during the departure of the comet from the sun, does not effect the instantaneous physical realities of either, despite changes to gravitational potential in either case. Therefore, because momentum is found by mass times velocity, the comet's mass can not remain invariant during the approach or departure from the Sun; as is required and explained by the GTE

  • George Nixon says:

    With regards to the question; what made the comet sing the magnetic song?, I would suggest the following possibilities that should be more deeply analysed.

    If the appearance of a pulsating magnetic field resulted from impact of high velocity particles in the solar wind ionizing molecules in the comet's corona, which surely would occur, then the comet's magnetic pulsations should become more pronounced during perihelion and for several months thereafter.
    I suggest the immediate above statement because when the magnetic phenomena was first observed, the solar wind was impacting the bow shock wave of the comet at a small angle to the left of the comet's trajectory. Thereby, locating most ionization and magnetic results mainly in the comet's sparsely populated shock wave and leading molecules of the comet's corona.
    During perihelion and some months thereafter, the solar wind will be impacting the comet's corona at close to ninety degrees relative to the comet's trajectory. The much closer proximity to the sun and occasions with little or no shock wave protection, the increased ionization and believed magnetic results, would be much more closely located on the surface of the comet. Due to the comet's trajectory, and sideways directed spillage of the bow shock wave resulting from the 12 hour rotational period of deflecting surfaces of an oddly shaped comet, any shock wave detected by Rosetta during the referred to period would constantly alternate in magnitude.

    With regards to the requirements of the GTE, there would be no activity from the comet until it was clear of the gravitational effect of the great planets, mainly Jupiter. When the comet gradually became subjected to the gravitational effect between its self and the sun, then every particle forming the mass of the comet and constituting the corona etcetera, would be loosing mass in proportion to the time rate of acceleration towards the sun. Such minute loss of mass undergoes a change of state to heat energy and to the beginning of an increasing magnitude of a pulsating magnetic field.

    When the comet begins to travel away from the sun, there is a reversal of that which happens during approach, and there is a gaining of mass in proportion to the time rate of acceleration relative to the sun. The gain in mass is at the expense of the rate of oscillations (temperature) of the molecules, and so a cooling in excess of radiation received from the sun. Due to the increasing mass, the comet would slow very slightly in excess of that resulting from Newtonian gravitation. In that regard, there would not be any pulsations of a magnetic field.

    The above referred to affect resulting from the GTE was in my opinion responsible for the measured slowing of Pioneer, and provided the majority of the excessive heat responsible for the volcanic activity on Jupiter's moon Io. With regards to Io, the changes to magnetic field would alternate according to Io's accelerating relative to Jupiter.

    When subjected to gravitation effect, all particles possessing mass respond as stated for the GTE.

  • George Nixon says:

    I am sorry for the mistake in the above post. For the words corona, please read coma.

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