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Today, we continue our series of reports on the latest
scientific papers from the ESA's Rosetta mission to Comet 67P.
In the previous episode, Dr. Franklin Anariba discussed the direct
measurements of electrical activity near the comet nucleus.
Data which may eventually prove to be a game-changer in comet science.
Today Dr. Anariba, a specialist in electrochemistry,
continues his analysis which suggests that electrochemical reactions
play a fundamental role in much comet activity.
In episode 2 of this series, I expand on the presence of
electrical activity in the vicinity of the nucleus of comet 67P.
This is done by commenting on two important recent reports.
In the manuscript titled, "The Rosetta ion and electron sensor (IES) measurement
of the development of pick up ions from Comet 67P/Churyumov-Gerasimenko"
by Goldstein et al., the plasma environment is scrutinized
at a heliocentric distance ~3.5 AU and at 28km from the nucleus of the comet.
In summary, the authors report: 1) Detection of low energy water cation
populations for the first time in a weakly "outgassing" comet far from the Sun.
2) Water cation populations detected display time-varying distributions.
3) No angular distribution is observed for the water cations populations,
suggesting that these ions are created locally and have not been accelerated
by the solar wind electric field as expected with mass loading.
4) Interestingly, mass-loaded ions and solar wind protons
move in opposite direction, while electrons remain unaffected.
The implications of these findings reside on the assumptions
that mass-loading plays an important role at such large
heliocentric distances on a comet with tenuous coma density.
For instance, it has been reported that the solar wind ions are deflected
but not slowed down as expected during a mass-loading process,
suggesting that mass loading is negligible.
Therefore, these findings instead can be interpreted to indicate a
complex and dynamical electron-rich environment near the nucleus
at such large heliocentric distances where mass loading is negligible,
suggesting another ion motion mechanism is dominant.
In this line of thinking, of utmost interest is the paper by Clark et al. titled,
"Suprathermal electron environment of Comet 67P/Churyumov-Gerasimenko:
observations from the Rosetta ion and electron sensor".
The term "suprathermal" implies highly heated electrons possessing high kinetic energy.
In the manuscript, according to the authors, suprathermal or fast-moving electrons
are "accelerated by an unknown mechanism from a few eV upward to hundreds of eV and play
an important role in the electron-neutral chemistry, as well as, in dust grain charging".
Moreover, previous studies showed that these accelerated electrons are ubiquitous,
but could not agree on a particular mechanism.
Several accelerating electrons mechanisms have been put forth:
1) "Suprathermal" electrons are likely associated with
upstream cyclotron wave fluctuations convecting tailward.
In other words, the accelerating energy is provided by mass-loaded ions
traveling from the direction of the Sun towards the tail of the coma.
2) "Suprathermal" electrons are accelerated in a similar manner
as electrons behind interplanetary collisionless bow shocks.
3) The Active Magnetospheric Particle Tracer Explorer (AMPTE) satellite showed that lower
"hybrid" cyclotron waves from mass-loaded ions were responsible for the electrons.
"Hybrid" refers to cyclotron frequencies in between a proton and an electron.
4) Some studies ascribed "suprathermal" electron acceleration to photoionization,
while other studies indicate the role of photoionization to be negligible.
It is then clear, that there is no agreement on the dominant mechanism
behind suprathermal electron acceleration, more so for Comet 67P,
which has a 100 times lower coma density than comet Halley.
In this context, the authors inform: 1) A highly dynamical "suprathermal" electron
cloud consistent with observations of peak comet activity near 1 AU.
The authors add: "It is surprising that a weakly outgassing comet at a large heliocentric
distance produces a dynamic and energetic suprathermal electron environment."
2) Electron densities near the nucleus are in the range of 10 to 100 cm³.
Taking into consideration that Comet 67P was ~3 AU at the time of the observations
and displays a tenuous coma, which contradicts all previous expectations,
it is very likely that another mechanism for
accelerating electrons near the nucleus, is in action.
The report continues on considering potential mechanisms for "which processes
account for the density (and flux) enhancements that IES observes are not obvious."
For instance: 1) Photoelectrons. Play a role by ionizing molecules
and creating energetic secondary ions via photoelectron impact dissociation.
2) Ambipolar electric field. The authors comment that
1) "if a well-structured electron field were present, then a well-defined peak
would be expected. Again this points to fine structure in the plasma environment."
The implications are that the electric field is filamentary in nature.
3) Acceleration caused by cross-shock potentials. Interesting enough,
a cross-shock potential, which has been observed to accelerate electrons,
can be thought of as a double layer in an electrochemical experiment.
However, there is no evidence for a bow shock,
or a reverse bow shock at a heliocentric distance of 3.5 AU.
4) Magnetic field compression. However, there was no data
on the direction and magnitude of the solar wind-associated magnetic field.
5) Lower hybrid cyclotron waves due to mass-loaded ions.
The authors settled for this explanation to explain their data.
Surprisingly, the authors conclude that the "suprathermal" electrons
are being accelerated by "an admixture of photoelectrons
and [cyclotron] waves produced by pick-up [or mass-loaded] ion instability."
Their conclusions is contradictory because in the text the authors report on a rich
and dynamical electron cloud, indicating a high density of electrons in the
vicinity of the nucleus, even though Comet 67P displays a tenuous coma density.
Hence, the question is: where are the electrons coming from?
Another contradiction is that mass loading
is associated with a solar wind mean speed deceleration.
However, the authors state that "the ion data do not show a significant deceleration
of the solar wind", suggesting that ion mass loading by the solar wind is negligible.
In addition, the authors write that "unfortunately, there is no instrument
onboard Rosetta that can measure electromagnetic waves at this frequency",
referring to lower hybrid cyclotron frequencies.
These observations might indicate that their conclusion is premature.
From an electrical perspective, electron acceleration is
due to the point charged sitting on an electric field,
which is an indication of the presence of a voltage differential.
To Be Continued...
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