A comet ionospheric model assuming the plasma moves radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has ...previously been tested in a series of case studies associated with the Rosetta mission at comet 67P/Churyumov-Gerasimenko. It has been found that at low activity and within several tens of kilometers from the nucleus such models (which originally were developed for such conditions) generally work well in reproducing observed electron number densities, in particular when plasma production through both photoionization and electron-impact ionization is taken into account. Near perihelion, case studies have, on the contrary, shown that applying similar assumptions overestimates the observed electron number densities at the location of Rosetta. Here we compare Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor-driven model results with Rosetta Plasma Consortium/Mutual Impedance Probe-derived electron number densities for an extended time period (2015 November through 2016 March) during the postperihelion phase with southern summer/spring. We observe a gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations to being in reasonable agreement during 2016 March.
ABSTRACT During 2015 January 9-11, at a heliocentric distance of ∼2.58-2.57 au, the ESA Rosetta spacecraft resided at a cometocentric distance of ∼28 km from the nucleus of comet ...67P/Churyumov-Gerasimenko, sweeping the terminator at northern latitudes of 43°N-58°N. Measurements by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure Sensor (ROSINA/COPS) provided neutral number densities. We have computed modeled electron number densities using the neutral number densities as input into a Field Free Chemistry Free model, assuming H2O dominance and ion-electron pair formation by photoionization only. A good agreement (typically within 25%) is found between the modeled electron number densities and those observed from measurements by the Mutual Impedance Probe (RPC/MIP) and the Langmuir Probe (RPC/LAP), both being subsystems of the Rosetta Plasma Consortium. This indicates that ions along the nucleus-spacecraft line were strongly coupled to the neutrals, moving radially outward with about the same speed. Such a statement, we propose, can be further tested by observations of H3O+/H2O+ number density ratios and associated comparisons with model results.
We present observations from the Rosetta Plasma Consortium of the effects of stormy solar wind on comet 67P/Churyumov‐Gerasimenko. Four corotating interaction regions (CIRs), where the first event ...has possibly merged with a coronal mass ejection, are traced from Earth via Mars (using Mars Express and Mars Atmosphere and Volatile EvolutioN mission) to comet 67P from October to December 2014. When the comet is 3.1–2.7 AU from the Sun and the neutral outgassing rate ∼1025–1026 s−1, the CIRs significantly influence the cometary plasma environment at altitudes down to 10–30 km. The ionospheric low‐energy (∼5 eV) plasma density increases significantly in all events, by a factor of >2 in events 1 and 2 but less in events 3 and 4. The spacecraft potential drops below −20 V upon impact when the flux of electrons increases. The increased density is likely caused by compression of the plasma environment, increased particle impact ionization, and possibly charge exchange processes and acceleration of mass‐loaded plasma back to the comet ionosphere. During all events, the fluxes of suprathermal (∼10–100 eV) electrons increase significantly, suggesting that the heating mechanism of these electrons is coupled to the solar wind energy input. At impact the magnetic field strength in the coma increases by a factor of 2–5 as more interplanetary magnetic field piles up around the comet. During two CIR impact events, we observe possible plasma boundaries forming, or moving past Rosetta, as the strong solar wind compresses the cometary plasma environment. We also discuss the possibility of seeing some signatures of the ionospheric response to tail disconnection events.
Key Points
Rosetta observations of corotating interaction region impacts on comet 67P/C‐G
CIR impacts cause, e.g., cold plasma density increase, pileup of B field, and acceleration of comet plasma
CIR impact could lead to plasma boundary formation or tail disconnection events
We report the detection of large‐amplitude, quasi‐harmonic density fluctuations with associated magnetic field oscillations in the region surrounding the diamagnetic cavity of comet 67P. Typical ...frequencies are ~0.1 Hz, corresponding to ~10 times the water and ≲0.5 times the proton gyro‐frequencies, respectively. Magnetic field oscillations are not always clearly observed in association with these density fluctuations, but when they are, they consistently have wave vectors perpendicular to the background magnetic field, with the principal axis of polarization close to field‐aligned and with a ~90° phase shift w.r.t. the density fluctuations. The fluctuations are observed in association with asymmetric plasma density and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. This is a new type of waves not previously observed at comets. They are likely Ion Bernstein waves, and we propose that they are excited by unstable ring, ring‐beam or spherical shell distributions of cometary ions just outside the cavity boundary. These waves may play an important role in redistributing energy between different particle populations and reshape the plasma environment of the comet.
We present Rosetta RPC case study from four events at various radial distance, phase angle and local time from autumn 2015, just after perihelion of comet 67P/Churyumov-Gerasimenko. Pulse like (high ...amplitude, up to minutes in time) signatures are seen with several RPC instruments in the plasma density (LAP, MIP), ion energy and flux (ICA) as well as magnetic field intensity (MAG). Furthermore the cometocentric distance relative to the electron exobase is seen to be a good organizing parameter for the measured plasma variations. The closer Rosetta is to this boundary, the more pulses are measured. This is consistent with the pulses being filaments of plasma originating from the diamagnetic cavity boundary as predicted by simulations.
We present a detailed study of the cometary ionospheric response to a cometary brightness outburst using in situ measurements for the first time. The comet 67P/Churyumov-Gerasimenko (67P) at a ...heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ~1000 UT on 19 February 2016, characterized by an increase in the coma surface brightness of two orders of magnitude. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of ~0.2 m s
-1
. The onset of the outburst was preceded by pre-outburst decreases in neutral gas density at Rosetta, in local plasma density, and in negative spacecraft potential at ~0950 UT. In response to the outburst, the neutral density increased by a factor of ~1.8 and the local plasma density increased by a factor of ~3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of ~2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (~5 nT) and an abrupt rotation (~36.4°) in response to the outburst. A weakening of 10–100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from ~1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.
The environment of a comet is a fascinating and unique laboratory to study plasma processes and the formation of structures such as shocks and discontinuities from electron scales to ion scales and ...above. The European Space Agency’s Rosetta mission collected data for more than two years, from the rendezvous with comet 67P/Churyumov-Gerasimenko in August 2014 until the final touch-down of the spacecraft end of September 2016. This escort phase spanned a large arc of the comet’s orbit around the Sun, including its perihelion and corresponding to heliocentric distances between 3.8 AU and 1.24 AU. The length of the active mission together with this span in heliocentric and cometocentric distances make the Rosetta data set unique and much richer than sets obtained with previous cometary probes. Here, we review the results from the Rosetta mission that pertain to the plasma environment. We detail all known sources and losses of the plasma and typical processes within it. The findings from in-situ plasma measurements are complemented by remote observations of emissions from the plasma. Overviews of the methods and instruments used in the study are given as well as a short review of the Rosetta mission. The long duration of the Rosetta mission provides the opportunity to better understand how the importance of these processes changes depending on parameters like the outgassing rate and the solar wind conditions. We discuss how the shape and existence of large scale structures depend on these parameters and how the plasma within different regions of the plasma environment can be characterised. We end with a non-exhaustive list of still open questions, as well as suggestions on how to answer them in the future.
Ion acoustic waves were observed between 15 and 30 km from the centre
of comet 67P/Churyumov–Gerasimenko by the Rosetta spacecraft during
its close flyby on 28 March 2015. There are two electron ...populations:
one cold at kBTe≈0.2 eV and one
warm at kBTe≈2 eV. The ions are
dominated by a cold (a few hundredths of electronvolt) distribution of
water group ions with a bulk speed of (3–3.7) km s−1. A
warm kBTe≈6 eV ion population,
which also is present, has no influence on the ion acoustic waves due
to its low density of only 0.25 % of the plasma density.
Near closest approach the propagation direction was within
50∘ from the direction of the bulk velocity. The waves, which
in the plasma frame appear below the ion plasma frequency
fpi≈2 kHz, are Doppler-shifted to the
spacecraft frame where they cover a frequency range up to
approximately 4 kHz.
The waves are detected in a region of space where the
magnetic field is piled up and draped around the inner part of the
ionised coma. Estimates of the current associated with the magnetic
field gradient as observed by Rosetta are used as input to
calculations of dispersion relations for current-driven ion acoustic
waves, using kinetic theory. Agreement between theory and observations
is obtained for electron and ion distributions with the properties
described above. The wave power decreases over cometocentric
distances from 24 to 30 km. The main difference between the
plasma at closest approach and in the region where the waves are
decaying is the absence of a significant current in the latter. Wave
observations and theory combined supplement the particle measurements
that are difficult at low energies and complicated by spacecraft
charging.
Abstract
The highly varying plasma environment around comet 67P/Churyumov–Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new ...operation modes, to enable high time resolution measurements of cometary ions. Two modes were implemented, one having a 4 s time resolution in the energy range 0.3–82 eV/q and the other featuring a 1 s time resolution in the energy range 13–50 eV/q. Comparing measurements made with the two modes, it was concluded that 4 s time resolution is enough to capture most of the fast changes of the cometary ion environment. The 1462 h of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84 per cent of these sequences into one of five categories, depending on their appearance in an energy–time spectrogram. The ion environment is generally highly dynamic, and variations in ion fluxes and energies are seen on time-scales of 10 s to several minutes.
In 2014 September, as Rosetta transitioned to close bound orbits at 30 km from comet 67P/Churyumov-Gerasimenko, the Rosetta Plasma Consortium Langmuir probe (RPC-LAP) data showed large systematic ...fluctuations in both the spacecraft potential and the collected currents. We analyse the potential bias sweeps from RPC-LAP, from which we extract three sets of parameters: (1) knee potential, that we relate to the spacecraft potential, (2) the ion attraction current, which is composed of the photoelectron emission current from the probe as well as contributions from local ions, secondary emission, and low-energy electrons, and (3) an electron current whose variation is, in turn, an estimate of the electron density variation. We study the evolution of these parameters between 4 and 3.2 au in heliocentric and cometocentric frames. We find on September 9 a transition into a high-density plasma region characterized by increased knee potential fluctuations and plasma currents to the probe. In conjunction with previous studies, the early cometary plasma can be seen as composed of two regions: an outer region characterized by solar wind plasma, and small quantities of pick-up ions, and an inner region with enhanced plasma densities. This conclusion is in agreement with other RPC instruments such as RPC-MAG, RPC-IES and RPC-ICA, and numerical simulations.
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