PS4.1

We perform a comprehensive statistical study of plasma wave activity observed in the electric field measurements obtained by the Langmuir probe instrument (RPC-LAP) onboard ESA's Rosetta spacecraft, which followed the comet 67P/Churyumov-Gerasimenko in its orbit around the sun for over two years in 2014-2016. We focus on waves in the range 1-30 Hz, roughly corresponding to the lower-hybrid frequency range. Here, electric field oscillations close to the local H₂O⁺ lower hybrid frequency are common and collocated with sharp plasma density gradients, suggesting generation by the lower hybrid drift instability. We compare statistically the properties of the waves to the theoretical predictions on lower-hybrid wave generation by the lower hybrid drift instability, regarding e.g. amplitude dependence on plasma density gradients. We also examine the data for waves that can be attributed to other instabilities, such as various velocity-space anisotropies that may occur in the cometary plasma. We correlate the comet-related parameters, (relative spacecraft position, solar distance, plasma and neutral gas density, etc.) with wave-related parameters, such as amplitude/spectral density and frequency. This investigation greatly helps to clarify the importance of the plasma waves in different regions of the cometary plasma environment. 

How to cite: Odelstad, E., Karlsson, T., Eriksson, A., and Johansson, F.: Plasma waves in Rosetta electric field observations in the plasma environment of comet 67P/Churyumov-Gerasimenko, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3437, https://doi.org/10.5194/egusphere-egu21-3437, 2021.

The Rosetta spacecraft arrived at comet 67P in August 2014 and then escorted it for 2 years along its orbit. Throughout this escort phase, two plasma instruments (Mutual Impedance Probe, MIP; and Langmuir Probe, LAP) measured a population of cold electrons (< 1 eV) within the coma of 67P (Engelhardt et al., 2018; Wattieaux et al, 2020; Gilet et al., 2020). These cold electrons are understood to be formed by cooling warm electrons through collisions with the neutral gas. The warm electrons are primarily newly-born and produced at roughly 10eV within the coma through ionisation. While it was no surprise that cold electrons would form near perihelion given the high density of the neutral coma, the persistence of the cold electrons up to a heliocentric distance of 3.8 au was highly unexpected. With the low outgassing rates observed at such large heliocentric distances (Q < 10²⁶ s^-1), there should not be enough neutral molecules to cool the warm electrons efficiently before they ballistically escape the coma.

We use a collisional test particle model to examine the formation of the cold electron population at a weakly outgassing comet. The electrons are subject to stochastic collisions with the neutral coma which can either scatter or cool the electrons. Multiple electron neutral collision processes are included such that the electrons can undergo elastic scattering as well as collisions inducing excitation and ionisation of the neutral species. The inputted electric and magnetic fields, which act on the test particles, are taken from a 3D fully-kinetic, collisionless Particle-in-Cell (PiC) model of the solar wind and cometary ionosphere (Deca et al., 2017; 2019), with the same neutral coma as used in our model. We use a pure water coma with spherical symmetry and a 1/r² dependence in the neutral number density to drive the production of cometary electrons and the electron-neutral collisions.

We first demonstrate the trapping of electrons in a potential well around the comet nucleus, formed by an ambipolar field. We show how this electron-trapping process can lead to more efficient cooling of electrons and the subsequent formation of a cold electron population, even at low outgassing rates.

How to cite: Stephenson, P., Galand, M., Deca, J., Henri, P., and Carnielli, G.: Electron cooling at a weakly outgassing comet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8616, https://doi.org/10.5194/egusphere-egu21-8616, 2021.

The formation and maintenance of the diamagnetic cavity around comets is a debated subject. For active comets such as 1P/Halley, the ion-neutral drag force is suggested to balance the outside magnetic pressure at the cavity boundary, but measurements made by Rosetta at the intermediately active comet 67P/Churyumov-Gerasimenko indicate that the situation might be different at less active comets. Measurements from the Langmuir probes and the Mutual Impedance Probe on board Rosetta, as well as modelling efforts, show ion velocities significantly above the velocity of the neutral particles, indicating that the ions are not as strongly coupled to the neutrals at comet 67P.

In this study we use low-energy high time resolution data from the Ion Composition Analyzer (ICA) on Rosetta to determine the bulk speeds and temperatures of the ions inside the diamagnetic cavity of comet 67P. The interpretation of the low-energy data is not straight forward due to the complicated influence of the spacecraft potential, but a newly developed method utilizing simulations with the Spacecraft Plasma Interaction Software (SPIS) software makes it possible to extract the original properties of the ion distribution. We use SPIS to model the influence of the spacecraft potential on the energy spectrum of the ions, and fit the energy spectrum sampled by ICA to the simulation results. This gives information about both the bulk speed and temperature of the ions.

The results show bulk speeds of 5-10 km/s, significantly above the speed of the neutral particles, and temperatures of 0.7-1.6 eV. The major part of this temperature is attributed to ions being born at different locations in the coma, and could hence be considered a dispersion rather than a temperature in the classical sense. The high bulk speeds support previous results, indicating that the collisional coupling between ions and neutrals is weak inside the diamagnetic cavity.

How to cite: Bergman, S., Stenberg Wieser, G., Wieser, M., Johansson, F. L., Vigren, E., Nilsson, H., Nemeth, Z., Eriksson, A., and Williamson, H.: Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4043, https://doi.org/10.5194/egusphere-egu21-4043, 2021.

The discrete aurorae on Mars were discovered with the SPICAM spectrograph on board Mars Express. Now, they have been analyzed in detail using the much more sensitive MAVEN/IUVS imaging spectrograph.

This presentation gives a summary of the very latest results obtained by Schneider et al. and Soret et al. on this topic.

The main conclusions are the following:

• the number of auroral event detections has considerably increased since the Mars Express observations;
• many detections have been made outside of the Southern crustal magnetic field structures;
• the MUV spectrum shows the same emissions as those observed in the dayglow, with similar intensity ratios;
• the Vegard-Kaplan bands of N₂ have been observed for the first time in the Martian aurora;
• the CO Cameron and the CO₂⁺ UVD emissions occur at the same altitude;
• the OI emission at 297.2 nm has been analyzed;
• the CO Cameron/CO₂⁺ UVD ratio is quasi-constant;
• intensities are higher in B-field regions;
• auroral emissions are more frequent in the pre-midnight sector;
• the altitude of the emission layer is independent of local time and presence or absence of a crustal magnetic field;
• the altitude of the emission layer varies moderately with season (atmospheric effect);
• the events are spatially correlated with an increase in the flux of energetic electrons simultaneously measured by the MAVEN/SWEA (Solar Wind Electron Analyzer) detectors;
• the peak altitude of the emission is in good agreement with that expected from the average electron energy.

How to cite: Soret, L., Milby, Z., Gérard, J.-C., Schneider, N., Jain, S., and Ritter, B.: All that is known about Mars discrete aurorae so far, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5187, https://doi.org/10.5194/egusphere-egu21-5187, 2021.

Solar Energetic Particle (SEP) and the Imaging UltraViolet Spectrograph (IUVS) instruments on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have discovered diffuse aurora that spans across nightside Mars, which resulted from the interaction of Solar Energetic Particles (SEPs) with Martian atmosphere [Schneider et al., 2015]. Previous models showed that 100 keV monoenergetic electron precipitation should have been at the origin of the low altitude (~60 km) peak of the limb emission, however, no models were able to reproduce the observed emission profiles by using the observed electron energy population [e.g. Haider et al., 2019]. Previous auroral emission models did not take into account the contribution of MeV proton precipitation, although MeV proton can penetrate down to ~60 km altitude as well [e.g., Jolitz et al., 2017]. This study aims to model SEP induced diffuse auroral emission by both electrons and protons.

We have developed a Monte-Carlo collision and transport model of SEP electrons and protons with magnetic fields on Mars. We calculated limb intensity profile of CO₂⁺ ultraviolet doublet (UVD) due to precipitation of electrons and protons with energy ranging 100eV-100keV and 100eV-5MeV, respectively, during December 2014 SEP event and September 2017 SEP event by using electron and ion fluxes observed by MAVEN/SEP, SWEA and SWIA.

The calculated peak limb intensity of CO₂⁺ UVD due to precipitation of protons is 3-5 times larger than that due to precipitation of electrons during both December 2014 and September 2017 SEP events, which suggests that protons can make brighter CO₂⁺ UVD emission than electrons. Peak altitude of limb intensity profiles of CO₂⁺ UVD due to precipitation of electrons and protons are both 10 - 20 km higher than the observation, a discrepancy could be explained by the uncertainty in the electron and proton fluxes that precipitate into the nightside Mars.

We have tested an effect of crustal field on the emission of CO₂⁺ UVD. CO₂⁺ UVD emission due to the precipitating electrons are depleted by a factor of 10 in the region of open crustal field and disappeared in the region of closed and parallel crustal field, whereas emission due to the precipitating protons does not change significantly. Further observations of diffuse aurora in the crustal field region should be needed to constrain the origin of diffuse aurora on Mars.

How to cite: Nakamura, Y., Terada, N., Nakagawa, H., Sakai, S., Hiruba, S., Kataoka, R., Murase, K., and Leblanc, F.: Modeling of SEP induced auroral emission at Mars: Contribution of precipitating protons and effects of crustal fields, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5649, https://doi.org/10.5194/egusphere-egu21-5649, 2021.