Space environments of unmagnetized or weakly magnetized solar system bodies and the effects of space weather on these systems

The ionospheres and (induced) magnetospheres of unmagnetized and weakly magnetized bodies with substantial atmospheres (e.g. Mars, Venus, Titan, Pluto and comets) are subject to disturbances due to solar activity, interplanetary conditions (e.g. solar flares, coronal mass ejections and solar energetic particles) or for moons parent magnetospheric activity. They interact similarly as their magnetized counterparts but with scientifically important differences.
As an integral part of planetary atmospheres, ionospheres are tightly coupled with the neutral atmosphere, exosphere and surrounding plasma environment, possessing rich compositional, density, and temperature structures. The interaction among neutral and charged components affects atmospheric loss, neutral winds, photochemistry, and energy balance within ionospheres.
This session invites abstracts concerning remote and in-situ data analysis, modelling studies, comparative studies, instrumentation and mission concepts for unmagnetized and weakly magnetized solar system bodies.

Co-organized by ST4
Convener: Martin Volwerk | Co-conveners: Charlotte Goetz, Pierre Henri
vPICO presentations
| Thu, 29 Apr, 11:45–12:30 (CEST)

vPICO presentations: Thu, 29 Apr

Tsubasa Kotani, Masatoshi Yamauchi, Hans Nilsson, Gabriella Stenberg-Wieser, Martin Wieser, Sofia Bergman, Satoshi Taguchi, and Charlotte Götz

The ESA/Rosetta spacecraft has studied the comet 67P/Churyumov-Gerasimenko for two years. Rosetta Plasma Consortium's Ion Composition Analyser (RPC/ICA) detected comet-origin water ions that are accelerated to > 100 eV.  Majority of them are interpreted as ordinary pick-up acceleration  by the solar wind electric field perpendicular to the magnetic field during low comet activity [1,2]. As the comet approaches the sun, a comet magnetosphere is formed, where solar winds cannot intrude.

However,  some water ions are accelerated to > 1 keV even in the magnetosphere [3]. Using RPC/ICA data during two years [4], we investigate the acceleration events > 1 keV where solar winds are not observed, and classify dispersion events with respect to the directions of the sun, the comet, and the magnetic field.  Majority of these water ions show reversed energy-angle dispersion. Results of the investigation also show that these ions are flowing along the (enhanced) magnetic field, indicating that the parallel acceleration occurs in the magnetosphere.

In this meeting, we show a statistical analysis and discuss a possible acceleration mechanism.


[1] H. Nilsson et al., MNRAS 469, 252 (2017), doi:10.1093/mnras/stx1491

[2] G. Nicolau et al., MNRAS 469, 339 (2017), doi:10.1093/mnras/stx1621

[3] T. Kotani et al., EPSC, EPSC2020-576 (2020),

[4] H. Nilsson et al., Space Sci. Rev., 128, 671 (2007), DOI: 10.1007/s11214-006-9031-z 

How to cite: Kotani, T., Yamauchi, M., Nilsson, H., Stenberg-Wieser, G., Wieser, M., Bergman, S., Taguchi, S., and Götz, C.: Statistical analysis of the accelerated H2O ions above 1 keV: the comet 67P/Churyumov–Gerasimenko observed by the Rosetta spacecraft., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-378,, 2020.

Elias Odelstad, Tomas Karlsson, Anders Eriksson, and Fredrik Johansson

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,, 2021.

Peter Stephenson, Marina Galand, Jan Deca, Pierre Henri, and Gianluca Carnielli

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,, 2021.

Sofia Bergman, Gabriella Stenberg Wieser, Martin Wieser, Fredrik Leffe Johansson, Erik Vigren, Hans Nilsson, Zoltan Nemeth, Anders Eriksson, and Hayley Williamson

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,, 2021.

Hayley Williamson, Hans Nilsson, Anja Moslinger, Sofia Bergman, and Gabriella Stenberg-Wieser

Defined as the region where the plasma interaction region of a comet goes from being solar wind-dominated to cometary ion-dominated, the cometopause is a region of comingling plasmas and complex dynamics. The Rosetta mission orbited comet 67P/Churyumov-Gerasimenko for roughly two years. During this time, the cometopause was observed by the Ion Composition Analyzer (ICA), part of the Rosetta Plasma Consortium (RPC), before and after the spacecraft was in the solar wind ion cavity, defined as the region where no solar wind ions were measured. Data from ICA shows that solar wind and cometary ions have similar momentum and energy flux moments during this transitional period, indicating mass loading and deflection of the solar wind. We examine higher order moments and distribution functions for the solar wind and cometary species between December 2015 and March 2016. The behavior of the solar wind protons indicates that in many cases these protons are deflected in a sunward direction, while the cometary ions continue to move predominately antisunward. By studying the distribution functions of the protons during these time periods, it is possible to see a non-Maxwellian energy distribution. This can inform on the nature of the cometopause boundary and the energy transfer mechanisms at play in this region.

How to cite: Williamson, H., Nilsson, H., Moslinger, A., Bergman, S., and Stenberg-Wieser, G.: Ion energy and momentum flux near the cometopause of Comet 67P/Churyumov-Gerasimenko, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14280,, 2021.

Richard Haythornthwaite, Andrew Coates, Geraint Jones, Anne Wellbrock, Hunter Waite, Véronique Vuitton, and Panayotis Lavvas


Titan is the largest moon of Saturn and has a thick extended atmosphere along with a large ionosphere. Titan's ionosphere contains a plethora of hydrocarbons and nitrile cations and anions as measured by the Ion Neutral Mass Spectrometer and Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft1.

Previous ion composition studies in Titan’s ionosphere by Cassini instruments revealed "families" of ions around particular mass values and a regular spacing of 12 to 14 u/q between mass groups 2. These are thought to be related to a carbon or nitrogen backbone that dominates the ion chemistry2. Previous studies also identified possible heavy ions such as naphthalene, anthracene derivatives and an anthracene dimer at 130, 170 and 335 u/q respectively1



               The CAPS Ion Beam Spectrometer3 is an electrostatic analyser that measures energy/charge ratios of ions. During the Titan flybys Cassini had a high velocity (~6 km/s) relative to the low ion velocities (< 230 m/s) observed in the ionosphere. The ions were also cold, having ion temperatures around 150K. The combination of these factors meant that the ions appeared as a highly-directed supersonic beam in the spacecraft frame. This means the ions appear at kinetic energies associated with the spacecraft velocity and the ion mass, therefore the measured energy spectra (eV/q) can be converted to mass spectra (u/q).


Results and Conclusions

Positive ion masses between 170 and 310 u/q are examined with ion mass groups identified between 170 and 275 u/q containing between 14 and 21 heavy (carbon/nitrogen/oxygen) atoms4. These groups are the heaviest positive ion groups reported so far from the available in situ ion data at Titan.

The ion group peaks are found to be consistent with masses associated with Polycyclic Aromatic Compounds, including Polycyclic Aromatic Hydrocarbon (PAH) and nitrogen-bearing polycyclic aromatic molecular ions. The ion group peak identifications are compared with previously proposed neutral PAHs5 and are found to be at similar masses, supporting a PAH interpretation. The spacing between the ion group peaks is also investigated, finding a spacing of 12 or 13 u/q indicating the addition of C or CH. Lastly, the occurrence of several ion groups is seen to vary across the five flybys studied, possibly relating to the varying solar radiation conditions observed across the flybys.

The discovery of these groups will aid future atmospheric chemical models of Titan through identification of prominent heavy positive ions and further the understanding between the low mass ions and the high mass negative ions, as well as the process of aerosol formation in Titan's atmosphere.


1. Waite et al., The Process of Tholin Formation in Titan’s Upper Atmosphere, Sci., 2007, doi:10.1126/science.1139727

2. Crary et al., Heavy ions, temperatures and winds in Titan's ionosphere: Combined Cassini CAPS and INMS observations, P&SS, 2009, doi:10.1016/j.pss.2009.09.006.

3. Young et al., Cassini Plasma Spectrometer Investigation. Space Sci. Rev., 2004, doi:10.1007/s11214-004-1406-4

4. Haythornthwaite et al., Heavy Positive Ion Groups in Titan's Ionosphere from Cassini Plasma Spectrometer IBS Observations, eprint arXiv:2009.08749

5. López-Puertas et al., Large Abundances of Polycyclic Aromatic Hydrocarbons in Titan's Upper Atmosphere, ApJ, 2013, doi:10.1088/0004-637X/770/2/132

How to cite: Haythornthwaite, R., Coates, A., Jones, G., Wellbrock, A., Waite, H., Vuitton, V., and Lavvas, P.: Heavy positive ion groups in Titan's ionosphere: Cassini Plasma Spectrometer IBS observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2726,, 2021.

Lauriane Soret, Zachariah Milby, Jean-Claude Gérard, Nick Schneider, Sonal Jain, and Birgit Ritter

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,, 2021.

Yuki Nakamura, Naoki Terada, Hiromu Nakagawa, Shotaro Sakai, Sayano Hiruba, Ryuho Kataoka, Kiyoka Murase, and François Leblanc

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,, 2021.

Mark Lester, Beatriz Sanchez-Cano, Daniel Potts, Rob Lillis, Marco Cartacci, Fabrizio Bernardini, Roberto Orosei, Matthew Perry, Nathaniel Putzig, Bruce Campbell, Pierre-Louis Blelly, Steve Milan, Hermann Opgenoorth, and Olivier Witasse

We present the first long-term characterization of the lower ionosphere of Mars, a region previously inaccessible to orbital observations, based on an analysis of radar echo blackouts observed by MARSIS on Mars Express and SHARAD on the Mars Reconnaissance Orbiter from 2006 to 2017.  A blackout occurs when the expected surface reflection is partly to fully attenuated for portions of an observation.  Enhanced ionization at altitudes of 60 to 90 km, below the main ionospheric electron density peak, results in the absorption of the radar signal, leading to a radar blackout.  MARSIS, operating at frequencies between 1.8 and 5 MHz suffered more blackouts than SHARAD, which has a higher carrier frequency (20 MHz).  More events are seen during solar maximum while  there is no apparent relationship between blackout occurrence and crustal magnetic fields. Blackouts do occur during both nightside and dayside observations, and have an interesting variation with solar zenith angle.   Analysis of MAVEN Solar Energetic Particle (SEP) electron counts between 20 and 200 keV during selected events demonstrates that these electrons are responsible for such events, and we investigate the minimum SEP electron fluxes required to ionize the lower atmosphere and produce  measurable attenuation.  When both radars observe a radar blackout at the same time, the SEP electron fluxes are at their highest. For certain events, we find that the average spectrum responsible for a blackout is particularly enhanced at the higher energy end of the spectrum, i.e. above 70 keV .   This study is, therefore, important for future communications for human exploration of Mars.

How to cite: Lester, M., Sanchez-Cano, B., Potts, D., Lillis, R., Cartacci, M., Bernardini, F., Orosei, R., Perry, M., Putzig, N., Campbell, B., Blelly, P.-L., Milan, S., Opgenoorth, H., and Witasse, O.: Energetic particles and radar blackouts at Mars, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11997,, 2021.

Laila Andersson, Scott Thaller, Christopher Fowler, Gina DiBraccio, and Kai Poh

How the heavy ionospheric ions escape the Martian atmosphere is still not solved. Missions such as the Mars Express (MEX) satellite have observed significant heavy ions (O2+ and Co2+) on the night side of the terminator. The hot oxygen corona when ionized gives rise to the pickup ions but they are of lighter mass.  With the more comprehensive instrumentation on the MAVEN mission, it is clear that cold heavy ions are transported down the tail of the planet. However, there has not yet been a good explanation of how heavy ions can reach into the Martian sheath in high density concentrations. In December 2020 the MAVEN satellite was observing on the dusk side tailward of the terminator with an orbital configuration allowing the density changes and the ion compositions to be followed. In this presentation the focus is on three subsequent orbits where a channel of heavy ions with high densities reaches out into the sheath. In this presentation we will argue for different possible processes that could explain the observations.

How to cite: Andersson, L., Thaller, S., Fowler, C., DiBraccio, G., and Poh, K.: Ionospheric Plasma Transported into the Martian Magnetosheath, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13920,, 2021.

David Andrews, Laila Andersson, Robert Ergun, Anders Eriksson, Marcin Pilinski, and Katerina Stergiopoulou

Recent Mars Express and MAVEN observations have shown the extent to
which Mars's crustal fields, though weak in absolute magnitude,
nevertheless exert significant control over the structure of the ionosphere
over a range of altitudes. However, quantifying this control remains
challenging given the generally dynamic nature of the Mars solar wind
interaction, and the therefore naturally varying densities and temperatures
of the upper ionosphere in particular. In this study we examine MAVEN
Langmuir Probe and Waves data, and show for the first time a very clear
correspondence between the structure of the crustal fields and both the
measured electron temperatures and densities. Electron temperatures are
shown to be systematically lower in regions of strong crustal fields over a
wide altitude range. We speculate on the origins of this deviation.

How to cite: Andrews, D., Andersson, L., Ergun, R., Eriksson, A., Pilinski, M., and Stergiopoulou, K.: Martian crustal magnetic fields: influences on the ionosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14397,, 2021.