Oral presentations and abstracts

Superthermal electron precipitation is one of the main sources supporting the Mars nightside ionosphere. It is expected that solar wind electron fluxes are to increase significantly during interplanetary coronal mass ejections (ICME) and therefore an enhanced nightside ionospheric density. This study is to quantify the variation of the precipitating and deposited electron fluxes during five of the most extreme ICMEs encountered by Mars Global Surveyor (MGS). We find energy fluxes correlate better with the upstream dynamic pressure proxy than number fluxes and electron fluxes increase more at high energies, which means electrons tend to have a lower peak production altitude during storm times. The precipitating and net/deposited fluxes are increased up to an order of magnitude from low to high dynamic pressures. The estimated total electron content (TEC) is a few times of 10¹⁴ m^-2 for quiet times and on the order of 10¹⁵ m^-2 for storm times, with an enhancement up to an order of magnitude locally near strong crustal fields. Crustal magnetic fields have an effect on the deposited fluxes with more prominent magnetic reflection over strong magnetic fields during quiet periods, which is significantly reduced during storm times. Lastly, we estimate a global energy input from downward fluxes of 1.3×10⁸ W and 5.5×10⁸ W and the globally deposited energy from net fluxes of 2.3×10⁷ W and 1.6×10⁸ W for quiet and storm time periods, a factor of 4 and 7 enhancement globally, respectively, but up to an order of magnitude locally near strong crustal fields.

How to cite: Xu, S., Curry, S., Mitchell, D., Luhmann, J., Lillis, R., and Dong, C.: Superthermal electron deposition on the Mars nightside during ICMEs, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-408, https://doi.org/10.5194/epsc2020-408, 2020.

We present an analysis of radar blackouts observed by MARSIS on Mars Express and SHARAD on Mars Reconnaissance Orbiter for the interval 2006 – 2017.  The period of interest encompasses the extended solar minimum between solar cycles 23 and 24 as well as the solar maximum of cycle 24.  Blackouts have been identified by eye through scanning daily plots of the surface reflection for both radars.  A blackout occurs when, for no apparent instrumental reason, the surface reflection normally expected is either not observed (total) or when the surface reflection is seen for only part of the orbit or the surface reflection is both weaker and spread over a significant time delay (partial).  Such blackouts are caused by enhanced ionisation at altitudes below the main ionospheric electron density peak resulting in increased absorption of the radar signal.  There are more occurrences observed by MARSIS than SHARAD, which is expected due to the lower absorption at the higher operating frequency of SHARAD.  We also observe more blackouts during solar maximum than solar minimum.  Indeed, there are no total blackouts during the extended solar minimum, although both radars do have partial blackouts.  There is no apparent relationship between blackout occurrence and crustal magnetic fields.  Following previous work, which has indicated that solar energetic particles, specifically electrons are responsible for the enhanced ionisation in the atmosphere, we also present the analysis of the MAVEN SEP electrons between 20 keV and 2 MeV during events when all three spacecraft were operational.  We find that the SEP electron flux-energy relationship is much enhanced during the total blackouts, in particular where both radars are impacted, while for partial blackouts the flux-energy spectrum is closer to those from orbits where no blackout occurs.  We also find that for certain events, the average spectrum which result in a blackout is particularly enhanced at the higher energy end of the spectrum, above 50 keV. The average spectra from each condition is presented.  We conclude that there is a higher probability of a radar blackout during solar maximum, that crustal magnetic fields play no apparent role in the their observational occurrence, that the higher energy (< 50 keV) electrons are responsible, and that for events where both radars observe a radar blackout the SEP electron fluxes are at their highest.

How to cite: Lester, M., Sanchez-Cano, B., Potts, D., Lillis, R., Orosei, R. I., Campbell, B., Witasse, O., Opgenoorth, H., Milan, S., Cartacci, M., Bernardini, F., Perry, M., Putzig, N., Blelly, P.-L., and Leblanc, F.: A Statistical Analysis of Radar Blackouts at Mars: MARSIS, SHARAD and MAVEN Observations, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-687, https://doi.org/10.5194/epsc2020-687, 2020.

The Martian interaction with the solar wind is unique due to the influence of remanent crustal magnetic fields. The recent studies by the Mars Express and Mars Atmosphere and Volatile Evolution missions underline the strong and complex influence of the crustal magnetic fields on the Martian environment and its interaction with the solar wind. Among them is the influence on the dynamic plasma boundaries that shape this interaction and on the bow shock in particular.

Compared to other drivers of the shock location (e.g. solar dynamic pressure, extreme ultraviolet fluxes), the influence of crustal magnetic fields are less understood, with essentially differences observed between the southern and northern hemispheres attributed to the crustal fields. In this presentation we analyze in detail the influence of the crustal fields on the Martian shock location by combining for the first time datasets from two different spacecraft (MAVEN/MEX). An application of machine learning techniques will also be used to increase the list of MAVEN shocks published to date. We show in particular the importance for analyzing biases due to multiple parameters of influence through a partial correlation approach. We also compare the impact of crustal fields with the other parameters of influence, and show that the main drivers of the shock location are by order of importance extreme ultraviolet fluxes and magnetosonic Mach number, crustal fields and then solar wind dynamic pressure.

How to cite: Garnier, P., Jacquey, C., Mazelle, C., Fang, X., Gruesbeck, J., Hall, B., Masunaga, K., Halekas, J., Espley, J., Jakosky, B., Genot, V., and Penou, E.: The influence of crustal magnetic fields on the Martian bow shock : a statistical analysis of Mars Volatile EvolutioN and Mars Express observations, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-819, https://doi.org/10.5194/epsc2020-819, 2020.

Next to its main constituent O₂⁺, the Martian ionosphere consists of several other ion species, like CO₂⁺, O⁺, CO⁺, HCO⁺, N2⁺, etc. The ionospheric escape is dominated by O₂⁺ and O⁺ ions, and as a result the escape of these species is well studied. The other, minor ion species are more difficult to measure in the escaping plasma, because their contribution is typically obscured in the mass spectra of ion instruments by the more abundant O₂⁺ peak.

In this study we use data from the SupraThermal And Thermal Ion Composition instrument (STATIC) on board MAVEN to investigate the escape of these ions. We use a peak-fitting method to separate the contribution of several ion species, including O₂⁺, CO₂⁺, O⁺ and ions with a mass between 28-30 AMU. Our method is validated against Neutral Gas and Ion Mass Spectrometer (NGIMS), also onboard MAVEN, and results in the ionosphere agree qualitatively very well.

We apply this method to STATIC data from January 2016 until May 2019 to perform a statistical study examining the escape of low energy (<100 eV) heavy (>=16 AMU) ions throughout the Martian magnetosphere and its surrounding. We find that CO₂⁺ ions do escape through the tail but at a very limited rate, namely at less than 1% of the O₂⁺ escape rate. Ions with a mass between 28-30 AMU, however, are found to constitute a significant part of the ionospheric outflow, with an escape rate 30% of the O₂⁺ rate and 15% of the total heavy ion escape.

How to cite: Maes, L., Fraenz, M., McFadden, J., and Benna, M.: The escape of CO2+ and other heavy minor ions from Mars, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-410, https://doi.org/10.5194/epsc2020-410, 2020.

Introduction

The Plasma Consortium Ion Composition Analyzer (RPC/ICA) onboard ESA's Rosetta mission has revealed that comet-origin water ions are accelerated in a more complicated manner than previously thought^[1]. Understanding these acceleration mechanisms in a cometary environment is an important issue in both cometary science and plasma physics. Very close to the comet, ambipolar diffusion accelerates newly ionized (by solar Extreme-UV (EUV) radiation, electron impact ionization, or charge-exchange) outgassing water molecules from the comet. If moderately far from the comet where solar winds can access, water ions are picked up by the solar wind and accelerated by the solar wind electric field^[1].

However, water ions are also accelerated in the magnetosphere, where solar wind ions cannot intrude and background neutrals may play some role. The acceleration mechanism there is not fully understood due to an unknown electric field environment. One common characteristic as observed by ICA near in the comet 67P/Churyumov-Gerasimenko is energy-angle dispersion when the acceleration is > 100 eV^[2,3].  If the acceleration is > 300 eV (particularly > 1 keV), we found that dispersion becomes reversed for almost all cases. In our presentation, we show two typical types of energy-angle dispersions. We also discuss a possible acceleration mechanism.

Instrument

RPC-ICA measures the distribution function of positively charged ions with mass-separation capability, has a field of view (FOV) of 360° with 22.5° resolution in azimuthal angle, and scanning 90° in the elevation direction over 192 sec with ~ 5° resolution for each elevation scan.  To survey all the comet-rendezvous data from 2014 August to 2016 September, we used one-hour quick-look (QL) data as shown in Figure 1 and Figure 3. The magnetometer RPC-MAG measures the three components of the magnetic field, with an accuracy of ~ ±3 nT per component.

Observation

1. Nose-dispersion:  Figure 1 shows the energy-time, energy-angle, and energy-mass spectrograms of positively charged ions from 7 December 2015, 23:00-24:00 UTC. In Figure 1, the direction of the energy-elevation dispersions is reversed between above 500 eV and below 200 eV, making the entire pattern nose-like.  This pattern is also seen in the integrated energy-elevation spectrograms in Figure 1b.  Figure 1c indicates that they are heavy ions whereas the solar wind ions (H⁺, He₂⁺, He⁺) are not observed. The energy of the "nose", where the direction of dispersion reversed, is about 300-500 eV. We find 68 events (in terms of 1-hour QL data) for this type of dispersion with maximum energy > 1 keV out of 12268 hours QL data during 2 years.

Next, we investigate where water ions come from with respect to the directions of the sun, the comet, and the magnetic field. We note that, although high-energy water ions (> 200 eV) are dispersed in the elevation plane, lower energy water ions (< 200 eV) are spread wider in the azimuthal direction than higher energy ions as shown in Figure 1d. Still, Figure 2 shows that the H₂O⁺ flow direction is close to the magnetic field direction, and the pitch-angles are larger at mid energy than low and high energies, causing the nose-dispersion as illustrated in Figure 5, in addition to different (and limited) gyrophase angles for different energy.

2. Sharp-dispersion: 

In Figures 3 and 4, we show another type of dispersion event with maximum energy > 1 keV.  In this example, water ions are dispersed more "sharply" in the elevation plane. The maximum energy exceeds 1 keV, and the energy where these "sharp dispersion" events seem to be reversed is also about 100-300 eV. We find 51 events during 2 years of observation (see, Table 1).  Figure 4 indicates that higher energy water ions (>300 eV) are dispersed in the elevation plane and lower energy water ions are spread widely in the azimuthal direction, which is in good agreement with the nose-like events. However, higher energy water ions come from the direction quasi-perpendicular to the magnetic field and lower energy water ions come from the nucleus of the comet along magnetic field. This means that sharp-like dispersion occurs separately in different energy ranges in the example.