Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020

Oral presentations and abstracts

TP7

Ionospheres are a fundamental part of planetary and cometary atmospheres that are formed by solar radiation and are affected by a myriad of different processes, such as space weather activity or neutral atmosphere variations. Moreover, ionospheres play an important role in controlling the dynamics of the system, as they are the link between the neutral atmosphere, exosphere and surrounding plasma environments (e.g. the solar wind for Mars, Venus, Pluto and comets, and the Kronian magnetosphere for Titan). Understanding how each unmagnetized body reacts to all these factors is a key in comparative aeronomy because although a priori all of them have a general similar behavior, they also have scientifically important differences caused by their different natures.

This session focuses on the ionospheres of Mars, Venus, Pluto, Titan, and comets, and solicits abstracts concerning remote and in situ data analysis, modeling studies, instrumentation and mission concepts. Topics may include, but are not limited to, day and night side ionospheric variability, sources and influences of ionization, ion-neutral coupling, current systems, comparative ionospheric studies, and solar wind-ionosphere interactions and responses of the ionized and neutral regimes to transient space weather events. Abstracts on general plasma and escape processes are also welcome.

Co-organized by OPS
Convener: Beatriz Sanchez-Cano | Co-conveners: Matteo Crismani, Niklas Edberg, Xiaohua Fang, Francisco González-Galindo

Session assets

Session summary

Chairperson: Beatriz Sanchez-Cano, Niklas Edberg
EPSC2020-408ECP
Shaosui Xu, Shannon Curry, David Mitchell, Janet Luhmann, Robert Lillis, and Chuanfei Dong

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.

Chairperson: Beatriz Sanchez-Cano, Niklas Edberg
EPSC2020-687
Mark Lester, Beatriz Sanchez-Cano, Daniel Potts, Rob Lillis, Roberto i Orosei, Bruce Campbell, Olivier Witasse, Hermann Opgenoorth, Steve Milan, Marco Cartacci, Fabrizio Bernardini, Matthew Perry, Nathaniel Putzig, Pierre-Louis Blelly, and Francois Leblanc

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.

Chairperson: Beatriz Sanchez-Cano, Niklas Edberg
EPSC2020-819
Philippe Garnier, Christian Jacquey, Christian Mazelle, Xiaohua Fang, Jacob Gruesbeck, Benjamin Hall, Kei Masunaga, Jasper Halekas, Jared Espley, Bruce Jakosky, Vincent Genot, and Emmanuel Penou

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.

Chairperson: Beatriz Sanchez-Cano, Niklas Edberg
EPSC2020-410
Lukas Maes, Markus Fraenz, James McFadden, and Mehdi Benna

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.

Chairperson: Beatriz Sanchez-Cano, Niklas Edberg
EPSC2020-576ECP
Tsubasa Kotani, Masatoshi Yamauchi, Hans Nilsson, Gabriella Stenberg-Wieser, Martin Wieser, Sofia Bergman, Satoshi Taguchi, and Charlotte Götz

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.

Interpretation

In previous studies where acceleration of water ions < several hundred eV is reported, dispersion direction is not reversed and therefore we could interpret it as acceleration by the electric field that is simply a transition from the solar wind electric field and the polarization (ambipolar type) electric fields[2, 4].  The sharp-dispersion event can be understood along this view, i.e., low energy water ions ( < several hundred eV) are accelerated along the magnetic field by the ambipolar electric field and the high energy water ions might be accelerated quasi-perpendicular to the magnetic field by the solar wind electric field. However, these electric fields are not enough to explain nose-like reversed dispersion. To explain the reversal of the dispersion direction, just magnetic curvature in not sufficient as illustrated in Figure 5a. In this configuration, we can explain the dispersion at lower energy with respect to pitch angle, but not the high energy part simultaneously. This means that we need more complicated non-uniformness in the magnetic field configuration. We must also note that gyroradius of 1 keV H2O with 20° pitch angles under 20 nT magnetic field is about 300 km (cf. Rosetta-comet distance is about 100 km) when considering such non-uniformity. 

References

How to cite: Kotani, T., Yamauchi, M., Nilsson, H., Stenberg-Wieser, G., Wieser, M., Bergman, S., Taguchi, S., and Götz, C.: Energy structure of the accelerated H2O ions above 1 KeV: the comet 67P/Churyumov-Gerasimenko observed by the Rosetta spacecraft, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-576, https://doi.org/10.5194/epsc2020-576, 2020.

Chairperson: Beatriz Sanchez-Cano, Niklas Edberg
EPSC2020-349ECP
Gwen Hanley, David Mitchell, James McFadden, Christopher Fowler, Shane Stone, Marcin Pilinski, Mehdi Benna, Meredith Elrod, Laila Andersson, Robert Ergun, and Bruce Jakosky

Background

The ion temperature is a key parameter controlling the structure of planetary ionospheres. In the lower ionosphere, where ion motion is dominated by collisions with neutrals, ion temperatures directly affect collision frequencies. At higher altitudes, in the collisionless regime, ion temperatures influence whether some fraction of the distribution can overcome the planet’s gravity to escape to space. Reliable knowledge of ion temperatures is critical to understanding current ionospheric structures as well as the long-term evolution of planetary atmospheres.  

For nearly 40 years, the only in-situ measurements of ion temperatures at Mars were obtained from ~375 km to ~120 km altitude during the descents of the Viking landers in 1976 [1]. These measurements were made by retarding potential analyzers. Assumptions about ion composition were required to derive temperatures, and all ion species were assumed to be the same temperature. While this assumption is reasonable in the collisional regime below the exobase (~200 km), mass-dependent electromagnetic acceleration can cause temperatures to diverge at higher altitudes.

Until recently, ion temperatures at Mars have primarily been investigated using models. The majority of work has focused on explaining elevated electron temperatures in the upper ionosphere, concluding that a topside heat source (e.g. the solar wind) is required to achieve agreement between modeled profiles and Viking observations [2, 3]. The Combined Atmospheric Photochemistry and Ion Tracing (CAPIT) model reproduced Viking profiles by tracing ion paths while accounting for magnetic fields, major chemical reactions, and wave heating [4]. Models of the solar wind interaction with Mars (e.g. [5],[6]) have traditionally adopted a simplified lower boundary assuming equal electron and ion temperatures, which is significantly in error above ~200 km, in contrast to findings by Matta et al. [7] that different ion species have different temperatures. Recently, Ma et al. [8] showed that the electron pressure gradient significantly affects ion temperatures.

Since 2014, comprehensive measurements of the ionosphere above 120 km have been made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter at all local times and most latitudes. Studies based on these data are beginning to revolutionize our understanding of the planet’s ionosphere. In this study, we use data collected by the MAVEN SupraThermal And Thermal Ion Composition (STATIC) instrument to present the first measurements of ion temperature at Mars since the Viking landers.

 

Methodology

We will present O2+ temperature profiles measured in the Martian ionosphere by STATIC. STATIC measures ion energies from 0.1 eV to 30 keV over a 360°x90° field of view, with mass resolution capable of distinguishing the main ionospheric and escaping species: CO2+, O2+, O+, and H+ [9]. Several instrumental effects contribute to the challenge of extracting ion temperatures from STATIC data, some of which require empirical corrections that are currently being calculated. Three different methods are used to calculate the O2+ temperature using different assumptions which are valid over different, overlapping altitude ranges. These calculations include corrections for spacecraft potential, ion suppression, and instrument response, as well as backscatter, straggling, and molecular fragmentation inside the instrument, among other effects. These calibrations have not been finalized, meaning that the O2+ temperatures presented require further correction.

Selection of the best temperature is based on characteristics of the measured distribution. If the population is beam-like (i.e. narrow in energy or angle), then the width of the beam indicates the temperature. However, if the distribution is broad, then calculating moments of the three-dimensional velocity distribution function provides a kinetic temperature, which can be higher than the thermal temperature of the Maxwellian core of the distribution. Once temperatures have been calculated, the data are processed automatically to determine which estimate is the most appropriate. The resulting temperature profiles are largely continuous from MAVEN’s periapsis near 150 km up to altitudes well above 350 km.

 

Results and Discussion

We present O2+ temperature profiles measured by STATIC when MAVEN’s periapsis occurred close to local noon, midnight, dawn, and dusk. These temperature profiles will be presented alongside the neutral Ar temperature measured by the Neutral Gas and Ion Mass Spectrometer, and the electron temperature measured by the Langmuir Probe and Waves experiment [10, 11]. We use profiles measured on the inbound segment of the orbit to minimize the effect of changing solar zenith angle. In addition, Ar temperatures are only available on the inbound segment. The Ar temperature is expected to be representative of the entire neutral population. At altitudes below ~250 km, the ion distribution is usually dominated by O2+, and we anticipate that all ions will converge to the O2+ temperature.

The ion, neutral, and electron temperatures are expected to converge below the exobase, where high collision rates force these populations to equilibrate. Although this thermalization begins to occur below ~200 km, temperature differences between Ar and O2+ persist down to MAVEN’s periapsis of 150 km. The electron temperature is several times higher than the ion temperature in this altitude range, contrary to the assumptions made in most modeling studies.

We note differences in profiles measured at different local times. Unsurprisingly, the median periapsis ion temperature at midnight is colder than at noon, reaching ~250 K at noon compared to ~175 K at midnight. Variations in temperature between successive orbits are larger on the nightside than the dayside, indicated by the larger range of temperatures measured on the nightside, which is to be expected due to the patchy, tenuous nature of the nightside ionosphere.

Measuring the cold thermal ion temperature at another planet poses many challenges. MAVEN STATIC is the first instrument capable of making such measurements at Mars–indeed, it is the first instrument capable of measuring the ion temperature since the Viking landers. Substantial progress has been made toward deriving ion temperatures from STATIC. Once the necessary calibrations are finalized, O2+ temperatures will be calculated for nearly the entire MAVEN mission, providing a new tool for systematically analyzing the Martian atmosphere.

References

[1] Hanson et al. https://doi.org/10.1029/JS082i028p04351

[2] Shinagawa & Cravens. https://doi.org/10.1029/JA094iA06p06506

[3] Cui et al. https://doi.org/10.1002/2014JE004726

[4] Andersson et al. https://doi.org/10.1016/j.icarus.2009.07.009

[5] Dong et al. https://doi.org/10.1002/2014GL059515

[6] Brecht & Ledvina. https://doi.org/10.1016/j.icarus.2009.04.028

[7] Matta et al. https://doi.org/10.1016/j.icarus.2013.09.006

[8] Ma et al. https://doi.org/10.1029/2019JA027091

[9] McFadden et al. https://doi.org/10.1007/s11214-015-0175-6

[10] Stone et al. https://doi.org/10.1029/2018JE005559

[11] Ergun et al. https://doi.org/10.1002/2015GL065280

How to cite: Hanley, G., Mitchell, D., McFadden, J., Fowler, C., Stone, S., Pilinski, M., Benna, M., Elrod, M., Andersson, L., Ergun, R., and Jakosky, B.: O2+ Temperature Profiles Measured in the Martian Ionosphere, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-349, https://doi.org/10.5194/epsc2020-349, 2020.