Modeling of SEP induced auroral emission at Mars: Contribution of precipitating protons and effects of crustal fields

<p>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.</p><p>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 CO<sub>2</sub><sup>+</sup> 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.</p><p>The calculated peak limb intensity of CO<sub>2</sub><sup>+</sup> 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 CO<sub>2</sub><sup>+</sup> UVD emission than electrons. Peak altitude of limb intensity profiles of CO<sub>2</sub><sup>+</sup> 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.</p><p>We have tested an effect of crustal field on the emission of CO<sub>2</sub><sup>+</sup> UVD. CO<sub>2</sub><sup>+</sup> 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.</p>

MAVEN observations of factors influencing the magnetotail twist at Mars

<p>At Mars, recent studies based on a combination of MAVEN data and modeling have determined the Martian magnetotail exhibits a ~45° twist, either clockwise or counterclockwise from the ecliptic plane, away from the nominal interplanetary magnetic field (IMF) draping morphology. An initial study by DiBraccio et al. [2018] employed MAVEN magnetic field measurements, coupled with MHD simulations, to indicate that the twist is likely a result of the sun-planetary interaction. Now with several more years of MAVEN data available, we augment this work using a statistical analysis of MAVEN magnetic field data from November 2014 through November 2019. We utilized ~6000 orbits, requiring that MAVEN observed both the magnetotail and the upstream IMF over a given orbit. For periods when the upstream IMF measurements were not available due to MAVEN’s orbit precession, we utilize an IMF proxy to determine characteristics of the upstream orientation. The location of the magnetotail lobes, identified in the data as the regions of magnetic field behind the planet directed towards and away from Mars, are analyzed as a function of the upstream IMF dawn-dusk component. In the previous DiBraccio et al. [2018] study, this dawn-dusk component was found to be the separating factor in the direction of magnetotail twisting. To quantify the degree of tail twisting for a given scenario, we determine the vector between the center of the towards/away tail lobes and calculate the angle between this vector and the expected direction for nominal IMF draping. This calculated tail twist angle is then assessed as a function of a variety of factors including strong crustal field location, Mars season, and downtail distance. In all cases, we determine that the degree of tail twisting is larger when the IMF is oriented in the duskward direction, suggesting enhanced coupling between the IMF and planetary crustal fields. Furthermore, we demonstrate that the degree of tail twisting exhibits different trends for crustal field orientation under dawnward versus duskward IMF configurations. Seasonal variations indicate that tail twisting may vary over the course of the Martian year, but additional data are needed during the northern fall and winter periods for confirmation. Finally, when assessing the tail twist with downtail distance we find that the degree of twisting increases with distance from the planet. This result is similar to Earth where observations of the magnetotail twist increases away from the planet as the torque exerted by the IMF on the planetary field increases. From these findings we confirm that the tail twist at Mars is likely a result of the direct interaction between the IMF and the planetary crustal fields; however, we find evidence suggesting that the degree of twisting is larger for duskward IMF orientations. This implies that magnetic reconnection on the dayside of Mars, between the IMF and crustal fields, may be favorable under specific IMF configurations.</p>

Was the moon magnetized by impact plasmas?

The crusts of the Moon, Mercury, and many meteorite parent bodies are magnetized. Although the magnetizing field is commonly attributed to that of an ancient core dynamo, a longstanding hypothesized alternative is amplification of the interplanetary magnetic field and induced crustal field by plasmas generated by meteoroid impacts. Here, we use magnetohydrodynamic and impact simulations and analytic relationships to demonstrate that although impact plasmas can transiently enhance the field inside the Moon, the resulting fields are at least three orders of magnitude too weak to explain lunar crustal magnetic anomalies. This leaves a core dynamo as the only plausible source of most magnetization on the Moon.

Implications of magnetic secular variation for interpretation of crustal field anomalies

<p>We usually think of crustal magnetic anomalies as static (barring some major seismic or thermal disruption).  But a significant portion of the crustal magnetic field is caused by the interaction of magnetic minerals with the Earth’s magnetic field.  This induced magnetic effect is dependent on the direction and magnitude of the ambient field.  So, of course, as the Earth’s magnetic field changes over time, the form and magnitude of induced magnetic anomalies will vary as well.  These changes will often be negligible for interpretation when compared with measurement and other interpretational uncertainties.  However, with the reduction of various sources of measurement noise and increased fidelity of interpretation, these temporal anomaly changes may need to be considered.</p><p>In addition to considerations relating to interpretation uncertainty, these temporal anomaly changes, if they are measured in multiple magnetic epochs, can theoretically provide valuable information for use in source inversion.  For example, since crustal magnetic anomalies arise from a combination of induced (dependent the ambient field) and remanent (not dependent on ambient field) magnetic sources, measurements of secular magnetic variation can assist in separating these two sources during inversion.</p><p>We will report modeling of the expected form and magnitude of predicted induced anomaly variations, the possible implications of these variations for data compilation and interpretation, and on the availability of relevant data for measuring them.  Recent research into the use of high-resolution magnetic anomaly maps for airborne magnetic navigation has also brought the issue of changing magnetic fields into focus.  Initial work indicates that changes in induced anomalies could affect navigation accuracy in certain situations.</p>

Crustal magnetic fields at Mars and ion escape

<p>Does an intrinsic field inhibits or enhances ion escape from planetary ionospheres is still an unsolved issue. Mars does not possess a global intrinsic magnetic field but instead has the strong crustal magnetic fields localized mainly in the southern hemisphere. The crustal magnetic field significantly influences the interaction of the solar wind with Mars adding features typical for planets with a global intrinsic magnetic field. Therefore it is interesting to compare ion losses from the ionosphere regions with and without strong crustal fields. Recently such studies were performed and have shown a protective effect of the crustal field on escape of the energized (E > 30 eV) oxygen ions (e.g. Fan et al., Geophysical Review Letters, 2019). However, the main bulk of escaping ions at Mars have energy lower than 30 eV. We will present the results of influence of the crustal magnetic field at Mars on the total losses of O<sup>+</sup> and O<sub>2</sub><sup>+</sup> ions. The global picture of ion escape occurs more complex. Effects of larger ionospheric areas above the crustal field sources exposed by solar wind compensate a shielding effect at lower altitudes. As a result, the ion losses from the southern ionosphere of Mars might be even higher than losses from the northern “unmagnetized” ionosphere.</p>

On the horizontal currents over the Martian magnetic cusp

<p>Localized crustal magnetization over heavily cratered southern hemisphere at Mars gives rise to open magnetic field configurations which interact with the solar wind magnetic field to form magnetic cusps.  The downward acceleration of energetic electrons in these cusps can produce aurora and an extended topside ionospheric structure over regions of magnetic anomalies.  We report plasma collisions with the neutral atmosphere at one of the Martian cusps located at 82<sup>o</sup>S and 108<sup>o</sup>E, where the crustal field is strong with a radial component ~30<sup>o</sup> from the local zenith.  We find that the dynamo region in the upper ionosphere of Mars is located between altitudes of 102 km and 210 km. The electrons in this region are constrained to gyrate along magnetic field lines while ions are dragged by neutrals and move along the direction of applied force.  In the absence of the electric field, the horizontal current in the Martian dynamo is generated by the differential motion of ions and electrons.  We find that the bulk of the current density is equatorward and confined within the Martian dynamo near the ionospheric peak with a magnitude of ~3.5 µA/m<sup>2</sup>.  We also find that the westward current density of magnitude ~0.4 µA/m<sup>2</sup> peaking near the upper boundary of the Martian dynamo is generated by magnetized ions in the -<strong>F</strong> x <strong>B</strong> direction.  The model details and results in comparison with other studies will be presented.        </p>