Superthermal electron deposition on the Mars nightside during ICMEs

<p class="p1">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 10<sup>14</sup> m<sup>-2</sup> for quiet times and on the order of 10<sup>15</sup> m<sup>-2</sup> 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 1.3×10<sup>8</sup> W and 5.5×10<sup>8</sup> W and the globally deposited energy from net fluxes of 2.3×10<sup>7</sup> W and 1.6×10<sup>8</sup> 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.</p>

Statistics of solar wind electron breakpoint energies using machine learning techniques

Solar wind electron velocity distributions at 1 au consist of a thermal “core” population and two suprathermal populations: “halo” and “strahl”. The core and halo are quasi-isotropic, whereas the strahl typically travels radially outwards along the parallel or anti-parallel direction with respect to the interplanetary magnetic field. Using Cluster-PEACE data, we analyse energy and pitch angle distributions and use machine learning techniques to provide robust classifications of these solar wind populations. Initially, we used unsupervised algorithms to classify halo and strahl differential energy flux distributions to allow us to calculate relative number densities, which are of the same order as previous results. Subsequently, we applied unsupervised algorithms to phase space density distributions over ten years to study the variation of halo and strahl breakpoint energies with solar wind parameters. In our statistical study, we find both halo and strahl suprathermal breakpoint energies display a significant increase with core temperature, with the halo exhibiting a more positive correlation than the strahl. We conclude low energy strahl electrons are scattering into the core at perpendicular pitch angles. This increases the number of Coulomb collisions and extends the perpendicular core population to higher energies, resulting in a larger difference between halo and strahl breakpoint energies at higher core temperatures. Statistically, the locations of both suprathermal breakpoint energies decrease with increasing solar wind speed. In the case of halo breakpoint energy, we observe two distinct profiles above and below 500 km s−1. We relate this to the difference in origin of fast and slow solar wind.

Collisionless electron dynamics in the expanding solar wind

<p><span>Observations of solar wind electron properties, as displayed in the T</span><sub><span>perp</span></sub><span>/T<sub>par</sub> vs β<sub>par</sub> plane, appear to be constrained both in the T<sub><span>perp</span></sub>/T<sub>par</sub></span><span> <1 and in the T<sub><span>perp</span></sub>/T<sub>par</sub></span><span> >1 regimes by the electron firehose instability (EFI) and by the whistler instability respectively [</span><span>Štverák 2008</span><span>]. The onset mechanism of the EFI is established: solar wind expansion results in an electron thermal anisotropy, which in turns promotes the development of the instability that contributes to limit that same anisotropy [Innocenti 2019a]. However, if this were the only mechanism at work in the expanding solar wind, electron observations would pool at the EFI marginal instability line. Instead, they populate the “stable” interval bound by EFI and whistler marginal instability lines. It is not fully clear which role fully kinetic processes have in lifting the observed data points above the EFI marginal stability line and into the “stable” area. Other competing processes redistributing excess parallel energy into the perpendicular direction, such as collisions, may be at work as well [Yoon 2019].</span></p><p><span>We investigate this issue with Particle In Cell, Expanding Box Model<span>  </span>simulations [Innocenti 2019b] of EFI developing self consistently in the expanding solar wind. Our results show that after the EFI marginal stability line is reached, further collisionless evolution brings our simulated data points in the “stable” area. We thus demonstrate that, at least under certain circumstances, purely collisionless processes may explain observed solar wind observations, without the need of invoking collisions as a way to channel excess parallel energy into the perpendicular direction.</span></p><p> </p><p><span>Štverák, Štěpán, et al. "Electron temperature anisotropy constraints in the solar wind." <em>Journal of Geophysical Research: Space Physics</em> 113.A3 (2008).</span></p><p><span>Innocenti, Maria Elena, et al. "Onset and Evolution of the Oblique, Resonant Electron Firehose Instability in the Expanding Solar Wind Plasma." <em>The Astrophysical Journal</em> 883.2 (2019): 146.</span></p><p><span>Yoon, P. H., et al. "Solar Wind Temperature Isotropy." <em>Physical review letters</em> 123.14 (2019): 145101.</span></p><p><span>Innocenti, Maria Elena, Anna Tenerani, and Marco Velli. "A Semi-implicit Particle-in-cell Expanding Box Model Code for Fully Kinetic Simulations of the Expanding Solar Wind Plasma." <em>The Astrophysical Journal</em> 870.2 (2019): 66.</span></p>