Whistler Wave Interactions With Superthermal Electrons on Martian Crustal Magnetic Fields: Bounce‐Averaged Diffusion Coefficients and Time Scales

Abstract

Mars has a unique space environment relative to other planets in our solar system. The Interplanetary Magnetic Field's interaction with the ionosphere of Mars sets up an induced magnetosphere, further complicated by the presence of localized crustal fields that rotate with the planet. Superthermal electrons, electrons with energies ranging from 1 to 1,000 eV, populate these crustal field lines during their time in the dayside hemisphere. These electrons primarily consist of photoelectrons, produced from photoionization of atmospheric neutrals, with peak production occurring around 130 km. Below the photoelectron exobase, found to be ∼150 km by S. Xu et al. (2016), collisions dominate and the electrons are lost locally. Above the photoelectron exobase, the particles are magnetized and can travel to high altitudes, eventually reaching the conjugate foot point of the crustal magnetic field. The magnetic field strength decreases with altitude and the electron's pitch angle becomes more field aligned as it travels to higher altitudes due to the conservation of the first adiabatic invariant. This source cone distribution is more pronounced for higher energy electrons as Coulomb collisions are proportional to 1/E 2 , where E is the energy of the electron. Figure, 1 of Shane et al. (2019) details this pitch angle distribution (PAD) evolution along a field line as a function of energy. Data from Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN; Jakosky et al., 2015) mission have shown that our assumptions of the PADs of superthermal electrons on the Martian crustal fields are incorrect and that more physics are involved than just collisions and single particle motion. Liemohn et al. (2003) looked at a case study between MGS data and modeled electron fluxes and found that MGS measured isotropic distributions for electrons with energies greater than 100 eV. Brain et al. (2007) looked at the PADs of 115 eV electrons measured with MGS and showed that isotropic and two-sided loss cones are the most common distributions on closed crustal field lines. Shane et al. (2019) performed a statistical study using MAVEN data of superthermal electron PADs on closed crustal field lines. They showed that electrons with energies less than 60 eV have PADs that are in agreement with modeling results and that adiabatic invariants and collisions describe the evolution of their distribution. Electrons with energies greater than 60 eV were not in agreement and electrons with energies between 100 and 500 eV

MAVEN Observations of Whistler-mode Waves Within the Magnetic Dips in the Martian Ionopause/Ionosphere

“…Through the resonant interaction with electrons, whistler-mode waves play a controlling role in the dynamics of Earth's magnetospheric electrons, such as radiation belt electrons, plasma sheet electrons, suprathermal electrons and so on. Similar to those at Earth, the pitch angle scattering and energy diffusion induced by whistler-mode waves at Mars also can have a significant impact on the electron distribution and precipitation on both open and closed magnetic field lines (Shane et al 2019;Shane & Liemohn 2021, 2022. However, the depression of magnetic field strength in the magnetic dip interiors at Mars provides a much weaker magnetic field environment from that at Earth.…”

Modeling Wave‐Particle Interactions With Photoelectrons on the Dayside Crustal Fields of Mars

“…We will be solving this equation for the bounce-averaged differential number flux along a Mars crustal field line. The magnetic field configuration, atmosphere conditions, and whistler wave parameters will be identical to the bounce-averaged calculations of Shane and Liemohn (2021), specifically Runs #1 and #2. The magnetic field is an ideal dipole with a field strength of ∼294 nT at the exobase (160 km) and 50 nT at the top of the field line (500 km).…”

“…Through resonant interactions, whistler waves can energize and pitch angle scatter electrons, which could explain both the perpendicular flux peak at high energies and the increased isotropy at all energies. Shane and Liemohn (2021) investigated the average plasma environment of the dayside-closed crustal fields to determine if the conditions are right for whistler mode waves to interact with electrons at the energies of interest. The characteristic energy, a function of the magnetic field strength and thermal electron density, is one quantity that determines the electron resonant energy.…”

“…The characteristic energy, a function of the magnetic field strength and thermal electron density, is one quantity that determines the electron resonant energy. MAVEN measures both quantities, and Shane and Liemohn (2021) used typical altitude profiles of the characteristic energy to calculate bounce-averaged diffusion coefficients of the wave-particle interaction. The wave frequency and wave normal angle were set using the observations made by Harada et al (2016) and Fowler et al (2020).…”

“…The equation, in both its theoretical formulation and numerical implementation, will be discussed in Section 2. We will describe our model configuration (Section 3) and show results from two simulations (Section 4) using the same bounce-averaged diffusion coefficients as calculated in Shane and Liemohn (2021). In Section 5, we will discuss the results and future work.…”