Bidirectional electron conic observations for photoelectrons in the Martian ionosphere

Abstract

An analysis is conducted focusing on the bidirectional electron conics at 19-55 eV on Mars q BECs are more likely observed on the nightside and near strong crustal magnetic anomalies q 19-55 eV BECs are formed due to day-tonight transport along cross-terminator magnetic field lines q

Photoelectron Butterfly Pitch-angle Distributions in the Martian Ionosphere Based on MAVEN Observations

“…First, from Figure 2(c), the photoelectron butterfly PADs are most likely to occur in the darkness at nearly all altitudes for almost all energies, except for 41.2 and 57.5 eV in the region above the altitude of 550 km. The occurrence rate in the eclipse is generally above 1%, and comparable to that reported by Cao et al (2020). These darkness observations are found far away from the source region, indicating that photoelectrons are significantly transported from the dayside to below 300 km at least on the nightside, via long magnetic loops (e.g., Xu et al 2016;Cao et al 2021).…”

“…According to the mechanism presented by Cao et al (2020), the generation of photoelectron butterfly PADs observed on the nightside might be related to the day-to-night transport of photoelectrons along closed cross-terminator magnetic field lines. They proposed that the adiabatic pitch-angle evolution of photoelectrons along converging/diverging magnetic field lines should take responsibility for the formation of the butterfly PADs observed in the nightside Martian ionosphere (more details about the mechanism are presented in Section 3.2).…”

“…These regions are: (1) field-aligned electrons (0°pitch angle); (2) antialigned electrons (180°pitch angle); and (3) trapped electrons (90°pitch angle). To generate the butterfly PAD from an isotropic distribution of photoelectrons, Cao et al (2020) proposed the day-to-night transport of photoelectrons along the closed cross-terminator magnetic field lines as the driver. They described the butterfly PAD as being formed by atmospheric absorption at each (1) field-aligned and (2) antialigned field configuration, combined with magnetic field expansions that move those particles into mid-pitch-angle regions and leave the trapped region empty.…”

“…Ulusen et al (2011) characterized the nightside butterfly PADs of 115 eV electrons detected by MGS at ∼400 km and 0200 LT, concluding that nightside electron butterfly PADs mostly occurred on moderately intense horizontal crustal magnetic field lines, and proposing two possible generation mechanisms, both indicating that the source particles come from solar wind electron precipitation. In contrast, a recent study of Cao et al (2020) identified the butterfly PADs for photoelectrons at lower energies, which are produced by photoionization of atmospheric neutrals due to solar extreme ultraviolet (EUV) irradiation. Using the normalized integrated electron fluxes over the energy channels of 19-55 eV measured by MAVEN, Cao et al (2020) presented the occurrence preference of photoelectron butterfly PADs on the nightside for the crustal magnetic anomaly region and, in particular, proposed the adiabatic evolution of photoelectrons that transport along the closed cross-terminator magnetic field lines as the possible driver.…”

“…As a follow-up investigation to Cao et al (2020), the present study is directed toward a comprehensive analysis of the Martian photoelectron butterfly PADs using MAVEN data sets. With the aid of our recently developed recognition model for accurately identifying any type of electron butterfly PAD (Luo et al 2022), we intend to establish a robust database of Martian photoelectron butterfly PADs with the electron flux information corrected using instantaneous spacecraft potential measurements, since it is well recognized that the values of spacecraft potential can affect the energies of the detected photoelectrons.…”