EPP

The photoelectron peak at 22–27 eV, a distinctive feature of the energetic electron distribution in the dayside Martian ionosphere, is a useful diagnostic of solar extreme ultraviolet (EUV) and X-ray ionization as well as of large-scale transport along magnetic field lines. In this work, we analyze the pitch angle distribution (PAD) of energetic electrons at 22–27 eV measured during several representative Mars Atmosphere and Volatile Evolution (MAVEN) orbits, based on the electron spectra gathered by MAVEN’s Solar Wind Electron Analyzer (SWEA) instrument. On the dayside, most photoelectron spectra show an isotropic PAD as is expected from production via solar EUV/X-ray ionization. The photoelectron spectra occasionally observed on the nightside show instead a strongly anisotropic PAD, indicative of cross-terminator transport along ambient magnetic field lines. This would in turn predict the presence of dayside photoelectrons, also with a strongly anisotropic PAD, which was indeed revealed in SWEA data. Comparison with magnetic field measurements made by the MAVEN Magnetometer suggests that on average the photoelectrons with anisotropic PAD stream away from Mars on the dayside and towards Mars on the nightside, further supporting the scenario of day-to-night transport. On both sides, anisotropic photoelectrons tend to be observed above the photoelectron exobase at ~160 km where photoelectron transport dominates over local production and energy degradation.

O⁺⁺ is an interesting species in the ionospheres of both the Earth and Venus. Recent measurements made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft provide the first firm detection of O⁺⁺ in the Martian ionosphere. This study is devoted to an evaluation of the dominant O⁺⁺ production and destruction channels in the dayside Martian ionosphere, by virtue of NGIMS data accumulated over a large number of MAVEN orbits. Our analysis reveals the dominant production channels to be double photoionization of O at low altitudes and photoionization of O⁺ at high altitudes, respectively, in response to the varying degree of O ionization. O⁺⁺ destruction is shown to occur mainly via charge exchange with CO₂ at low altitudes and with O at high altitudes. In the dayside median sense, an exact balance between O⁺⁺ production and destruction is suggested by the data below 200 km. The apparent discrepancy from local photochemical equilibrium at higher altitudes is interpreted as a signature of strong O⁺⁺ escape on Mars, characterized by an escape rate of 6×10²² s^–1.

The $ {\rm{CO}}_2^+$ ultraviolet doublet (UVD) emission near 289 nm is an important feature of dayside airglow emission from planetary upper atmospheres. In this study, we analyzed the brightness profiles of $ {\rm{CO}}_2^+$ UVD emission on Mars by using the extensive observations made by the Imaging Ultraviolet Spectrograph on board the recent Mars Atmosphere and Volatile Evolution spacecraft. Strong solar cycle and solar zenith angle variations in peak emission intensity and altitude were revealed by the data: (1) Both the peak intensity and altitude increase with increasing solar activity, and (2) the peak intensity decreases, whereas the peak altitude increases, with increasing solar zenith angle. These observations can be favorably interpreted by the solar-driven scenario combined with the fact that photoionization and photoelectron impact ionization are the two most important processes responsible for the production of excited-state $ {\rm{CO}}_2^+$ and consequently the intensity of $ {\rm{CO}}_2^+$ UVD emission. Despite this, we propose that an extra driver, presumably related to the complicated variation in the background atmosphere, such as the occurrence of global dust storms, is required to fully interpret the observations. In general, our analysis suggests that the $ {\rm{CO}}_2^+$ UVD emission is a useful diagnostic of the variability of the dayside Martian atmosphere under the influences of both internal and external drivers.

We investigated the variations of equatorial plasma bubbles (EPBs) in the East-Asian sector during a strong geomagnetic storm in October 2016, based on observations from the Beidou geostationary (GEO) satellites, Swarm satellite and ground-based ionosonde. Significant nighttime depletions of F region in situ electron density from Swarm and obvious nighttime EPBs in the Beidou GEO observations were observed on 13 October 2016 during the main phase. Moreover, one interesting feature is that the rare and unique sunrise EPBs were triggered on 14 October 2016 in the main phase rather than during the recovery phase as reported by previous studies. In addition, the nighttime EPBs were suppressed during the whole recovery phase, and absent from 14 to 19 October 2016. Meanwhile, the minimum virtual height of F trace (h’F) at Sanya (18.3°N, 109.6°E, MLAT 11.1°N) displayed obvious changes during these intervals. The h’F was enhanced in the main phase and declined during the recovery phase, compared with the values at pre- and post-storm. These results indicate that the enhanced nighttime EPBs and sunrise EPBs during the main phase and the absence nighttime EPBs for many days during the recovery phase could be associated with storm-time electric field changes.