We use observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to show how superthermal electron fluxes and crustal magnetic fields affect ion densities in the nightside ionosphere of Mars. We find that due to electron impact ionization, high electron fluxes significantly increase the CO 2+, O+, and O 2+ densities below 200 km but only modestly increase the NO+ density. High electron fluxes also produce distinct peaks in the CO 2+, O+, and O 2+ altitude profiles. We also find that superthermal electron fluxes are smaller near strong crustal magnetic fields. Consequently, nightside ion densities are also smaller near strong crustal fields because they decay without being replenished by electron impact ionization. Furthermore, the NO+/O 2+ ratio is enhanced near strong crustal fields because, in the absence of electron impact ionization, O 2+ is converted into NO+ and not replenished. Our results show that electron impact ionization is a significant source of CO 2+, O+, and O 2+ in the nightside ionosphere of Mars.

We provide an overview of the composition, vertical structure, and variability of the nightside ionosphere of Mars as observed by Mars Atmosphere and Volatile EvolutioN (MAVEN)'s Neutral Gas and Ion Mass Spectrometer (NGIMS) through 19 months of the MAVEN mission. We show that O 2+ is the most abundant ion down to ∼130 km at all nightside solar zenith angles (SZA). However, below 130 km NO+ is the most abundant ion, and NO+ densities increase with decreasing altitude down to at least 120 km. We also show how the densities of the major ions decrease with SZA across the terminator. At lower altitudes the O 2+ and CO 2+ densities decrease more rapidly with SZA than the NO+ and HCO+ densities, which changes the composition of the ionosphere from being primarily O 2+ on the dayside to being a mixture of O 2+, NO+, and HCO+ on the nightside. These variations are in accord with the expected ion‐neutral chemistry, because both NO+ and HCO+ have long chemical lifetimes. Additionally, we present median ion density profiles from three different nightside SZA ranges, including deep on the nightside at SZAs greater than 150° and discuss how they compare to particle precipitation models. Finally, we show that nightside ion densities can vary by nearly an order of magnitude over monthlong timescales. The largest nightside densities were observed at high northern latitudes during winter and coincided with a major solar energetic particle event.

The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has been orbiting Mars since September 21, 2014, with a primary mission to study the behavior of the upper atmosphere and the escape of its constituent gases to space (Jakosky et al., 2014). At the time of these observations, MAVEN orbited Mars on a 4.5-h elliptical orbit with a closest approach to Mars' surface at periapse of 150-200 km and an apoapse ranging from 6,200 km to 4,400 km over the mission. MAVEN carries one remote sensing instrument for the study of Mars' upper atmosphere: the Imaging UltraViolet Spectrograph (IUVS) (McClintock et al., 2015). The instrument captures spectra of the planet and its atmosphere in the far-UV (FUV) from 110 to 190 nm and mid-UV (MUV) from 180 to 340 nm, ideal for recording well-known atmospheric emissions from CO 2 and its dissociation and ionization products. The instrument is mounted on an Articulated Payload Platform (APP), which can orient IUVS's field of view relative to Mars depending on spacecraft location, orientation and desired viewing geometry. IUVS was designed to observe the Mars dayglow, nightglow, hydrogen corona, D/H ratio, and stellar occultations, and is also sensitive to auroral emissions. Mars exhibits at least three types of aurora (Figure 1). The SPICAM instrument on Mars Express discovered discrete aurora: small, short-lived patches of aurora related to the crustal magnetic fields in Mars' southern hemisphere (Bertaux et al., 2005). MAVEN/IUVS discovered a second type called diffuse aurora (Schneider,

The Mars thermosphere holds clues to the evolution of the Martian climate and has practical implications for spacecraft visiting Mars, which often use it for aerobraking upon arrival, or for landers, which must pass through it. Nevertheless, it has been sparsely characterized, even when past accelerometer measurements and remote observations are taken into account. The Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter, which includes a number of instruments designed to characterize the thermosphere, has greatly expanded the available thermospheric observations. This paper presents new and unanticipated measurements of density and temperature profiles (120–200 km) derived from solar occultations using the MAVEN Extreme Ultraviolet (EUV) Monitor (EUVM). These new measurements complement and expand MAVEN's intended thermospheric measurement capacity. In particular, because the local time is inherently fixed to the terminator, solar occultations are ideally suited for characterizing long‐term and latitudinal variability. Occultation measurements are made during approximately half of all orbits, resulting in thousands of new thermospheric profiles. The density retrieval method is presented in detail, including an uncertainty analysis. Altitude‐latitude maps of thermospheric density and temperature at perihelion and aphelion are presented, revealing structures that have not been previously observed. Tracers of atmospheric dynamics are also observed, including (1) a high altitude polar warming feature at intermediate latitudes, (2) cooler temperatures coinciding with increased gravity wave activity, and (3) an apparent thermostatic response to solar EUV heating during a solar rotation, which shows heating at high altitudes that is accompanied by cooling at lower altitudes.

The peak electron density in the dayside Martian ionosphere is a valuable diagnostic of the state of the ionosphere. Its dependence on factors like the solar zenith angle, ionizing solar irradiance, neutral scale height, and electron temperature has been well studied. The Mars Atmosphere and Volatile EvolutioN spacecraft's September 2015 “deep dip” orbits, in which the orbital periapsis was lowered to ~125 km, provided the first opportunity since Viking to sample in situ a complete dayside electron density profile including the main peak. Here we present peak electron density measurements from 37 deep dip orbits and describe conditions at the altitude of the main peak, including the electron temperature and composition of the ionosphere and neutral atmosphere. We find that the dependence of the peak electron density and the altitude of the main peak on solar zenith angle are well described by analytical photochemical theory. Additionally, we find that the electron temperatures at the main peak display a dependence on solar zenith angle that is consistent with the observed variability in the peak electron density. Several peak density measurements were made in regions of large crustal magnetic field, but there is no clear evidence that the crustal magnetic field strength influences the peak electron density, peak altitude, or electron temperature. Finally, we find that the fractional abundance of O2+ and CO2+ at the peak altitude is variable but that the two species together consistently represent ~95% of the total ion density.