Nightside ionosphere of Mars: Composition, vertical structure, and variability

Abstract

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.

On the response of Martian Ionosphere to the Passage of a Corotating Interaction Region: MAVEN Observations

“…The maximum dynamic pressure during the solar event in the present study is more than the peak dynamic pressure observed by MAVEN during the 8 March 2015 interplanetary CME event Thampi et al, 2018). The higher standard deviations for the nightside profile indicate that the nightside ionosphere of Mars is more variable compared to the dayside which corroborates with the previous observations (Girazian et al, 2017). (For the MAVEN data presented here, a new orbit number starts when the instrument is at the geometric periapsis, and is the same for the outbound and the next inbound sectors, and incremented at the next periapsis).…”

The Morphology of the Topside Martian Ionosphere: Implications on Bulk Ion Flow

“…Here we consider four ion species including O , O + , HCO + , and NO + in decreasing order of nightside median density at 500 km, with the remaining species displayed in Figure neglected due to their relatively low densities at all altitudes sampled by the NGIMS. The nightside median density profiles for the four species are shown in Figure at 180–500 km, which are generally consistent with the published nightside NGIMS ion density results (e.g., Girazian, Mahaffy, Lillis, Benna, Elrod, Jakosky, et al, ; Girazian, Mahaffy, Lillis, Benna, Elrod, Fowler, et al, ). Except for regions above 450 km, these species roughly present an exponential decay with increasing altitude, though their density scale heights are not as constant as on the dayside.…”

“…The solar cycle variation in ion outflow was also obtained by Cravens et al () using a semiempirical approach and predicted to scale with the square root of the solar ionizing flux. The inferred dayside plasma outflow either helps to maintain a substantial nightside ionosphere on Mars (e.g., Brain et al, ; Cui et al, ; Girazian, Mahaffy, Lillis, Benna, Elrod, Jakosky, ; Němec et al, ; Withers et al, ) or leads to considerable plasma escape down the tail (e.g., Barabash et al, ; Dubinin, Fraenz, Pätzold, McFadden, et al, ; Dubinin, Fraenz, Pätzold, Andrews, et al, ; Edberg et al, ; Lundin et al, ; Ramstad et al, ). The relative contributions of these two channels are comparable based on existing magnetohydrodynamic calculations (e.g., Ma et al, ).…”

“…This situation has been significantly improved by the Mars Atmosphere and Volatile Evolution (MAVEN) mission (Jakosky et al, ) with its Neutral Gas and Ion Mass Spectrometer (NGIMS) providing, in the open source ion (OSI) mode, a wealth of information on the densities of many important species of the Martian ionosphere covering a broad mass range of 2–150 Da (Mahaffy, Benna, King, et al, ). Meanwhile, the available NGIMS measurements extend well beyond the terminator (e.g., Girazian, Mahaffy, Lillis, Benna, Elrod, Jakosky, et al, ; Girazian, Mahaffy, Lillis, Benna, Elrod, Fowler, et al, ), allowing a thorough investigation of the topside Martian ionosphere on the nightside.…”

“…The data acquired by the NGIMS were used to explore the characteristics of the Martian ionosphere under a range of external conditions (e.g., Benna, Mahaffy, Grebowsky, Fox, et al, ; Benna, Mahaffy, Grebowsky, Plane, et al, ; Girazian, Mahaffy, Lillis, Benna, Elrod, Jakosky, et al, ; Girazian, Mahaffy, Lillis, Benna, Elrod, Fowler, et al, ; Vogt et al, ; Withers, Vogt, Mahaffy, et al, ; Withers, Vogt, Mayyasi, et al, ). In the present study, the NGIMS OSI level 2 data from the arrival of MAVEN at Mars on 21 September 2014 to 14 February 2018 are used.…”