Statistical properties of planetary heavy‐ion precipitations toward the Martian ionosphere obtained from Mars Express

Abstract

[1] The interplanetary magnetic field (IMF) embedded in the solar wind interacts with the Martian crustal magnetic field and atmosphere. The IMF orientation is one of the important parameters to control the acceleration and precipitation of planetary heavy ions (PHIs). We statistically investigate the effects of the IMF orientation on PHI precipitations toward the ionosphere based on observations by Mars Express (MEX). We identified 59 PHI precipitation events between July 2007 and September 2009. To estimate the IMF orientation without magnetometer that MEX does not carry, we used the velocity distribution of exospheric-origin pickup protons. We estimated the IMF orientation without its polarity for 10 events. The results show that the precipitations of PHIs tend to be observed around pole regions in the MSE (Mars-centered, solar electrical) coordinates determined from the solar wind electric field (E sw ), in which the pole axis directs to the parallel or antiparallel to E sw due to the ambiguity in the IMF polarity determination. The observed precipitating PHIs are accelerated only up to a few keV. This feature may reflect the short distance from the picked-up region. For one of these 10 events, we estimated the IMF polarity by comparing the velocity distribution of exospheric-origin pickup protons observed by MEX with those obtained from statistical trajectory tracing simulations under two cases of possible IMF polarity conditions. The estimated polarity indicates that the PHI precipitation in this event is observed in the downward electric field hemisphere in MSE, where E sw points to Mars in the pole region.Citation: Hara, T

MAVEN observations on a hemispheric asymmetry of precipitating ions toward the Martian upper atmosphere according to the upstream solar wind electric field

“…However, the absence of a magnetometer on board MEX prevented the direct investigation of the effects of the background convection electric field on ions precipitating toward the Martian upper atmosphere. Hara et al [] attempted to address the dependence of the solar wind electric field on precipitating heavy ions based on MEX observations by estimating the IMF orientation from the characteristics of velocity distribution functions of picked up exospheric protons around Mars, because they are generally gyrating in the plane perpendicular to the IMF direction [e.g., Yamauchi et al , , ]. They confirmed that the locations where precipitating heavy ions were observed tend to be well organized along the inferred solar wind electric field direction [ Hara et al , ].…”

O+ ion beams reflected below the Martian bow shock: MAVEN observations

“…The second population is low phase space density precipitating O + ion beams (see Figures 2e, 4e, and 8e). We suggest that this population may escape Mars through the plume or the tail escape channel in the +E hemisphere as shown in Dong et al [2015] but precipitate into the Martian ionosphere in the ÀE hemisphere as shown in Hara et al [2013]. Since the first population has a higher phase space density and smaller energy than those of the second and the third populations, the source of the first population is most likely located near the observation location, namely, in the magnetosheath.…”

“…The ring distribution has been observed at Mars before [Dubinin et al, 2006;Hara et al, 2013]. For example, pickup ions show a characteristic ion VDF.…”

“…For example, pickup ions show a characteristic ion VDF. Hara et al [2013] estimated the IMF direction from the proton ring distribution observed outside the bow shock. The ring distribution has been observed at Mars before [Dubinin et al, 2006;Hara et al, 2013].…”

Statistical analysis of the reflection of incident O+ pickup ions at Mars: MAVEN observations

“…For the incoming ions in Figure b, we can see a strong inward O + ion fluxes in the −E hemisphere because denser O + ions precipitate toward Mars in the −E hemisphere. This feature is consistent with previous studies [ Luhmann and Kozyra , ; Hara et al ., ]. The fluxes in Figures c and d are calculated by applying the plume removal method explained in section 2 (the plume removal case).…”