Evidence for Magnetic Reconnection as the Precursor to Discrete Aurora at Mars

Abstract

Discrete aurora at Mars are characterized as localized, short‐lasting ultraviolet emissions on the nightside. They are caused by the precipitation of accelerated electron along open magnetic field lines and their collision with the Martian atmosphere. Discrete auroral emissions detected by the Imaging Ultraviolet Spectrograph (IUVS) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft occur most frequently over the strongest crustal magnetic anomalies at Mars and under particular local time and upstream interplanetary magnetic field (IMF) conditions. These trends suggest that the onset of discrete aurora is controlled by the processes that govern the interaction between the draped IMF and the crustal magnetic anomalies. Here, we analyze MAVEN magnetometer measurements over regions of strong crustal fields during 49 discrete auroral events observed by IUVS. Our results indicate that the draped IMF orientations during each discrete auroral event show a clear tendency to be anti‐parallel to the underlying crustal anomaly magnetic fields near the location of the emission onset. This suggests that magnetic reconnection, a process that favors anti‐parallel magnetic field regimes, between the crustal fields and the draped IMF plays a role in discrete aurora formation. Of the 49 discrete auroral events analyzed, 42 (86%) occurred under draped IMF conditions that are susceptible to reconnection with the underlying crustal anomalies. This investigation produces strong evidence linking discrete aurora to magnetic reconnection at Mars and provides insights into other trends in discrete auroral activation such as local time and upstream IMF conditions.

EMM EMUS Observations of FUV Aurora on Mars: Dependence on Magnetic Topology, Local Time, and Season

“…Figure 6d shows the auroral occurrence as a function of local solar time, and we can notice that there is a dawn-dusk asymmetry, with more aurora occurring in pre-midnight local times as compared to post-midnight local times. This is also consistent with the previous observations by MAVEN/IUVS instrument on pre-midnight versus post-midnight detection frequencies (Bowers et al, 2023;Johnston et al, 2023).…”

Evidence for a Current System and Potential Structure in the Martian Magnetotail

“…Reconnection is associated with a variety of plasma phenomena, including the possibility that it might be associated with discrete aurora on Mars. The body of evidence in support of this hypothesis is growing (e.g., Bowers, DiBraccio, Slavin, Johnston, et al, 2023;Xu, Poppe, et al, 2019). Our study adds to this evidence because we observe signatures associated with magnetic reconnection, and counterstreaming electrons, which are observed above active auroral arcs at Earth.…”

The Unique Seasonal Influence of the Interplanetary Magnetic Field on Martian Proton Aurora

“…Reconnection could increase occurrence rates by enabling direct penetration of solar wind protons into the atmosphere along field lines. The possible influence of reconnection on proton aurora informs our understanding of the Martian magnetic field environment and Sun-planet interactions, as reconnection plays a crucial role in many wide-ranging processes such as energy/plasma transfer (Halekas et al, 2009;Harada et al, 2015Harada et al, & 2017, ion/atmospheric escape (Brain et al, 2010), discrete aurora activity (Bowers et al, 2023b;Johnston et al, 2023), and the twisting of the structure of the magnetotail at different planets (DiBraccio et al, 2018(DiBraccio et al, & 2022Romanelli et al, 2022;Tsyganenko et al, 2015). These results suggest that the strength and location of the crustal fields may play an important role in proton aurora occurrence at Mars.…”