Sinuous Aurora at Mars: A Link to the Tail Current Sheet?

We examine the newly discovered phenomena of sinuous aurora on the nightside of Mars, using images of 130.4 and 135.6 nm oxygen emission measured by the Emirates Mars Mission EMUS ultraviolet spectrograph, and upstream measurements from the MAVEN and Mars Express spacecraft. They are detected in similar to 3% of observations, totaling 73 clear detections. These emissions are narrow, elongated (1,000-6,000 km), cross Mars' UV terminator, and are oriented generally toward the anti-solar point, clustering into north, south, east, and west-oriented groups. Diverse morphologies are observed, though some spatial features, such as broad curves, may in some cases be due to temporal aliasing of aurora motion as each image is built up over 15-20 min. Sinuous aurora form away from Mars' strongest crustal magnetic fields and can be interrupted by moderate crustal fields. Sinuous aurora occurrence increases strongly with solar wind pressure, though brightness shows only a weak positive dependence on pressure. Interplanetary magnetic field (IMF) clock angle affects their occurrence and orientation: sinuous aurora show a broad range of orientations centered on the solar wind convection electric field (Econv) direction and forming in the +Econv hemisphere, although with moderate clockwise and counterclockwise average "twists" for westward and eastward IMF, respectively. From these features we infer a link between sinuous aurora and electron energization in Mars' magnetotail current sheet, where field geometry on the +Econv side of the sheet is more organized and symmetric. Determination of specific triggering conditions for sinuous aurora requires further investigation. Sinuous aurora are narrow, extended patterns of UV emission caused by long, thin channels of energized electrons striking Mars' nightside upper atmosphere. We study images of these aurora taken by the Emirates Mars Mission EMUS instrument. 73 cases of sinuous auroras were found (similar to 3% occurrence rate) with lengths ranging from 1,000 to 6,000 km. These auroras usually cross Mars' day-night boundary and extend in the direction opposite to the Sun. They tend to cluster into groups oriented toward the north, south, east, and west directions with a diverse array of shapes. Sinuous aurora generally form away from Mars' strongest crustal magnetic fields. They occur more frequently for higher solar wind pressure. Their orientations are affected by the interplanetary magnetic field (IMF), displaying a broad range of orientations centered on the direction of the electric field in the solar wind, and forming in the hemisphere to which this electric field points, although with moderate counterclockwise and clockwise average "twists" for eastward and westward IMF, respectively. From these features we infer a link between sinuous aurora and a sheet of current in Mars magnetic tail, wherein the aurora-causing electrons may be energized before falling into the upper atmosphere to produce aurora. These narrow emission features form away from strong crustal fields, oriented anti-sunward, cross the terminator, are detected in 3% observations Occurrence increases with solar wind pressure, certain interplanetary magnetic field orientations, and in the positive motional electric field hemisphere Sinuous aurora may be related to magnetotail asymmetry and electron energization processes occurring in the tail current sheet