Interplanetary Hydrogen Observations of the Emirates Ultraviolet Spectrometer Onboard the Emirates Mars Mission

Abstract

The Emirates Mars Ultraviolet Spectrometer (EMUS) aboard the Emirates Mars Mission (EMM) has been studying backscattered interplanetary hydrogen (interplanetary hydrogen (IPH)) Lyman emissions with dedicated observation strategies viz., U-OS3b and U-OS4b. These observation techniques involve looking away from Mars from the EMM's orbit. U-OS3b provides broader coverage of the Martian sky, while U-OS4b is designed to achieve high signal-to-noise measurements. Here we present analysis of the interplanetary hydrogen emission distribution across the sky, particularly at the Lyman-$\alpha$ and $\beta$ emission wavelengths. EMUS observes a well-known reduction in the brightness of interplanetary hydrogen as the observation angle increases across the sky from the hydrogen bulk flow upwind direction. Our modeled emission intensities indicate that when Mars is around aphelion, the observed emissions are dominated by IPH rather than Martian exospheric hydrogen. Our modeled intensities for H Ly-$\alpha$ emission are smaller by a factor of 1.6 compared to the observations, and they are consistent with the observations for H Ly-$\beta$ emission. The H Ly-$\alpha$ intensities measured by the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile EvolutioN mission were found to be 45% lower than those from EMUS measurements, likely owing to differences in the instruments' calibration factors. During the observation period, differences in the measured intensities between the Solar Wind ANisotropy (SWAN) instrument aboard the Solar and Heliospheric Observatory and EMUS were also found to vary. These discrepancies can be attributed not only to instrumental factors but also to the relative positions of Earth and Mars.