Due to their proximity to the Sun and lack of magnetic shielding, the Martian and Venusian atmospheres are strongly affected by solar variability and space weather events, which has important consequences for atmospheric evolution (e.g. Barabash et al. 2007, Luhmann et al. 2008, Futaana et al. 2008, McEnulty et al. 2010, Dubinin et al. 2011, Dubinin et al. 2013, Jakosky et al. 2015b, Brain et al. 2016). However, many aspects of these space weather interactions are currently unknown or poorly constrained. This is particularly true for the Venusian nightside ionosphere, where the number of observations has been very limited.
Venus presents us with an ideal laboratory with which to study these interactions because unlike Mars, Venus is purely unmagnetized. In addition, its thicker atmosphere and closer proximity to the Sun means that atmospheric variability due to solar events is typically more pronounced (e.g. Nordheim et al. 2015). The presence of aurora is an important manifestation and tracer of the interaction between the solar wind and a planetary ionosphere. The OI (1S-1D) 557.7 nm (oxygen green line) is a bright auroral line in the terrestrial atmosphere and is detected on the Venusian nightside after major solar storms as a diffuse, global aurora (Gray et al. 2014, Gray 2015). Mars exhibits a similar global diffuse aurora after intense solar storms, though the oxygen green line has yet to be detected on Mars.
In general, emission is not present outside a major solar event such as a coronal mass ejection (CME) or solar flare. However, weak emission was detected twice outside of solar storms; December 27, 2010 and Dec 12, 2013. Both of these detections coincided with the passage of a dense solar wind stream interaction region (SIR, Gray 2015). It has been proposed that the Venusian green line is a proton aurora occurring deep in the Venusian ionospheres, due to protons with energies ~100 keV impacting near 125 km (e.g. Gray et al. 2017).
Here, we present the results of a monitoring campaign conducted at Apache Point Observatory (APO) from July 28–Aug 13, 2020 using the Astrophysical Research Consortium Echelle Spectrograph (ARCES, R~35,000) to study the effects of SIR passages over Venus. While there are presently no dedicated spacecraft observing the interaction between Venus and the solar wind, we compare proton energies measured by the NASA’s Advanced Composition Explorer (ACE) spacecraft. Additionally, we present results from a ground-based coordinated campaign taken from APO during the Parker Solar Probe flyby of Venus in July 2020.