Mars Photoelectron Energy and Pitch Angle Dependence on Intense Lower-Atmospheric Dust Storms

We have conducted a survey of the Mars Global Surveyor electron data across all the pitch angles (PAs) of 12 usable energy bins (10 eV~650 eV) for dayside photoelectron observations over regions of strong crustal fields. Studies have shown that solar EUV flux is the main controlling factor but dust storms play an important role as well. Our study of different energies and pitch angles has shown that the unusual bimodal solar flux dependence is not a common feature but mainly found in low energies (< 53 eV) and a couple of higher energy channels. By multiplying time-history dust opacity with a solar EUV proxy as a new controlling function, the statistically significant increase of the correlation of photoelectron flux against this function indicates that dust storms have a long-lasting influence on high-altitude (~400 km) photoelectron fluxes, especially at low energies. Furthermore, by isolating the huge global dust storm in Earth year 2001-2002 from the rest, the results suggest that this storm is entirely responsible for the second solar flux dependent trend. It leads to the hypothesis that there is a threshold of dust opacity that low-altitude dust's influence on high-altitude photoelectron fluxes begins to be significant.