Mars photoelectron energy and pitch angle dependence on intense lower atmospheric dust storms

Abstract

We have conducted a survey of the Mars Global Surveyor (MGS) electron data across all the pitch angles of 12 usable energy bins (11-746 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 and a few bins 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 photoelectron fluxes, especially at low energies and the pitch angle source regions of high-energy channels. The correlation increases experienced by the pitch angle source regions of all examined energy channels suggest that dust storms' influence most likely happens in the thermosphere-ionosphere source region of the photoelectrons, rather than at exospheric altitudes at or above MGS. Furthermore, by isolating the global-scale dust storm in Mars year 25 (2001) from the rest, the results suggest that this storm is entirely responsible for the second solar flux-dependent trend. While not excluding the possibility of this phenomenon being a one-time event, we hypothesize that there is a threshold of dust opacity at which the low-altitude dust's influence on high-altitude photoelectron fluxes begins to be significant.

Mars nightside electrons over strong crustal fields

“…Measurements from sectors 8, 10, and 11 were discarded due to high fluxes being recorded frequently, regardless of field line orientations [Xu et al, 2014a] With the magnetic field information from the magnetometer (MAG), these angular distributions can be converted into PADs . The field of view spanned 360 ∘ × 14 ∘ and was divided into sixteen 22.5 ∘ sectors.…”

“…Xu et al [2014aXu et al [ , 2014b conducted the analogous studies for the dayside strong crustal field regions. All measurements with a solar zenith angle of less than 90 ∘ were excluded to limit this study to the nightside.…”

Superthermal electron transport model for Mars

“…The detailed description of the instrument is given in Acuña et al []. To isolate dayside photoelectron measurements over the strong crustal field regions, the same method is applied as was done by Xu et al []. To briefly summarize their method, a spatial constraint of 160°–200° east longitude and 30°–70° south latitude is applied, along with a magnetic field magnitude minimum constraint (35 nT), to ensure data selected over the strong crustal fields.…”

Solar wind electron precipitation into the dayside Martian upper atmosphere through the cusps of strong crustal fields

“…One thing not addressed in the previous analysis is the possible contamination by spacecraft photoelectrons and other secondary electrons. The anode sectors (sectors 5–12) of MGS ER more likely to detect these spacecraft electrons look toward zenith and along the spacecraft bus [ Mitchell et al , ; Xu et al , ]. Since the data set of this study is confined to more vertical magnetic fields lines, these contaminated sectors are converted into upward modified pitch angles while the downward modified pitch angles should be barely affected.…”