The Martian Photoelectron Boundary as Seen by MAVEN

• Published in: Journal of geophysical research. Space physics 2017-10, Vol.122 (10), p.10,472-10,485
• Main Authors: Garnier, P., Steckiewicz, M., Mazelle, C., Xu, S., Mitchell, D., Holmberg, M. K. G., Halekas, J. S., Andersson, L., Brain, D. A., Connerney, J. E. P., Espley, J. R., Lillis, R. J., Luhmann, J. G., Sauvaud, J.‐A., Jakosky, B. M.
• Format: Article
• Language: English
• Subjects: Altitude; Atmospheric evolution; Cross-sections; Electron density; Electron flux; Interplanetary magnetic field; Ionopause; Ionosphere; Ionospheric plasma; Magnetic fields; Mars; Mars atmosphere; Mars missions; Parameter identification; Photoelectrons; Photoionization; Photons; Planetary atmospheres; Planetary magnetic fields; plasma boundary; plasma escape; Sciences of the Universe; Solar wind; solar wind interaction; Statistical analysis; Topology; Tracers
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2017JA024497

Photoelectron peaks in the 20–30 eV energy range are commonly observed in the planetary atmospheres, produced by the intense photoionization from solar 30.4 nm photons. At Mars, these photoelectrons are known to escape the planet down its tail, making them tracers for the atmospheric escape. Furthermore, their presence or absence allow to define the so‐called photoelectron boundary (PEB), which separates the photoelectron dominated ionosphere from the external environment. We provide here a detailed statistical analysis of the location and properties of the PEB based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) electron and magnetic field data obtained from September 2014 to May 2016 (including 1696 PEB crossings). The PEB appears as mostly sensitive to the solar wind dynamic and crustal fields pressures. Its variable altitude thus leads to a variable wake cross section for escape (up to ∼+50%), which is important for deriving escape rates. The PEB is not always sharp and is characterized on average by the following: a magnetic field topology typical for the end of magnetic pileup region above it, more field‐aligned fluxes above than below, and a clear change of the altitude slopes of both electron fluxes and total density (that appears different from the ionopause). The PEB thus appears as a transition region between two plasma and fields configurations determined by the draping topology of the interplanetary magnetic field around Mars and much influenced by the crustal field sources below, whose dynamics also impacts the estimated escape rate of ionospheric plasma. Key Points We determined the influence of the main driving parameters on the altitude of the photoelectron boundary (PEB) We identified clear plasma and magnetic field characteristics of the PEB and discuss its nature with respect to the ionopause We show how the PEB dynamics modifies the tail cross section used for estimating the photoelectrons (and associated ions) escape rate