Citation:
Cao, Y. T., Cui, J., Wu, X. S., and Zhong, J. H. (2020). Photoelectron pitch angle distribution near Mars and implications on cross terminator magnetic field connectivity. Earth Planet. Phys., 4(1), 17–22.. http://doi.org/10.26464/epp2020008
2020, 4(1): 17-22. doi: 10.26464/epp2020008
Photoelectron pitch angle distribution near Mars and implications on cross terminator magnetic field connectivity
| 1. | Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China |
| 2. | School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, China |
| 3. | School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai Guangdong 519082, China |
| 4. | Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei 230026, China |
The photoelectron peak at 22–27 eV, a distinctive feature of the energetic electron distribution in the dayside Martian ionosphere, is a useful diagnostic of solar extreme ultraviolet (EUV) and X-ray ionization as well as of large-scale transport along magnetic field lines. In this work, we analyze the pitch angle distribution (PAD) of energetic electrons at 22–27 eV measured during several representative Mars Atmosphere and Volatile Evolution (MAVEN) orbits, based on the electron spectra gathered by MAVEN’s Solar Wind Electron Analyzer (SWEA) instrument. On the dayside, most photoelectron spectra show an isotropic PAD as is expected from production via solar EUV/X-ray ionization. The photoelectron spectra occasionally observed on the nightside show instead a strongly anisotropic PAD, indicative of cross-terminator transport along ambient magnetic field lines. This would in turn predict the presence of dayside photoelectrons, also with a strongly anisotropic PAD, which was indeed revealed in SWEA data. Comparison with magnetic field measurements made by the MAVEN Magnetometer suggests that on average the photoelectrons with anisotropic PAD stream away from Mars on the dayside and towards Mars on the nightside, further supporting the scenario of day-to-night transport. On both sides, anisotropic photoelectrons tend to be observed above the photoelectron exobase at ~160 km where photoelectron transport dominates over local production and energy degradation.
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