O+ ion beams reflected below the Martian bow shock: MAVEN observations

K Masunaga; K Seki; D. A Brain; Y Dong; X Fang; B. M Jakosky; J. P McFadden; J. S Halekas; J. E. P Connerney

doi:10.1002/2016JA022465

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O+ ion beams reflected below the Martian bow shock: MAVEN observations

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O+ ion beams reflected below the Martian bow shock: MAVEN observations

K Masunaga, K Seki, D. A Brain, Y Dong, X Fang, B. M Jakosky, J. P McFadden, J. S Halekas and J. E. P Connerney

Journal of geophysical research. Space physics, Vol.121(4), pp.3093-3107

04/2016

DOI: 10.1002/2016JA022465

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Abstract

We investigate a generation mechanism of O+ ion beams observed above the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. In the solar wind near Mars, MAVEN often observes energetic O+ ion beams (~10 keV or higher). Accompanied with the O+ ion beam events, we sometimes observe characteristic ion VDFs in the magnetosheath: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O+ ions of the magnetosheath. Thus, the partial ring distribution is most likely produced by the reflection of pickup O+ ions precipitating from the upstream solar wind below the bow shock. After being injected into the magnetosheath from the solar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O+ ions are reflected back to the solar wind, generating O+ beams in the solar wind. The beams direct quasi‐sunward near the subsolar region but have large angle with respect to the sunward direction at high solar zenith angles (>50°). The reflected O+ beams are accelerated by the convection electric field of the solar wind and may escape Mars. Key Points Energetic O+ beams are often observed in the solar wind above the Martian bow shock O+ ions in the magnetosheath have a partial ring distribution Reflection of precipitating O+ ions below the bow shock generate the O+ beams in the solar wind

ion reflection

Mars

MAVEN

pickup ion

precipitating ion

solar wind interaction

Details

Title: Subtitle: O+ ion beams reflected below the Martian bow shock: MAVEN observations
Creators: K Masunaga - The University of Tokyo
K Seki - The University of Tokyo
D. A Brain - University of Colorado Boulder
Y Dong - University of Colorado Boulder
X Fang - University of Colorado Boulder
B. M Jakosky - University of Colorado Boulder
J. P McFadden - University of California, Berkeley
J. S Halekas - University of Iowa
J. E. P Connerney - Goddard Space Flight Center
Resource Type: Journal article
Publication Details: Journal of geophysical research. Space physics, Vol.121(4), pp.3093-3107
DOI: 10.1002/2016JA022465
ISSN: 2169-9380
eISSN: 2169-9402
Number of pages: 15
Grant note: Grant‐in‐Aid for Scientific Research (B) (15H0731) NASA through the Mars Exploration Program Grant‐in‐Aid for JSPS Fellows (14J03613) Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers (G2602)
Language: English
Date published: 04/2016
Academic Unit: Physics and Astronomy
Record Identifier: 9984199681402771

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