[P-EM19] Dynamics of the Inner Magnetospheric System

[PEM19-05] Magnetosonic waves driven by proton ring distributions in the Martian magnetosphere

★Invited Papers

*原田 裕己¹、Ruhunusiri Suranga²、Halekas Jasper²、Espley Jared³、DiBraccio Gina³、McFadden James⁴、Mitchell David⁴、Mazelle Christian⁵、Collinson Glyn³、Brain David⁶、原 拓也⁴、能勢 正仁⁷、生松 聡¹、山本 和弘¹、Jakosky Bruce⁶ (1.京都大学大学院理学研究科、2.Department of Physics and Astronomy, University of Iowa、3.NASA Goddard Space Flight Center、4.Space Sciences Laboratory, University of California, Berkeley、5.IRAP, University of Toulouse, CNRS, UPS, CNES、6.LASP, University of Colorado at Boulder、7.名古屋大学宇宙地球環境研究所)

キーワード：Mars、magnetosonic waves、proton ring distributions

We present MAVEN observations of magnetosonic waves at the local proton cyclotron frequency in the Martian magnetosphere. As a consequence of the lack of a global magnetic field of internal origin, the upper atmosphere of Mars directly interacts with the solar wind, thereby forming an induced magnetosphere. The majority of the solar wind plasma diverts around the obstacle, but some fraction of the hot magnetosheath protons have access to the upper ionosphere of Mars due to their large gyroradii with respect to the size of the induced magnetosphere. As a result, ring/shell-like proton velocity distribution functions are formed just below the induced magnetospheric boundary in the presence of cold, dense protons of ionospheric origin.

Below the induced magnetospheric boundary, MAVEN detected narrowband compressional waves at the local proton cyclotron frequency (and its harmonics for some cases) typically in the presence of both hot ring protons and cold ionospheric protons. The occurrence of the waves at the local proton cyclotron frequency is strongly controlled by solar extreme ultraviolet (EUV) conditions, showing a distinct preference for high EUV. The wave properties and their dependence on local and upstream conditions can be mostly explained by a proton Bernstein mode instability driven by a positive perpendicular slope in proton velocity distribution functions. The wave properties and their association with ring protons exhibit striking similarities to the magnetosonic waves observed in the terrestrial inner magnetosphere. These "Martian magnetosonic waves" potentially facilitate energy transport from the precipitating hot protons of magnetosheath origin to the cold ionospheric protons.