Stable ring beam of solar wind He2+ in the magnetosheath. Issue 2 (8th February 2016)

• Record Type: Journal Article
• Authors: Tsubouchi, K.; Nagai, T.; Shinohara, I.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 2(2016:Feb.)
• Page Start: 1233
• Page End: 1248
• Publication Date: 2016-02-08
• Subjects: bow shock -- magnetosheath -- solar wind helium; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2015JA021769
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Heavier, or large mass‐per‐charge, ion species like helium ions (He 2+ ) in the solar wind are decelerated less than protons at the Earth's bow shock. The resulting velocity difference in the magnetosheath forms a ring beam around the core protons. In this study, we identified the presence of such a ring beam composed of He 2+ ions in the magnetosheath from observations from the Geotail spacecraft. Previous studies have shown that the ring distribution induces ion cyclotron waves that rapidly scatter the He 2+ ring beam, leading to evolution into a shell or to a Maxwellian distribution. In contrast, the present event suggested that the He 2+ ring beam persisted for the order of 10 min, i.e., hundreds of ion gyroperiods. One‐dimensional hybrid simulations of a quasi‐perpendicular, low Mach number shock ( θ = 85°, M A ∼ 2.5) showed that the lower density of He 2+ ions (less than 1% to protons) is the crucial condition that accounts for the long‐duration property of the ring beam. Weaker wave activity was insufficient to scatter He 2+ ions, whereas two‐dimensional simulations showed a rapid scattering of the ring beam as suggested in the previous studies. Comparison between these results indicated that there are still controversial issues concerning the dynamics of minor ions in the magnetosheath, whether the He 2+ ring beam is really stable, and what parameters control the stability condition. Key Points: Velocity distribution of solar wind helium ions shows a ring beam after crossing the bow shock Contrary to previous studies, the ring beam remains stable in the magnetosheath Smaller density ratio of helium ions to proton dominates the stability of the helium ring beam