The plasma environment inside geostationary orbit: A Van Allen Probes HOPE survey. Issue 9 (4th September 2017)

• Record Type: Journal Article
• Authors: Fernandes, Philip A.; Larsen, Brian A.; Thomsen, Michelle F.; Skoug, Ruth M.; Reeves, Geoffrey D.; Denton, Michael H.; Friedel, Reinhard H. W.; Funsten, Herbert O.; Goldstein, Jerry; Henderson, Michael G.; Jahn, Jörg‐Micha; MacDonald, Elizabeth A.; Olson, David K.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 9(2017)
• Page Start: 9207
• Page End: 9227
• Publication Date: 2017-09-04
• Subjects: Van Allen Probes -- inner magnetosphere -- magnetospheric composition -- plasma convection -- plasma access -- UBK modeling; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2017JA024160
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: The two full precessions in local time completed by the Van Allen Probes enable global specification of the near‐equatorial inner magnetosphere plasma environment. Observations by the Helium‐Oxygen‐Proton‐Electron (HOPE) mass spectrometers provide detailed insight into the global spatial distribution of electrons, H +, He +, and O + . Near‐equatorial omnidirectional fluxes and abundance ratios at energies 0.1–30 keV are presented for 2 ≤ L ≤ 6 as a function of L shell, magnetic local time (MLT), and geomagnetic activity. We present a new tool built on the UBK modeling technique for classifying plasma sheet particle access to the inner magnetosphere. This new tool generates access maps for particles of constant energy for more direct comparison with in situ measurements, rather than the traditional constant μ presentation typically associated with UBK. We present for the first time inner magnetosphere abundances of O + flux relative to H + flux as a function of Kp, L, MLT, and energy. At L = 6, the O + /H + ratio increases with increasing Kp, consistent with previous results. However, at L < 5 the O + /H + ratio generally decreases with increasing Kp . We identify a new "afternoon bulge" plasma population enriched in 10 keV O + and superenriched in 10 keV He + that is present during quiet/moderate geomagnetic activity ( Kp < 5) at ~1100–2000 MLT and L shell 2–4. Drift path modeling results are consistent with the narrow energy and approximate MLT location of this enhancement, but the underlying physics describing its formation, structure, and depletion during higher geomagnetic activity are currently not understood. Key Points: Two‐dimensional L‐MLT plots of average electron, H +, He +, and O + fluxes and ion flux ratios show significant structure and dependence on energy and Kp A new afternoon bulge plasma population enriched in 10 keV O + and He + is observed for Kp < 5 at ~1100–2000 MLT and L shell range 2–4 A new tool built on UBK modeling classifies plasma sheet particle access to the inner magnetosphere Plain Language Summary: We present a detailed study of the composition of the equatorial plane of the inner magnetosphere using 40 months of electron and ion data from the HOPE mass spectrometers on the Van Allen Probes spacecraft. Specifically, we present 2‐D distributions of fluxes of electrons, H +, O +, and He +, as well as flux ratios of O + /H +, He + /H +, and He + /O + . This study builds on historical characterization of the equatorial plasma environment. These previous studies have various shortcomings, including limited 2‐D coverage due to the nature of their spacecraft orbits and severe contamination of measurements due to the significant background induced by penetrating radiation inherent in the radiation belts. This study advances the field by presenting the most comprehensive measurements of electrons and ion species to date using instrumentation specifically designed to mitigate background effects. In this study, we establish a new tool built upon well‐established modeling and use this tool to analyze the data by classifying regions to which certain particle populations may or may not have access. Our analysis identifies a new plasma population enriched in both 10 keV O + and He + that we call the "afternoon bulge" that is persistently present near the Earth during geomagnetically quiet conditions.