The plasma environment inside geostationary orbit: A Van Allen Probes HOPE survey

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.