Multipoint Density Measurements of Geocoronal Pickup Ions

Abstract

Observations of the plasma environment outside of the Earth's magnetosphere by the Magnetospheric Multiscale Mission Hot Plasma Composition Analyzers (MMS‐HPCAs) allow density measurements of geocoronal hydrogen pickup ions (PUIs). A study of this environment from December 4, 2019 to December 6, 2019, part of a single orbital period of the MMS spacecraft, revealed an inverse square dependence on PUI density. This dependence corresponds well with an inverse‐cube dependence on the neutral hydrogen density within the geocorona.

One Solar Cycle of Heliosphere Observations with the Interstellar Boundary Explorer: Energetic Neutral Hydrogen Atoms Observed with IBEX-Lo from 10 eV to 2 keV

“…However, if this hypothesis were true the ENA intensities in energy bin 4, independent of the transformed energy, should be by far the highest (see peak position in Figure 14). This disagrees with the observed time series in the spacecraft reference frame (Figure 10) and inertial reference frame (Figure 11 (local H + densities ∼10 −3 cm −3 at 20-30 Earth radii distance from Earth according to Gomez et al (2021)). However, it is unclear why this signal would be restricted to the years around 2015, whereas the orbital configuration of IBEX has remained identical from 2011 onward, and why it would be restricted to 50-200 eV energies.…”

Variation of Hydrogen Energetic Neutral Atom Flux in the Subsolar Magnetosheath as a Function of Solar Cycle

“…The overall change in the standoff distance of the bow shock and magnetopause to the Earth is of an order of a few Earth's radii during that time. Assuming a variation in the geocorona hydrogen density with distance to the Earth as n(r) ∝ r −3 (e.g., Baliukin et al, 2019;Chamberlain, 1963;Gomez et al, 2021), a change of 1 R E moves the subsolar magnetosheath into a region of about 20% higher concentration of hydrogen. Thus, the increase in ENA flux observed by IBEX in the second half of the Solar Cycle 24 is related to the increased production of ENAs due to the shocked solar wind probing a region with higher geocoronal density, as shown by Connor et al (2021) in the study of ENA response to the sudden enhancements of the solar wind dynamic pressure in the dayside magnetosphere.…”

“…A small fraction of these protons charge exchange with atoms of Earth's geocorona (Shklovsky, 1959). The neutral hydrogen geocorona extends beyond the bow shock up to the Moon's orbit (e.g., Baliukin et al., 2019; Cucho‐Padin et al., 2022; Gomez et al., 2021; Kameda et al., 2017) and thus fills in the dayside magnetosheath. The geocoronal hydrogen atoms exchange electrons with the solar wind protons in the subsolar magnetosheath creating ENAs (Figure 2).…”