Neutral Densities in the Outer Exosphere Near the Subsolar Magnetopause. Issue 23 (6th December 2021) - Sibeck, D. G.
Abstract: In a seminal analysis, Fuselier et al. (2020) combined subsolar in situ magnetosheath plasma observations with remote energetic neutral atom (ENA) observations to derive an exospheric neutral density of 11 cm −3 in the vicinity of the subsolar magnetopause. This commentary extends the analysis of Fuselier et al. (2020) to (a) employ predicted and observed magnetosheath plasma parameters at times other than those immediately following an MMS magnetopause crossing, (b) take into account the component of the plasma bulk flow transverse to the ENA spacecraft's line‐of‐sight through the flank magnetosheath, and (c) consider the cooling that results from the antisunward expansion of the magnetosheath plasma. With these additional considerations, subsolar exospheric neutral densities rise by a factor of 4.1–4.5 to 45–50 cm −3, consistent with results from previous studies. Plain Language Summary: Because the number of energetic neutral atoms generated when protons exchange electrons with hydrogen atoms depends on the number of protons available for charge exchange and the neutral density, neutral densities in the Earth's outer atmosphere can (with the use of models) be deduced from simultaneous remote observations of energetic neutral atoms and in situ plasma observations. When proton parameters typical of the time interval studied and the motion of the protons are taken into account, neutral densities in the vicinity of the subsolar point on the Earth's magnetopause are found to be on the order of 50 cm −3 . Key Points: The strength of energetic neutral atom and soft X‐ray emissions from the magnetosheath depends upon exospheric neutral densities An augmentation of a previous MMS/IBEX study indicates that neutral densities may significantly exceed those originally derived Exospheric neutral densities suffice to ensure the success of future soft X‐ray imaging missions