The Unknown Hydrogen Exosphere: Space Weather Implications

Recent studies suggest that the hydrogen (H) density in the exosphere and geocorona might differ from previously assumed values by factors as large as 2. We use the SAMI3 (Sami3 is Also a Model of the Ionosphere) and Comprehensive Inner Magnetosphere- Ionosphere models to evaluate scenarios where the hydrogen density is reduced or enhanced, by a factor of 2, relative to values given by commonly used empirical models. We show that the rate of plasmasphere refilling following a geomagnetic storm varies nearly linearly with the hydrogen density. We also show that the ring current associated with a geomagnetic storm decays more rapidly when H is increased. With respect to these two space weather effects, increased exosphere hydrogen density is associated with reduced threats to space assets during and following a geomagnetic storm. Plain Language Summary Commonly used empirical models of the hydrogen density in the outer reaches of Earth's atmosphere, the exosphere, may be off by factors as large as two. We use the SAMI3 (Sami3 is Also a Model of the Ionosphere) and CIMI (Comprehensive Inner Magnetosphere- Ionosphere) models to evaluate situations where the model hydrogen density is reduced or enhanced, by a factor of 2, relative to these empirical models. We show that the rate at which the inner magnetosphere plasma density recovers following a geomagnetic storm varies nearly linearly with the hydrogen density. We also show that the ring current associated with a geomagnetic storm decays more rapidly when there is more hydrogen. Overall, increased exosphere hydrogen density is associated with reduced threats to space assets during and following a geomagnetic storm.