A New Mechanism for Early-Time Plasmaspheric Refilling: The Role of Charge Exchange Reactions in the Transport of...

Cold H⁺ produced via charge exchange reactions between ring current ions and exospheric neutral hydrogen constitutes an additional source of cold plasma that further contributes to the plasmasphere and affects the plasma dynamics in the Earth's magnetosphere system; however, its production and associated effects on the plasmasphere dynamics have not been fully assessed and quantified. In this study, we perform numerical simulations mimicking an idealized three-phase geomagnetic storm to investigate the role of heavy ion composition in the ring current (O⁺ vs. N⁺) and exospheric neutral hydrogen density in the production of cold H⁺ via charge exchange reactions. It is found that ring current heavy ions produce more than 50% of the total cold H⁺ via charge exchange reactions, and energetic N⁺ is more efficient in producing cold H⁺ via charge exchange reactions than O⁺. Furthermore, the density structure of the cold H⁺ is highly dependent on the mass of the parent ion; that is, cold H⁺ deriving from charge exchange reactions involving energetic O⁺ with neutral hydrogen, populates the lower L-shells, while cold H⁺ deriving from charge exchange reactions involving energetic N⁺ with neutral hydrogen populates the higher L-shells. In addition, the density of cold H⁺ produced via charge exchange reactions involving N⁺ can be peak at values up to one order of magnitude larger than the local plasmaspheric density, suggesting that solely considering the supply of cold plasma from the ionosphere to the plasmasphere can lead to a significant underestimation of plasmasphere density.