Simulation of field-aligned H⁺ and He⁺ dynamics during late-stage plasmasphere refilling

Abstract. The refilling of the plasmasphere for 3≤L≤4 following a model storm is simulated over long times (days) using the NRL ionosphere code SAMI2 (Sami2 is Another Model of the Ionosphere). Refilling is dependent on the supply of topside H⁺ and He⁺ ions with the result that H⁺ refilling rates decrease and He⁺ refilling rates generally increase with increasing F10.7 index. Both early- and late-stage refilling are affected by net ion flows from the warmer to the colder geomagnetic hemisphere. When these flows are strong, the ability of the "winter helium bulge" to increase He⁺ refilling rates is suppressed. When neutral winds are not included, refilling rates fall, typically by a factor of two. In most cases, late-stage He⁺ refilling is proportional to H⁺ refilling, with typical He⁺/H⁺ density ratios of 2% for solar minimum and 10% for solar maximum. For high values of F10.7, He⁺ refilling exhibits a strong diurnal variation so that the He⁺/H⁺ density ratio varies by as much as a factor of two during late-stage refilling. Finally if the plasmasphere is left undisturbed, the H⁺ density can refill for as long as five weeks at L=3 and ten weeks at L=4, with saturation densities nearly an order of magnitude greater than typical observed densities. This confirms that the plasmasphere at these L values rarely obtains saturation.