Simulation of field-aligned H<sup>+</sup> and He<sup>+</sup> dynamics during late-stage plasmasphere refilling

Abstract

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 F 10.7 index. Both early-and latestage 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 F 10.7, 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.

Recent Progress in Physics-Based Models of the Plasmasphere

“…In less than 4 hours a field-aligned density distribution exceeding that of the EE model is set up along all refilling flux tubes. These ionospheric particles build up a dynamical field-aligned density distribution like that studied in the dynamical refilling model of Krall et al (2008). The pitch angle distribution of the ions is expected to be initially strongly anisotropic and peaked in the field-aligned direction, i.e., confined to the source and loss cones.…”

“…These two complementary approaches were also used to model the solar wind and the polar wind (Lemaire and Echim 2008). In the following, we briefly overview how these models have been applied in the past to describe the refilling of plasmaspheric flux tubes that have been emptied or almost during a substorm (Horwitz and Singh 1991;Krall et al 2008, and references therein).…”

“…Therefore, EE models or the corresponding distribution obtained for a Lorentzian VDF (Pierrard and Lemaire 1996) are minimum models. They should be considered as initial boundary conditions in dynamical plasmaspheric refilling models like those proposed by Krall et al (2008) and others. At any time between peeling off event and the long term saturation time, the fieldaligned density distributions should be somewhere in between the two extreme EE and DE models.…”

“…Ion flows across magnetic field lines have been taken into account in several time dependent models and simulations for various given convection electric field models. The effects of counter-streaming of H + and He + along plasmaspheric tubes have been comprehensively studied by Rasmussen and Schunk (1988) and Krall et al (2008).…”

Geospace imaging using Thomson scattering

“…Thus, the plasmaspheric column emission rate for Thomson scattering is 0.1-2 R, again a measurable quantity with current technology. A caveat in this scaling of He + images is that significant variability can occur in the relative abundance of helium ions to the total plasma (Craven et al, 1997;Krall et al, 2009). It is, indeed, this variability that motivates direct measurements of the actual electron density in order to mitigate reliance on proxies.…”