Two-dimensional mapping of the time-variable plasmasphere using Van Allen Probes observations.

The low energy plasmasphere is an important contributor to magnetospheric dynamics by virtue of its overlap with the ring current and the radiation belts. The plasmasphere is not only dynamically tied to magnetospheric dynamics (changes in content, structure, and extent), but also magnetically tied to the ionosphere below. Because of their fast gradient-curvature drift motion, the energetic ring current and radiation belt populations are comparatively simple to characterize experimentally across the whole inner magnetosphere using a single radial measurement profile in the equatorial plane and a good global magnetic field model. The same is not true for the plasmasphere, where radial profiles are more strongly MLT dependent. An experimental 3D characterization of the plasmasphere with a single spacecraft is therefore difficult. We demonstrate that the Van Allen Probes orbit design is surprisingly well-suited to overcome this difficulty. Three aspects of the Van Allen Probes orbits make this possible: (1) The "follow-the-leader" design, with both spacecraft constantly lapping each other every few months, allows for the study of short-term plasmasphere variations on near-identical orbits. (2) The slow differential precession of both spacecraft's apogee results in a ~1 hr MLT separation at mission end, increasing the ability to investigate smaller-scale MLT structures in the plasmasphere at high cadence. (3) The 9-hour orbit period results in a 3-day orbit repetition in the fixed plasmasphere reference frame, meaning that the Van Allen Probes spacecraft cover the whole plasmasphere with two sets of orbits each 72 hours. These orbit sets consist of eight equally spaced petals, providing a 2D map of the plasmasphere over any 72-hour period. The combination of relative orbit precession and the follow-the leader dynamics also moves those two orbit-petals with respect to each other. Based on these properties we discuss the possibilities and limitations of mapping the 2D structure of plasmasphere density and ion composition as a function of time for both a representative quiet and disturbed time period during the Van Allen Probes era. We also discuss the implications for building an empirical, time-variable 2D (equatorial plane) model of the plasmasphere.