Simulation of the POLAR-observed Geomagnetic Ion Energy Spectrometer

Abstract

[1] Observations by polar-orbiting spacecraft of outward streaming ionospheric O+ ions in the polar cap magnetosphere often show decreasing field-aligned streaming energy with antisunward distance from their apparent dayside source, the Cleft Ion Fountain (CIF). In this paper we use the UT Arlington Dynamic Fluid-Kinetic (DyFK) ionospheric plasma transport model to simulate the transport of CIF generated ions along convecting magnetic flux tubes. We compare these simulations to observations by the Thermal Ion Dynamics Experiments (TIDE) on board the Polar spacecraft, for periods when the Polar orbit was aligned parallel to the noon-midnight direction. When the Polar spacecraft traversed from high altitudes on the dayside to lower altitudes on the nightside, the peak O+ streaming energy decreased from above 100 eV to below 5 eV. For the case in which the Polar satellite traveled from the nightside higher altitude to the dayside lower altitude, the O+ energy remained relatively stable, ranging between 20 eV and about 50 eV. Using the DyFK model, we simulate the ionospheric plasma transport and, in particular, the energy spectrometer effects under the geophysical circumstances of the observations, and compare the simulated and observed streaming energies. The results show that the simulated O+-streaming energy variations in the noon-midnight direction were in reasonable agreement with those of the Polar/TIDE observations, independent of whether Polar was moving sunward or antisunward, for realistic choices of geophysical parameters. The altitude and the distance to the CIF are the two primary O+ parameters influencing the O+ energy spectrometer variations, with the antisunward distance from the CIF being the principal controlling parameter.