Estimation of radial gradients of phase space density from POLAR observations during a quiet period prior to a sudden solar wind dynamic pressure enhancement

Abstract

We have analyzed POLAR electron flux data to estimate the radial gradients of electron phase space density (PSD) immediately prior to a sudden solar wind dynamic pressure enhancement at ∼1135 UT on 12 August 2001. In this event, the instantaneous flux changes from the magnetospheric compression at L ∼ 7.6 in the postmidnight magnetic local time sector are observed to be both energy and pitch angle (PA) dependent: a substantial flux increase for lower energies (<∼200 keV) particularly at smaller local PA (<∼50°) and a large flux decrease for relativistic energies that is more pronounced for a larger PA. For this event, because of the extremely steady and quiet solar wind conditions prior to the pressure impact, we can reasonably assume that the temporal variation is negligible and so deduce the spatial and pitch angle distribution of initial electron fluxes to construct the event‐specific electron flux model for a localized region of the nightside outer radiation belt. PSD radial profiles are then estimated for 6.5 < L < 8.5 using the constructed electron flux model and Tsyganenko magnetospheric model for quiet times. The estimates show a positive PSD gradient for low energies (<∼a few 100 s keV at the POLAR location right after the pressure impact) and a negative gradient for relativistic energies. In addition, a rather gradual transition of the radial gradient from highly positive to more negative is found with increasing energy. The expected immediate flux responses using these estimated PSD radial gradients are qualitatively consistent with those observed, partially validating the veracity of the estimated profiles.