Polar cap electron density distribution from IMAGE radio plasma imager measurements: Empirical model with the effects of solar illumination and geomagnetic activity

Abstract

We present a statistical study of the relative importance of solar illumination and geomagnetic activity dependences of the electron density ( N e ) distribution in the polar cap magnetosphere based on 5 years of electron density measurements made by the radio plasma imager (RPI) on board the IMAGE spacecraft. This study covers a geocentric distance of R = 1.4–5.0 R E , and the polar cap is defined by an empirical boundary model that takes into account the dynamic nature of the location and size of the polar cap. The RPI N e data show that the electron density distribution within the polar cap depends on the geocentric distance, R , geomagnetic activity level, e.g., measured by the Kp index, and solar illumination (solar zenith angle) at the footprints of the geomagnetic field lines. Our analysis of RPI N e data shows that although an increase in geomagnetic activity leads to an enhanced N e , the enhancement is found to be altitude‐dependent such that it is most pronounced at higher altitudes and less significant at lower altitudes. At geocentric distance of R = 4.5 R E , an increase in the geomagnetic activity level from Kp < 2 to ∼5 results in an N e increase by a factor of ∼5. On the other hand, the observations show a strong solar illumination control of N e at lower altitudes and not at higher. RPI N e data show that at geocentric distance of about 2 R E in the polar cap, the average N e is larger on the sunlit side than on the darkside by a factor of 3–4 for both quiet and disturbed conditions. At geocentric distance of R ≈ 2.5 R E the effects of these two factors on N e appear to be comparable. Similar to previous polar cap density models, the new empirical model of N e developed in this study takes the form of a power law. While in the previous N e functional representations the power index is a constant, the power index in our representation of N e distribution is a function of Kp and solar zenith angle.