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 (Ne) 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 RE, 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 Ne 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 Ne data shows that although an increase in geomagnetic activity leads to an enhanced Ne, 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 RE, an increase in the geomagnetic activity level from Kp < 2 to ∼5 results in an Ne increase by a factor of ∼5. On the other hand, the observations show a strong solar illumination control of Ne at lower altitudes and not at higher. RPI Ne data show that at geocentric distance of about 2 RE in the polar cap, the average Ne 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 RE the effects of these two factors on Ne appear to be comparable. Similar to previous polar cap density models, the new empirical model of Ne developed in this study takes the form of a power law. While in the previous Ne functional representations the power index is a constant, the power index in our representation of Ne distribution is a function of Kp and solar zenith angle.