On the polar cap plasma properties and processes: Electron density distribution, flow, and driving forces

The polar cap is a region of the Earth's magnetosphere in which the magnetosphere and ionosphere are directly coupled and hence it is very important in space physics. It is also a region that is very sensitivity to disturbances from the Sun. The motion and distribution of charge particles (e.g. electrons) in this region is therefore affected by a whole variety of solar and geomagnetic phenomena. Many spacecrafts carrying electronic instruments going through this region in space are bombarded by these charge particles. A study of the velocity and density distribution of these charged particles in the polar cap region is presented.; A method is developed to derive the velocity profile along the field line in the polar region. At altitudes from the peak of the F2 ionosphere layer (∼250--400 km) upto ∼1400 km, the velocity of the polar cap electrons moving upward along the geomagnetic field lines, normalized by the base velocity is derived using the mass conservation equation as a function of electron and neutral species densities. The normalized parallel velocity profiles are calculated using ISIS 2 electron density observations and neutral species density profiles from the MSIS model. From the cases analyzed, the derived velocity profiles show similar trends of (a) a region of slow acceleration starting at the density peak height and up to about 150 km above it, (b) a region of fast acceleration (∼500 km--1100 km), and (c) another region of weak or no acceleration (>∼1100 km). The maximum acceleration height is found to be in the range of ∼600--1300 km. The parallel velocity can increase on average by a factor of up to 9 within the acceleration region. A simple analytical function is proposed to describe the normalized parallel velocity profile.; A comprehensive empirical model of plasma distribution in the polar cap is also derived. At higher altitudes, R = 1.4--5.0 RE geocentric distance, where RE is the Earth radius (∼6400 km), 5 years of electron density measurements by the radio plasma imager (RPI) on the IMAGE spacecraft are used for a statistical study of the relative importance of solar illumination (solar zenith angle) and geomagnetic activity (Kp index) dependences of electron density (N e) distribution in the polar cap magnetosphere. Analysis of the RPI Ne data shows that an altitude-dependent enhancement in Ne can result from increase in geomagnetic activity. The observed enhancement in Ne is most pronounced at higher altitudes and less significant at lower altitudes. On the other hand a strong solar illumination control of Ne at lower altitudes than at higher altitudes is observed. At geocentric distance of R ≈ 2.5 RE the control of solar illumination and geomagnetic activity on Ne appear to be comparable. While previous Ne models used a power law with a power index that is constant to represent the polar cap N e, in this study the power index is found to correlate with (and hence a function of) the Kp index and the solar zenith angle (SZA).