The electron density distribution in the polar cap: Its variability with seasons, and its response to magnetic activity

Abstract Using forty-five months of spacecraft potential measurements gathered with the double probe electric field antenna on the Polar satellite, we study the electron density in the polar cap and investigate its dependence on distance (2 R E E ), season, and geomagnetic activity. The polar cap density increases with magnetic activity at all distances; it is larger during local summer than during local winter, by up to two orders of magnitude at distances of around 2 R E . The density profile follows a power law of the form r −a . In winter, a =3.3–3.7, which tends to suggest that the escaping ions are accelerated ( a >3). However, as the ion composition (particularly, the H + /O + ratio) is known to vary with altitude, the occurrence of ion acceleration cannot be deduced from our observations. In summer, the exponent reaches values of about 6.0, which implies that the density of the polar topside ionosphere is high and the heavy ions are gravitationally bound at low altitudes.