Electron density variations in the upper D region at high latitudes according to in situ measurements

A large set of rocket measurements of the electron concentration [e] in the upper D region at four rocket sites (Molodezhnaya (Antarctic), Heiss Islan d (Arctic), Syowa (Antarctic), and Andoya/Kiruna (Arctic)) is considered. T he dependence of [e] on the solar zenith angle chi and geomagnetic activity index Sigma Kp is analyzed. It has been shown that, the spread in [e] valu es reaches two orders of magnitude at any assigned value of chi. Neverthele ss, it is possible to draw a lower envelope for the entire set of points an d to obtain the [e] variation with chi in undisturbed conditions. In an ana lysis of the envelopes, it is seen that a corpuscular ionization source (pr esumably, electrons with an energy of above 40 keV) exists in the nonsunlit D region even in quiet conditions. An analysis of the electron concentrati on dependence on the daily sum of Sigma Kp indices (Sigma Kp) shows that, a t all heights considered, increases [e] with increasing Sigma Kp to a certa in boundary value Sigma Kp and then remains either constant (with a rather wide spread in the data) or even decreases. This saturation effect may be c aused by the auroral oval equatorward motion. As a result of this motion th e site position may change: it may move toward the boundary between the aur oral oval and polar cap or even be within the latter. The slope k of the [e ] variation plotted versus Sigma Kp at different chi is considered. It is s hown that k grows with increasing chi and is maximal at chi > 100 degrees.