Ad. Danilov et al., Electron density variations in the upper D region at high latitudes according to in situ measurements, GEOMAG AERO, 41(5), 2001, pp. 623-631
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.