F-region ionospheric perturbations in the low-latitude ionosphere during the geomagnetic storm of 25-27 August 1987

摘要

We have presented a comparison between the modeled and, and and which were observed at the equatorial anomaly crestand close to the geomagnetic equator simultaneously by the Akita, Kokubunji,Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and bythe middle and upper atmosphere (MU) radar (34.85° N, 136.10° E)during the 25-27 August 1987 geomagnetically storm-time period at low solaractivity near 201°, geomagnetic longitude. A comparison between theelectron and ion temperatures measured by the MU radar and those produced bythe model of the ionosphere and plasmasphere is presented. The correctionsof the storm-time zonal electric field, E, from 16:30 UT to21:00 UT on 25 August bring the modeled and measured into reasonableagreement. In both hemispheres, the meridional neutral wind, W, taken fromthe HWW90 wind model and the NRLMSISE-00 neutral temperature, T, anddensities are corrected so that the model results agree with the ionosphericsounders and MU radar observations. The geomagnetic latitude variations in on 26 August differ significantly from those on 25 and 27 August. Theequatorial plasma fountain undergoes significant inhibition on 26 August.This suppression of the equatorial anomaly on 26 August is not due to areduction in the meridional component of the plasma drift perpendicular tothe geomagnetic field direction, but is due to the action of storm-timechanges in neutral winds and densities on the plasma fountain process. Theasymmetry in W determines most of the north-south asymmetry in and on 25 and 27 August between about 01:00-01:30 UT and about 14:00 UT when theequatorial anomaly exists in the ionosphere, while asymmetries in W,T, and neutral densities relative to the geomagnetic equator areresponsible for the north-south asymmetry in and on 26 August. Atheory of the primary mechanisms causing the morning and evening peaks inthe electron temperature, T, is developed. An appearance, magnitudevariations, latitude variations, and a disappearance of the morning Tpeaks during 25-27 August are caused by variations in E,thermospheric composition, T, and W. The magnitude of the eveningT peak and its time location are decreased with the lowering of thegeomagnetic latitude due to the weakening of the effect of the plasma driftcaused by W on the electron density. The difference between 25 August and26-27 August in an appearance, magnitude and latitude variations, and adisappearance of the evening T peak is caused by variations in W, thethermospheric composition, T, and E.