The ionospheric F2-region at low geomagnetic latitudes during the geomagnetic storms of 22–26 April 1990: Comparison of observed and modeled response

Abstract

A comparison between the modeled NmF2 and hmF2 and NmF2 and hmF2, which were observed by the Kokubunji, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper (MU) atmosphere radar, have been used to study the time-dependent response of the low-latitude ionosphere to geomagnetic forcing during a time series of geomagnetic storms from 22 to 26 April 1990. The reasonable agreement between the model results and data requires the modified equatorial meridional E×B plasma drift, the modified HWM90 wind, and the modified NRLMSISE-00 neutral densities. We found that changes in a flux of plasma into the nighttime equatorial F2-region from higher L-shells to lower L-shells caused by the meridional component of the E×B plasma drift lead to enhancements in NmF2 close to the geomagnetic equator. The equatorward wind-induced plasma drift along magnetic field lines, which cross the Earth equatorward of about 20° geomagnetic latitude in the northern hemisphere and about −19° geomagnetic latitude in the southern hemisphere, contributes to the maintenance of the F2-layer close to the geomagnetic equator. The nighttime weakening of the equatorial zonal electric field (in comparison with that produced by the empirical model of Fejer and Scherliess [Fejer, B.G., Scherliess, L., 1997. Empirical models of storm time equatorial zonal electric fields. J. Geophys. Res. 102, 24047–24056] or Scherliess and Fejer [Scherliess, L., Fejer, B.G., 1999. Radar and satellite global equatorial F region vertical drift model. J. Geophys. Res. 104, 6829–6842) in combination with corrected equatorward nighttime wind-induced plasma drift along magnetic field lines in the both geomagnetic hemispheres are found to be the physical mechanism of the nighttime NmF2 enhancement formation close to the geomagnetic equator over Manila during 22–26 April 1990. The model crest-to-trough ratios of the equatorial anomaly are used to study the relative role of the main mechanisms of the equatorial anomaly suppression for the 22–26 April 1990 geomagnetic storms. During the most part of the studied time period, a total contribution from geomagnetic storm disturbances in the neutral temperature and densities to the equatorial anomaly changes is less than that from meridional neutral winds and variations in the E×B plasma drift. It is shown that the latitudinal positions of the crests are determined by the E×B drift velocity and the neutral wind velocity.