Responses of the low-latitude ionosphere to very intense geomagnetic storms

In this work, we investigate the ionospheric responses to exceptionally high-intensity and long-duration magnetic storms over Brazil. Disturbed ionospheric F-region vertical drifts and peak electron density changes observed at the equatorial station Fortaleza - Fz (3 degrees 55'S; 38 degrees 25'W; dip - 3.5 degrees) and the low-latitude station Cachoeira Paulista - CP (22 degrees 41'S; 45 degrees 00'W; dip 24 degreesS), for three magnetospheric storm events that occurred in December 1980, April 1981 and September 1982, are analyzed. These storms had minimum Dst indexes -240, -311 and -289 nT, respectively. The interplanetary magnetic field (Bz) data from the ISEE-3 satellite, the auroral activity index AE, and the ring current index Dst are used as indicators of the magnetospheric conditions. The ionospheric response features are analyzed using the F-layer critical parameters h'F, hpF2 and foF2, from ionograms obtained at Fz and CP. The Bz and the AE index variations were much higher than those in many previous studies. Therefore, many of the observations reported here either have not been observed or are not readily explained by current models for predicting the penetration/dynamo disturbance electric fields. The altitude of the nocturnal ionospheric F-layer at low latitudes may undergo significant variations during storm-time, caused by magnitude variations on the local zonal component of the F-region electric field intensity. During the period studied here, clear association of the F-layer rise (vertical velocity and altitude) and spread-F occurrence is observed. It is shown that the storm-time layer rise has a dominant role on the equatorial spread-F. An attempt is made to identify the origin of electric fields responsible for the disturbed F-layer alterations. The main conclusions of this study are that (a) some effects on the F-layer height and peak electron concentrations are consistent with model predictions. Some others are in discrepancy or have not been either predicted by model studies or experimentally detected, (b) the F-layer rise over Fz played a major role in the generation of spread-F, (c) the maximum disturbance electric field intensity observed was about 1.09 mV m s(-1), (d) in some cases, foF2 increases (decreases) over CP were seen to be related to increases (decreases), in the fountain effect mechanism, (e) storm-time-induced h'F post-sunset height rise inhibitions over Fz may extend for at least 4 days, as observed, (f) daytime foF2 depressions of amplitude up to -9 MHz are observed over Fz, (g) in particular, a rather unexpected disturbance rise in h'F over Fortaleza, on September 8 at 08 LT, does not seem directly associated with either disturbance winds or penetration electric fields with origin at high latitude, where the convection remained low during the preceding 10h. (C) 2001 Elsevier Science Ltd. All rights reserved.