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

In this work, we investigate the ionospheric responses to exceptionally hig h-intensity and long-duration magnetic storms over Brazil. Disturbed ionosp heric F-region vertical drifts and peak electron density changes observed a t the equatorial station Fortaleza - Fz (3 degrees 55'S; 38 degrees 25'W; d ip - 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 s torm 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 s atellite, the auroral activity index AE, and the ring current index Dst are used as indicators of the magnetospheric conditions. The ionospheric respo nse features are analyzed using the F-layer critical parameters h'F, hpF2 a nd foF2, from ionograms obtained at Fz and CP. The Bz and the AE index vari ations were much higher than those in many previous studies. Therefore, man y of the observations reported here either have not been observed or are no t readily explained by current models for predicting the penetration/dynamo disturbance electric fields. The altitude of the nocturnal ionospheric F-l ayer at low latitudes may undergo significant variations during storm-time, caused by magnitude variations on the local zonal component of the F-regio n electric field intensity. During the period studied here, clear associati on of the F-layer rise (vertical velocity and altitude) and spread-F occurr ence 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 hei ght 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 m ajor role in the generation of spread-F, (c) the maximum disturbance electr ic field intensity observed was about 1.09 mV m s(-1), (d) in some cases, f oF2 increases (decreases) over CP were seen to be related to increases (dec reases), 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 o bserved, (f) daytime foF2 depressions of amplitude up to -9 MHz are observe d over Fz, (g) in particular, a rather unexpected disturbance rise in h'F o ver Fortaleza, on September 8 at 08 LT, does not seem directly associated w ith 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.