Low latitude ionosphere response to severe geomagnetic storms

During the occurrence of a geomagnetic storm a series of disturbances can affect the ionosphere at all latitudes and longitudes. Over the low latitudes, the ionospheric F layer height can be affected mainly by magnetospheric electric fields that penetrate to the low latitudes and by disturbance dynamo electric fields. Disturbed meridional winds have minor effect over low latitudes and disturbed zonal winds are important only near sunset. The ionospheric heights are directly affected by the disturbed electric fields through the effect of the ExB drift. The density distribution and the equatorial ionization anomaly are also affected in the process due to occurrence of a disturbed vertical drift. Changes in the atmospheric composition due to disturbance winds are also responsible for part of the electron density changes during magnetic storms. The solar events that occurred in the end of October 2003 gave rise to very strong geomagnetic disturbances that peaked twice with Dst values reaching less that -300 nT between 00:00 UT on the 29th and 04:00 UT on the 30th, and between 22:00 UT on the 30th and 01:00 UT on the 31st. Several disturbances were observed in the ionospheric stations over the Brazilian region, but the most severe of them occurred on the evening (around 21 UT) of October 30th. A very strong vertical drift shifted the ionosphere over the equatorial station São Luís to heights above 800 km. The effect was observed also at the low- and sub-tropical stations Fortaleza and Cachoeira Paulista, but it was less intense over those latitudes, due to the higher efficiency of the ExB drift at the magnetic equator. The ionospheric effects of these very strong geomagnetic storms over the Brazilian region are investigated in this work, and the results are compared to previous results of other very intense storms.