Effects of intense geomagnetic storms (Dst index <-100) to low latitude F region irregularities in the post-sunset hours are statistically studied by the total electron content (TEC) phase fluctuation observations obtained from the ground-based global positioning system (GPS) receivers in the Brazilian longitude sector during a 7.5-year period of 1 April 1998-31 October 2005. Results show that the low latitude irregularities are significantly suppressed during the main and recovery phases of the intense geomagnetic storms in the months (September-March) that normally have high irregularity activity. In contrast, the post-sunset triggering signatures are pronounced in the low irregularity activity season (April-August) during storms. To further understand the storm-produced behaviors in the low latitude F region irregularity in the post-sunset hours, the relationship between the intensity of GPS phase fluctuations and the occurrence time of the of IMF Bz turnings is examined and discussed.
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