Gilda González

We analyze the day-to-day variability in the total electron content (TEC) measured at five GPS stations at the equatorial region for 90 days in 2008, February 1 (day 32) to May 2 (day 122). TEC data was obtained from GPS receivers at five stations in the equatorial region of South America: Belem Du Para, Maraba, Guajara-Mirim, La Paz and Ancón. The days considered were characterized by very low solar activity (F10.7 index between 70 and 85) and quiet geomagnetic conditions (Kp between 1 and 4). The GPS data used has a resolution of 10, 15 and 30s depending on the station. For spectral analysis, the Lomb-Scargle periodogram was used. The geophysical conditions were obtained from the OMNI database (http://omniweb.gdfc.nasa.gov). The day-to-day variability of solar wind speed (Sw), solar wind density (Nn), IMF Bz component, Kp geomagnetic activity index and F10.7 index were analyzed.
The spectral analysis reveled the presence of peaks of 13.8 and 9 days for Sw, Nn and Kp, whereas Bz have periods of 27.6 y 13.8 days. In the other hand, TEC variations show periods of 9, 13-14.5 and 21-22 days, for all stations. These periods suggest that variations in the speed and density of the solar wind are the most effective sources of TEC variability in the equatorial region during the days considered. For the five stations studied, a spectral peak of 21-22 days occurs, but this periodicity is not observed in the solar wind, neither in IMF nor in the Kp index. This periodicity probably does not originate in the geomagnetic activity or in the solar wind conditions. We suggest that it may be related to meteorological effects, in particular to planetary waves that propagate through the mesosphere and the thermosphere producing disturbances in the ionospheric region.