Variability of Total Electron Content at Low Latitude Region During Solar Minimum 2008: Impact of High Speed Streams, HSSs

Abstract:

The solar minimum period of solar cycle 23 was unusually long and quiet in comparison to other solar minima in last century. Several reports have analyzed its features and its impact under diverse points-of-view. In this work, we analyze the low latitude ionosphere behavior in Brazil and its response during this peculiar period. The ionospheric variation is analyzed through typical parameters such as vertical total electron content (VTEC), the peak height of F2 layer and its critical frequency, hmF2 and foF2, in 2008, around the southern crest of the Equatorial Ionization Anomaly (EIA), in Cachoeira Paulista (22.5^o S, 45.0 ^o W, mag. lat: 16 ^o S, dip angle: -32.3^o) and at an equatorial station, São Luís (2.33^o S, 44.2^o W, dip angle: −6.7^o). VTEC values present a semiannual variation pattern and two well-defined peaks in March and October. Daily maximum values are observed around 15:00 LT. It was observed periodicities observed of 9, 13.5 and 27 days in VTEC and hmF2, mainly at the first and the second half of 2008. These periods match with the observed periods in solar and geomagnetic indexes such as Vsw, Kp and AE and are associated with occurrence of high speed streams (HSS) coming from solar coronal holes. A complex response of the low latitude ionosphere is observed, with prominent increases and decreases of VTEC at daytime during the interval of occurrence of HSSs. It is suggested that a combination of several factors such as prompt penetration of electric field, disturbed dynamo electric field, meridional winds, thermal expansion of thermosphere and composition changes of neutral atmosphere are responsible for the high day-to-day variability of the ionosphere