An unusual night time ionospheric phenomenon observed at Tucumán, Argentina, under quiet geomagnetic conditions

S.M. RADICELLA, L. CIRAOLO, M. MOSERT, O. ABARCA, B. ZOLESSI, M. PEZZOPANE, R. EZQUER AND M. CABRERA

Crete, Greece

Simposio; 12th International Symposium on Equatorial Aeronomy; 2008

University of Crete

An Advanced Ionospheric Sounder, built at "Istituto Nazionale di Geofisica e Volcanologia", Rome, Italy, was installe at Tucumán, Argentina (26.5 S; 64.7W) particularly interesting for its location under the Southern peak of the ionospheric equatorial anomaly. Both the ionograms an the basic ionospheric characteristics automatically scaled are available real-time on line. An unusual phenomenon was observed on 1 October, 2007 at night (day 274), a period geomagnetically quiet. After few hours values of foF2 risen to 8.7 Mhz at 6.00 UT corresponding to 2.00 LT to decrease  in two hours to below 2 Mhz. Comparisons were made with the behavior on a quiet day before the day of the event (day 272). Measurements of total electron content (TEC) taken at locations near Tucumán and to the north toward the geomagnetic equator allowed to study the evolution of TEC in the region during the phenomenon and to calculated the slab thickness above Tucumán. The variations of this parameter during the night were unusual because it dropped to a value of 119 km at 2.15  LT but risen to 713 km at 4.30 LT to drop again to 238 km at 7.15 LT. These observations are confirmed by the analysis of the electron density profiles obtained from the Tucumán ionograms. They allow a careful descriptions of the movements of ionizations and expansions and contractions of the ionosphere in the region of the southern peak of the equatorial anomaly over the South America difficult to explain.