Opposite hemispheric asymmetries during the ionospheric storm of 29-31 August 2004

By making use of multiple ground-based and spaceborne instruments, we study ionospheric and thermospheric behavior during the moderately intense geomagnetic storm of 29-31 August 2004 (minimum Dst excursion of -128 nT). Although this storm was far from the strongest in solar cycle 23, it provoked quite interesting effects in the ionosphere, such as opposite hemispheric asymmetries in the ionospheric F layer and in the topside ionosphere and a development of the ionospheric superfountain effect in the postsunset sector. Data from ground-based GPS receivers and ionosondes revealed large increase in total electron content (TEC) and in N_mF₂ in the southern hemisphere, whereas in the northern hemisphere, very weak or no effect was observed. On the contrary, the topside measurements indicated the occurrence of a positive storm in the northern hemisphere. Overall, the strongest storm time disturbances were observed in the postsunset sector (∼20:30-21:30 LT), where satellite radar altimeters TOPEX and Jason 1, along with the CHAMP satellite showed ∼250-400% TEC increase in the middle- and low-latitude regions. The signatures of the ionospheric plasma enhancement were seen up to the height of the Defense Meteorological Satellite Program (DMSP) satellites (∼840 km). As for the thermospheric storm, data of the Gravity Recovery and Climate Experiment (GRACE) satellite mission revealed no asymmetry in neutral density data in the evening sector (∼17 UT); however, very strong hemispheric asymmetry was observed in the postsunset sector by CHAMP (∼21 UT). Overall, neutral density increase in the postsunset sector was found to be much stronger than in the evening sector.