Night−day imprints of ionospheric slab thickness during geomagnetic storm

Abstract

Spatial maps of the ionosphere–plasmasphere slab thickness ( τ) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 1° spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 35° to 70°N, and longitudes −10° to 40°E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS–TEC and ionosonde observations at solar minimum (1995–1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor ( w) is introduced as a ratio of disturbance to quietness representing area mean TEC, foF2, and τ for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and τ maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but τ enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS–TEC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300 km by night.