Total electron content behavior over Japan during geomagnetic storms

Abstract

The total electron content (TEC) obtained from GPS signals is used to study ionospheric dynamics over Japan during geomagnetically disturbed conditions. The numerous TEC measurements are averaged in cells with a size 1.5° × 1.5° geographic scale and formatted as time series within the years 2000–2002. To extract the storm time changes of TEC, the diurnal and 27‐day periodicities are subsequently removed. Diurnal variations are removed by replacing absolute TEC values in each cell with their relative deviations (RTEC) from medians. The hourly RTEC values from all cells within the central 4°‐wide band over Japan area are then approximated by a plane surface. This surface is represented by two parameters: its value at the center ( rt ) and the slope ( b ) along the main axis, taken as constants of the linear regression. The 27‐day periodicity was approximated by Fourier waves with main period of 640 hours and two harmonics separately for rt and b and subtracted from them. The analysis of rt and b behavior during a number of geomagnetic storms allowed us to reveal several repeatable features of average TEC behavior. It was found that TEC behavior during the storms is similar to that of f o F 2 at the F region and was local time–dependent. A marked poleward expansion of the equatorial ionosphere (crest region) at the end of recovery phase is persistently observed feature, produced probably by intensified eastward zonal winds. Such an expansion of equatorial ionosphere is observed also during isolated substorms, outside main geomagnetic storms. An oscillation‐like change of positive and negative disturbances with period of 24 hours is observed during a 4‐day period, following a moderate storm. In the absence of geomagnetic activity driver that effect is probably caused by the alternative expansion and contraction of equatorial ionosphere.