Asymmetry of the Ionosphere Variability in the North and South Auroral Zones at the Extreme Geomagnetic AE and Apo Indices

Differences in geomagnetic and ionospheric activity are investigated for the maximum monthly–hourly values of the auroral electrojet AE index, measured on a network of magnetometers above 60° in the Northern hemisphere from 1995 to 2019. The selected extreme AE indices were compared with the time–matched 1-h Apo indices observed in the sub-auroral zone from 1995 to the present. A high correlation of 300 selected values of AE and Apo indices (cc = 0.69) was obtained for the period of their synchronous observations in 1995–2019. For a comparison, variations of the ionospheric zonal dispersion (Net Volume, NT) are considered designating the difference between the positive and negative deviations of TEC from the quiet state in the selected zone. The NT is produced from TEC-based W-index values at the grid in the auroral zones of the Northern and Southern hemispheres for the geomagnetic latitudes exceeding ±60°. The NT values were estimated from JPL maps of the total electron content, GIM–TEC, and the corresponding W-index maps converted from geographic to geomagnetic coordinates. We observed an asymmetry of the ionospheric variability in the Northern and Southern auroral zones with the dominance of the positive (negative) NT values in the local winter (summer). At the same time, the seasonal variation of the geomagnetic AE and Apo indices recorded mainly in the Northern Hemisphere shows changes similar to the ionospheric variations of NT in the Southern Hemisphere with a decrease in the amplitude by the winter solstice. The analytical dependences of NT indices on the day of year in the North and South auroral zones were derived suitable for estimating the ionospheric variability in the operational forecasting models of the ionosphere.