Arctic–Antarctic asymmetry of the ionospheric weather

Abstract

The state of the ionosphere is investigated on a global scale, in the Arctic and Antarctic regions poleward of 60° magnetic latitudes. The Total Electron Content, TEC, enhancement (as represented by the global WU, polar WUN and WUS indices) and depletion (WL, WLN and WLS), and the respective ranges (WE, WEN and WES) are evaluated from global GIM-W index maps. GIM-W maps are derived from JPL GIM-TECs converted to geomagnetic coordinate frame for Solar Cycles 23–24. Using a superimposed epoch analysis, we examined 380 geo-storms caused by high speed solar wind stream, Vsw, with time t0 set at onset of the Dst storm and Dstmin ≤ −50 nT. Single-sign positive and negative storm effects are detected in both Arctic and Antarctic, when the polar WUN, WLN, WUS or WLS index is equal to zero for 10–20% of the pre-storm period, 5–25% for the main storm phase, and 7–25% during recovery. These asymmetric single-sign conditions violate the correlation between WUS (WUN) and WLN (WLS) data and global Dst, WU, WL and WE indices, demonstrated for the extreme storm on 15–17 July 2000 (Dst = −301 nT). The persistent single-sign conditions are observed in the polar segments for the both disturbed and quiet times from 1994 to 2021. The peak of the monthly single-sign conditions up to 50% of time occurs during the polar summer when the solar wind – polar ionosphere interaction at the open field lines facing the Sun is strengthened due to reduced magnetosphere shielding the solar wind. A long-term Arctic–Antarctic asymmetry is observed from 1994 to 2021, with larger values of daily polar indices WUN, WUS, WUN and WLS found for the winter hemisphere (the ‘winter anomaly’) while contribution of the multiple single-sign zeros yields the reduced monthly values at summer. The asymmetry is likely caused by the difference in the solar irradiance that is minimized at the dominant high-latitude winter night-time, compared to the sunlit summer conditions in the opposite hemisphere. Higher values of asymmetry index AIWU and AIWL shows dominant North Hemisphere variance over South Hemisphere during December solstice reversed towards the June solstice presumably related with dynamical influences of the lower atmosphere.