Persistent perturbations of ionosphere at diminution of solar and geomagnetic activity during 21–24 solar cycles

Abstract Moderate ionospheric disturbances combined with storm signatures are characterized by the positive ionospheric index (WU), the negative index (WL), and the range (WE = WU – WL). We assign weights to these indices as a function of the magnitude of the W-index, the observation latitude of the critical frequency of the F2 layer, foF2 (the F2 layer peak electron density, NmF2) or total electron content (TEC). It is shown that moderate ionospheric irregularities (W = ±2) occur more than 20% of the time, while ionospheric storms (W = ±3 and ± 4) are observed in about 5% of Global Ionosphere Maps (GIM-TEC) observations for the period 1994–2020. The response of WU, WL and WE indices are provided for the intense storm on 14–15 December 2006 measured by solar wind and interplanetary magnetic field parameters, and geomagnetic Kp, ap, aa and Dst-indices. Sustained perturbations in WU, WL and WE indices are observed with GIM-TEC observations from 1994 to 2020 in contrast to the geomagnetic Kp-index depletion. The sustained perturbations are also observed with foF2-based 3-h local ionospheric indices WUf, WLf, and WEf at Moscow, Slough-Chilton and Canberra from 1945 to 2020, despite the trends of decreasing in solar activity, Kp, ap, aa and Dst-indices, and GIM-TEC-based ionospheric storms. These persistent irregularities appear to be due to moderate-level ionospheric perturbations, a fact that did not receive sufficient attention within the research community so far.