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

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