Drop of solar wind at the end of the 20th century
Variations in the solar wind (SW) parameters with scales of several years are an important characteristic of solar activity and the basis for a long-term space weather forecast. We examine the behavior of interplanetary parameters over 21-24 solar cycles (SCs) on the basis of OMNI database (https://spdf.gsfc.nasa.gov/pub/data/omni). Since changes in parameters can be associated both with changes in the number of different large-scale types of SW, and with variations in the values of these parameters at different phases of the solar cycle and during the transition from one cycle to another, we select the entire study period in accordance with the Catalog of large-scale SW types for 1976-2019 (See the site http://www.iki.rssi.ru/pub/omni, [Yermolaev et al., 2009]), which covers the period from 21 to 24 SCs, and in accordance with the phases of the cycles, and averaging the parameters at selected intervals. In addition to a sharp drop in the number of ICMEs (and associated Sheath types), there is a noticeable drop in the value (by 20-40%) of plasma parameters and magnetic field in different types of solar wind at the end of the 20th century and a continuation of the fall or persistence at a low level in the 23-24 cycles. Such a drop in the solar wind is apparently associated with a decrease in solar activity and manifests itself in a noticeable decrease in space weather factors.