Influence of solar activity on red sprites and on vertical coupling in the system stratosphere-mesosphere
The positive downward propagating streamers of sprites are considered as factors of vertical coupling in middle atmosphere. Sprites are initiated in the lower ionosphere (at 75-85 km) and their streamers propagate in the mesosphere and upper stratosphere where the solar activity (SA) can have significant influence. The problem considered by us is whether sprites are sensitive to the solar activity. Different possible ways of such influence are considered. They concern: i) relations between solar activity and the occurrence of sprite-producing lightning discharges; ii) sensitivity of streamer inception to solar variability; iii) 11-year variations of conductivity in the night-time mesosphere and stratosphere during solar cycle due to modulation of the galactic cosmic ray flux by solar activity, which can lead to changes in sprite-driving electric fields, and therefore, in sprites. Accounting for the effects of sprites on minor constituents (in particular NOx), a link between SA level and the che^mical balance in the mesosphere and stratosphere is considered, as well. With respect to this we study by modeling the response of the sprite-driving electric fields to SA variations with the account to a complex of parameters of sprite-producing lightning discharges and atmospheric conductivity. The lightning-driven electric fields needed for streamer propagation show minor dependence on conductivity changes caused by variations in cosmic ray flux during a solar cycle. The long-term changes in sprite's lower boundary by different parameters of lightning discharges and atmospheric conductivity parameters are estimated. During solar minimum, of the vertical dimension of sprites increases by up to 1.5 km than those during solar maximum. We estimate also the effect of the reduction of conductivity in thunderclouds with respect to the adjacent air. Reduction of cloud conductivity by a factor of 5-10 leads to larger vertical dimension of sprites due to descending of the sprite lower boundary by up to 5 km related to the case of unmodified cloud conductivity. The solar variability has significantly bigger effect on the sprite vertical dimension by larger charge moment change of the parental lightning discharge and by large reduction of the cloud conductivity.