Topside ionosphere during solar cosmic ray bursts and Forbush decreases in galactic cosmic rays

The paper considers the behavior of the upper ionosphere at heights of the F2 layer during Forbush decreases in galactic cosmic rays (GCR FDs) and solar cosmic ray (SCR) bursts. We use the results of long-term continuous observations of cosmic rays and the ionosphere in Novosibirsk for the period from 1968 to 2021. The ionospheric disturbances in the F2 layer during GCR FDs, which were accompanied by a magnetic storm, took the form of an ionospheric storm negative phase. The scale of the negative phase of the ionospheric F-layer disturbance increases with increasing Dst index of the geomagnetic storm. This increase in the amplitude of the ionospheric disturbance becomes more and more significant depending on the magnitude of Forbush decreases. A burst of the amplitude of the daily variation in the F2-layer critical frequency occurred eight days after SCR bursts and GCR FD front. We assume that this burst might have been caused by disturbances in the lower atmosphere due to significant variations in the intensity of SCR and GCR fluxes.