Parameters of the Geomagnetic Activity, Thermosphere, and Ionosphere for the Ultimately Intense Magnetic Storm

Equations of regression are derived for the intense magnetic storms of 1957-2016. They reflect the nonlinear relation between Dst(min) and the effective index of geomagnetic activity Ap(tau) with a timeweighted factor tau. Based on this and on known estimations of the upper limit of the magnetic storm intensity (Dst(min) =-2500 nT), the maximal possible value Ap(tau)(max) similar to 1000 nT is obtained. This makes it possible to obtain initial estimates of the upper limit of variations in some parameters of the thermosphere and ionosphere that are due to geomagnetic activity. It is found, in particular, that the upper limit of an increase in the thermospheric density is seven to eight times larger than for the storm in March 1989, which was the most intense for the entire space era. The maximum possible amplitude of the negative phase of the ionospheric storm in the number density of the F-2-layer maximum at midlatitudes is nearly six times higher than for the March 1989 storm. The upper limit of the F-2-layer rise in this phase of the ionospheric storm is also considerable. Based on qualitative analysis, it is found that the F-2-layer maximum in daytime hours at midlatitudes for these limiting conditions is not pronounced and even may be unresolved in the experiment, i.e., above the F-1-layer maximum, the electron number density may smoothly decrease with height up to the upper boundary of the plasmasphere.