Abnormal Diurnal Variation of Total Ionization

Abstract

IT has long been known that magnetic storms may be associated with major changes in the F₂ region of the ionosphere. Recently interest has been concentrated on the relationship between the level of magnetic activity, and three ionospheric parameters, namely N_T, the total electron content; N_M, the maximum ionization density; and T=N_T/N_M, the “equivalent slab thickness”. The exact nature of the relationship is not yet clear, and published results are to some extent conflicting. It has been shown, however^1,2, that a moderate or severe storm is often accompanied by the following effects, (a) During the first day after the start—a “sudden commencement” or “gradual commencement” — N_T is unusually high. It may remain so for 12–24 h, and then change fairly rapidly to become unusually low. It may then take as long as 24 – 36 h to return to a normal value. (b) N_M follows a similar pattern to that of N_T, although the effect is not always so pronounced, (c) The slab thickness increases considerably during the day after the commencement of a storm. The amount and duration of the increase are very variable.