The “calm before the storm” in CIR/magnetosphere interactions: Occurrence statistics, solar wind statistics, and magnetospheric preconditioning

Intervals of extreme geomagnetic calm just prior to recurring high‐speed‐stream‐driven storms are noted and studied. These calm intervals may be important for preconditioning the magnetosphere for the ensuing storms. It is argued that this preconditioning causes (1) a decay of the number density of relativistic electrons in the outer radiation belt, (2) a mass loading of the convection of the middle magnetosphere during the early phase of the storms, (3) dumping of the outer electron radiation belt by the formation of a plasmaspheric drainage plume at the onset of the storm, and (4) a contribution to the inner plasma sheet and ring current by the convection of cool dense plasma into the dipole. Calm intervals tend to occur when uncompressed slow wind passes the Earth immediately prior to the CIR. Using 73 years of geomagnetic data, it is shown that the occurrence rate of calms before recurring storms is substantially greater than random occurrence probability would predict, even though the properties of the wind that drives calms before storms are statistically similar to the properties of the wind that drives quiet intervals during solar minimum. The recurrence of storms and calms are studied and it is found that both tend to recur with a 27‐day period (strongest during declining phase) and that the persistence of calm recurrence (∼3 solar rotations) is greater than the persistence of storm recurrence (∼2 solar rotations). An argument is given as to why there is a tendency for calms to occur just prior to high‐speed‐stream‐driven storms.