On the Relationship Between the Transit Time of ICMEs and Strength of the Initiated Geomagnetic Storms

Abstract

More than 140 isolated non-recurrent geomagnetic storms (GMSs) of various intensities from extreme to weak are considered; these are reliably identified with solar eruptive sources (coronal mass ejections, CMEs). The analysis aims to obtain a possibly complete picture of the relationship between the transit time of propagation of CMEs and interplanetary coronal mass ejections (ICMEs) from the Sun to the Earth (more precisely, the time interval $\Delta t_{p}$ from the moment of an eruption until the peak of the corresponding GMS) and the maximum intensity of this GMS, as measured by the disturbance storm time geomagnetic index Dst. Two groups of events are singled out: one includes GMSs, the source of which was an eruption from an active region (AR events), the other GMSs caused by filament eruptions from quiescent areas of the Sun located outside ARs (QS events). The distribution of the large number of the analyzed events on a $\Delta t_{p}$ – Dst plane confirms and substantially clarifies the known regularities. The AR events are characterized by a shorter transit time ($\Delta t_{p} \approx 1$–4 days) and much stronger GMSs (Dst up to –600 nT mainly) in comparison with the QS events ($\Delta t_{p} \approx 3$–5 days, Dst > –200 nT). For events of both groups, the shorter the transit time of CMEs/ICMEs, the more intense the GMSs; in particular, for AR events when $\Delta t_{p}$ declines from 4 to 1 day, Dst decreases on average from –100 to –470 nT and can reach – 900 nT. From the point of view of the nature of GMSs and their sources on the Sun, the obtained results mean that both the speed of CMEs/ICMEs and the strength of the magnetic field transported by them are largely determined by the parameters of the corresponding eruptions, in particular, by the eruptive magnetic flux and the released energy.