Geomagnetic response to magnetic clouds

Abstract

Results are given of superposed epoch analyses of hourly values of the equatorial Dst geomagnetic index and the B_z component of the interplanetary magnetic field for 19 magnetic clouds (bubbles) observed at Earth. This study confirms remarks given first by Burlaga et al. (1981, J. geophys. Res.86, 6673) and later by Crooker (1983, Solar Wind Five, NASA Conference Publication 2280, p. 303) who suggested that magnetic clouds, a manifestation of coronal mass ejections directed earthward, may be associated with geomagnetic storms (identified as large decreases in Dst) and that clouds without preceding shocks (groups B and C) may be associated with gradual commencements; it also demonstrates that magnetic bubbles described in Thomsen (1985, Sci. News127, 326) do cause a disturbance in the Earth's magnetosphere.

Statistical testing reveals that one can be assured of a genuine association between magnetic clouds at Earth and geomagnetic storms at >99% confidence. Dependent upon the threshold level of Dst that one selects for identifying storms, one finds that 75% of the clouds had a maximum Dst ⪅ −30 gammas and 95% of the clouds had a maximum Dst ⪅ −16 gammas. The main phase and recovery phase of the geomagnetic storms associated with clouds follow the well-established description of storm behavior; namely the Dst index simultaneously decreases to a large negative value (indicating the presence of a geomagnetic disturbance at Earth) at the onset of a large, sustained southward magnetic field (within 3 h of cloud onset), with recovery starting when the magnetic field becomes northward.