Capture of the heliospheric current sheet by a coronal hole, strong deformation of the interplanetary magnetic sector, and the largest geomagnetic storm in 1999

The solar sources and interplanetary disturbances responsible for the most severe geomagnetic storm (Kp(max) = 9, a(pmax) = 207 nT, Dst(min) = 220 nT) in 1999 (October 22) are considered. The complex solar source is shown to have been an extremely rare combination of a coronal hole (CH), an active f ilament, and the heliospheric current sheet (HCS) with the CH crossed by th e neutral line of solar magnetic field on the source surface. The HCS acted as a piston that produced a strong deformation of the IMF sector. The seve re geomagnetic storm was directly caused by a geoefficient (B-z less than o r equal to -20 nT) structural disturbance element in front of the HCS. This element was either (i) a nonlinear fast magnetoacoustic wave attributable to fluctuations of the dynamic pressure on the HCS or (ii) an isolated tube of a strong magnetic field (magnetic cloud) actively affecting the ambient medium.