Solar wind disturbances responsible for geomagnetic storms

Abstract

The superposed epoch method is used to construct a typical behavior of solar wind parameters before and during strong isolated geomagnetic storms. A total of 130 geomagnetic storms during the period of 1966–2000 were analyzed. The results obtained show that the averaged disturbance in the solar wind responsible for geomagnetic storms is associated with the compression of ambient solar wind plasma and interplanetary magnetic field (IMF) ahead of a high‐speed plasma flow. The magnetic field strength and plasma density start to increase several hours before geomagnetic storm onset. However, the negative IMF Bz (which is responsible for the onset of strong negative Dst) starts to increase approximately 4–5 hours after the maximum variations in plasma density and IMF By. The time delays between peaks in negative IMF Bz and plasma density as well as IMF ∣By∣ (in the solar‐magnetospheric coordinate system) may be a result of the compression and three‐dimensional draping of the ambient magnetic field about high‐speed flows as discussed earlier by some authors. The combined effect of the compression and draping of the ambient magnetic field may lead to an increase in plasma density and IMF ∣By∣ ahead of a high‐speed flow followed by an increase in IMF ∣Bz∣ in its central portion. The motion of such structure leads to a time delay between the peaks in the IMF ∣By∣ and ∣Bz∣ observed at satellites.