Magnetic field at geosynchronous orbit during high‐speed stream‐driven storms: Connections to the solar wind, the plasma sheet, and the outer electron radiation belt

[1] Superposed‐epoch analysis is performed on magnetic field measurements from five GOES spacecraft in geosynchronous orbit during 63 high‐speed stream‐driven storms in 1995–2005. The field strength and the field stretching angle are examined as functions of time and local time, and these quantities are compared with the properties of the solar wind, the plasma sheet, and the outer electron radiation belt. Compression of the dayside magnetosphere coincides with an increased solar wind ram pressure commencing before the arrival of the corotating interaction region (CIR). Stretching of the nightside magnetosphere occurs in two phases: a strong‐stretching phase early in the storm followed by a modest‐stretching phase lasting for days. The strong‐stretching phase coincides with the occurrence of the superdense plasma sheet, implying that ion pressure causes the strong stretching. This nightside strong‐stretching perturbation corresponds to a ∼25% contribution to Dst*. The relativistic electron flux at geosynchronous orbit has a dropout recovery temporal profile that matches the strong‐stretching temporal profile; however, the number density dropout and recovery of the electron radiation belt has a profile that leads the stretching profile. A comparison of geosynchronous field strengths and magnetopause field strengths indicates that magnetopause shadowing plays a role in the radiation belt dropout. Temporal fluctuations of the geosynchronous magnetic field are examined via 1 min changes of the GOES magnetic field vectors. Fluctuation amplitudes increase at all local times at storm onset and then slowly decay during the storms. The amplitude is linearly related to the Kp, PCI, and MBI indices, except during the strong‐ stretching phase of the storms.