Cluster observations in the magnetotail during sudden and quasiperiodic solar wind variations

A clear bipolar (negative/positive) signature in the E_y component was observed by three spacecraft on Cluster in the magnetotail during the passage of a solar wind discontinuity on October 11, 2001 (day 284), which caused a sudden commencement (se) on the ground. The positive E_y perturbation was accompanied by the northward/dawnward plasma flow and the B_x enhancement, which is the dominant magnetic field component. The estimated E_y from the plasma flow and magnetic field was in good agreement with the observed E_y Thus the positive E_y perturbation can be interpreted to be the signature of inward plasma motions, corresponding to the compression of the magnetopause. During the interval of the negative E_y perturbation, the magnetic field increase was mainly due to the increase in B_y and B_z, not in B_x. Unlike the positive E_y perturbation, the negative E_y perturbation was not consistent with the estimated E_y Therefore the negative E_y signature does not seems to have a direct connection with plasma motions. The observed field and flow variations may be associated with a deformation of the magnetotail due to the solar wind discontinuity moving tailward. The direction and shape of the sc front propagating tailward can be confirmed by three spacecraft of Cluster. We also observed quasiperiodic geomagnetic perturbations at the low-latitude ground station Kakioka (L = 1.25) following the sc event. They were highly correlated with the magnetic field perturbations at Cluster in the magnetotail (X_gse = ∼12 R _E). We show that the source of these perturbations is the quasiperiodic solar wind variations superposed on the increased solar wind pressure behind the interplanetary discontinuity.