In situ evidence for interplanetary magnetic field induced tail twisting associated with relative displacement of conjugate auroral features

Abstract

We provide in situ evidence for a twisted near-Earth tail configuration that is responsible for the time sequence of conjugate auroral features associated with relative interhemispheric displacement during a weak substorm, as reported by Motoba et al. (2010). We analyzed the magnetic field data observed using four Cluster satellites in the vicinity of 11-14 R_E central downtail, in close conjunction with the Iceland-Syowa conjugate optical auroral features. Interestingly, we found that the variations in the magnetic field y component (B_y) at all satellites correlated moderately well with the variations in the time-shifted interplanetary magnetic field (IMF) clock angle ($\theta$_CA). The correlation coefficients (0.56 ∼ 0.61) between the B_y field at Cluster and IMF $\theta$_CA peaked at a time delay of 52 ± 1 min from the dayside magnetopause, probably corresponding to the time scale for the reconfiguration of the IMF $\theta$_CA related B_y field in the near-Earth tail. The IMF $\theta$_CA related B_y variation at Cluster, regarded as a manifestation of the twisting magnetotail configuration, also roughly coincided with the relative magnetic local time displacement of nightside conjugate auroral forms. These results provide strong evidence that the reconfiguration (twisting) process of the near-Earth tail on a relatively longer time scale controls the nightside conjugate auroral locations in both ionospheres.