Auroral Electrojet Activity During Long-Duration Radial IMF Intervals

In near-Earth space, the interplanetary magnetic field (IMF) sometimes deviates from the common Parker spiral configuration and follows either nearly radial or azimuthal or vertical orientation (either Bx- or By- or Bz-dominated IMF conditions in the geocentric solar magnetospheric coordinate system) on a timescale of more than several hours. Although the contribution of IMF By and Bz components (that is, the IMF clock angle) to auroral electrojet activity has been extensively studied by estimating solar wind coupling functions for auroral electrojet indices (AU, AL, and AE indices), its IMF Bx contribution is still poorly understood. In this study, we statistically examine the auroral electrojet indices for long-duration (lasting for more than four hours) radial IMF events identified in the period from 1996 to 2017 and compare with those for long-duration azimuthal IMF events. It is found that for both IMF orientation events, auroral electrojet activity tends to increase with increasing IMF Bz magnitude and solar wind dynamic pressure when the IMF Bz is negative, while the activity remains low irrespective of values in these two parameters when the IMF Bz is positive. Comparison of the auroral electrojet indices for the radial IMF events with those for the azimuthal IMF events in terms of IMF Bz and solar wind dynamic pressure reveals that the auroral electrojet activity under radial IMF conditions is lower than azimuthal IMF conditions in general. Based on these results, we discuss magnetospheric/ionospheric conditions during prolonged intervals of the radial IMF.