On the azimuthal location of auroral breakup: Hemispheric asymmetry

We study the effects of the azimuthal component of interplanetary magnetic field (IMF) and the Earth's dipole tilt angle (larger DTA means northern winter) on the magnetic local time of auroral breakup using 2539 previously published events. It is found that in the Northern Hemisphere the location of auroral breakup occur the earliest for the IMF By+/DTA+ paring and the latest for the IMF By−/DTA− pairing. The ordering is opposite in the Southern Hemisphere for the same conditions. A similar result is obtained if the DTA is replaced by the solar zenith angle. We did not find a strong IMF clock angle effect on the north–south azimuthal onset displacement as reported previously. We interpret the observed hemispheric asymmetry as a result of an additional By field in the plasma sheet associated with IMF By and dipole tilt and/or a day‐night gradient in ionospheric conductivity, skewing of the tail field azimuthally.