Hemispheric Asymmetry of the Premidnight Aurora Associated With the Dawn-Dusk Component of the Interplanetary Magnetic Field

摘要

It has been known for decades that the nightside aurora in the Northern Hemisphere (NH) tends to be brighter when the interplanetary magnetic field (IMF) measured at Earth has a dawnward (negative y) component compared to a duskward (positive y) component. This asymmetric response to the polarity of IMF B_y has been explained by an interhemispheric current flowing out of the NH due to a nonuniform “penetration” of IMF B_y onto the magnetotail. If such a hypothesis is correct, it should predict a brighter aurora in the nightside Southern Hemisphere (SH) for positive IMF B_y than negative IMF B_y. Here we investigate this hypothesis using Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics/Global Ultraviolet Imager data. The present study not only reproduces the result previously found in NH but also shows an opposite change to its Northern Hemispheric counterpart in SH in response to the different IMF B_y polarity. When comparing north to south, for negative IMF B_y, the premidnight auroral energy flux is greater in NH than that in SH. The result becomes opposite for positive IMF B_y. This result is consistent with the hypothesis of the existence of an interhemispheric field-aligned current.