Seasonal and Interhemispheric Variations of the Afternoon Auroral Responses to the Interplanetary Magnetic Field B y Polarity

This work investigates seasonal and interhemispheric variations of the afternoon auroral responses to the interplanetary magnetic field (IMF) B y effects. The auroral observations are adopted from the global ultraviolet imager instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite during 2002–2007. The results show that in both summer and winter solstices, the stronger afternoon auroral intensity is associated with negative IMF B y ( B y 0 in the southern hemisphere. This suggests stronger contributions from the upward field‐aligned currents (FACs), which can be induced by the B y ‐associated north‐south oriented electric field and the B y ‐associated flow shear in the ionosphere. In addition, the strongest afternoon aurora occurs in summer in each hemisphere. In summer, the absolute difference between the auroral peak intensity under the two B y polarities is greater and occurs earlier than in winter, which may be related to changes in FACs and conductivity from winter to summer. Differently, in equinoxes stronger auroral intensity favors B y conditions associated with more frequent occurrence of southward IMF B z , such as B y 0 conditions in March and in September, respectively. Therefore, in equinoxes the effects of the favorable B y , which were seen in solstices, are masked. We suggest that these are caused by the Russell‐McPherron effect, which leads to more southward B z conditions, resulting in more energy deposited and subsequent stronger aurora in polar ionosphere. These results contribute to our deeper understanding of the asymmetrical phenomena in the Earth's magnetosphere‐ionosphere induced by IMF B y . Aurora is mainly controlled by coupling among the solar wind, magnetosphere and ionosphere. The afternoon aurora can vary with the change of the north‐south components (positive‐negative B z ) and east‐west components (positive‐negative B y ) of interplanetary magnetic field (IMF). Generally, compared with northward B z , stronger afternoon aurora occurs when B z is southward. It also has B y orientation preference from case studies and statistics. In this study, quantitative and statistical analysis was performed on the afternoon aurora. We confirmed that in summer and winter of the northern hemisphere, the afternoon auroral intensity is stronger for negative B y than that for positive B y , while the opposite trend, that is, the st