This work investigates seasonal and interhemispheric variations of the afternoon auroral responses to the interplanetary magnetic field (IMF) By 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 By (By < 0) in the northern hemisphere, and with By > 0 in the southern hemisphere. This suggests stronger contributions from the upward field‐aligned currents (FACs), which can be induced by the By‐associated north‐south oriented electric field and the By‐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 By 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 By conditions associated with more frequent occurrence of southward IMF Bz, such as By < 0 and By > 0 conditions in March and in September, respectively. Therefore, in equinoxes the effects of the favorable By, which were seen in solstices, are masked. We suggest that these are caused by the Russell‐McPherron effect, which leads to more southward Bz 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 By. Plain Language Summary: 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 Bz) and east‐west components (positive‐negative By) of interplanetary magnetic field (IMF). Generally, compared with northward Bz, stronger afternoon aurora occurs when Bz is southward. It also has By 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 By than that for positive By, while the opposite trend, that is, the stronger afternoon aurora under positive By, occurs in the southern hemisphere. This hemispheric change of the afternoon auroral response to By is most probably associated with the resultant field‐aligned currents. However, the afternoon auroral intensity is stronger for negative By in both hemispheres in March and is stronger for positive By in both hemispheres in September. In each equinox, the stronger afternoon auroral intensity prefers the By orientation that is associated with greater occurrence frequency of southward Bz. Key Points: In summer and winter solstices, the afternoon auroral responses to By polarities are opposite between the northern and southern hemispheresThe absolute difference in auroral peak intensity between different By polarities is greater and occurs earlier in summer than in winterIn equinoxes, stronger afternoon auroral intensity is mostly associated with the Russell‐McPherron effect, regardless of the hemisphere [ABSTRACT FROM AUTHOR]