Hemispheric Asymmetries in the Afternoon Aurora due to Solar Wind and IMF Variations

Using global auroral images from Polar UVI in the southern hemisphere and IMAGE FUV in the north, we have analyzed the conjugacy of the afternoon aurora over a five-month period. From over 50 concurrent intervals we observe several asymmetries, periods for which auroral structure at one pole diverges from the other. Initial results suggest that this is due to the interplanetary magnetic field (IMF) and the solar wind velocity. Should the velocity fall below a certain threshold (~ 500 km/s), any fluctuation or discontinuity in the IMF appears to have no affect on auroral activity. Above this threshold, however, when the IMF is southward and a significant dawnward component exists, there is an enhancement in the afternoon aurora in the northern hemisphere. If, on the other hand, there is a significant duskward component to the IMF, then an enhancement in the afternoon aurora in the southern hemisphere is observed. The results of this study indicate that these features are driven by an instability at the magnetopause (such as a Kelvin-Helmholtz instability). In addition, these findings show that the dayside magnetosphere can respond asymmetrically to discontinuities in the IMF.