Auroral Asymmetries in the Conjugate Hemispheres During Strong IMF Bx

Simultaneous UV images of the conjugate aurora, taken from the IMAGE and Polar spacecraft reveal prominent differences between the two hemispheres. During a period of 1.5 hours of continuous measurements, we observe: 1) An intense arc in the dusk sector which extends further towards midnight in the southern hemisphere than in the northern hemisphere, 2) A much more severe poleward expansion in the southern hemisphere during a substorm growth phase, and 3) An asymmetric intensity distribution, with the most intense aurora at dusk in the southern hemisphere, and at dawn in the northern hemisphere. Throughout this period, the IMF was stable, with Bz negative (≈-4 nT), By slowly increasing from ≈-2 to ≈ 2 nT, and Bx positive (≈ 10 nT). Previous studies suggest that penetration of positive IMF Bx into the magnetosphere may explain at least part of the observations presented here.