Aurora conjugacy during substorms: Coordinated antarctic ground and polar ultraviolet observations

Simultaneous optical observations at the Antarctic station Mimy (phi' = -77 .4(0) Corrected geomagnetic latitude (CGL)) and Ultraviolet Imager images o f the northern auroral oval from the POLAR satellite were used to determine the displacement of nightside aurorae in the Nor-them Hemisphere relative to those in the Southern Hemisphere during 10 (1.5 to 3 hour long) time int ervals including auroral substorms. Displacements of the poleward edge of t he auroral bulge can be significant, up to 5(0) in CGL. The sense and magni tude of displacements are not related to the dipole tilt angle or differing ionospheric conductivity in the two hemispheres but rather to interplaneta ry magnetic field (IMF) orientations in the ecliptic plane. Aurorae reach h igher latitudes in the Northern Hemisphere when Bx <0 and By >0 but occur a t higher latitudes in the Southern Hemisphere when Bx >0 and By <0. Perturb ations in conjugate ground magnetometers display a similar asymmetry. By co ntrast, displacements of the aurora are small when the IMF strength is weak , the IMF orientation fluctuates, or the IMF has an orthospiral orientation . Even when the latitudes are similar, local bright auroral forms and trans ient intensifications often occur in only one hemisphere, Our results testi fy to the effective penetration of the equatorial component of the IMF into the magnetosphere.