Aurora conjugacy during substorms: Coordinated Antarctic ground and Polar Ultraviolet observations

Abstract

Simultaneous optical observations at the Antarctic station Mirny (Φ'= -77.41 Corrected geomagnetic latitude (CGL)) and Ultraviolet Imager images of the northern auroral oval from the POLAR satellite were used to determine the displacement of nightside aurorae in the Northern Hemisphere relative to those in the Southern Hemisphere during 10 (1.5 to 3 hour long) time intervals including auroral substorms. Displacements of the poleward edge of the auroral bulge can be significant, up to 50 in CGL. The sense and magnitude of displacements are not related to the dipole tilt angle or differing ionospheric conductivity in the two hemispheres but rather to interplanetary magnetic field (IMF) orientations in the ecliptic plane. Aurorae reach higher latitudes in the Northern Hemisphere when Bx<0 and By>0 but occur at higher latitudes in the Southern Hemisphere when Bx>0 and By<0. Perturbations in conjugate ground magnetometers display a similar asymmetry. By contrast, 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 transient intensifications often occur in only one hemisphere. Our results testify to the effective penetration of the equatorial component of the IMF into the magnetosphere. Copyright 2001 by the American Geophysical Union.