Asymmetric auroral intensities in the Earth’s Northern and Southern hemispheres

Abstract

New evidence suggests that the commonly held idea that the aurora borealis (in the Northern Hemisphere) and aurora australis (in the south) are mirror images of each other is not always the case. Aurora do tend to occur at the same time in northern and southern polar regions at locations connected by geomagnetic field lines, a connection that might be expected to link patterns, positions and timing of the two aurora. On 12 May 2001 two Earth observation spacecraft — IMAGE and Polar — were well placed to observe the two poles simultaneously. The resulting images have now been analysed and they provide clear evidence that the auroras can be asymmetric. The asymmetry may be caused by inter-hemispheric currents predicted to arise from conductivity differences, but not previously demonstrated. It is commonly assumed that the aurora borealis (Northern Hemisphere) and the aurora australis (Southern Hemisphere) are mirror images of each other. Here, observations are reported that clearly contradict this common assumption: intense spots are seen at dawn in the Northern summer Hemisphere, and at dusk in the Southern winter Hemisphere. It is commonly assumed that the aurora borealis (Northern Hemisphere) and aurora australis (Southern Hemisphere) are mirror images of each other because the charged particles causing the aurora follow the magnetic field lines connecting the two hemispheres. The particles are believed to be evenly distributed between the two hemispheres, from the source region in the equatorial plane of the magnetosphere. Although it has been shown that similar auroral features in the opposite hemispheres can be displaced tens of degree in longitude1,2 and that seasonal effects can cause differences in global intensity3,4, the overall auroral patterns were still similar. Here we report observations that clearly contradict the common assumption about symmetric aurora: intense spots are seen at dawn in the Northern summer Hemisphere, and at dusk in the Southern winter Hemisphere. The asymmetry is interpreted in terms of inter-hemispheric currents related to seasons, which have been predicted5,6 but hitherto had not been seen.