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.
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We are indebted to the IMAGE and Polar teams for the design and successful operations of the two missions. We especially thank S. B. Mende for the use of IMAGE FUV WIC data and J. B. Sigwarth for the use of Polar VIS Earth Camera data. This study was supported by the Norwegian Research Council, through the IPY-ICESTAR project 176045/S30
Author Contributions N.Ø is responsible for the project planning, K.M.L. is responsible for the data processing. The data were identified and analysed by K.M.L. and N.Ø. The manuscript was written by K.M.L. and N.Ø.
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Laundal, K., Østgaard, N. Asymmetric auroral intensities in the Earth’s Northern and Southern hemispheres. Nature 460, 491–493 (2009). https://doi.org/10.1038/nature08154