Figure 1

Azimuthal equidistant projection maps of auroral substorm onset occurrence frequency for the (a) Northern Hemisphere (NH) and (b) Southern Hemisphere (SH). The occurrence frequency is derived from 2659 auroral substorm onsets and averaged in equal-area bins (~5° in latitude). Contours of geographic latitudes are drawn every 5° starting from 45° (−45° in the SH) and longitudes are drawn every 15°. Most of the auroral substorm onsets are initiated between 60° and 75° (−60° and −75° in SH) magnetic latitudes, which are plotted in black contours. Continents are overlapped in black. In NH, the peak onset frequency is ~3.5% and is located west of Hudson Bay, center around Churchill, Canada, and Khatanga, Siberia. In SH the peak onset frequency is ~4.5% and is located in the Antarctic ocean between the Australia and the Antarctic continents.