Occurrence of red-green low latitude aurora during large geomagnetic storms: Two case studies

Low latitude auroras are seen during large geomagnetic disturbances when precipitation of electrons from the inner magnetosphere occurs. Stable Auroral Red (SAR) arcs formed from the overlap region between outer plasmasphere electrons and ring current ions contribute to 630 nm (red) emission in the equatorward edge of the auroral oval. Broadband flux of low energy electrons from the inner magnetosphere also causes both red and green auroras at low latitudes. However, the red emissions alone are usually seen in lower latitudes because the green emissions are hidden below the horizon. Recent observations of the low latitude auroras at Rikubetsu (MLat, 34.70 N) in Japan in the rising phase of solar cycle 25 have shown comparable red-green intensity, which are unusual compared to previous observations. Two such events occurred during the geomagnetic storms on February 27, 2023 and April 24, 2023 (F10.7 = 157.9 and 135.5, SYM-HMin = -161 nT and -233 nT). This study investigates the two events using all sky imager, background solar wind-IMF parameters, magnetometer measurements and ionospheric (NOAA and DMSP) satellites. Satellites pass over the location at the time of low latitude auroras show that the auroral oval expanded equatorward to ~480 N magnetic latitude. The solar wind dynamic pressure shows sudden similar rise at the impact of the corresponding ICMEs in both auroral events. The high energy electron flux measured in the order of 100 eVs seems to cause the comparable green and red intensities.