Abstract Previous studies indicate that specific polar cap auroral forms frequently occur in the postnoon in summer due to the influence of the interplanetary magnetic field (IMF) By component. However, regardless of the season and IMF conditions, similar auroral forms rarely occur in the prenoon. The reason for this asymmetry remains a mystery. By comparing concurrent auroral observations in both hemispheres, we found that when the polarity of the IMF By (positive), which favors the formation of the specific postnoon auroral forms, is reversed (negative), a phenomenon called Prenoon Auroral Oval Poleward Shift (PAOPS) can be often observed in the summer hemisphere. This highlights a new finding about prenoon‐postnoon auroral asymmetry. PAOPS occurrence exhibits solar cycle dependence, seasonal preference, and IMF control. Energy‐dispersed ions suggest PAOPS originates from magnetic reconnection between IMF and geomagnetic fields in the lobe region.
Observations from the Little Bunker
Last updated: August 18, 2025 04:13 PM
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