The Critical Factor in Controlling the Auroral Intensity in the Cusp Region As Revealed by a Statistical Study on Midday Gap and Non‐Gap Events

Taking advantage of the high spatial-resolution and global coverage of Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observations, we investigated the critical interplanetary factors in controlling the cusp auroral emission by dividing the midday auroras into the gap (weak emission) and non-gap (intense emission) events. Although the cusp auroral intensity is essentially determined by a parameter related to the IMF (interplanetary magnetic field) direction, IMF magnitude (vertical bar B-y vertical bar), and solar wind speed (V), we found that the cusp aurora is statistically weak during the southward IMF but intense when the V and IMF vertical bar B-y vertical bar are greater. Further, we confirmed that even with V > 600 km/s, the intense-aurora event still shows a minimum occurrence near the IMF vertical bar B-y vertical bar = 0. However, when the IMF vertical bar B-y vertical bar is greater, the V becomes less significant for the intense-aurora occurrence. These results demonstrate that the IMF vertical bar B-y vertical bar is critical in controlling the cusp auroral intensity, most likely by producing an electric field which is given as V x B-y.