IMF By and Dipole Tilt Control of the Auroral Cusp and Electron Aurora in the Dayside Polar Cap During Multiple Transpolar Arcs—A Statistical Study

Abstract It is well‐known that the auroral cusp appears poleward of the dayside oval during northward IMF, and that transpolar arcs (TPAs) are typically cusp‐aligned. Using Lyman‐Birge‐Hopfield band short (LBHs) images from the Defense Meteorological Satellite Program, we investigate electron aurora that occur approximately in the same region as the ion cusp. For this purpose, we examine 140 LBHs images during which multiple TPAs appear simultaneously in the polar cap, and a region of bright electron precipitation is visible close to the tip of the TPAs. In eight cases, the ion cusp could be identified from particle data. We find that the electron precipitation region in most cases overlaps with the cusp, but also extends dawnward of it. This dawnside part is identified as the auroral signature of upward flowing northward Bz currents in the polar cap. Studying the spatial distribution of the 140 events, a clear dipole tilt dependence of cusp and connected electron precipitation region is observed. In the summer hemisphere, they appear on higher latitudes and in winter on lower latitudes close to the dayside oval. While the electron precipitation region extends nearly always to the dawnside, the winter hemisphere cusp appears in the northern (southern) hemisphere at postnoon (prenoon). IMF By controls the dawn‐dusk location of cusp and electron precipitation region only in the summer hemisphere. In the winter By influences instead its latitude. As TPAs typically merge with the electron precipitation region, these observations might also explain the recently reported explicit By dependence of initial TPA location.