Cusp aurora dependence on interplanetary magnetic field Bz

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Abstract
The Imager for Magnetopause to Aurora Global Exploration (IMAGE) Spectrographic Imager (SI12) detects Doppler‐shifted Lyman alpha emissions created by charge exchange and de‐excitation of precipitating protons in the atmosphere. At high latitudes near local noon, emissions consistent with the location of the cusp foot point have distinct interplanetary magnetic field (IMF) dependence. For northward IMF a spot of emissions is observed poleward of the dayside auroral oval. By tracing the magnetic field lines from this spot using a model magnetosphere it is shown that the cusp foot point maps to a narrow region on the high‐latitude magnetopause where antiparallel magnetic reconnection may be occurring. As the IMF turns southward, the spot merges into the auroral oval, producing a broad region of intense emissions centered near local noon. By tracing the magnetic field lines from this broad region using a model magnetosphere it is shown that the cusp foot point maps to a relatively broad region on the dayside magnetopause where component magnetic reconnection may be occurring.

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