We demonstrate that high-resolution 557.7-nm all-sky images are useful tools for investigating the spatial and temporal evolution of merging on the dayside magne-topause. Analysis of ground and satellite measurements leads us to conclude that high-latitude merging events can oc-cur at multiple sites simultaneously and vary asynchronously on time scales of 30 s to 3 min. Variations of 557.7 nm emis-sions were observed at a 10 s cadence at Ny Alesund on 19 December 2001, while significant changes in the IMF clock angle were reaching the magnetopause. The optical pat-terns are consistent with a scenario in which merging occurs around the rim of the high-latitude cusp at positions dictated by the IMF clock angle. Electrons energized at merging sites represent plausible sources for 557.7 nm emissions in the cusp. Polar observations at the magnetopause have directly linked enhanced fluxes of ≥0.5 keV electrons with merging. Spectra of electrons responsible for some of the emissions, measured during a DMSP F15 overflight, exhibit " inverted-V " features, indicating further acceleration above the iono-sphere. SuperDARN spectral width boundaries, character-istic of open-closed field line transitions, are located at the equatorward edge of the 557.7 nm emissions. Optical data suggest that with IMF B Y >0, the Northern Hemisphere cusp divides into three source regions. When the IMF clock angle was ∼150 • structured 557.7-nm emissions came from east of the 13:00 MLT meridian. At larger clock angles the emis-sions appeared between 12:00 and 13:00 MLT. No signifi-cant 557.7-nm emissions were detected in the prenoon MLT sector. MHD simulations corroborate our scenario, showing that with the observed large dipole-tilt and IMF clock angles, Correspondence to: N. C. Maynard (email@example.com) merging sites develop near the front and eastern portions of the high-altitude cusp rim in the Northern Hemisphere and near the western part of the cusp rim in the Southern Hemi-sphere.
Maynard, N. C., Moen, J., Burke, W. J., Lester, M., Ober, D. M., Scudder, J. D., … Balogh, A. (2004). Temporal-spatial structure of magnetic merging at the magnetopause inferred from 557.7-nm all-sky images. Annales Geophysicae, 22(8), 2917–2942. https://doi.org/10.5194/angeo-22-2917-2004