24-10-2012, 03:49 PM
Temporal-Spatial Structure of Magnetic Merging at the Magnetopause Inferred from 557.7-nm All-Sky Images
ABSTRACT
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 magnetopause. Analysis of ground and satellite measurements leads us to conclude that
high-latitude merging events can occur at multiple sites simultaneously and vary asynchronously on time scales of 30 s to 3
min. Variations of 557.7 nm emissions 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 patterns 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 \g or = 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
ionosphere. SuperDARN spectral width boundaries, characteristic of open-closed field line transitions, are located at the
equatorward edge of the 557.7nm emissions. Optical data suggest that with IMF B(sub Y) \g 0, the Northern Hemisphere cusp
divides into three source regions. When the IMF clock angle was approx. 150 deg structured 557.7-nm emissions came from
east of the 13:00 MLT meridian. At larger clock angles the emissions appeared between 12:00 and 13:00 MLT. No significant
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, 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 Hemisphere.