Temporal-spatial structure of magnetic merging at the magnetopause inferred from 557.7-nm all-sky images

Abstract

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-Ålesund 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 ≥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 557.7 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 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, Correspondence to: N. C. Maynard (nmaynard@mrcnh.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 Hemisphere.

Characteristics of merging at the magnetopause inferred from dayside 557.7-nm all-sky images: IMF drivers of poleward moving auroral forms

“…At the same time Cluster was crossing the pre-noon cusp in the Northern Hemisphere. The combined observations confirm results of a previous study that merging events can occur at multiple sites simultaneously and vary asynchronously on time scales of 10 s to 3 min (Maynard et al, 2004). The intensity of the emissions and the merging rate appear to vary with changes in the IMF clock angle, IMF B X and the dynamic pressure of the solar wind.…”

“…5p. Typically, the emissions intensify at a particular location for 30 s to several minutes, a phenomenology similar to that seen above Ny-Ålesund (Maynard et al, 2004). Multiple sites often activate simultaneously.…”

“…Electrons exiting merging regions along inner separatracies intersect the ionosphere very close to the open-closed field-line boundary, where they excite structured 557.7 nm auroral emissions in the ionosphere (Maynard et al, 2004). Because their high speeds allow quick reflection from the ionosphere, tail electrons within the magnetosphere move in both directions along field lines Maynard et al, 2005).…”

“…Since the lifetime of the 1 S→ 1 D (557.7 nm) transition is 1 s, Maynard (2003) suggested that all-sky images of the cusp provide "television pictures" of large-scale merging at the magnetopause. Subsequently Maynard et al (2004) showed that 557.7 nm cusp emissions above Ny-Ålesund are localized rayed structures that appear and vanish at particular locations on time scales ranging from 30 s to several minutes. Structures may occur simultaneously, but asynchronously, at multiple locations, then change positions in local time in response to changes in the interplanetary magnetic field (IMF) clock angle θ.…”

“…Geophys., 24, 2006 www.ann-geophys.net/24/3071/2006/ N. C. Maynard et al: Temporal and spatial dependence of merging 3073 Maynard et al (2004) estimated that merging activations last between ≈30 s and several minutes from the durations of 557.7 nm cusp emissions. However, their spatial scales were smaller than those postulated by Cowley and Lockwood, and many were detected simultaneously but asynchronously.…”