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

Abstract

Abstract. We combine in situ measurements from Cluster with high-resolution 557.7 nm all-sky images from South Pole to investigate the spatial and temporal evolution of merging on the dayside magnetopause.Variations of 557.7 nm emissions were observed at a 6 s cadence at South Pole on 29 April 2003 while significant changes in the Interplanetary Magnetic Field (IMF) clock angle were reaching the magnetopause. Electrons energized at merging sites are the probable sources for 557.7 nm cusp emissions. 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. Most poleward moving auroral forms (PMAFs) reflect responses to changes in interplanetary medium rather than to local processes. The changes in magnetopause position required by increases in dynamic pressure are mediated by merging and result in the generation of PMAFs. Small (15-20%) variations in dynamic pressure of the solar wind are sufficient to launch PMAFs. Changes in IMF B X create magnetic flux compressions and rarefactions in the solar wind. Increases (decreases) in IMF B X Correspondence to: N. C. Maynard (nelson.maynard@unh.edu) strengthens |B| near northern (southern) hemisphere merging sites thereby enhancing merging rates and triggering PMAFs. When correlating responses in the two hemispheres, the presence of significant IMF B X also requires that different lagtimes be applied to ACE measurements acquired ∼0.1 AU upstream of Earth. Cluster observations set lag times for merging at Northern Hemisphere sites; post-noon optical emissions set times of Southern Hemisphere merging. Allsky images and magnetohydrodynamic simulations indicate that merging occurs in multiple discrete locations, rather than continuously, across the dayside for southward IMF conditions in the presence of dipole tilt. Matching optical signatures with clock-angle, B X , and dynamic pressure variations provides new insights about interplanetary control of dayside merging and associated auroral dynamics.

Cluster observations of fast shocks in the magnetosheath launched as a tangential discontinuity with a pressure increase crossed the bow shock

“…When a solar wind pressure increase occurs, the magnetopause position must move inward to maintain pressure balance. The removal, or erosion, of the magnetic flux between the old and new positions is accomplished by merging [e.g., Siscoe et al , 2002; Maynard et al , 2006; Ober et al , 2007]. The timing of the initiation of these changes is determined by the arrival of the fast shock generated when the solar wind pressure increase crossed the bow shock.…”

Interaction of the bow shock with a tangential discontinuity and solar wind density decrease: Observations of predicted fast mode waves and magnetosheath merging

“…However, the bursty nature of the accelerated ions indicates that intervals of merging occurred, each lasting <1 min. Maynard et al [2004b, 2006] used 557.7 nm observations of the dayside cusp to show that merging events at the magnetopause last from 30 s to a several minutes at any given spot. Such short intervals usually lack enough points to provide significant statistical results.…”

Magnetohydrodynamic simulations of transient transpolar potential responses to solar wind density changes

“…It seems likely in nature that any compression of the magnetosphere during conditions of southward IMF must be transient and in time removed by erosion. In an investigation of the dayside auroral response to small changes in solar wind dynamic pressure, Maynard et al [2006] concluded that changes in the magnetopause position demanded by dynamic pressure changes were accomplished by merging.…”