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

• Published in: Annales Geophysicae
• Main Authors: N. C. Maynard, W. J. Burke, Y. Ebihara, D. M. Ober, G. R. Wilson, K. D. Siebert, J. D. Winningham, L. J. Lanzerotti, C. J. Farrugia, M. Ejiri, H. Rème, A. Balogh, A. Fazakerley
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2006
• Subjects: Science; Q; Physics; QC1-999; Geophysics. Cosmic physics; QC801-809; South Pole; South pole
• Online Access: https://doi.org/10.5194/angeo-24-3071-2006; https://doaj.org/article/0bb2ccaf2566405b86320ab41753083a

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 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 lag-times 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. All-sky images and magnetohydrodynamic simulations indicate that merging occurs in multiple discrete locations, rather than continuously, across the dayside ...