|
|
 |
|
|
Ann. Geophys., 24, 3071-3098, 2006
www.ann-geophys.net/24/3071/2006/
© European Geosciences Union 2006
Characteristics of merging at the magnetopause inferred from dayside 557.7-nm all-sky images: IMF drivers of poleward moving auroral forms
N. C. Maynard1, W. J. Burke2, Y. Ebihara3, D. M. Ober4,*, G. R. Wilson4,**, K. D. Siebert4,***, J. D. Winningham5, L. J. Lanzerotti6, C. J. Farrugia1, M. Ejiri3, H. Rème7, A. Balogh8, and A. Fazakerley9
1Space Science Center, University of New Hampshire, Durham, NH, USA
2Air Force Research Laboratory, Hanscom Air Force Base, MA, USA
3National Institute of Polar Research, Tokyo, Japan
4ATK Mission Research, Nashua, NH, USA
5Southwest Research Institute, San Antonio, TX, USA
6New Jersey Institute of Technology, NJ, USA
7CESR/CNRS, Avenue du Colonel Roche, 31028 Toulouse, Cedex 4, France
8Imperial College, Exhibition Road, London SW7 2BW, UK
9Mullard Space Science Laboratory, Dorking, Surrey RH5 6NT, UK
*now at: Boston College, Chestnut Hill, MA, USA
**now at: Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL, USA
***now at: SPARTA, Inc., Nashua, NH, USA
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 BX 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 BX create
magnetic flux compressions and rarefactions in the solar wind. Increases
(decreases) in IMF BX 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 BX 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 for
southward IMF conditions in the presence of dipole tilt. Matching optical
signatures with clock-angle, BX, and dynamic pressure variations provides
new insights about interplanetary control of dayside merging and associated
auroral dynamics.
Full Article in PDF (3430 KB) |
|
|
