2013 THEMIS SCIENCE NUGGETS
Westward Traveling Surges: Sliding Along Boundary Arcs and Distinction from Onset Arc Brightening
by Larry Lyons, Atmospheric & Oceanic Sciences, UCLA
Introduction
Substorms are perhaps the most dramatic of auroral phenomena and are associated with large disturbances within the Earth's magnetosphere-ionosphere system. The onset of substorms is identifiable as a brightening of an auroral arc near the equatorward boundary of the auroral oval. Following an onset, the region of active aurora expands poleward towards the auroral poleward boundary. It is not uncommon for the poleward expansion to lead to a bulge region of activity that expands westward, forming what is referred to as the “westward traveling surge”. A westward traveling surge is not observed during all substorm expansions, though we are not aware of studies showing just how common they are. However, the westward traveling surge is an interesting and, based on the intense auroral emissions, energetically important aspect of many substorm expansions.
Results
Included as part of the THEMIS program is an extensive array of ground-based all-sky cameras that covers much of the North American auroral zone. Combining images from these cameras has provided high spatial and temporal pictures of auroral activity that are well beyond what was available previously and have led to much that is new and important. In this study, by using these cameras, we have found expanded our understanding of westward traveling surges and how they relate to other features of substorms.
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| Figure 1. Selected mergers of the auroral images from the THEMIS cameras for the time interval covering the 0801:30 UT substorm onset on 6 February 2007. Yellow-green arrows identify the onset and ensuing westward broadening of the onset arc brightening, and the orange arrows indicate the inferred polar-cap flow channel adjacent to the polar cap boundary arc. Red plus signs give the locations of ground magnetometer stations used in our analysis, and white text identifies each camera. The cyan line marks magnetic midnight, and longitude lines are spaced 1 hr in MLT apart. Moonlight and light contamination are identified by magenta arrows in the first panel. |
Discussion
Features that we have identified are illustrated from the time sequence of mergers of camera images shown in the figure above. After onset, auroral brightening rapidly spreads azimuthally along the onset arc near the auroral equatorward boundary. The region of auroral brightening also expands poleward, and can contact an arc along the poleward boundary of the auroral oval. When this happens, we find that poleward expansion of the region of active aurora accelerates, forming a bulge region that protrudes into the pre-existing polar cap and expands westward as the westward traveling surge. This westward motion is slower than, and initiates later than, the azimuthal spreading of the onset arc brightening. The poleward boundary auroral arc remains connected to the surge as the surge slides along the equatorward moving edge of this boundary arc, and this connection appears to set off the westward traveling, poleward protruding activity of the surge via localized polar cap flow adjacent to the boundary arc.
Ground magnetic signatures show a relatively weak magnetic depression moving westward with the onset arc brightening, and a delayed substantially larger depression moving with the westward propagating surge that must be associated with a substantially larger westward ionospheric current. The clear distinction between onset arc brightening and the westward traveling surge seen in auroral and magnetic (electrical current) signatures implies that their underlying processes are very different. We suggest that flow channels adjacent to the boundary arc feed surge development, and that the activity associated with the surge affects the magnetosphere-ionosphere system much more than does the activity associated with the brightening of the onset arc.
Reference
Lyons, L. R., Y. Nishimura, B. Gallardo-Lacourt, Y. Zou, E. Donovan, S. Mende, V. Angelopoulos, J. M. Ruohoniemi, and K. McWilliams (2013), Westward traveling surges: Sliding along boundary arcs and distinction from onset arc brightening, J. Geophys. Res. Space Physics, 118, 7643–7653, doi:10.1002/2013JA019334.Biographical Note
Larry Lyons is a Distinguished Professor at UCLA and Fellow of the American Geophysical Union. He works on all aspects of magnetospheric and auroral physics, including coupling to the ionosphere and upper atmosphere, and guides an active group of talented younger researchers and graduate students.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu