Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm41a1679m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM41A-1679
Physics
Plasma Physics
[2704] Magnetospheric Physics / Auroral Phenomena, [2744] Magnetospheric Physics / Magnetotail, [2764] Magnetospheric Physics / Plasma Sheet, [7836] Space Plasma Physics / Mhd Waves And Instabilities
Scientific paper
We do not know how auroral arcs are formed, whether or not there are different types of arcs (meaning different underlying physics), nor what arcs correspond to in the magnetosphere. Given the ubiquity of arcs and their obvious importance to MI coupling, including specific processes such as the substorm, resolving the questions we have about arcs is one of the key objectives in space physics. We have carried out a survey of a two-year subset of the THEMIS ASI image data set. In this survey, we have classified the ~150M images in terms of viewing conditions, presence or absence of aurora, and auroral type. One of the consequences of this survey is that we have amassed what is arguably (to date) the largest set of images of auroral arcs. This set of arcs spans all auroral latitudes and magnetic local times except for a few hours around local noon. In this paper we use this auroral survey, together with the results of a similar survey of the proton aurora (from the NORSTAR Meridian Scanning Photometer array) and magnetic pulsations (published by Baker et al. [JGR, Volume 108, doi:10.1029/2002JA009801, 2003]), to elucidate some new quantitative and qualitative results including the following: 1) auroral arcs occur on field lines that are poleward of the ion isotropy boundary; 2) the orientation of arcs in geomagnetic coordinates suggest that arcs are an ionospheric projection of a gradient of some as yet unidentified magnetospheric parameter; 3) although some arcs oscillate in ways that are compellingly suggestive of their generation via field line resonances, most auroral arcs do not oscillate; 4) the magnetic local time where auroral arc occurrence peaks corresponds to a minimum in the occurrence of Pc5 pulsations and field line resonances. We will conclude with a discussion of the implications of these results for models of auroral arc generation. MLT occurrence distributions of arcs (gray histogram) and FLRs in the Pc5 spectral band (transparent histogram with dark border).
Baker George
Barnetson K.
Donovan Eric
Jackel Brian J.
Knudsen David J.
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