Heliophysics Science Division
Sciences and Exploration Directorate - NASA's Goddard Space Flight Center

April 8, 2011, 12:00 pm - 1:00 pm

April 8, 2011, 12:00 pm - 1:00 pm

Auroral Arc Theories--A Missing Link



Dr. David Knudsen, University of Calgary, Canada, and NASA Goddard Space Flight Center

In-situ and optical observations of visible auroral arcs - the most common pre-midnight auroral form - place tight restrictions on candidate theories. For example, electron energies within inverted-V arcs can reach 20-30 keV; these energies vary characteristically and often symmetrically with latitude; they have typical widths of order 20 km; they are highly elongated in longitude; they can occur as far south as the inner edge of the plasma sheet and as far north as the polar cap (though at lower energies); they often appear in multiple parallel sets of two to five or more; and although they sometimes oscillate, more often they exist with no basic change in form for several to tens of minutes. These basic properties have been well-known for many decades, yet most auroral theories address only a small subset - often only one - and no theory accounts for them all. As a result, there remains no consensus as to the essential mechanism(s) responsible for auroral arcs. One barrier to progress is a surprising lack of statistical information on basic arc behaviors. This talk describes initial work intended to remedy this situation, and discusses implications for theories of auroral arcs, focusing in particular on zero-frequency electromagnetic structures known as "stationary inertial Alfven waves" [Knudsen, JGR, 1996].