Low-frequency waves in auroral arcs: optical echo of magnetotail dynamics (Invited)
Abstract
We report new results of our analysis of longitudinally propagating low-frequency (period 0.5 - 2.0 min) wave oscillations in optical auroral arcs, with the emphasis on the underlying physics. Several alternative wave mechanisms are considered, including a variety of wave modes in the ionosphere and the auroral acceleration region, field line resonances, and plasma waves in the magnetotail. A representative set of well-documented episodes of wave activity recorded by ground-based and in situ THEMIS instruments is studied in this context. Our analysis rules out three of the four wave locations listed above. The ionospheric origin of the arc wave is unlikely considering the measured phase velocities which require unrealistically strong convection electric fields. The acceleration region is able to produce fast-traveling localized auroral features such as folds and rays, but not for the observed range of wavelengths (up to ~400 km in the ionospheric frame), and not for the observed propagation direction (predominantly westward at both southern and northern edges of pre-midnight arcs). It is noteworthy that the majority of wave events are not accompanied by field line resonances, indicating that this effect is not the primary cause of the arc wave, contrary to some recent claims. The only interpretation consistent with our quantitative analysis of the relevant data tests is that the arc waves are a manifestation of an azimuthally propagating wave in the magnetotail. We present several examples of a nearly one-to-one correspondence between low-frequency tail waves (e.g. flapping motion) and the auroral arc waves, the parameters of the optical wave being consistent with theoretical predictions for drift wave oscillations in a thin current sheet. Although the detailed physical picture of the underlying magnetosphere -ionosphere interaction is still lacking, our results strongly suggest that further investigation of optical arc waves will provide a valuable piece of information on spatial propagation of plasma instabilities associated with substorm growth and expansion phases as well as other magnetospheric conditions. Detrended ewogram representation (MLON vs UT, color-coded optical intensity) of a longitudinally propagating arc wave prior to 8:04 03/03/08 substorm onset observed by the Fort Simpson ASI. Parameters of several representative pre-onset arc wave events. Letters N and S correspond to northern and southern arcs in double-arc systems. Numbers in brackets are expected magnetotail values from T89 mapping.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMSM52B..03U
- Keywords:
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- 2704 MAGNETOSPHERIC PHYSICS / Auroral phenomena;
- 2744 MAGNETOSPHERIC PHYSICS / Magnetotail;
- 2772 MAGNETOSPHERIC PHYSICS / Plasma waves and instabilities;
- 2790 MAGNETOSPHERIC PHYSICS / Substorms