Evolution of Wave Numbers in Auroral Structures at a Substorm Onset

Details Evolution of Wave Numbers in Auroral Structures at a Substorm Onset Evolution of Wave Numbers in Auroral Structures at a Substorm Onset

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusmsm22a..02s&link_type=abstract

American Geophysical Union, Spring Meeting 2009, abstract #SM22A-02

Physics

2704 Auroral Phenomena (2407), 2736 Magnetosphere/Ionosphere Interactions (2431), 2752 Mhd Waves And Instabilities (2149, 6050, 7836), 2790 Substorms

Scientific paper

Recent developments of optical instruments for ground-based auroral observations, e.g. THEMIS All-Sky Imagers array, reveal existences of fine-scale auroral structures at substorm expansion-phase onsets, which we could not identify previously due to limitation of both temporal and spatial coverage/resolution. Donovan et al. [2006] reported that a pseudo auroral break up occurred on a pre-existing aurora consisting of eastward propagating beads with a wavelength of ~100 km. Similar auroral structures with wavelengths of 50-200 km at substorm expansion-phase onsets were reported by Friedrich et al. [2001] and Liang et al. [2008]. These authors pointed out that the fine structures imply the dynamics of some plasma instabilities associated with substorm onsets in the magnetosphere were projected onto the structures of the onset auroras. In this presentation, we show time evolutions of auroral structures observed at a pseudo substorm onset on January 15, 2008 at Gillam, Canada (56.4N, 265.4E, dipole geomagnetic latitude 65.6N). The auroral initial brightening started at 2223 MLT just in the center of a field of view of a panchromatic (white-light) all-sky camera with a sampling rate of 30 Hz. The fast-sampling images enable us to analyze differential images every sampling interval (0.03 s). The differential images show that the initial brightening at the first 1 second had a longitudinal size of ~30 km. One of the brightening edges expanded westward with an average speed of ~20 km/s at the first 10 s, forming a longitudinal ripple-like structure. The speed gradually decreased to ~10 km/s in the following 10 s. Two-dimensional Fourier analyses of the wave number are applied for auroral images with horizontal sizes of 256 km x 256 km. The results show that the brightening auroras are subdivided into smaller scales, especially during the first 10 s of the onset. The first harmonic structures with a wave number k=3, or a wavelength ~80 km, appeared in 4 s after the onset, and the second one with a wave number k=6, or a wavelength ~40 km, appeared in 8 s after the onset. These harmonic structures are similar to those indicated by a simulation of inertial Alfven waves in the ionospheric Alfven resonator by Lysak and Song [2008]. Considering the excitation of higher modes, a decrease in the auroral expanding (phase) speed as above is consistent with the dispersion relation of inertial Alfvén waves. We suppose that auroral fine structures just after the onset can be formed by interferences of inertial Alfven waves in higher altitudes.

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