Splitting elemental arcs of aurora and their association with inertial Alfvén waves

Ground based optical observations of aurora reveal fine scale structures with brightness width less than 10 kms in the direction perpendicular to B. These fine scale structures exhibit a phenomenon called arc splitting, also known as bifurcating elemental arcs or packets. One can witness the arc splitting in the auroral image when elemental arcs peel away from a central bright arc which is then followed by the generation of new arcs. Dispersive Alfvén waves have been suggested as a possible generation mechanism for this phenomenon. Semeter et al., JGR 2008 interpreted the observations of splitting elemental arcs with respect to inertial Alfvén waves. They suggested that the energy of the precipitating electrons should decrease as the arc packets move away from the central bright arc. We tested this theory by using the data from the multi spectral imager called ASK (Auroral Structure and Kinetics) stationed on Svalbard. The energy and flux of the precipitating electrons are calculated from the ASK data and Southampton ionospheric model. We used empirical number density and IGRF (International Geomagnetic Reference Field) model to calculate the properties of the inertial Alfvén waves. A comparison between the splitting elemental arcs and Alfvén waves indicates that the wave particle interaction between Alfvén waves and the precipitating electrons is a possible generation mechanism for the production of these splitting elemental arcs. From our data and calculations, we infer an acceleration height of precipitating electrons just under 3000 km.

 

References:

Semeter, J., M. Zettergren, M. Diaz, and S. Mende (2008), Wave dispersion and the discrete aurora:

New constraints derived from high-speed imagery, J. Geophys. Res., 113, A12208.