Small-Scale Dynamic Aurora

Abstract

Small-scale dynamic auroras have spatial scales of a few km or less, and temporal scales of a few seconds or less, which visualize the complex interplay among charged particles, Alfvén waves, and plasma instabilities working in the magnetosphere-ionosphere coupled regions. We summarize the observed properties of flickering auroras, vortex motions, and filamentary structures. We also summarize the development of fundamental theories, such as dispersive Alfvén waves (DAWs), plasma instabilities in the auroral acceleration region, ionospheric feedback instabilities (IFI), and the ionospheric Alfvén resonator (IAR).

Supplementary information: The online version contains supplementary material available at 10.1007/s11214-021-00796-w.

Keywords: Auroral breakup; Auroral phenomena; Dispersive Alfven waves; Flickering aurora; Ionospheric Alfven resonator; Ionospheric feedback instabilities; Magnetosphere-ionosphere coupled region.

Fig. 1

Inverted-V electrons and auroral arcs observed by the Reimei satellite on 2005-12-26, moving across the auroral band structure with a latitudinal width of about 100 km. Multiple small-scale auroral arcs showed sheared motions as shown in Movie 1

Fig. 2

Cartoon of the magnetic zenith-view of small scale auroral structures. (After Hallinan ; Dahlgren et al. 2010). The packet structures are compared against vortex motion (Semeter et al. ; Semeter 2012)

Fig. 3

Ground-observations of fine scale auroral phenomena with fields-of-view as shown: a) Auroral curls (Vogt et al. 1999), b) “Flaming” auroral filaments (Dahlgren et al. 2013), c) Auroral “packets” (Semeter et al. 2008), d) Decameter filaments produced by HAARP (Kendall et al. 2010)

Fig. 4

Auroral electron and magnetically conjugate camera observations of rapidly evolving auroral forms returned from the Remei spacecraft. (a) Electron energy spectrogram revealing the electron energy fluxes responsible for auroral emission at 670 nm shown for (b) Alfvénic and (c) Quasi-static aurora (After Chaston et al. 2010)

Fig. 5

Snapshots in the evolution of an auroral current sheet for plasma parameters that provide M_sub_A > 1 and auroral ‘curls’ (After Chaston and Seki 2010). a) and e) show vertical structure, c) and d) show horizontal slices of the current and potential through the acceleration region while g) and h) show energy flux at the ionosphere. Spatial scales vary with altitude

Fig. 6

Observations of likely tearing instability in a discrete feature from the Reimei spacecraft (After Chaston 2015a,b). a) shows intensity at 670 nm while b) shows the corresponding optical vorticity with arrows indicating the direction of optical flow

Fig. 7

Color contours of the vorticity distributions on the ionosphere (lower plane) and the magnetic equator (upper plane) at three different time steps of the nonlinear simulation, where the vertical scale is shortened just for clarity of the plots. (After Watanabe 2010)