Auroral bulge - last update: 25 November 1998, 1330 UT (RR)

The most spectacular form of discrete auroras is the substorm related auroral bulge that grows out from a quite ("breakup") arc that is deformed. The bulge is formed close to the midnight sector, and it is characterized by rapid poleward motion. It was first recognized from all-sky cameras and magnetometers by Akasofu et al. (1965, 1966). Auroral bulge is closely related with substorm current wedge, i.e., localized field-aligned currents.

There are three characteristic types of discrete aurora within a bulge (Nakamura et al., 1993):

  1. a surge (westward termination of the bulge, also known as the Westward travelling surge, WTS)
  2. north-south (N-S) aligned auroras
  3. eastward propagating auroras

The poleward expansion of the bulge is not continuous, but stepwise with several intensifications. Furthermore, new arcs form at the poleward boundary of the surge (Yahnin et al., 1990; see substorm finestructure). Comparison of the precipitation particle spectra at the substorm onset and within the surge has shown that the onset precipitation is more energetic (20-30 keV vs. 10 keV; Olsson et al., 1996) than is typical for discrete auroras.

The north-south auroras were originally observed by Rostoker et al. (1987) from the Viking images. Recently (Fukunishi and Takahashi, ICS-4 meeting, 1998) proposed that the N-S auroral region is channel through which plasma sheet plasma is injected into the ring current. (Are these connected to the "azimuthally spaced auroral forms (AAF)" of Elphinstone et al. (1995)?)


See also: