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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
There are three characteristic types of discrete aurora
within a bulge (Nakamura et al., 1993):
- a surge (westward termination of the bulge, also known as the Westward travelling surge,
- north-south (N-S) aligned auroras
- 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)?)
- Akasofu, S.-I., D. S. Kimball, and C.-I. Meng, Dynamics of the aurora, II, Westward
traveling surges, J. Atmos. Terr. Phys., 27, 173, 1965.
- Akasofu, S.-I., C.-I. Meng, and D. S. Kimball, Dynamics of the aurora, IV, Polar
magnetic substorm and westward traveling surges, J. Atmos. Terr. Phys., 28, 489, 1966.
- Elphinstone, R. D., D. J. Hearn, L. L. Cogger, J. S. Murphree, H. Singer, V. Sergeev, K.
Mursula, D. M. Klumpar, G. D. Reeves, M. Johnson, S. Ohtani, T. A. Potemra, I. Sandahl, E.
Nielsen, M. Persson, H. Opgenoorth, P. T. Newell, and Y. I. Feldstein, Observations in the
vicinity of substorm onset: Implications for the substorm process, J. Geophys. Res., 100,
- Nakamura, R., T. Oguti, T. Yamamoto, and S. Kokubun, Equatorward and poleward expansion
of the auroras during auroral substorms, J. Geophys. Res., 98, 5743-5759, 1993.
- Olsson, A. M. A. L. Persson, H. J. Opgenoorth, and S. Kirkwood,
Particle precipitation in auroral breakups and westward traveling surges, J. Geophys. Res., 101, 24661-24673, 1996.
- Rostoker, G., A. T. Lui, C. D. Anger, and J. S. Murphree, North-south structures
in the midnight sector auroras as viewed by the Viking imager, Geophys. Res. Lett.,
14, 407-410, 1987.
- Yahnin, A. G., T. Bösinger, J. Kangas, and R. D. Belian, Some implications on substorm
dynamics inferred from correlations between multiple flux peaks of drifting high-energy proton
clouds and ground observations, Ann. Geophysicae, 8, 327-336, 1990.