Analysis of substorm expansion and surge development

Abstract

We have used simultaneous magnetogram, radar, and Viking imager data from evening local time to study several substorm expansion phase onsets and an expansion phase auroral surge that formed near the radar field of view. At the onsets, auroral brightenings developed within the region of the electric field gradient of the Harang discontinuity, and if not already present, a westward electrojet began poleward of the eastward electrojet. Immediately following onsets, the poleward boundary of the westward electrojet moved poleward along with the poleward boundary of the aurora; however, no consistent motion of the Harang discontinuity was found in the ionosphere. The auroral surge formed along the poleward boundary of the westward electrojet (and of the aurora). We argue that the distortion of the poleward boundary of the aurora associated with the surge was a manifestation of a distortion of the separatrix between open and closed magnetic field lines, the distortion being caused by the field‐aligned currents within the head of the surge. We suggest that rapid surge development may result from a tapping of the westward electrojet current at the surge head. We also suggest that the separatrix distortion could lead to the escape of trapped plasma sheet particles along open field lines in the tail.

Magnetospheric solitary structure maintained by 3000 km/s ions as a cause of westward moving auroral bulge at 19 MLT

“…The pre-midnight bulge part stayed equatorward of this poleward arc, i.e., stayed far inside the separatrix. Thus, this bulge belongs to the type reported by Murphree and Cogger (1992), and is not the type proposed by Lyons et al (1990) or Ober et al (2001). The double-arc structure is seen in the entire evening sector, one at around 60 • geomagnetic latitude (GMLat), and another at 70 • ∼75 • GMLat.…”

“…Among those, the magnetospheric signature of westward propagating auroral bulge in the evening sector has not been studied much compared to the other magnetospheric signature of the substorm except Roux et al (1991). Most of the past observations of the westward moving auroral bulge in the evening sector (Opgenoorth et al, 1989;Lyons et al, 1990;Fujii et al, 1994;Weimer et al, 1994;Sanchez et al, 1996;Gjerloev et al, 2007) are limited to low altitudes and not near the equatorial plane. Inversely, most of the magnetospheric observation of the auroral bulge is limited to midnight sectors where phenomena is convoluted by many effects such as Harang discontinuity (e.g., Lyons et al, 2005).…”

“…In the model, the westward motion of evening aurora has been considered as the result either by ionospheric Hall current and conductivity gradient in the magnetosphere-ionosphere coupled electromagnetic system (Kan and Sun, 1985;Lyons et al, 1990), or result of magnetic drift (summation of gradient-B drift and curvature drift) of plasma sheet ions (Vasyliunas, 1970;Wolf, 1970). The magnetosphere plays relatively a passive role in the former models, whereas the magnetospheric drift plays active role in the latter models.…”

Estimate of Global and Regional Distributions of Ionospheric Electrodynamic Quantities from EISCAT Radar and Ground Magnetometer Data.

“…For example, Robinson and V ondark (1990) showed that the amoral surge is formed within a region of the northward electric field, i.e., south of the convection reversal regions, and that the northward component is reduced within the surge. On the contrary, Lyons et al (1990) argued that the auroral brightening occurs within the region several degrees poleward of the electric field reversal. Both of them did rely on radar measurements as well as on ground-based and satellite-based observations of amoral images which provide simultaneous data of the electric field and particle precipitations, but it was rather difficult to differentiate spatial and temporal changes in individual cases of radar observations.…”

An empirical model of the substorm current wedge

“…Vasilyev et al [ 1986] were first to relate the magnetic effects of the SCW to the formation of the auroral bulge. They mapped equatorial cross sections of the near tail, running from dawn to dusk, by using a background model field with added disturbance field from a simple line-current wedge model, and found a bulge-like deformation of the ionospheric projection, caused by the twisting effect of the Birkeland currents upon the magnetic field line [see also Kaufmann et al, 1990;Lyons et al, 1990Lyons et al, , 1991. Hilmer and Voigt [1995] suggested modeling the magnetic effects of the SCW by adding to the background magnetic field model a disturbance field from an eastward current sheet with a limited extension in the dawn-dusk and tailwardsunward directions.…”