A substorm observed by EISCAT and other ground-based instruments — evidence for near-Earth substorm initiation

Abstract

We present observations of the development of a substorm in the ionosphere made by the EISCAT radar, optical and magnetic instruments. A typical growth phase started 30 min before the substorm onset with equatorward drifting arcs. The arc which crossed the EISCAT beam drifted equatorward with approximately the same velocity as the ambient plasma and it approached the most equatorward discrete arc. The most equatorward discrete arc (the breakup arc) stopped its drift at L = 5.3. Arcs about 1° of latitude poleward of the breakup arc continued their equatorward motion with no significant changes in intensity after the onset of the substorm, which was timed on the basis of the explosive intensification in the breakup arc. Similarly, no dramatic changes in electric fields or other plasma parameters measured by EISCAT were observed poleward of the breakup arc. The observations indicate that the instability that triggered the substorm onset was localized in the near-Earth magnetotail. A westward travelling surge (WTS) developed in the breakup arc and moved westward with a very high velocity, 13 ± 3 km s −1. Extremely high conductances were measured by EISCAT from the poleward boundary of the poleward expanding bulge with a maximum value of Σ H = 214 S obtained with a time resolution of 0.2 s. The westward electrojet (WEJ) was observed to be latitudinally very inhomogenous and concentrated near conductivity enhancements, especially close to the head of the WTS. The localization of the WEJ close to discrete arcs and consequent motions with the arcs gave the charasteristic spiky appearance of the magnetic X-component in the pre-midnight sector.