At. Aikio et Ku. Kaila, A SUBSTORM OBSERVED BY EISCAT AND OTHER GROUND-BASED INSTRUMENTS - EVIDENCE FOR NEAR-EARTH SUBSTORM INITIATION, Journal of atmospheric and terrestrial physics, 58(1-4), 1996, pp. 5
We present observations of the development of a substorm in the ionosp
here made by the EISCAT radar, optical and magnetic instruments. A typ
ical growth phase started 30 min before the substorm onset with equato
rward drifting arcs. The are which crossed the EISCAT beam drifted equ
atorward with approximately the same velocity as the ambient plasma an
d it approached the most equatorward discrete are. The most equatorwar
d discrete are (the breakup are) stopped its drift at L=5.3. Arcs abou
t 1 degrees of latitude poleward of the breakup are continued their eq
uatorward motion with no significant changes in intensity after the on
set of the substorm, which was limed on the basis of the explosive int
ensification in the breakup are. Similarly, no dramatic changes in ele
ctric fields or other plasma parameters measured by EISCAT were observ
ed poleward of the breakup are. The observations indicate that the ins
tability that triggered the substorm onset was localized in the near-E
arth magnetotail. A westward travelling surge (WTS) developed in the b
reakup are and moved westward with a very high velocity, 13+/-3 km s(-
1). Extremely high conductances were measured by EISCAT from the polew
ard boundary of the poleward expanding bulge with a maximum value of S
igma(H) = 214 S obtained with a time resolution of 0.2 s. The westward
electrojet (WEJ) was observed to be latitudinally very inhomogenous a
nd concentrated near conductivity enhancements, especially close to th
e head of the the WTS, The localization of the WEJ close to discrete a
rcs and consequent motions with the arcs gave the charasteristic spiky
appearance of the magnetic X-component in the pre-midnight sector.