Jk. Hargreaves et al., A STUDY OF SUBSTORM-ASSOCIATED NIGHTSIDE SPIKE EVENTS IN AURORAL ABSORPTION USING IMAGING RIOMETERS AT SOUTH-POLE AND KILPISJARVI, Journal of atmospheric and solar-terrestrial physics, 59(8), 1997, pp. 853-872
The short-duration 'spike' events which are a common feature of substo
rm-associated auroral radio absorption in the midnight sector are obse
rved both at Kilpisjarvi (69.05 degrees N, 20.79 degrees E, L = 5.9) i
n the auroral zone and at the much higher latitude of the South Pole (
90.00 degrees S, L = 13). These events have been known for many years,
but it is only recently, since the evolution of the imaging riometer,
that it has become possible to measure their size, shape and movement
s. It is found that the spike events are remarkably similar at the two
latitudes studied. They are usually elliptical in shape with the majo
r axis generally along rather than across the L shells; median dimensi
ons are 167 km by 74 km at the South Pole, and 190 km by 80 km at Kilp
isjarvi. It may be significant that in each case the perturbed region
of the ionosphere maps to an almost circular region at the magnetosphe
ric equatorial plane, and that the total magnetic flux included within
the event is similar at each latitude. The velocities of the events a
re variable in the range of several 100 m/s to 2 or 3 km/s; the direct
ion of motion tends to be poleward at the beginning of a precipitation
event, and is often equatorward towards the end. The east-west compon
ent has not shown any consistency of direction at Kilpisjarvi, though
its magnitude may be as large as the north-south one. It is shown that
the true duration of a spike event is only 1-2 min. The effect of the
spike event's limited extent on wide-beam measurements is investigate
d, and it is shown that a typical wide-beam riometer underestimates th
e absorption by a factor of 2 to 3 for spikes occurring at the onset o
f a substorm. This does not necessarily apply to the later phases of t
he substorm when the precipitation is likely to be more widespread. Th
e slowly moving absorption bay which may precede the intense precipita
tion at substorm onset has been detected over Kilpisjarvi. It is ident
ified as an are-like feature extending east-west across the entire fie
ld of view, but containing structure. Its typical north-south extent i
s 60-100 km, and its equatorward speed is a little over 100 m/s. (C) 1
997 Elsevier Science Ltd.