A study of substorm-associated nightside spike events in auroral absorption using imaging riometers at South Pole and Kilpisjärvi

Abstract The short-duration ‘spike’ events which are a common feature of substorm-associated auroral radio absorption in the midnight sector are observed both at Kilpisjarvi (69.05 °N, 20.79 °E, L = 5.9) in the auroral zone and at the much higher latitude of the South Pole (90.00 °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 movements. It is found that the spike events are remarkably similar at the two latitudes studied. They are usually elliptical in shape with the major axis generally along rather than across the L shells; median dimensions are 167 km by 74 km at the South Pole, and 190 km by 80 km at Kilpisjarvi. It may be significant that in each case the perturbed region of the ionosphere maps to an almost circular region at the magnetospheric equatorial plane, and that the total magnetic flux included within the event is similar at each latitude. The velocities of the events are variable in the range of several 100 m/s to 2 or 3 km/s; the direction of motion tends to be poleward at the beginning of a precipitation event, and is often equatorward towards the end. The east-west component 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 investigated, and it is shown that a typical wide-beam riometer underestimates the absorption by a factor of 2 to 3 for spikes occurring at the onset of a substorm. This does not necessarily apply to the later phases of the substorm when the precipitation is likely to be more widespread. The slowly moving absorption bay which may precede the intense precipitation at substorm onset has been detected over Kilpisjarvi. It is identified as an arc-like feature extending east-west across the entire field of view, but containing structure. Its typical north-south extent is 60–100 km, and its equatorward speed is a little over 100m/s.