Sammendrag
The generation mechanism for so called naturally enhanced ion-acoustic echoes, observed at mid- and high-latitude Incoherent Scatter observatories, is still debated. One important issue in this regard is how these enhancements are related to auroral activity.
All events, where the EISCAT Svalbard Radar (ESR) observed enhanced ion-acoustic echoes simultaneous as high-resolution narrow field-of-view auroral imagers were making observations, have been collected and studied.
Characteristic for all the events is the appearance of very dynamic rayed aurora, and some of the intrinsic features of these auroral displays are identified. Several of these identified features are directly related to a presence of low energy (10 - 100 eV) precipitating electrons in addition to the higher energy population producing most of the associ- ated light. The low energy contribution is vital for the for- mation of the ion-acoustic echoes. We argue that this type of aurora is sufficient for the generation of naturally enhanced ion-acoustic echoes.
In one event two imagers was used to observe the auroral rays simultaneously, one from the radar site and one 7 km away. The data from those imagers shows that the auroral rays and the strong back-scattering filaments (where the enhanced echoes are produced) are located on the same field line, which is in contrast to earlier statements in the litterature that they should be separated.
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