Interferometric radar observations of filamented structures due to plasma instabilities and their relation to dynamic auroral rays.
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https://hdl.handle.net/10037/546Date
2004-04-02Type
Journal articlePeer reviewed
Tidsskriftartikkel
Author
Grydeland, Tom; Hagfors, T.; Trondsen, T.S.; Blixt, Erik Mårten; Løvhaug, Unni Pia; La Hoz, CesarAbstract
Several explanations have been proposed for
Naturally Enhanced ion-acoustic Echoes observed at midand
high-latitude Incoherent Scatter observatories. A decisive
measure for distinguishing between these explanations
is whether or not simultaneously observed up- and downshifted
enhancement occur simultaneously, or if they are the
result of temporal and/or spatial averaging.
The EISCAT Svalbard Radar has two antennas in the same
radar system, which can be used as an interferometer when
pointed parallel. In observations from 17 January 2002, between
06:46:10 and 06:46:30 UT, we used this possibility,
in combination with direct sampling of the received signals,
to yield measurements of “naturally enhanced ion-acoustic
echoes” with sufficiently high resolution to resolve such averaging,
if any. For the first time, radar interferometry has
been employed to estimate the sizes of coherent structures.
The observations were coordinated with an image intensified
video camera with a narrow field of view. Together, this
forms the initial study on the causal relationships between
enhanced echoes and fine structure in the auroral activity on
sub-kilometer, sub-second scales.
The results confirm that the enhanced echoes originate
from very localised regions ( 300m perpendicular to the
magnetic field at 500 km altitude) with varying range distribution,
and with high time variability ( 200 ms). The
corresponding increase in scattering cross section, up to
50 dB above incoherent scattering, eliminates theoretical explanations
based on marginal stability. The simultaneously
observed up- and down-shifted enhanced shoulders, when
caused by sufficiently narrow structures to be detected by
the interferometer technique, originate predominantly from
the same volume. These results have significant impact on
theories attempting to explain the enhancements, in particular
it is found that the ion-electron two-stream mechanism
favoured by many authors is an unlikely candidate to explain
the observations. The video data has helped establish a clear correlation between the enhanced echoes and auroral activity,
on sub-second time scales, showing a threshold connection
between the auroral intensity and the triggering of the
radar enhancements. It appears that the up- and down-shifted
enhanced echoes correlate with fine auroral structures in different
ways.
Publisher
European Geosciences UnionSeries
Annales Geophysicae 22(2004) pp 1115–1132Metadata
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