| Several explanations have been proposed for Naturally Enhanced
ion-acoustic Echoes observed at mid- and high-latitude Incoherent Scatter
observatories. A decisive measure for distinguishing between these
explanations is whether or not simultaneously observed up- and down-shifted 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
500km altitude) with varying range distribution, and with high
time variability (≈200ms).
The corresponding increase in scattering cross section, up to 50dB
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.
Key words. Ionosphere (auroral ionosphere; plasma waves
and instabilities) – Radio science (interferometry) |