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Ann. Geophys., 21, 983-996, 2003
www.ann-geophys.net/21/983/2003/
© European Geosciences Union 2003
A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements
G. Chisham and M. P. Freeman
British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
Abstract. Accurately measuring the
location and motion of the polar cap boundary (PCB) in the high-latitude
ionosphere can be crucial for studies concerned with the dynamics of the polar
cap, e.g. the measurement of reconnection rates. The Doppler spectral width
characteristics of backscatter received by the SuperDARN HF radars have been
previously used for locating and tracking the PCB in the cusp region. The
boundary is generally observed in meridional beams of the SuperDARN radars and
appears as a distinct change between low spectral width values observed
equatorward of the cusp region, and high, but variable spectral width values
observed within the cusp region. To identify the spectral width boundary (SWB)
between these two regions, a simple algorithm employing a spectral width
threshold has often been applied to the data. However, there is not, as yet, a
standard algorithm, or spectral width threshold, which is universally applied.
Nor has there been any rigorous assessment of the accuracy of this method of
boundary determination. This study applies a series of threshold algorithms to
a simulated cusp-region spectral width data set, to assess the accuracy of
different algorithms. This shows that simple threshold algorithms correctly
identify the boundary location in, at the most, 50% of the cases and that the
average boundary error is at least ~ 1–2 range gates (~ 1° latitude). It
transpires that spatial and temporal smoothing of the spectral width data (e.g.
by median filtering), before application of a threshold algorithm can increase
the boundary determination accuracy to over 95% and the average boundary error
to much less than a range gate. However, this is sometimes at the cost of
temporal resolution in the motion of the boundary location. The algorithms are
also applied to a year’s worth of spectral width data from the cusp
ionosphere, measured by the Halley SuperDARN radar in Antarctica. This analysis
highlights the increased accuracy of the enhanced boundary determination
algorithm in the cusp region. Away from the cusp, the resulting SWB locations
are often dependent on the choice of threshold. This suggests that there is not
a sharp latitudinal SWB in regions of the dayside ionosphere away from the
cusp, but that there is a shallower latitudinal gradient in spectral width near
the boundary location.
Key words. Ionosphere (instruments
and techniques) – Magnetospheric physics (magnetopause, cusp and boundary
layers; magnetosphere-ionosphere interactions)
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