A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements

Abstract

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 latituCorrespondence to: G. Chisham (G.Chisham@bas.ac.uk) dinal 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.

Measuring the dayside reconnection rate during an interval of due northward interplanetary magnetic field

“…6), and varies little. During southward IMF conditions the equatorward edge of the region of high Doppler spectral width backscatter in the cusp correlates well with the equatorward edge of cusp particle precipitation and so is often used as a proxy for the ionospheric projection of the merging line (which equates to the polar cap boundary under those conditions) Chisham and Freeman, 2003). On many occasions, the equatorward edge of the high spectral width region also marks the equatorward edge of the observed backscatter.…”

“…Before the fitting, the SuperDARN line-of-sight velocity data have been preprocessed using the methods of Chisham and Pinnock (2002) to remove all non-F -region backscatter. The ionospheric convection during this interval, as measured by the SuperDARN radar network, was first studied by Huang et al (2000b), although not using the same data processing that we employ here.…”

A statistical comparison of SuperDARN spectral width boundaries and DMSP particle precipitation boundaries in the morning sector ionosphere

“…These techniques involve choosing a spectral width threshold value above which the spectral width values are more likely to originate from the distribution of spectral width values typically found in the cusp, and developing an algorithm that searches poleward along a radar beam until this threshold is exceeded. Chisham and Freeman (2003) showed that this technique can be inaccurate in its basic form as the probability distributions of the spectral width values poleward and equatorward of the SWB are typically broad and have considerable overlap (see also Freeman and Chisham, 2004). Chisham and Freeman (2003) showed that the inclusion of additional rules in the threshold algorithm, such as spatially and temporally median filtering the spectral width data, increased the accuracy of the estimation of the SWB location.…”

“…Chisham and Freeman (2003) showed that this technique can be inaccurate in its basic form as the probability distributions of the spectral width values poleward and equatorward of the SWB are typically broad and have considerable overlap (see also Freeman and Chisham, 2004). Chisham and Freeman (2003) showed that the inclusion of additional rules in the threshold algorithm, such as spatially and temporally median filtering the spectral width data, increased the accuracy of the estimation of the SWB location. This led to the development of the C−F threshold technique, described in detail in , who also showed that the technique objectively identified SWBs at all MLTs.…”

Space- and ground-based investigations of solar wind–magnetosphere–ionosphere coupling

“…de la Beaujardiere et al, 1991;Blanchard et al, 2001), and identification of the spectral width boundary observed by coherent radars (e.g. Chisham and Freeman, 2003). Ideally, a combination of some or all of these measurement techniques can be used to improve accuracy (Milan et al, 2003).…”