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Ann. Geophys., 20, 1631-1645, 2002
www.ann-geophys.net/20/1631/2002/
© European Geosciences Union 2002
SuperDARN radar HF propagation and absorption response to the substorm expansion phase
J. K. Gauld1, T. K. Yeoman1, J. A. Davies1, S. E. Milan1, and F. Honary2
1Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
2Department of Communication Systems, University of Lancaster, Bailrigg, Lancaster, LA1 4YR, UK
Correspondence to: T. K. Yeoman
(tim.yeoman@ion.le.ac.uk)
Abstract. Coherent scatter HF ionospheric radar
systems such as SuperDARN offer a powerful experimental technique for the
investigation of the magnetospheric substorm. However, a common signature in
the early expansion phase is a loss of HF backscatter, which has limited the
utility of the radar systems in substorm research. Such data loss has generally
been attributed to either HF absorption in the D-region ionosphere, or the
consequence of regions of very low ionospheric electric field. Here
observations from a well-instrumented isolated substorm which resulted in such
a characteristic HF radar data loss are examined to explore the impact of the
substorm expansion phase on the HF radar system. The radar response from the
SuperDARN Hankasalmi system is interpreted in the context of data from the EIS-CAT
incoherent scatter radar systems and the IRIS Riometer at Kilpisjarvi, along
with calculations of HF absorption for both IRIS and Hankasalmi and ray-tracing
simulations. Such a study offers an explanation of the physical mechanisms
behind the HF radar data loss phenomenon. It is found that, at least for the
case study presented, the major cause of data loss is not HF absorption, but
changes in HF propagation conditions. These result in the loss of many
propagation paths for radar backscatter, but also the creation of some new,
viable propagation paths. The implications for the use of the characteristics
of the data loss as a diagnostic of the substorm process, HF communications
channels, and possible radar operational strategies which might mitigate the
level of HF radar data loss, are discussed.
Key words. Ionosphere (ionosphere-magnetosphere
interactions). Magnetospheric physics (storms and substorms). Radio science
(radio wave propagation)
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