ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-27-2623-2009 Excitation thresholds of field-aligned irregularities and associated ionospheric hysteresis at very high latitudes observed using SPEAR-induced HF radar backscatter Wright D. M. 1 Dhillon R. S. 1 Yeoman T. K. 1 Robinson T. R. 1 Thomas E. C. 1 Baddeley L. J. 1 2 Imber S. 1 3 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK Department of Geophysics, The University Centre in Svalbard (UNIS), P.O. Box 156, 9171 Longyearbyen, Norway NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA 01 07 2009 27 7 2623 2631 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/27/2623/2009/angeo-27-2623-2009.html The full text article is available as a PDF file from http://www.ann-geophys.net/27/2623/2009/angeo-27-2623-2009.pdf

On 10 October 2006 the SPEAR high power radar facility was operated in a power-stepping mode where both CUTLASS radars were detecting backscatter from the SPEAR-induced field-aligned irregularities (FAIs). The effective radiated power of SPEAR was varied from 1–10 MW. The aim of the experiment was to investigate the power thresholds for excitation (<I>P<sub>t</sub></I>) and collapse (<I>P<sub>c</sub></I>) of artificially-induced FAIs in the ionosphere over Svalbard. It was demonstrated that FAI could be excited by a SPEAR ERP of only 1 MW, representing only 1/30th of SPEAR's total capability, and that once created the irregularities could be maintained for even lower powers. The experiment also demonstrated that the very high latitude ionosphere exhibits hysteresis, where the down-going part of the power cycle provided a higher density of irregularities than for the equivalent part of the up-going cycle. Although this second result is similar to that observed previously by CUTLASS in conjunction with the Tromsø heater, the same is not true for the equivalent incoherent scatter measurements. The EISCAT Svalbard Radar (ESR) failed to detect any hysteresis in the plasma parameters over Svalbard in stark contract with the measurements made using the Tromsø UHF.

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