ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-21-1553-2003 Interhemispheric comparison of spectral width boundary as observed by SuperDARN radars Hosokawa K. 1 Woodfield E. E. 2 Lester M. 2 Milan S. E. 2 Sato N. 3 Yukimatu A. S. 3 Iyemori T. 4 Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan Department of Physics and Astronomy, University of Leicester, Leicester, UK National Institute of Polar Research, Tokyo, Japan Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, Kyoto, Japan 31 07 2003 21 7 1553 1565 Copyright: © 2003 K. Hosokawa et al. 2003 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/21/1553/2003/angeo-21-1553-2003.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/21/1553/2003/angeo-21-1553-2003.pdf

Previous studies have shown that dayside equatorward edge of coherent HF radar backscatter having broad Doppler spectral width is coincident with the equatorward edge of the cusp particle precipitation. This enables the boundary between broad and narrow spectral width backscatters (spectral width boundary) in the dayside magnetic local time sector to be used as a proxy for the open/closed field line boundary. The present case study employs magnetically conjugate SuperDARN coherent HF radars to make an inter-hemispheric comparison of the location and variation of the spectral width boundaries. Agreement between the magnetic latitudes of the boundaries in both hemispheres is remarkable. Correlation coefficients between the latitudes of the boundaries are larger than 0.70. Temporal variation of the spectral width boundary follows the same equatorward trend in both hemispheres. This is consistent with the accumulation of open flux in the polar cap by dayside low-latitude magnetopause reconnection, expected when IMF B<sub>z</sub> is negative. Boundaries in both hemispheres also exhibit short-lived poleward motions superposed on the general equator-ward trend, which follows the onset of substorm expansion phase and a temporary northward excursion of IMF B<sub>z</sub> during substorm recovery phase. There is an interhemispheric difference in response time to the substorm occurrence between two hemispheres. The spectral width boundary in the Southern Hemisphere starts to move poleward 10 min earlier than that in the Northern Hemisphere. We discuss this difference in terms of interhemispheric asymmetry of the substorm breakup region in the longitudinal direction associated with the effect of IMF B<sub>y</sub>.<br><br><b>Key words. </b>Ionosphere (ionosphere-magnetosphere interactions; plasma convection) – Magnetospheric physics (magnetopause, cusp, boundary layers)