Seasonal variation in the latitude of geomagnetically conjugate points observed with imaging riometers in the auroral zone

Abstract

Seasonal variation in the latitude of geomagnetically conjugate point was obtained statistically using the imaging riometers at Syowa-Tjornes conjugate pair stations. The conjugate point was evaluated experimentally from the relative location of band-type CNA (Cosmic Noise Absorption) appearing at the conjugate stations, associated with auroral breakups in premidnight hours. The statistical analyses were made for poleward moving CNA-bands observed during February 1992 to December 1993. Following characteristics are obtained. (1) The poleward expanding speed of the CNA-bands differs significantly between the two hemispheres : it is faster at Syowa than Tjornes in Iceland by 15% on average. This difference may be attributable to an interhemispherical asymmetry of the magnetic flux tube radii mapped to the ionospheres, which is caused by the differences in the geomagnetic field strength and dip angles between the both stations. (2) The conjugate point of Syowa mapped to the northern hemisphere moves most equatorward in June and most poleward in December from Tjornes. The latitudinal range of the seasonal displacement is about 200km for the average geomagnetic activities of Kp=4. (3) The seasonal displacement agrees well with the calculations of the conjugate points using the combination of IGRF 1995 and Tsyganenko 1989 geomagnetic field models.