Interhemispheric conjugacy of auroral poleward expansion observed by conjugate imaging riometers at 67゜ and 75゜- 77゜ invariant latitude

• Main Authors: Hisao Yamagishi, Yuiti Fujita, Natsuo Sato, Masanori Nishino, Peter Stauning, Ruiyuan Liu, Th. Saemundsson
• Format: Report
• Language: English
• Published: National Institute of Polar Research/Nagano JRC Co./National Institute of Polar Research/Solar-Terrestrial Environment Laboratory, Nagoya University/Danish Meteorological Institute/Polar Research Institute of China/Science Institute, University of Iceland 2000
• Subjects: Danmarkshavn; Greenland; Longyearbyen; Svalbard; Syowa Station; Zhongshan; Zhongshan Station; Antarc*; Antarctica; Iceland; Advances in polar upper atmosphere research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=6300; http://id.nii.ac.jp/1291/00006300/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=6300&item_no=1&attribute_id=18&file_no=1

Interhemispheric conjugacy of auroral poleward expansion was studied using conjugate imaging riometers at ～67° inv. (invariant latitude) (Syowa Station, Antarctica, and Tjomes, Iceland) and at 75-77° inv. (Zhongshan Station, Antarctica; Longyearbyen, Svalbard and Danmarkshavn, Eastern Greenland). Ninety poleward expansion events at ～67° inv. observed in 1992-1993 were analyzed. The expansion at this latitude was characterized by a continuous poleward motion of cosmic noise absorption (CNA) bands with a velocity distribution that peaked at 1-1.5 km/s at a polar ionospheric altitude of ～90 km. The velocity distribution has another apparent peak at 3-5 km/s when measurements are made along the meridian. However, ～60% of the events forming this peak are overestimated because the motion of the CNA region's front was observed while the front was tilted away from the east-west direction. The difference in expansion velocities between conjugate stations remained within ±30% for half of the events. The time lag between the passage of a CNA band over conjugate stations was typically between 30-60 s. The product of this time lag and the poleward expanding velocity provides the location of the actual conjugate point. For 87% of the total number of events, the conjugate point for Syowa was located within ±200 km of Tjornes. The poleward expansion observed at 75°-77° inv. was characterized by a stepwise progression of CNA bands to higher latitudes. In other words, new CNA bands formed 50-180 km poleward of the preceding CNA band. The appearance of a new CNA band was often associated with an equatorward motion of the preceding CNA band. Equatorward moving CNA bands are thought to be located in a closed fieldline. The persistence of this band for several minutes was explained by pitch angle scattering of the trapped electrons as a result of a very stretched fieldline configuration in which the fieldline curvature was comparable to the Larmor radius of the trapped electrons. Although the conjugate pair used at these latitudes ...