Studies of conjugate plasma convection in the vicinity of the Harang discontinuity

Two case studies are presented of the large-scale nightside ionospheric plasma convection observed simultaneously in the two polar regions using HF backscatter radars. The case studies occur during geomagnetically quiet conditions, and for one of the two periods the interplanetary magnetic field (IMF) is known to be northwards. For both cases plasma generally convects westward in the evening and eastward in the morning. The times at which the nightside flow reversal (the Harang discontinuity) occurs is observed to differ by several hours between the two hemispheres, and between the two study periods. For the case where IMF data are available, the nightside plasma flow is shown to respond to a step change in the IMF y-component (becoming less negative), with a time delay of about 25 min. In the southern hemisphere, the flow reversal appeared simply to be shifted to later magnetic local times, whilst for the northern hemisphere the evening westward flow was disrupted by the occurrence of eastward flow ahead of the Harang discontinuity. The general features of the observed convection are not consistent with the Heppner and Maynard (1987, J. geophys. Res.92, 4467) convection models, which exclude any significant association between the Harang discontinuity and the state of the IMF. The results are more in agreement with the MHD simulations for northward IMF of Oginoet al. (1985, J. geophys. Res.90, 10835). The latter invoke nightside reconnection cells whose behaviour is dependent upon the IMF y-component in the same sense as for our observations.