Abrupt changes in the solar wind dynamic pressure can greatly affect the Earth’s magnetosphere‐ionosphere system. We present an ionospheric flow vortex in the morning side during the sudden decrease in the solar wind dynamic pressure. The flow vortex was clearly observed by both the Hankasalmi radar and the azimuthal scan mode of the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR). The flow vortex was first seen in the eastern field of view (FOV) of the Hankasalmi radar, and then propagated poleward and westward into the FOV of the ESR. During the passage of the flow vortex, a gradual decrease of electron density was observed by the field-aligned ESR 42 m antenna. When the equatorward directed ionospheric flow reached the ESR site, weak and visible increases in the electron density and electron temperature were observed. This impact was likely caused by soft electron precipitation associated with the clockwise flow vortex and upward field-aligned current. The azimuthal scan mode of the ESR 32 m radar at low elevation angle (30°) allowed us to measure key ionospheric parameters over a larger area (6° in latitude and 120° in azimuthal angle). The latitudinal scan of the electron temperature was used to derive the equatorward auroral boundary, which shows that the flow vortex was located in the subauroral region. We further demonstrated that it is possible to study the weak increase of electron density by using GPS total electron content (TEC) data. A minor TEC increase was observed near the center of the flow vortex.

The Heppner-Maynard Boundary (HMB) represents the equatorward extent of the ionospheric convection pattern and can be used as a proxy for the low latitude of the auroral oval. We present a statistical study of the radial interplanetary magnetic field (IMF) effects on the HMB midnight latitude calculated from SuperDARN measurements between January 2002 and December 2017. We found the average values of HMB midnight latitude during both sunward and anti-sunward radial IMF are higher than 65.5º. There is a negative correlation between the magnitude of Bx and HMB midnight latitude, although this effect is not obvious. Moreover, the seasonal variation of Bx-HMB correlation coefficients is different with the existence of single lobe reconnection. At the anti-sunward radial period, the correlation coefficient is up to 0.54 in wintertime. It would be caused by the enhanced lobe reconnection rates, which related to the special configuration between the solar-wind and magnetopause. This is the first long-term statistical study focused on HMB during radial IMF conditions in the context of solar wind-magnetosphere-ionosphere coupling. The results suggest that the effect of IMF Bx should not be ignored in the northern hemisphere wintertime especially during the anti-sunward radial IMF conditions.