HMB Variations Measured by SuperDARN During the Extremely Radial IMFs: Is the Coupling Function Applicable in Radial IMF?

The effect of radial interplanetary magnetic field (IMF BX ${\mathrm{B}}_{\mathrm{X}}$) has been ignored in the solar wind-magnetosphere-ionosphere (S-M-I) coupling. We present a statistical study of IMF Bx effects on the Heppner-Maynard Boundary (HMB) midnight latitude calculated from SuperDARN measurements between January 2002 and December 2017. The HMB represents the equatorward extent of the ionospheric convection pattern and can be used as a proxy for the equatorward boundary of the auroral oval. That is, the HMB expands toward lower latitude when the level of S-M-I coupling is enhanced. It is found that the averaged HMB midnight latitudes during both sunward and antisunward radial IMFs are around 64-65 degrees magnetic latitude (MLAT). HMB midnight latitude is negatively correlated with the magnitude of IMF Bx as well as solar wind speed, while no significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. For antisunward radial IMF, the relationship between upstream parameters and HMB is more dependent than in sunward radial direction. As the first long-term statistical study focused on HMB during radial IMF conditions, this work provides observational evidence that the IMF Bx plays an important role in the coupling process and the traditional coupling function could not be applicable in the case of radial IMF.