HMB Variations Measured by SuperDARN During the Extremely Radial IMFs: Is the Coupling Function Applicable in Radial IMF?. Issue 2 (29th January 2022)

• Record Type: Journal Article
• Authors: Wang, Zhiwei; Lu, Jianyong; Hu, Hongqiao; Liu, Jianjun; Hu, Zejun; Wang, Ming; Li, Bin; Chen, Xiangcai; Wu, Yewen; Zhang, Hua; Guan, Haiyan
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-29
• Subjects: radial IMF -- Heppner‐Maynard boundary -- IMF Bx -- S‐M‐I coupling; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA029589
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: The effect of radial interplanetary magnetic field (IMF B X ${\mathrm{B}}_{\mathrm{X}}$ ) 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° 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. Plain Language Summary: The magnetic reconnection between solar wind and magnetosphere can drive a convection structure in the high‐latitude ionosphere. Heppner and Maynard developed a method to calculate the equatorward boundary of the ionospheric convection, which is called Heppner‐Maynard Boundary (HMB). HMB could be used as the monitor of the interaction process between the solar wind and geomagnetic field. In this paper, we report the effects of the radial (sunward or antisunward) interplanetary magnetic field (IMF) on HMB in the northern hemisphere. For normal cases, the Bz component o IMF is the most important. But our statistical results indicate that there is a decreasing trend between the strength of IMF Bx and HMB latitudes at midnight. No significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. Moreover, the results provide observational evidence the Bx term should be considered in the coupling function during radial IMF. Key Points: The HMB midnight latitude has a negative trend on the IMF Bx, especially for negative IMF Bx The averaged values of HMB during radial IMFs are larger than those for substorm periods and a long‐term period of 17 years The traditional coupling function may not be applicable in the case of radial IMF