Coronal Hole Boundary Region: Multiple Facets and Connection to the Slow Solar Wind

Abstract It is commonly recognized that a coronal hole (CH) boundary region exists where the slow solar wind (SW) is formed by more than one physical process. The physical properties at the CH boundary region play an important role in dictating the slow SW properties by providing conditions for the relevant processes to work upon. However, how the CH boundary region is defined is still a work under debate. We present investigations on two large CHs that crossed the central meridian during 2015 August 17–21. We use imaging, spectroscopic and photospheric magnetic field observations by Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly, Hinode/Extreme-ultraviolet Imaging Spectrometer, and SDO/Helioseismic and Magnetic Imager to investigate the spatial and temporal variation in coronal emission, electron temperature, and the photospheric magnetic field across the CH and its vicinity to search for how a CH boundary region can be defined. The collective evidence from this study suggests that the CH/open field area is significantly larger than what is commonly identified by CH identification schemes. We find that the coronal line intensity at the peak of its intensity distribution within an area enclosing the CH and nearby quiet Sun/active region structures is a good indicator for the outer CH boundary where the mostly open field area meets with the quiet Sun/active region dominated by close loops. We discuss the role of this CH boundary region in the slow SW formation. The results also provide a solution for alleviating the open flux problem and serve as an empirical constraint for refining global coronal field models.