11:15 AM - 11:30 AM
[PEM11-12] What makes the solar coronal hole dark? Insights from numerical simulations on the open flux problem
Keywords:Solar Coronal Hole, Interplanetary Magnetic Field, Solar Coronal Heating, Numerical Simulation
The "open flux problem," inconsistency between the two estimates of the interplanetary magnetic flux, by magnetic flux inside coronal holes and by in-situ satellite observations, has recently been a topic of active debate. The coronal hole is one of the most striking features in X-ray/EUV images of the Sun. Usually, the coronal hole is assumed to be dark because it is magnetically open. The coronal plasma cannot be hot enough as most energy input is spent to accelerate the solar wind. This study suggests a counter-evidence to this assumption using a series of three-dimensional radiative magnetohydrodynamic simulations of magnetically-open solar corona. The numerical box is spatially local, 50 (horizontal) x 50 (horizontal) x 210 (vertical) Mm^3, from the upper solar convection zone (10 Mm below the photosphere) to the corona (200 Mm above the photosphere). Iijima et al. (2023, ApJL, 951, L47) describe more details of our numerical model. We controlled simulations by changing the open magnetic flux (strictly conserved in our horizontal periodic box) and the magnitude of the small-scale dynamo (by the thermal convection in the Sun). Most coronal wave energy passes through our top boundary, mimicking the magnetically open solar corona. If we follow the classical assumption, the coronal brightness from all simulations should be dark. However, we found that the simulated corona can be bright, like a quiet Sun (outside the coronal hole), depending on the magnetic configuration in the photosphere. The result implies a possibility that a major part of open magnetic flux is supplied from regions other than coronal holes. We will present a detailed analysis and discuss the physical origin of coronal holes and the potential impact on the open flux problem.