Presentation #202.07 in the session Observing and Inferring Solar Chromospheric and Coronal Magnetic Fields I.
The past decade has seen a revolution in our understanding of the coronal magnetic field in that it is now widely accepted that the field’s topology is highly complex, and that this complexity is essential for understanding solar activity. The so-called S-Web model for the slow wind relies explicitly on topological complexity to account for the observations. In closed field regions the topology is more complex and likely to play a critical role, as well. A major problem, however, is that primary topological quantities such as separatrices are difficult to determine directly even in numerical models, because they are non-local quantities. We propose that observations of characteristic plasma flows, determined from UV – XUV spectroscopic data, can be used to determine the network of coronal magnetic separatrices (the S-Web) of closed field regions at the photosphere. The key point is that a magnetic flux tube very near a separatrix must have extremely strong (singular at the separatrix itself) cross-sectional area variation along its coronal length, which leads to large characteristic plasma flows along the flux tube. We use the HYDRAD 1D loop model code to calculate the equilibrium state of a flux tube (a coronal loop) near the fan-spine separatrix of a simple embedded bipole topology. The coronal loop shows extreme area expansion at its closest approach to a null point in the corona. We find that strong upflows result near the inner spine and downflows near the fan. We discuss the implications of our results for planning and interpreting high-resolution spectroscopic observations from upcoming missions such as EUVST and MUSE.
This work was supported by the NASA Living With a Star Program.