The Magnetic Skeleton of the Solar Corona Over Several Solar Rotations: Features, Analysis, and Community Availability

Abstract:

The magnetic field in the solar corona is thought to be the main driver for solar eruptive events, such as flares and coronal mass ejections. The coronal magnetic field is therefore important to study, but it is difficult to measure directly. Usually, it is studied through extrapolations based on photospheric magnetograms. As the corona is thought to be mostly in a state of equilibrium, equations of low-beta equilibria are often used in order to study the structure of the field, or to estimate the magnetic energy. One of the complications that arise from this approach is that the solar photosphere itself is not a low-beta equilibrium.

Images of the solar corona in extreme ultraviolet (EUV) do not directly measure the magnetic field; however, they do reveal structures from which information about magnetic field can be inferred. For example, coronal loops are thought to trace out magnetic field lines, coronal cavities are bounded by magnetic surfaces, coronal holes are areas of magnetic flux that is open to the heliosphere, and plasma flows are also thought to follow lines of magnetic field. In other wavelengths, coronal spectropolarimetry (SP) can provide us with proxies for magnetic field strength and reveal plasma flows along the line of sight, off the limb. The EUV images and SP data are frequently used to validate magnetic field models. Additionally, new models are emerging which can use these data directly as additional constraints.

We aggregate available relevant features seen in EUV and SP data for several solar rotations. We apply existing techniques to infer 3D constraints on the magnetic field from these data. The result is an interactive 3D model based on these constraints for a full rotation, or a "magnetic skeleton". It is modular, so individual constraints can be easily added, or only selected constraints can be used. The features could be exported in either graphical or numerical form. The possible uses of our approach include validation of magnetic models that are based on extrapolations alone. Some models allow for using additional coronal constraints directly. These 'skeletons' can also be used in non-magnetic-modeling applications, as a simple, interactive reference for features seen in a given rotation. We make the models available to the community and show how to obtain and use them.