Toward a Quantitative Comparison of Magnetic Field Extrapolations and Observed Coronal Loops

Abstract

It is widely believed that loops observed in the solar atmosphere trace out magnetic field lines. However, the degree to which magnetic field extrapolations yield field lines that actually do follow loops has yet to be studied systematically. In this paper we apply three different extrapolation techniques -a simple potential model, a NLFF model based on photospheric vector data, and a NLFF model based on forward fitting magnetic sources with vertical currents -to 15 active regions that span a wide range of magnetic conditions. We use a distance metric to assess how well each of these models is able to match field lines to the 12,202 loops traced in coronal images. These distances are typically 1-2 . We also compute the misalignment angle between each traced loop and the local magnetic field vector, and find values of 5-12 • . We find that the NLFF models generally outperform the potential extrapolation on these metrics, although the differences between the different extrapolations are relatively small. The methodology that we employ for this study suggests a number of ways that both the extrapolations and loop identification can be improved.

The FitCoPI Code: Iterative Determination of the 3D Density and Temperature Configuration in the Active-Region Corona

“…Different from an approach linking observed loops and magnetic-field lines, our method is not restricted to locations where loops can be traced with sufficient accuracy (Malanushenko et al, 2012;Warren et al, 2018). It produces results in the entire volume filled by extrapolated field lines.…”

“…On the other hand, the loop line linking can use more robust and well-tested methods for retrieving the plasma conditions in the loop, such as analysing the differential emission measure (Kashyap and Drake, 1998;Hannah and Kontar, 2012). As a combination of the two approaches, one could think of linking some of the extrapolated field lines and traced loops, as in Warren et al (2018), in a first step. The sample points along this field lines can obtain their monitored temperature and density directly derived from the observations.…”

“…In this way, the field line provides the 3D-structure of the loop, while the physical conditions can be retrieved from the observed loop, for example via the analysis of the differential emission measure (Kashyap and Drake, 1998;Hannah and Kontar, 2012). Warren et al (2018) derived a method to tie field lines extrapolated from photospheric vector magnetograms to traced loops. Malanushenko et al (2012) used observable loops for determining the coronal field lines.…”

Assessing the Quality of Models of the Ambient Solar Wind

“…They found the error in total energy to be of the same order as the estimated total free energy. This suggests that to be useful in supporting models of the ambient solar wind, force‐free models should be supported by imaging analysis of active region loops, as suggested by Peter et al () and tested by Chifu et al (; see also; Warren et al, ), or should be extended to include both gas pressure gradients and gravity as in magnetohydrostatic equilibrium models (Zhu & Wiegelmann, ).…”

Spectroscopic detection of coronal plasma flows in loops undergoing thermal non-equilibrium cycles

“…The angle between tho loop and the LOS could be estimated using magnetic field extrapolation and loop tracing methods in EUV images (see e.g. Warren et al 2018 and references therein). The result would be different for every observed loop as it depends on the loop geometry and the LOS position.…”