On the Area Expansion of Magnetic Flux Tubes in Solar Active Regions

Abstract

We calculated the 3D distribution of the area expansion factors in a potential magnetic field extrapolated from the high-resolution Hinode/SOT magnetogram of a quiescent active region NOAA 11482. Retaining only closed loops within the computational box, we show that the distribution of area expansion factors show significant structure. Loop-like structures characterized by locally lower values of the expansion factor are embedded in a smooth background. These loop-like flux-tubes have squashed cross-sections and expand with height. The distribution of the expansion factors show overall increase with height, allowing an active region core characterized by low values of the expansion factor to be distinguished. The area expansion factors obtained from extrapolation of the SOT magnetogram are compared to those obtained from an approximation of the observed magnetogram by a series of 134 submerged charges. This approximation retains the general flux distribution in the observed magnetogram, but removes the small-scale structure in both the approximated magnetogram and the 3D distribution of the area expansion factors. We argue that the structuring of the expansion factor can be a significant ingredient in producing the observed structuring of the solar corona. However, due to the potential approximation used, these results may not be applicable to loops exhibiting twist neither to active regions producing significant flares.

Spectroscopic Observations of a Coronal Loop: Basic Physical Plasma Parameters Along the Full Loop Length

“…• We measured the diameter of the loop along its length using the images obtained in different spectral lines and found that the loop has a non-uniform cross-section (see Figure 14, see also Dudík et al 2014). We also estimated filling factors along the loop length and found that values obtained are similar to those reported previously.…”

The role of asymmetries in coronal rain formation during thermal non-equilibrium cycles

“…This can be explained by the low resolution of the magnetogram, which reduces and spreads out the magnetic field intensity. Line E98 has an area expansion of 98, which is higher than typical active region values (Dudík et al 2014 report a maximum value of 80 in a given active region). Overall, potential magnetic field extrapolation from a low resolution magnetogram only gives a very coarse estimation of the magnetic field, especially in active regions.…”

Delayed Development of Cool Plasmas in X-Ray Flares from the Young Sun-like Star κ 1 Ceti

“…Solar coronal loops, whether quiescent or flaring, do not necessarily show an expansion of the loop width along their lengths (Klimchuk et al 1992;Klimchuk 2000;Klimchuk & DeForest 2020, and references therein). However, the magnetic field strength falls off with height in the corona, implying that there should be an expansion of the cross-sectional area of the loops (e.g., Dudík et al 2014), and models are unable to reproduce both hot and cool emission simultaneously without an area expansion (Warren et al 2010;Reep et al 2022). The loop expansion reduces the thermal conductivity near the footpoints.…”