Are Nonthermal Velocities in Active Region Coronal Loops Anisotropic?

Abstract

We have measured line widths in active region coronal loops in order to determine whether the nonthermal broadening is anisotropic with respect to the magnetic field direction. These nonthermal velocities are caused by unresolved fluid motions. Our analysis method combines spectroscopic data and a magnetic field extrapolation. We analyzed spectra from the Extreme Ultraviolet Imaging Spectrometer on Hinode. A differential emission measure analysis showed that many spectral lines that are commonly considered to be formed in the active region have a substantial contribution from the background quiet Sun. From these spectra we identified lines whose emission was dominated by the active region loops rather than background sources. Using these lines, we constructed maps of the nonthermal velocity. With data from the Helioseismic Magnetic Imager on the Solar Dynamics Observatory and the Coronal Modeling System nonlinear force-free magnetic field reconstruction code, we traced several of the magnetic field lines through the active region. Comparing the spectroscopic and magnetic data, we looked for correlations of the nonthermal velocity with the viewing angle between the line of sight and the magnetic field. We found that nonthermal velocities show a weak anticorrelation with the viewing angle. That is, the tendency is for the nonthermal velocity to be slightly larger in the parallel direction. This parallel broadening may be due to acoustic waves or unresolved parallel flows.

Spectroscopic Observations of the Solar Corona during the 2017 August 21 Total Solar Eclipse: Comparison of Spectral Line Widths and Doppler Shifts between Open and Closed Magnetic Structures

“…A few studies of the anisotropy of the nonthermal velocity confirm the existence of ξ ∥ but found different relationships between the two components ξ ∥ and ξ ⊥ , including ξ ∥ > ξ ⊥ (Hahn et al 2023) and ξ ∥ < ξ ⊥ (Hara & Ichimoto 1999). The possible candidates to create ξ ∥ and the relatively constant nonthermal widths in closed fields include reconnection (e.g., jets and nanoflares) or slow mode waves (Hahn et al 2023), but it is still difficult to reach a firm conclusion (Brooks & Warren 2016).…”

Observations of Nonthermal Velocities and Comparisons with an Alfvén Wave Turbulence Model in Solar Active Regions

“…The emission of the spectral lines we use, from Fe XIV, XV, and XVI, represents an integration along the LOS and therefore potentially contains contributions from both the active region loop arcade and unresolved foreground and background emission. Hahn et al (2023) have quantified the unresolved component contribution to these lines in an active region, indicating substantial contributions, with an increasing impact as the line formation temperature decreases. This contribution may also be activeregion-dependent (Brown et al 2008).…”