Microscopic Nonthermal Plasma Motions of Coronal Loops in a Solar Active Region

Abstract

We present a spectroscopic observation of a solar active region NOAA 7590 with a coronagraph at the Norikura Solar Observatory, which provides high-resolution spectra of the visible coronal emission lines (Fe X j6374, Fe XIV j5303, Ca XV j5694) with a spatial sampling of Nonthermal veloci-2A .0 ] 2A .3. ties (m) estimated from Fe X j6374, Fe XIV j5303, and Ca XV j5694 in this observation are 14È20, 10È18, and 16È26 km s~1, respectively. The Ðrst two results are consistent with the results obtained by Cheng et al. and others. Even in the Ca XV structures the present observation does not conÐrm the large nonthermal velocity of 40È60 km s~1 to be typical value. The hypothesis that the nonthermal width in coronal emission lines is due to coronal waves is tested by carefully examining the relationship Alfve n between the width of coronal emission lines and orientation of coronal loops to the line-of-sight direction. From the comparison between edge-on loops in which the direction of magnetic Ðeld is nearly parallel to the line-of-sight direction and face-on loops in which the magnetic Ðeld is almost perpendicular to the line-of-sight direction, the full width at half-maximum (FWHM) of coronal emission lines for the edge-on loop appears to become smaller near the loop top than that for the face-on loops. The obvious decrease of FWHM of 0.04È0.07 (*m \ 3È5 km s~1) is found in the Fe XIV edge-on loops. A Although this may be evidence for the waves in coronal loops, the velocity amplitude seems to be Alfve n too small to explain all the nonthermal velocity reported so far.

The Heating of Solar Coronal Loops by Alfvén Wave Turbulence

“…The value of VNTH averaged over the image is about 27 km s −1 , nearly independent of resolution. This is somewhat higher than the nonthermal velocity of 17.6 ± 5.3 km s −1 measured with EIS (Brooks & Warren 2016), the most-probable velocity of 15 km s −1 observed with IRIS (Testa et al 2016), and the values of 14 -26 km s −1 observed at the Norikura Solar Observatory (Hara & Ichimoto 1999). Therefore, the transverse waves in our model have relatively high velocities that are only marginally consistent with the available spectroscopic observations.…”

“…We predict that the Alfvén waves should be detectable as variations in Doppler shift, provided the instrument has sufficiently high spatial resolution (FWHM < 0.5"). The rms value of the non-thermal velocity is predicted to be nearly independent of spatial resolution, and is about 27 km s −1 , which is high compared to the observed values (Brooks & Warren 2016;Testa et al 2016;Hara & Ichimoto 1999). Therefore, despite not providing enough heating, the AWT model is already injecting as much energy into the corona as is consistent with spectroscopic observations.…”

Magnetohydrodynamic Shock Heating of the Solar Corona

“…More often it is defined as an Alfvén flux, v 2 v A (Priest 1982;Foukal 1990;Saba & Strong 1991), for a wave propagating at the Alfvén speed with amplitude v. Variations in the definition of the Alfvén flux use the observed nonthermal speed (Hara & Ichimoto 1999) or the mean-square velocity toward the observer hv 2 k i (Erdélyi et al 1998) in the place of v. These Alfvén flux expressions differ from the one used in this work in two respects. They do not include the second term in equation (14) for the flux of the longitudinal mode, and they assume that the propagation occurs at the Alfvén speed, instead of at the shock speed (v sh > v A ), as obtained in these simulations.…”

“…The majority of nonthermal velocities, derived from spectral line broadenings and attributed to random turbulence, flows, or unpolarized wave amplitudes, lie in the 1 6 ð ÞÂ10 4 m s À1 range (Feldman & Behring 1974;Saba & Strong 1991;Erdélyi et al 1998;Banerjee et al 1998;Hara & Ichimoto 1999;Ruderman 1999). Exceptionally larger values of about 1 Â 10 5 m s À1 are reported (Acton et al 1981;Withbroe et al 1985), although some reservations exist (Saba & Strong 1991;Hara & Ichimoto 1999). Observations that consider the anisotropy in the velocity distributions produce wave amplitudes on the order of 10 5 m s À1 perpendicular to the magnetic field in coronal holes (Kohl et al 1998;Leer & Marsch 1999).…”

Coronal loops heated by turbulence-driven Alfvén waves: A two fluid model

“…We study Alfvén waves as they carry a large energy flux due to the high Alfvén velocity in the corona. Studies by Hara & Ichimoto (1999) and other authors strongly imply that Alfvén waves may be present in observational data of coronal loops and play a key role in coronal heating. We therefore simulate asymmetric plasma flow (Noci & Zuccarello 1983;Craig & McClymont 1986) with Alfvén wave dissipation acting as the driving force.…”