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 … Show more
“…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.…”
Section: Effect Of the Waves On Spectral Line Profilessupporting
confidence: 57%
“…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.…”
In this paper we further develop a model for the heating of coronal loops by Alfvén wave turbulence (AWT). The Alfvén waves are assumed to be launched from a collection of kilogauss flux tubes in the photosphere at the two ends of the loop. Using a three-dimensional magneto-hydrodynamic (MHD) model for an active-region loop, we investigate how the waves from neighboring flux tubes interact in the chromosphere and corona. For a particular combination of model parameters we find that AWT can produce enough heat to maintain a peak temperature of about 2.5 MK, somewhat lower than the temperatures of 3 -4 MK observed in the cores of active regions. The heating rates vary strongly in space and time, but the simulated heating events have durations less than 1 minute and are unlikely to reproduce the observed broad Differential Emission Measure distributions of active regions. The simulated spectral line non-thermal widths are predicted to be about 27 km s −1 , which is high compared to the observed values. Therefore, the present AWT model does not satisfy the observational constraints. An alternative "magnetic braiding" model is considered in which the coronal field lines are subject to slow random footpoint motions, but we find that such long period motions produce much less heating than the shorter period waves launched within the flux tubes. We discuss several possibilities for resolving the problem of producing sufficiently hot loops in active regions.
“…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.…”
Section: Effect Of the Waves On Spectral Line Profilessupporting
confidence: 57%
“…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.…”
In this paper we further develop a model for the heating of coronal loops by Alfvén wave turbulence (AWT). The Alfvén waves are assumed to be launched from a collection of kilogauss flux tubes in the photosphere at the two ends of the loop. Using a three-dimensional magneto-hydrodynamic (MHD) model for an active-region loop, we investigate how the waves from neighboring flux tubes interact in the chromosphere and corona. For a particular combination of model parameters we find that AWT can produce enough heat to maintain a peak temperature of about 2.5 MK, somewhat lower than the temperatures of 3 -4 MK observed in the cores of active regions. The heating rates vary strongly in space and time, but the simulated heating events have durations less than 1 minute and are unlikely to reproduce the observed broad Differential Emission Measure distributions of active regions. The simulated spectral line non-thermal widths are predicted to be about 27 km s −1 , which is high compared to the observed values. Therefore, the present AWT model does not satisfy the observational constraints. An alternative "magnetic braiding" model is considered in which the coronal field lines are subject to slow random footpoint motions, but we find that such long period motions produce much less heating than the shorter period waves launched within the flux tubes. We discuss several possibilities for resolving the problem of producing sufficiently hot loops in active regions.
“…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.…”
Section: Energy Fluxesmentioning
confidence: 95%
“…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 MHD waves excited by perturbations of magnetic field lines propagate upward, carrying with them the energy from the excitation. Under favorable conditions shocks form, and part of the wave energy is converted to plasma heating and motion. We use numerical simulations to accurately follow the shock formation and subsequent energy release. The model includes an adiabatic energy equation for the explicit evaluation of temperature increases and energy fluxes contributed by the shocks. Transverse, plane-polarized excitations are considered; they can be periodic, as in Alfvén wave trains, or pulsed, as might result from nanoflares. The model is tested with a set of validation runs that produce good agreement with theoretical predictions. Our results show that nonlinear waves moving along large magnetic fields with low plasma , with field amplitudes comparable to the background field, develop shocks that form important amounts of plasma heating and that mass outflow may occur. Fast and slow magnetoacoustic shocks are generated, each one making its own contribution. Most of the heating takes place in the low corona, but long-range distributed heating still occurs up to heights of several solar radii. The energy fluxes for the stronger cases are sufficient to compensate for thermal and convective losses, consistent with observations. We conclude that large-amplitude MHD shocks in low-regions could be a viable mechanism for coronal heating and wind acceleration in regions of open magnetic field lines.
“…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.…”
Abstract. We present the results of a thorough parameter study of coronal loop models in the aim to explore the mechanism behind coronal heating. The two-fluid coronal loops described in this paper have lengths from 10 Mm to 600 Mm and consist of protons and electrons. The loops are treated with our unique, self-consistent, steady state dynamic loop model to derive the basic parameters (as introduced by Li & Habbal 2003, ApJ, 598, L125). The only heating mechanism assumed is turbulently generated Alfvén waves that carry the necessary flux from the chromosphere to energize the coronal plasma through preferential heating of the proton gas. Strong Coulomb coupling allows energy to pass efficiently from protons to electrons. We have control over the independent variables, driving scale (l) and Alfvén amplitude (ξ), which influence the dissipation and flux of these resonant waves. We find "mapping" the loop parameter response to varying l with fixed ξ a useful tool to find where certain conditions for each loop length exist. From this, we are able to pin-point where the coldest solution lies. For a loop of L = 10 Mm, the coolest loops have a maximum temperature of T = 0.75 MK. We also focus on a L = 40 Mm loop and vary both l and ξ so we can compare results with existing work. From this parameter mapping we can categorise the loop heating profiles. Our model indicates the existence of footpoint, non-uniformly and quasi-uniformly heated profiles. There is also strong evidence to suggest the same mechanism may apply to hot, SXT loops.
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