Observational study of chromospheric heating by acoustic waves

Aims. Our aim is to investigate the role of acoustic and magneto-acoustic waves in heating the solar chromosphere. Observations in strong chromospheric lines are analyzed by comparing the deposited acoustic-energy flux with the total integrated radiative losses. Methods. Quiet-Sun and weak-plage regions were observed in the Ca II 854.2 nm and Hα lines with the Fast Imaging Solar Spectrograph (FISS) at the 1.6-m Goode Solar Telescope on 2019 October 3 and in the Hα and Hβ lines with the echelle spectrograph attached to the Vacuum Tower Telescope on 2018 December 11 and 2019 June 6. The deposited acoustic energy flux at frequencies up to 20 mHz was derived from Doppler velocities observed in line centers and wings. Radiative losses were computed by means of a set of scaled non-local thermodynamic equilibrium 1D hydrostatic semi-empirical models obtained by fitting synthetic to observed line profiles. Results. In the middle chromosphere (h = 1000–1400 km), the radiative losses can be fully balanced by the deposited acoustic energy flux in a quiet-Sun region. In the upper chromosphere (h > 1400 km), the deposited acoustic flux is small compared to the radiative losses in quiet as well as in plage regions. The crucial parameter determining the amount of deposited acoustic flux is the gas density at a given height. Conclusions. The acoustic energy flux is efficiently deposited in the middle chromosphere, where the density of gas is sufficiently high. About 90% of the available acoustic energy flux in the quiet-Sun region is deposited in these layers, and thus it is a major contributor to the radiative losses of the middle chromosphere. In the upper chromosphere, the deposited acoustic flux is too low, so that other heating mechanisms have to act to balance the radiative cooling.

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Diagnostics of Non-thermal Particles in Solar Chromospheric Flares

Symposium - International Astronomical Union ◽

10.1017/s0074180900182087 ◽

2004 ◽

Vol 219 ◽

pp. 171-175

Author(s):

C. Fang ◽

Z. Xu ◽

M. D. Ding

Keyword(s):

Total Energy ◽

Energy Flux ◽

Atmospheric Condition ◽

Particle Beam ◽

Profile Measurement ◽

Solar Chromosphere ◽

Line Profiles ◽

Total Energy Flux ◽

Hydrogen Lines ◽

Semi Empirical

Particle beam bombardment on the solar chromosphere produces non-thermal ionization and excitation. The effect on hydrogen lines is investigated by using non-LTE theory and semi-empirical flare models. It has been found that in the case of electron bombardment, the Hα line is widely broadened and enhanced. Significant enhancements at the wings of Lyα and Lyβ lines are also predicted. In the case of proton bombardment, less strong broadening and less central reversal are expected. We found that the total energy flux of the particle beam and the atmospheric condition give much influence on the line profiles, which, however, are less sensitive to the power index. Based on the Hα line profile measurement, a method to deduce the total energy flux of the particle beam is proposed.

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Numerical determination of the cutoff frequency in solar models

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038387 ◽

2020 ◽

Vol 640 ◽

pp. A4 ◽

Cited By ~ 1

Author(s):

T. Felipe ◽

C. R. Sangeetha

Keyword(s):

Magnetic Field ◽

Wave Propagation ◽

Magnetic Fields ◽

Numerical Simulations ◽

Acoustic Waves ◽

Cutoff Frequency ◽

Quiet Sun ◽

Radiative Losses ◽

The Magnetic Field

Context. In stratified atmospheres, acoustic waves can only propagate if their frequency is higher than the cutoff value. The determination of the cutoff frequency is fundamental for several topics in solar physics, such as evaluating the contribution of the acoustic waves to the chromospheric heating or the application of seismic techniques. However, different theories provide different cutoff values. Aims. We developed an alternative method to derive the cutoff frequency in several standard solar models, including various quiet-Sun and umbral atmospheres. The effects of magnetic field and radiative losses on the cutoff are examined. Methods. We performed numerical simulations of wave propagation in the solar atmosphere using the code MANCHA. The cutoff frequency is determined from the inspection of phase-difference spectra computed between the velocity signal at two atmospheric heights. The process is performed by choosing pairs of heights across all the layers between the photosphere and the chromosphere to derive the vertical stratification of the cutoff in the solar models. Result. The cutoff frequency predicted by the theoretical calculations departs significantly from the measurements obtained from the numerical simulations. In quiet-Sun atmospheres, the cutoff shows a strong dependence on the magnetic field for adiabatic wave propagation. When radiative losses are taken into account, the cutoff frequency is greatly reduced and the variation of the cutoff with the strength of the magnetic field is lower. The effect of the radiative losses in the cutoff is necessary to understand recent quiet-Sun and sunspot observations. In the presence of inclined magnetic fields, our numerical calculations confirm that the cutoff frequency is reduced as a result of the reduced gravity experienced by waves that propagate along field lines. An additional reduction is also found in regions with significant changes in the temperature, which is due to the lower temperature gradient along the path of field-guided waves. Conclusions. Our results show solid evidence that the cutoff frequency in the solar atmosphere is stratified. The cutoff values are not correctly captured by theoretical estimates. In addition, most of the widely used analytical cutoff formulae neglect the effect of magnetic fields and radiative losses, whose role is critical for determining the evanescent or propagating nature of the waves.

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Chromospheric heating during flux emergence in the solar atmosphere

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732027 ◽

2018 ◽

Vol 612 ◽

pp. A28 ◽

Cited By ~ 12

Author(s):

Jorrit Leenaarts ◽

Jaime de la Cruz Rodríguez ◽

Sanja Danilovic ◽

Göran Scharmer ◽

Mats Carlsson

Keyword(s):

Linear Polarization ◽

Active Regions ◽

Spatial And Temporal Variation ◽

Solar Chromosphere ◽

Gas Temperature ◽

Horizontal Magnetic Field ◽

Magnetic Elements ◽

Spatially Extended ◽

Non Local ◽

Radiative Losses

Context. The radiative losses in the solar chromosphere vary from 4 kW m−2 in the quiet Sun, to 20 kW m−2 in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aim. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region. Methods. We have used observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα, and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line profiles and in addition perform multi-line, multi-species, non-local thermodynamic equilibrium (non-LTE) inversions to estimate the spatial and temporal variation of the chromospheric structure. Results. We investigate which spectral features of Ca II K contribute to the frequency-integrated Ca II K brightness, which we use as a tracer of chromospheric radiative losses. The majority of the radiative losses are not associated with localised high-Ca II K-brightness events, but instead with a more gentle, spatially extended, and persistent heating. The frequency-integrated Ca II K brightness correlates strongly with the total linear polarization in the Ca II 854.2 nm, while the Ca II K profile shapes indicate that the bulk of the radiative losses occur in the lower chromosphere. Non-LTE inversions indicate a transition from heating concentrated around photospheric magnetic elements below log τ500 = −3 to a more space-filling and time-persistent heating above log τ500 = −4. The inferred gas temperature at log τ500 = −3.8 correlates strongly with the total linear polarization in the Ca II 854.2 nm line, suggesting that that the heating rate correlates with the strength of the horizontal magnetic field in the low chromosphere.

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Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038538 ◽

2020 ◽

Vol 642 ◽

pp. A215

Author(s):

Vasco M. J. Henriques ◽

Chris J. Nelson ◽

Luc H. M. Rouppe van der Voort ◽

Mihalis Mathioudakis

Keyword(s):

Fine Structure ◽

Acoustic Waves ◽

Large Scale ◽

Small Scale ◽

Empirical Modelling ◽

Early Portion ◽

Spectral Profiles ◽

Non Local ◽

Semi Empirical ◽

First Time

Context. The chromosphere of the umbra of sunspots features an assortment of dynamic fine structures that are poorly understood and often studied separately. Small-scale umbral brightenings (SSUBs), umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral dark fibrils are found in any observation of the chromosphere with sufficient spatial resolution performed at the correct umbral flash stage and passband. Understanding these features means understanding the dynamics of the umbral chromosphere. Aims. We aim to fully understand the dynamics of umbral chromosphere through analysis of the relationships between distinct observed fine features and to produce complete models that explain both spectral profiles and the temporal evolution of the features. We seek to relate such understanding to umbral flashes. Methods. We studied the spatial and spectral co-evolution of SDFs, SSUBs, and umbral flashes in Ca II 8542 Å spectral profiles. We produced models that generate the spectral profiles for all classes of features using non-local thermodynamic equilibrium radiative transfer with a recent version of the NICOLE inversion code. Results. We find that both bright SSUBs and dark SDF structures are described with a continuous feature in the parameter space that is distinct from the surroundings even in pixel-by-pixel inversions. We find a phase difference between such features and umbral flashes in both inverted line-of-sight velocities and timing of the brightenings. For umbral flashes themselves we resolve, for the first time in inversion-based semi-empirical modelling, the pre-flash downflows, post-flash upflows, and the counter-flows present during the umbral flash phase. We further present a simple time-dependent cartoon model that explains the dynamics and spectral profiles of both fine structure, dark and bright, and umbral flashes in umbral chromospheres. Conclusions. The similarity of the profiles between the brightenings and umbral flashes, the pattern of velocities obtained from the inversions, and the phase relationships between the structures all lead us to put forward that all dynamic umbral chromospheric structures observed to this date are a locally delayed or locally early portion of the oscillatory flow pattern that generates flashes, secondary to the steepening large-scale acoustic waves at its source. Essentially, SSUBs are part of the same shock or merely compression front responsible for the spatially larger umbral flash phenomenon, but out of phase with the broader oscillation.

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Absorption of sound by homogeneous turbulence

Journal of Fluid Mechanics ◽

10.1017/s0022112078001299 ◽

1978 ◽

Vol 86 (3) ◽

pp. 593-608 ◽

Cited By ~ 6

Author(s):

D. T. Noir ◽

A. R. George

Keyword(s):

Kinetic Energy ◽

Acoustic Waves ◽

Isotropic Turbulence ◽

Turbulent Energy ◽

Acoustic Energy ◽

Homogeneous Turbulence ◽

Aerodynamic Noise ◽

Reynolds Stresses ◽

Order Of Magnitude ◽

Semi Empirical

The following problem is treated: given a plane acoustic wave propagating through an unbounded field of turbulence, calculate the amount of acoustic energy converted into turbulent kinetic energy. The fluid velocities due to the acoustic waves and the turbulence are assumed to be small compared with the speed of sound. Thus the sound-turbulence interaction is weak and the turbulent field may be considered to be incompressible. The analysis is based on the interaction of two opposite effects: the acoustic distortion of the turbulence (producing anisotropic Reynolds stresses) and the redistribution of the kinetic energy among components (tendency towards isotropy) and among wavenumbers (energy cascade and dissipation). These phenomena are described using semi-empirical turbulence arguments. It is seen that the simplest model for the redistribution among components is not sufficient for unsteady flows. A more complete model is used which is modified to agree with the exact instantaneous distortion analysis of Ribner [map ] Tucker to first order. Owing to the two redistribution effects, the Reynolds stress behaves inelastically and is out of phase with the acoustic field. Thus there is an average production of turbulent energy corresponding to the absorption of acoustic energy and attenuation of the incident wave. For nearly isotropic turbulence, the attenuation coefficient is found to be proportional to the rate of viscous dissipation and independent of the frequency.In order to compare the theory with experiment several constants involved in the semi-empirical model of the turbulence must be found. Owing to the lack of better information these constants are estimated here by order-of-magnitude considerations. No existing experiments correspond to the homogeneous turbulence assumed by the theory. Comparison with the few reasonably applicable experiments shows qualitative agreement though the importance of the turbulent absorption is generally of nearly the same order as the measurement error. Several discrepancies between jet noise experiments and aerodynamic noise predictions may be roughly explained using the above analysis.

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