The signature of chromospheric heating in Ca II H spectra

Beck, C.; Schmidt, W.; Rezaei, Reza; Rammacher, W.

doi:10.1051/0004-6361:20078410

Cited by 63 publications

(83 citation statements)

References 47 publications

(79 reference statements)

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“…As an explanation for the BGs the (upwards) propagation of acoustic waves has been suggested soon after their first detection, because the BGs can often be seen to appear in the line wings already about 50-100 sec before the emission in the core (see Fig. 1, or Liu 1974;Beck et al 2008), which supports the idea of a wave propagation. Due to their formation above the photosphere in a layer with mostly NLTE conditions, the theoretical treatment and the A&A 510, A66 (2010) interpretation of the Ca line behavior in observations is difficult.…”

Section: Introductionmentioning

confidence: 72%

Linear wavelength correlation matrices of photospheric and chromospheric spectral lines

Beck¹,

Rammacher²

2010

A&A

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Context. The process that heats the solar chromosphere is a difficult target for observational studies because the assumption of local thermal equilibrium (LTE) is not valid in the upper solar atmosphere, which complicates the analysis of spectra. Aims. We investigate the linear correlation coefficient between the intensities at different wavelengths in photospheric and chromospheric spectral lines because the correlation can be determined directly for any spectra from observations or modeling. Waves which propagate vertically through the stratified solar atmosphere affect different wavelengths at different times when the contribution functions for each wavelength peak in different layers. This leads to a characteristic pattern of (non-)coherence of the intensity at various wavelengths with respect to each other which carries information on the physical processes. Methods. We derived the correlation matrices for several photospheric and chromospheric spectral lines from observations. We separated locations with a significant photospheric polarization signal and thus magnetic fields from those without a polarization signal. For comparison with the observations, we calculated correlation matrices for spectra from simplified LTE modeling approaches, 1-D NLTE simulations, and a 3-D MHD simulation run. We applied the correlation method also to temperature maps at different optical depth layers derived from a LTE inversion of Ca ii H spectra.Results. We find that all photospheric spectral lines show a similar pattern: a pronounced asymmetry of the correlation between line core and red or blue wing. The pattern cannot be reproduced with a simulation of the granulation pattern, but with waves that travel upwards through the formation heights of the lines. The correct asymmetry between red and blue wing only appears when a temperature enhancement occurs simultaneously with a downflow velocity in the wave simulation. All chromospheric spectral lines show a more complex pattern. The 1-D NLTE simulations of monochromatic waves produce a correlation matrix that qualitatively matches the observations near the very core of the Ca ii H line. The photospheric signature is well reproduced in the correlation matrix derived from the 3-D MHD simulation. Conclusions. The correlation matrices of observed photospheric and chromospheric spectral lines are highly structured with characteristic and different patterns in every spectral line. The comparison with matrices derived from simulations and simple modeling suggests that the main driver of the detected patterns are upwards propagating waves. Application of the correlation method to 3-D temperature cubes seems to be a promising tool for a detailed comparison of simulation results and observations in future studies.

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Section: Introductionmentioning

confidence: 72%

Linear wavelength correlation matrices of photospheric and chromospheric spectral lines

Beck¹,

Rammacher²

2010

A&A

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“…above a magnetic canopy with a strong damping of impinging acoustic oscillation, whereas Ca ii H forms below the canopy in QS (cf. Beck et al 2008;their Fig. 22).…”

Section: Difference Between Temperature Stratifications In Quiet Sun mentioning

confidence: 98%

“…All these lines are classified as chromospheric. However, other chromospheric lines, especially Ca ii H and K in the near-UV, show large spatiotemporal fluctuations from reversal-free absorption profiles to moderate emission in the line cores (Liu & Smith 1972;Cram & Dame 1983;Rezaei et al 2008;Beck et al 2008). While the temporal evolution is naturally explained in dynamical simulations, persistent emission is only reproduced in static 1D models with a permanent chromospheric temperature rise.…”

Section: Additional References)mentioning

confidence: 99%

The energy of waves in the photosphere and lower chromosphere

Beck

Rezaei

Puschmann

2013

A&A

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Context. Most semi-empirical static one-dimensional (1D) models of the solar atmosphere in the magnetically quiet Sun (QS) predict an increase in temperature at chromospheric layers. Numerical simulations of the solar chromosphere with a variable degree of sophistication, i.e. from 1D to three-dimensional (3D) simulations; assuming local thermal equilibrium (LTE) or non-LTE (NLTE), on the other hand, only yielded an increase in the brightness temperature without any stationary increase in the gas temperature.Aims. We investigate the thermal structure in the solar chromosphere as derived from an LTE inversion of observed Ca ii H spectra in QS and active regions (ARs).Methods. We applied an inversion strategy based on the SIR (Stokes inversion by response functions) code to Ca ii H spectra to obtain 1D temperature stratifications. We investigated the temperature stratifications on differences between magnetic and field-free regions in the QS, and on differences between QS and ARs. We determined the energy content of individual calcium bright grains (BGs) as specific candidates of chromospheric heating events. We compared observed with synthetic NLTE spectra to estimate the significance of the LTE inversion results. Results. The fluctuations of observed intensities yield a variable temperature structure with spatio-temporal rms fluctuations below 100 K in the photosphere and between 200 and 300 K in the QS chromosphere. The average temperature stratification in the QS does not exhibit a clear chromospheric temperature rise, unlike the AR case. We find a characteristic energy content of about 7 × 10 18 J for BGs that repeat with a cadence of about 160 s. The precursors of BGs have a vertical extent of about 200 km and a horizontal extent of about 1 Mm. The comparison of observed with synthetic NLTE profiles partly confirms the results of the LTE inversion that the solar chromosphere in the QS oscillates between an atmosphere in radiative equilibrium and one with a moderate chromospheric temperature rise. Two-dimensional x − z temperature maps exhibit nearly horizontal canopy-like structures with an extent of a few Mm around photospheric magnetic field concentrations at a height of about 600 km. Conclusions. The large difference between QS regions and ARs and the better match of AR and NLTE reference spectra suggest that magnetic heating processes are more important than commonly assumed. The temperature fluctuations in QS derived by the LTE inversion do not suffice on average to maintain a stationary chromospheric temperature rise. The spatially and vertically resolved information on the temperature structure allows one to investigate in detail the topology and evolution of the thermal structure in the lower solar atmosphere.

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“…The continuum map in each channel was used to compensate for the differential refraction in the Earth's atmosphere (Filippenko 1982;Reardon 2006;Beck et al 2008;Felipe et al 2010). To this end, the same method as in Beck et al (2008) was applied to the data.…”

Section: Data Alignmentmentioning

confidence: 99%

“…To this end, the same method as in Beck et al (2008) was applied to the data. After that, all maps were aligned to the Ca ii H map, which has the smallest field of view.…”

Section: Data Alignmentmentioning

confidence: 99%

Multiwavelength spectropolarimetric observations of an Ellerman bomb

Rezaei

Beck

2015

A&A

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Context. Ellerman bombs (EBs) are enhanced emission in the wings of the Hα line in the solar spectrum. Aims. We study the structure of an EB in the photosphere and chromosphere.Methods. We analyze simultaneous observations of four chromospheric lines (Hα, Ca ii H, Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines (Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170 nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the Helioseismic and Magnetic Imager (HMI) are used to trace the temporal evolution of the magnetic structure.Results. We identify the EB by excess emission in the wings of the Hα line, a brightening in the UV continuum, and large emission peaks in the core of the two Ca ii lines. The EB shows a blueshift in all chromospheric lines, while no shifts are observed in the photospheric lines. The blueshift in the chromospheric layer causes very asymmetric emission peaks in the Ca ii H line. The photospheric Si i spectral line shows a shallower line depth at the location of the EB. The UV continuum maps show that the EB was substantially brighter than its surroundings for about 30 min. The continuum contrast of the EB from 170 nm to 1080 nm shows a power-law dependency on the wavelength. The temperature enhancement amounts to 130 K in the low photosphere and 400 K at the temperature minimum level. This temperature excess is also seen in an LTE inversion of the Ca ii spectra. The total thermal and radiative energy content of the EB is about 10 20 J and 10 18 J in the photosphere and chromosphere, respectively. The HMI data hints at a photospheric magnetic flux cancellation as the driver of the EB. Conclusions. Ellerman bombs release the energy in a height range of several pressure scale heights around the temperature minimum such that they affect both the photosphere and the lower chromosphere.

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The signature of chromospheric heating in Ca II H spectra

Cited by 63 publications

References 47 publications

Linear wavelength correlation matrices of photospheric and chromospheric spectral lines

Linear wavelength correlation matrices of photospheric and chromospheric spectral lines

The energy of waves in the photosphere and lower chromosphere

Multiwavelength spectropolarimetric observations of an Ellerman bomb

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