Definition and significance of average temperatures in time-dependent solar chromosphere models

Puschmann

2012

A&A

Context. The Ca ii H line is one of the strongest lines in the solar spectrum, and it provides continuous information on the solar atmosphere from the photosphere to the lower chromosphere.Aims. We describe an inversion approach that reproduces observed Ca ii H spectra by assuming local thermal equilibrium (LTE). Methods. We developed an inversion strategy based on the SIR code that reproduces Ca ii H spectra in the LTE approximation. The approach uses a two-step procedure with an archive of pre-calculated spectra to fit the line core and a subsequent iterative modification to improve the fit mainly in the line wing. Simultaneous spectra in the 630 nm range can optionally be used to fix the continuum temperature. The method retrieves one-dimensional (1D) temperature stratifications while neglecting lateral radiative transport. Lineof-sight velocities are included post facto with an empirical approach.Results. An archive of about 300 000 pre-calculated spectra is more than sufficient to reproduce the line core of observed Ca ii H spectra both in the quiet Sun and in active regions. The subsequent iterative adjustment of the thermodynamical stratification matches observed and best-fit spectra to a level of about 0.5% of I c in the line wing and about 1% of I c in the line core. Conclusions. The successful application of the LTE inversion strategy suggests that inversion schemes based on pre-calculated spectra allow a reliable and relatively fast retrieval of solar properties from observed chromospheric spectra. The approach can be easily extended to a 1D non-LTE (NLTE) case by a simple exchange of the pre-calculated archive spectra. Using synthetic NLTE spectra from numerical three-dimensional (3D) simulations instead will finally allow one to extend the approach from the static 1D-case to dynamical atmosphere models, including the complete 3D radiative transport.

Section: Introductionmentioning

confidence: 99%

The energy of waves in the photosphere and lower chromosphere

Beck

Puschmann

2012

A&A

“…While the C6, FALA, and FALC models look too hot, with strong emission peaks, the Holweger-Müller model fails due to its very deep calcium core. The dynamical models of Carlsson & Stein (1994 and Rammacher & Cuntz (2005) have episodes (significantly) cooler than the Holweger-Müller model with correspondingly lower line core intensities (Uitenbroek 2002;Rammacher 2005). It should be remembered, however, that the chromospheric radiative transfer in those simulations is not "consistent": important cooling agents are not represented at all and deviations from various equilibrium states are ignored.…”

Section: Synthetic Calcium Profilesmentioning

confidence: 95%

Reversal-free Ca II H profiles: a challenge for solar chromosphere modeling in quiet inter-network

Bruls

Schmidt

et al. 2008

A&A

Aims. We study chromospheric emission to understand the temperature stratification in the solar chromosphere.Methods. We observed the intensity profile of the Ca ii H line in a quiet Sun region close to the disk center at the German Vacuum Tower Telescope. We analyze over 10 5 line profiles from inter-network regions. For comparison with the observed profiles, we synthesize spectra for a variety of model atmospheres with a non local thermodynamic equilibrium (NLTE) radiative transfer code.Results. A fraction of about 25% of the observed Ca ii H line profiles do not show a measurable emission peak in H 2v and H 2r wavelength bands (reversal-free). All of the chosen model atmospheres with a temperature rise fail to reproduce such profiles. On the other hand, the synthetic calcium profile of a model atmosphere that has a monotonic decline of the temperature with height shows a reversal-free profile that has much lower intensities than any observed line profile. Conclusions. The observed reversal-free profiles indicate the existence of cool patches in the interior of chromospheric network cells, at least for short time intervals. Our finding is not only in conflict with a full-time hot chromosphere, but also with a very cool chromosphere as found in some dynamic simulations.

“…Carlsson 2007;Wedemeyer-Böhm & Carlsson 2011). A complete treatment of this regime in a non-LTE (NLTE) formalism still exceeds the computing power of even the most recent computer clusters (Auer & Mihalas 1969;Ayres & Wiedemann 1989;Rammacher & Cuntz 1991;Carlsson & Stein 1992Uitenbroek 2000;Asensio Ramos et al 2003;Leenaarts et al 2007;Wedemeyer-Böhm & Carlsson 2011;Carlsson & Leenaarts 2012;Martínez-Sykora et al 2012). …”

Section: Additional References)mentioning

confidence: 99%

The energy of waves in the photosphere and lower chromosphere

Beck

Puschmann

2013

A&A

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.