Heating and Dynamics of the Solar Chromosphere
Final Report Abstract
In the course of this project, we were able to show that the acoustic spectra imposed at the photospheric boundary of a simulation can be varied over substantial ranges without significantly changing the structure of the resulting chromosphere and its Ca n emissions. The precise form of the acoustic spectrum turned out to be of minor importance for the chromosphere, whereas the total energy flux integrated over all frequencies plays a major role. The observational data obtained in the June 2007 campaign proved to be very fruitful because of their high spatial and temporal resolution and the excellent seeing conditions. For the same reason these data make great demands on theoretical models. One of the essential results of the analysis is that the observed intensity ftuctuations generated at different heights are highly correlated with each other. Our ID simulations with vertically propagating shock fronts cannot reproduce this feature. Moreover, these types of calculations also fail to reproduce the new observations of fluctuations of the Si i continuum, as Fossum & Carlsson (2005a,b; 2006) have shown. On the other hand, even the best currently available 3D models of convection zone, photosphere, and chromosphere show that significant amounts of acoustic waves are generated and steepen into to shock waves when they propagate into the chromosphere. As a rule, these shock fronts are not oriented vertically to the gravity vector, but can be inclined under any angle. Consequently shock fronts should be inclined as a rule. In principle, such inclined shock fronts could also explain the observed high correlations and should be observable as (fast) horizontal movements of bright points. In June 2007, we observed such fast horizontal variations of the intensity with time for both the Ca II H and Ca II 8662 lines. Several types of variations occur: pulsations (a high-intensity region grows and shrinks without a change of position), collisions (two regions of high intensity, initially separated, touch and merge) and horizontal shifts. Horizontal front velocities up to 30 km/s were detected. A statistical analysis of our data shows that such fast horizontal motions are rare events - contrary to the model predictions. The results that we achieved so far and the new papers by Fossum & Carlsson show that plane-parallel waves fail in many ways to match the observations. The approach of applying cone geometries will be further pursued at a low priority, but is now considered less promising since here, too, there are only horizontal shock fronts in ID calculations. Both observations and 3D simulations, however, point towards oblique shock fronts. Incidentally, such oblique fronts would even be observed if the shocks had 3D cone geometries, since a vertical cut (with the spectrograph slit) through a cone corresponds to an oblique front. 3D codes can not yet realistically simulate the NLTE radiative effects in such oblique shocks. The observational data collected during this project provides an excellent and rich basis for further investigations of the statistical properties of bright points in the chromosphere of the Sun.
Publications
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Fast method for calculating chromospheric Ca ll and Mg II radiative losses, Astrophysical Journal, Vol. 631, p. 1113-1119(2005)
Rammacher, W., Fawzy, D., Ulmschneider, P. & Musielak, Z.
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How strong is the dependence of the solar chromosphere upon the convection zone? in: Danesy, D. (ed.): Proceedings of the conference Chromospheric and Coronal Magnetic Fields held at the Max Planck Institute for Solar System Research, Katlenburg-Lindau, 2005, ESA SP - 596 (2005)
Rammacher, W.
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On the validity of acoustically heated chromosphere models, Astrophysical Journal Letters, Vol. 631, p. L155-L158 (2005)
Ulmschneider, P., Rammacher, W., Musielak, Z. & Kalkofen, W.
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Acoustic Heating of the Solar Chromosphere: Present Indeed and Locally Dominant, Astrophysical Journal Letters, Vol. 657, p. L57-L60 (2007)
Cuntz, M., Rammacher, W. & Musielak, Z.
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Chromospheric Dynamics and Line Formation, in S.S. Hasan and D. Banerjee (Eds.): Kodai School on Solar Physics, held at Kodaikanal Observatory, 10-22 Dec. 2006, AIP Conference Proceedings Vol. 19, pp. 138-172 (2007)
Hammer, R., and Ulmschneider, P.
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Observations and Simulations of Solar Ca ll H and Ca ll 8662 Lines in: Heinzel, R, Dorotovic,l., Rutten, R.J. (eds.): The Physics of Chromospheric Plasmas, Coimbra Solar Physics Meeting, held 9-13 October 2006, ASP Conference Series 368, p. 147-150 (2007)
Rammacher, W., Schmidt, W. & Hammer, R.
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Simultaneous observations of solar Ca ll H and Ca ll 8662 lines and numerical simulations of these lines in: Astronomische Nachrichten Vol. 328, p. 657-658 (2007)
Rammacher, W., Schmidt, W. & Hammer, R.
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What is heating the quiet-Sun chromosphere? in: Heinzel, P., Dorotovic, l., Rutten, R.J. (eds.): The Physics of Chromospheric Plasmas, Coimbra Solar Physics Meeting, held 9-13 October 2006, ASP Conference Series 368, p. 93-102 (2007)
Wedemeyer, S., Steiner, O., Bruls, J. & Rammacher, W.
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Simultaneous Maps of the Chromosphere for Ca ll H and Ca ll 8662 in: Electronic proceedings of the 12th European Solar Physics Meeting 8-12 September 2008 Freiburg, Germany, ADS 2008 ESPM ..., 12.2.40R (2008)
Rammacher, W., Schmidt, W. & Hammer, R.
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The signature of chromospheric heating in Ca ll H spectra, Astronomy and Astrophysics, Vol. 479, p. 213-227 (2008)
Beck, C., Schmidt, W., Rezaei, R. & Rammacher, W.
