J. H. M. J. Bruls

Publications (3)0 Total impact

• Source

  Available from: Wolfgang Schmidt

  Article: Reversal-free CaIIH profiles: a challenge for solar chromosphere modeling in quiet inter-network

  R. Rezaei, J. H. M. J. Bruls, W Schmidt, C. Beck, W. Kalkofen, R. Schlichenmaier
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  ABSTRACT: We study chromospheric emission to understand the temperature stratification in the solar chromosphere. We observed the intensity profile of the CaIIH 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. A fraction of about 25% of the observed CaIIH 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. 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. Comment: 8 pages, accepted in A&A
  04/2008;
• Source

  Available from: Wolfgang Schmidt

  Article: Relation between photospheric magnetic field and chromospheric emission

  R. Rezaei, R. Schlichenmaier, C. A. R. Beck, J. H. M. J. Bruls, W Schmidt
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  ABSTRACT: We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm and the intensity profile of the chromospheric Ca II H line at 396.8 nm in a quiet Sun region at a heliocentric angle of 53 deg. We perform a statistical analysis of network and inter-network properties.The H-index is the integrated emission in a 0.1 nm band around the Ca core. We separate a non-magnetically, H_non, and a magnetically, H_mag, heated component from a non-heated component, H_co in the H-index. The average network and inter-network H-indices are equal to 12 and 10 pm, respectively. The emission in the network is correlated with the magnetic flux density, approaching a value of H 10 pm for vanishing flux. The inter-network magnetic field is dominated by weak field strengths with values down to 200 G and its mean absolute flux density is 11 Mx cm$^{-2}$. We find that a dominant fraction of the calcium emission caused by the heated atmosphere in the magnetic network, has non-magnetic origin (H_mag = 2pm, H_non = 3pm). Considering the effect of straylight, the contribution from an atmosphere with no temperature rise to the H-index (H_co = 6pm) is about half of the observed H-index in the inter-network. The H-index in the inter-network is not correlated to any property of the photospheric magnetic field, suggesting that magnetic flux concentrations have a negligible role in the chromospheric heating in this region. The height range of the thermal coupling between the photosphere and low/mid chromosphere increases in presence of magnetic field. In addition, we demonstrate that a poor signal-to-noise level in the Stokes profiles leads to a significant over-estimation of the magnetic field strength.
  03/2007;
• Article: Reversal-free Ca II H profiles: a challenge for solar chromosphere modeling in quiet inter-network

  R. Rezaei, J. H. M. J. Bruls, W Schmidt, C. Beck, W. Kalkofen, R. Schlichenmaier
  [show abstract] [hide abstract]
  ABSTRACT: Aims. We study chromospheric emission to understand the temperature stratification in the solar chromosphere.Methods. We observed the intensity profile of the $\ion{Ca}{ii}$ 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 $\ion{Ca}{ii}$ H line profiles do not show a measurable emission peak in $H_{{\rm 2v}}$ and $H_{{\rm 2r}}$ 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.
  http://dx.doi.org/10.1051/0004-6361:20079050.