Using high-precision spectroscopy to scrutinize solar model atmospheres
Final Report Abstract
Within this project, we carried out ultra-precise solar spectroscopy of selected solar spectral lines to investigate the convective blue shift and the line asymmetries, and the center-to-limb variation of these quantities. We used the LARS instrument at the Vacuum-Tower-Telescope of the KIS on Tenerife, consisting of a fiber-fed spectrograph with a field of view of either 10 or 3 arcseconds, and a laser frequency comb for absolute wavelength calibration. A major upgrade from a prototype to a turnkey system, which was made in the first year of this project, but with independent funding, converted LARS into an instrument with nearly 100% duty cycle. This allowed us to carry out several successful observing campaigns with more than 300 hours of observations, recording a few dozens of spectral lines in the wavelength range between 5200 and 6600 Å, with a spectral resolution of l/Dl=700,000. The main results are summarized below. The convective blue shift of solar spectral lines changes systematically with line strength: The blueshift of the line minima decreases almost linearly as a function of line depth. From the deepest to the highest layers of the photosphere, the systematic convective blueshift decreases by approximately 700 m/s. We also find a systematic trend of the center-to-limb variation of the convective blue shift. The weakest lines which are attributed to the lower photosphere feature the strongest blueshift with a monotonic decrease toward the solar limb. On the contrary, line minima formed in the upper photosphere to chromosphere exhibit hardly any blueshift or even a slight redshift, with a less pronounced center-to-limb variation. A comparison of our measurements with Dopplergrams from the full-disk imager Helioseismic Magnetic Imager (HMI) reveals a systematic discrepancy (up to 200 m/s) between the convective blueshifts measured by HMI and by LARS. We find that the center-to-limb variation of HMI suffers from an additional blueshift that is incompatible with our results. We also compare our results with predictions of the center-to-limb variation of existing numerical hydrodynamical models and find good agreement at disk center and close to the limb. However, at intermediate disk positions, the line synthesis, or the 3D hydrodynamic model itself, require modification, to properly reproduce the measured center-to-limb variation of the convective blueshift. We find that sunspot umbrae yield strongly reduced convective blueshifts around −30 m/s. This value is not constant but is anticorrelated with the magnitude of the magnetic field strength, and correlated with the brightness of the umbra. In the darkest umbrae, the residual blue shift may reach a value of zero. Finally, we analyzed the convective blueshift and its center-to-limb variation of the strong Fraunhofer lines Na I 5896 Å (Na D1) and H I 6563 Å (Ha). We demonstrate that Ha is largely of photospheric origin except for the innermost line core, while the NaD1 line is mostly chromospheric.
Publications
- 2017, LARS: An Absolute Reference Spectrograph for solar observations. Upgrade from a prototype to a turn-key system, Astronomy & Astrophysics, 607, A12
Löhner-Böttcher, J.; Schmidt, W.; Doerr, H.-P.; Kentischer, T.; Steinmetz, T.; Probst, R. A.; Holzwarth, R.
(See online at https://doi.org/10.1051/0004-6361/201731164) - 2018, Absolute velocity measurements in sunspot umbrae, Astronomy & Astrophysics, 617, A19
Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.; Doerr, H.-P.; Steinmetz, T.; Holzwarth, R.
(See online at https://doi.org/10.1051/0004-6361/201832886) - 2018, Convective blueshifts in the solar atmosphere II. High-accuracy observations of the Fe I 6173.3Å line and deviations of full-disk Dopplergrams, 2019, Astronomy & Astrophysics 622, A34
Stief, Franziska; Löhner-Böttcher, Johannes; Schmidt, Wolfgang; Steinmetz, Tilo; Holzwarth, Ronald
(See online at https://doi.org/10.1051/0004-6361/201834538) - 2018, Convective blueshifts in the solar atmosphere. I. Absolute measurements with LARS of the spectral lines at 6302 Å, Astronomy & Astrophysics, 611, A4
Löhner-Böttcher, J.; Schmidt, W.; Stief, F.; Steinmetz, T.; Holzwarth, R.
(See online at https://doi.org/10.1051/0004-6361/201732107) - (2019). Convective blueshifts in the solar atmosphere-II. High-accuracy observations of the Fe I 6173.3 Å line and deviations of full-disk Dopplergrams. Astronomy & Astrophysics, 622, A34
Nebelo, E; Löhner-Böttcher, J.; Schmidt, W.; Steinmetz, T.; Holzwarth, R.
(See online at https://doi.org/10.1051/0004-6361/201834538) - 2019, Convective blueshifts in the solar atmosphere III. High-accuracy observations of spectral lines in the visible, Astronomy & Astrophysics, 624, A57
Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.; Steinmetz, T.; Holzwarth, R.
(See online at https://doi.org/10.1051/0004-6361/201834925)
