摘要：Spectroscopic observations of a solar eclipse can provide unique information for solar
and exoplanet research; the huge amplitude of the Rossiter-McLaughlin (RM) effect during
solar eclipse and the high precision of solar radial velocities (RVs) allow detailed
comparison between observations and RV models, and they provide information about the
solar surface and about spectral line formation that are otherwise difficult to obtain. On
March 20, 2015, we obtained 159 spectra of the Sun as a star with the solar telescope and
the Fourier Transform Spectrograph at the Institut für Astrophysik Göttingen, 76 spectra
were taken during partial solar eclipse. We obtained RVs using I2 as wavelength
reference and determined the RM curve with a peak-to-peak amplitude of almost
1.4 km s-1 at
typical RV precision better than 1 m s-1. We modeled the disk-integrated solar RVs using
well-determined parameterizations of solar surface velocities, limb darkening, and
information about convective blueshift from 3D magnetohydrodynamic simulations. We confirm
that convective blueshift is crucial to understand solar RVs during eclipse. Our best
model reproduced the observations to within a relative precision of 10% with residuals
lower than 30 m s-1. We cross-checked parameterizations of velocity fields
using a Dopplergram from the Solar Dynamics Observatory and conclude that disk-integration
of the Dopplergram does not provide correct information about convective blueshift
necessary for m s-1
RV work. As main limitation for modeling RVs during eclipses, we identified limited
knowledge about convective blueshift and line shape as functions of solar limb angle. We
suspect that our model line profiles are too shallow at limb angles larger than
μ =
0.6, resulting in incorrect weighting of the
velocities across the solar disk. Alternative explanations cannot be excluded, such as
suppression of convection in magnetic areas and undiscovered systematics during eclipse
observations. To make progress, accurate observations of solar line profiles across the
solar disk are suggested. We publish our RVs taken during solar eclipse as a benchmark
curve for codes calculating the RM effect and for models of solar surface velocities and
line profiles.
