Kolloquium: Ekaterina Dineva (AIP)
Sun-as-a-Star with PEPSI: Tracing Solar Activity in Strong Chromospheric Absorption Lines
The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) is a state-of-the-art, high-resolution spectrograph for the Large Binocular Telescope (LBT) at Mt. Graham, Arizona, USA. During daytime, the 11-millimeter aperture binocular Solar Disk-Integrated (SDI) telescope feeds PEPSI with disk-unresolved sunlight. This produces a quasi-continuous daily record of the Sun-as-a-star spectrum in the broad wavelength range 3830 – 9140 Å and with a spectral resolution of R ≈ 250 000. The Sun is the only star, where spectra can be related to surface structure and magnetic fields. We present high-resolution, high-fidelity spectra that were recorded with PEPSI/SDI on 184 & 82 days in 2018 & 2019 and derive the Ca II H & K emission ratio, i.e., the S-index. In addition, we compile excess brightness and area indices based on full-disk Ca II K line-core filtergrams of the Chromospheric Telescope (ChroTel) at Observatorio del Teide, Tenerife, Spain and full-disk UV 1600 Å images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Thus, Sun-as-a-star spectral indices are related to their counterparts derived from resolved images of the solar chromosphere. Combining these three types of indices provides evidence that the minimum towards the end of Solar Cycle 24 is very deep and represents the lowest level of activity recorded with contemporary instruments, with a strong activity asymmetry between the two hemispheres. Furthermore, the spectral profiles of the Fraunhofer Na I D doublet at λ5890/5896 Å was observed with PEPSI during the 2017 August 21 total solar eclipse. Sun-as-a-star spectra are mainly affected by limb darkening, solar differential rotation, convective motions, and magnetic activity. During a solar eclipse, the contribution from these features is modified by the passage of the Moon, thus causing a transit spectrum signature. The temporal evolution of the Na I D doublet is analyzed employing contrast profiles which illustrate subtle changes in the line profile. The line bisectors are used to study the height dependent signature of convective motions. These observations are compared to synthetic Na I D spectra computed with the state-of-the-art Bifrost code combined with a geometrically accurate model of the solar eclipse.
4. November 2021, 14:30 – 15:30