Absolute velocity measurements in sunspot umbrae

Abstract

Context. In sunspot umbrae, convection is largely suppressed by the strong magnetic field. Previous measurements reported on negligible convective flows in umbral cores. Based on this, numerous studies have taken the umbra as zero reference to calculate Doppler velocities of the ambient active region. Aims. To clarify the amount of convective motion in the darkest part of umbrae, we directly measured Doppler velocities with an unprecedented accuracy and precision. Methods. We performed spectroscopic observations of sunspot umbrae with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the high-resolution spectrograph and absolute wavelength positions for 13 observation sequences. A thorough spectral calibration, including the measurement of the reference wavelength, yielded Doppler shifts of the spectral line Ti i 5713.9 Å with an uncertainty of around 5 m s −1 . A bisector analysis gave the depth-dependent line asymmetry. Results. The measured Doppler shifts are a composition of umbral convection and magneto-acoustic waves. For the analysis of convective shifts, we temporally average each sequence to reduce the superimposed wave signal. Compared to convective blueshifts of up to −350 m s −1 in the quiet Sun, sunspot umbrae yield a strongly reduced convective blueshifts around −30 m s −1 . We find that the velocity in a sunspot umbra correlates significantly with the magnetic field strength, but also with the umbral temperature defining the depth of the Ti i 5713.9 Å line. The vertical upward motion decreases with increasing field strength. Extrapolating the linear approximation to zero magnetic field reproduces the measured quiet Sun blueshift. In the same manner, we find that the convective blueshift decreases as a function of increasing line depth. Conclusions. Simply taking the sunspot umbra as a zero velocity reference for the calculation of photospheric Dopplergrams can imply a systematic velocity error reaching 100 m s −1 , or more. Setting up a relationship between vertical velocities and magnetic field strength provides a remedy for solar spectropolarimetry. We propose a novel approach of substantially increasing the accuracy of the Doppler velocities of a sunspot region by including the magnetic field information to define the umbral reference velocity.

The CARMENES search for exoplanets around M dwarfs

“…Thus, the solar convective blueshift is used as a reference. As an estimation of its absolute value, in our case a solar blueshift of 300 m s −1 (Dravins et al 1981;Cavallini et al 1985;Löhner-Böttcher et al 2018) was subtracted from all CS values shown in this work. Therefore, the absolute values of CS given in this work have a dependence on the precise determination of the convective blueshift from CIFIST 3D models.…”

Temporal Evolution of the Inverse Evershed Flow

“…The zero point of the velocity scale was defined separately for each observation as the average velocity value across the full IBIS aperture. This velocity calibration or forcing the average umbral velocity to zero (Cabrera Solana et al 2007;Löhner-Böttcher et al 2018) gave nearly identical results within ±0.5 km s −1 . For the two observations Nos.…”

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…These conspire to dramatically reduce the signal-to-noise ratio of the observations, particularly when spectropolarimetric measurements are needed to determine the magnetic-field configuration along with the flow. Interestingly, umbral dots may not be uniformly distributed throughout the umbra, with recent measurements suggesting the existence of very dark, fully non-convective umbral regions (Löhner-Böttcher et al, 2018). DKIST's off-axis design and consequent low scattered-light properties will allow high-precision high-resolution Doppler and spectropolarimetric measurements of even the darkest regions of the umbra.…”