Absolute velocity measurements in sunspot umbrae
J. L\"ohner-B\"ottcher, W. Schmidt, R. Schlichenmaier, H.-P. Doerr, T., Steinmetz, R. Holzwarth

TL;DR
This study precisely measured Doppler velocities in sunspot umbrae, revealing significantly reduced convection compared to the quiet Sun and highlighting the importance of considering magnetic field effects for accurate velocity references.
Contribution
We used laser frequency comb calibration to accurately measure Doppler shifts in sunspot umbrae, showing reduced convection and its correlation with magnetic field strength and temperature.
Findings
Sunspot umbrae show much smaller convective blueshifts (~-30 m/s) than the quiet Sun (~-350 m/s).
Vertical upward motion decreases with increasing magnetic field strength.
Extrapolation to zero magnetic field aligns with quiet Sun Doppler shifts.
Abstract
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. To clarify the amount of convective motion in the darkest part of umbrae, we directly measured Doppler velocities with an unprecedented accuracy and precision. 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. A thorough spectral calibration, including the measurement of the reference wavelength, yielded Doppler shifts of the spectral line Ti i 5713.9 {\AA} with an uncertainty of…
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