Suppression of convection in sunspot umbrae

In the dark umbra of a sunspot, the convective motion is largely suppressed by the strong magnetic field. In the past, numerous studies have reported on negligible convective flows (e.g., Beckers 1977). Based on this, the umbra typically served as the zero-velocity reference for the calculation of Doppler velocities of an active region.

The darkest core of sunspot umbrae (left panel, red 3“ region) was spectroscopically observed with LARS. The Doppler shift of the neutral titanium line at 5713.9 Å (right panel) yielded the umbral velocity with an accuracy of a few m/s. The laser frequency comb (LFC, green curve) and the titanium hollow cathode lamp (HCL, black curve) enabled the absolute velocity calibration.

To verify the exact velocities in the darkest part of umbrae, KIS scientists (Löhner-Böttcher et al. 2018) performed spectroscopic observations with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency comb enabled the absolute wavelength calibration of the high-resolution solar spectra with an unprecedented accuracy of 1 m/s. The absolute Doppler velocity of 13 sunspot umbrae was spectroscopically measured with the neutral titanium line at 5713.9 Å. Compared with convective velocities (blueshift) of up to 350 m/s in the quiet Sun, the velocity in sunspot umbrae is strongly reduced to around 30 m/s. However, this umbral velocity value is not constant, but varies significantly with the magnetic field strength and temperature. The upward velocities decrease with increasing field strength. For magnetic configurations with a field strength of larger than 2.8kG, the convective motion is entirely absent. In conclusion, simply taking any sunspot umbra as a zero-velocity reference for the calculation of photospheric Dopplergrams can imply a systematic velocity error of the order of 100 m/s. With the new insights, the accuracy of the velocity calibration of active region Dopplergrams can be increased substantially. This will become a crucial step for the correct interpretation of small-scale motions in high-resolution Dopplergrams as planned to be observed with the Visible Tunable Filter (VTF) at the new Daniel K. Inouye Solar Telescope (DKIST).

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