  Betelgeuse, a red supergiant star of semi-regular variability, reached a
historical minimum brightness in February 2020, known as the Great Dimming.
Even though the brightness has returned to the values prior to the Great
Dimming now, it continues to exhibit highly unusual behavior. Our goal is to
study long-term dynamics of the photosphere, including during the Great
Dimming. We applied the tomographic method, which allows different layers in
the stellar atmosphere to be probed in order to reconstruct depth-dependent
velocity fields. The method is based on the construction of spectral masks by
grouping spectral lines from specific optical depths. These masks are
cross-correlated with the observed spectra to recover the velocity field inside
each atmospheric layer. We obtained about 2800 spectra over the past 15 years
that were observed with the STELLA robotic telescope in Tenerife. We analyzed
the variability of five different layers of Betelgeuse's photosphere. We found
phase shift between the layers, as well as between the variability of velocity
and photometry. The time variations of the widths of the cross-correlation
function reveal propagation of two shockwaves during the Great Dimming. For
about two years after the dimming, the timescale of variability was different
between the inner and outer photospheric layers. By 2022, all the layers were
pulsating with higher frequency corresponding with the first overtone. The
combination of the extensive high-resolution spectroscopic data set with the
tomographic method revealed the variable velocity fields in the photosphere of
Betelgeuse, for the first time in such detail. Our results demonstrate that
powerful shocks are the triggering mechanism for episodic mass-loss events,
which may be the missing component to explain the mass-loss process in red
supergiants.
