  The so-called great dimming event of alpha Ori in late 2019 and early 2020
sparked our interest in the behaviour of chromospheric activity during this
period. To study the timeline of chromospheric activity, we derive a S_MWO time
series of TIGRE and Mount Wilson values, and we compare this long time series
with photometric data from the AAVSO database. In addition, we determine the
absolute and normalised excess flux of the Ca II H&K lines. To do so, we
estimate the changing effective temperature from TIGRE spectra and find a clear
drop of about 80 K between November 2019 and February 2020, which coincides
with the minimum of visual brightness. During the same period, the S-index
increased significantly, yet this is a mere contrast effect, because the
normalised excess flux of the Ca II H&K lines did not change significantly.
However, the latter dropped immediately after this episode. Comparing the
combined S_MWO values and visual magnitude time series, we find a similar
increase in the S-index during another noticeable decrease in the visual
magnitude of alpha Ori, which took place in 1984 and 1985. To also probe the
dynamics of the upper photosphere, we analysed the lines in 6251-6263 A and
found core distance varies which shows a relation with the great dimming event.
This type of variation could be caused by rising and sinking cool plumes as a
temporary spill-over of convection on alpha Ori. Based on our study, we
conclude that the cause for the great dimming is located in the photosphere.
Furthermore, the long-term spectroscopic and photometric time series suggests
that this great dimming does not appear to be a unique phenomenon, but rather
that such dimmings do occur more frequently, which motivates further monitoring
of alpha Ori with facilities such as TIGRE.
