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The so-called great dimming event of alpha Ori in late 2019 and early 2020sparked our interest in the behaviour of chromospheric activity during thisperiod. To study the timeline of chromospheric activity, we derive a S_MWO timeseries of TIGRE and Mount Wilson values, and we compare this long time serieswith photometric data from the AAVSO database. In addition, we determine theabsolute and normalised excess flux of the Ca II H&K lines. To do so, weestimate the changing effective temperature from TIGRE spectra and find a cleardrop of about 80 K between November 2019 and February 2020, which coincideswith the minimum of visual brightness. During the same period, the S-indexincreased significantly, yet this is a mere contrast effect, because thenormalised excess flux of the Ca II H&K lines did not change significantly.However, the latter dropped immediately after this episode. Comparing thecombined S_MWO values and visual magnitude time series, we find a similarincrease in the S-index during another noticeable decrease in the visualmagnitude of alpha Ori, which took place in 1984 and 1985. To also probe thedynamics of the upper photosphere, we analysed the lines in 6251-6263 A andfound 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 atemporary spill-over of convection on alpha Ori. Based on our study, weconclude that the cause for the great dimming is located in the photosphere.Furthermore, the long-term spectroscopic and photometric time series suggeststhat this great dimming does not appear to be a unique phenomenon, but ratherthat such dimmings do occur more frequently, which motivates further monitoringof alpha Ori with facilities such as TIGRE.

Chromospheric activity and photospheric variation of $\alpha$ Ori during the great dimming event in 2020