  Understanding stellar activity in M dwarfs is fundamental to improving our
knowledge of the physics of stellar atmospheres and for planet search
programmes. High levels of stellar activity (also with flare events) can cause
additional variations in the stellar emission that contaminate the signal
induced by a planet and that need to be corrected. The study of activity
indicators in active stars can improve our capability of modelling this signal.
Our aim is to understand the behaviour of stellar chromospheres of M stars,
studying the more sensitive chromospheric activity indicators, characterising
their variability and on finding the correlations among these indicators to
obtain information on the origin of the magnetic activity in low-mass stars. We
studied the main optical activity indicators (Ca II H&K, Balmer lines, Na I
D$_{1,2}$ doublet, He I D$_3$ and other helium lines) measured for AD Leonis
using the data provided by HARPS-N in 2018 and by HARPS in 2006. We measured
excess flux of the selected activity indicators and analysed the correlation
between the different indicators as well as the temporal evolution of fluxes. A
stellar flare was identified during the 2018 observing run and the H$\alpha$,
H$\beta$, He I 4471 A and He I 5876 A lines were analysed in detail by fitting
the line profiles with two Gaussian components. We found that the Ca II H&K
flux excesses are strongly correlated with each other, but the Ca II H&K
doublet is generally less correlated with the other indicators. Moreover,
H$\alpha$ is correlated with Na I doublet and helium lines. Analysing the time
variability of flux of the studied lines, we found a higher level of activity
of the star during the observations in 2018 than in 2006, while Ca II H&K
showed more intense emission on spectra obtained during the observations in
2006. We investigated the flare evaluating the mass motion during the event.
