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We present a spectroscopic analysis of a 1-year intensive monitoring campaignof the 22-Myr old planet-hosting M dwarf AU Mic using the HARPS spectrograph.In a companion paper, we reported detections of the planet radial velocity (RV)signatures of the two close-in transiting planets of the system, withrespective semi-amplitudes of 5.8 $\pm$ 2.5 m/s and 8.5 $\pm$ 2.5 m/s for AUMic b and AU Mic c. Here, we perform an independent measurement of the RVsemi-amplitude of AU Mic c using Doppler imaging to simultaneously model theactivity-induced distortions and the planet-induced shifts in the lineprofiles. The resulting semi-amplitude of 13.3 $\pm$ 4.1 m/s for AU Mic creinforces the idea that the planet features a surprisingly large innerdensity, in tension with current standard models of core accretion. Ourbrightness maps feature significantly higher spot coverage and lower level ofdifferential rotation than the brightness maps obtained in late 2019 with theSPIRou spectropolarimeter, suggesting that the stellar magnetic activity hasevolved dramatically over a $\sim$1-yr time span. Additionally, we report a3-$\sigma$ detection of a modulation at 8.33 $\pm$ 0.04 d of the He I D3(587.562 nm) emission flux, close to the 8.46-d orbital period of AU Mic b. Thepower of this emission (a few 10$^{17}$ W) is consistent with 3Dmagnetohydrodynamical simulations of the interaction between stellar wind andthe close-in planet if the latter hosts a magnetic field of $\sim$10 G.Spectropolarimetric observations of the star are needed to firmly elucidate theorigin of the observed chromospheric variability.

One year of AU Mic with HARPS: II -- stellar activity and star-planet interaction