Analyzing the long-term evolution of active regions (ARs) permits to quantify the link between their atmospheric emission (from optical to X-rays) and the magnetic field. Multi-wavelength studies provide the full story, and not just a snapshot, of the phenomena and they allow us analyze how the atmosphere changes as the field strength decreases (with the dispersion of the AR).

The evolution of the emitted flux in various wavelength ranges contains information on the heating mechanism(s) of the different atmospheric levels. I review what we can learn from such long-term (months) multi-wavelength observations, and compare the results to other solar and stellar studies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html