Nature of solar behaviour

Current knowledge of the present state of the sun and of its evolution since formation as a main sequence star 4.7 billion years ago is reviewed as an astrophysical background to studies of the fossil record of solar behavior. Attention is given to the internal structure of the sun as a reflection of the thermonuclear energy generation processes and convective energy dissipation processes occurring there, and to the magnetic fields caused by motions in the convective zone, which are responsible for solar activity. The long-term variability of the periodic changes in solar magnetic fields explained by kinematic dynamo theory is then considered in light of the sunspot record, terrestrial records, and the relationship between magnetic activity, solar luminosity and terrestrial climate. The evident magnetic properties of the sun are compared with those observed in other stars, and it is concluded that solar activity may have been 10 to 100 times more vigorous than at present when the sun reached the main sequence, and even more violent before then