This chapter describes the magnetic activity of the Sun throughout most of its history. We begin with a summary of the activity of the Sun as described in historical records, i.e. going back at most a few centuries. The picture of the Sun in time can be completed by complementing solar observations with stellar data. This chapter focuses on the period from a solar age of roughly 0.1 Gyr onward. The formation of the Sun and its solar system and the very earliest phases of their joint evolution are discussed in Chapters 3 and 4.
The Sun's magnetic activity and the associated changes in its extended atmosphere evolve on time scales that range from minutes up to billions of years. The coupling of these scales requires that we describe the patterns of the solar magnetic field from the smallest currently observable structures to the global dipole field; for the longest time scales, we need to include a discussion of stellar evolution. This chapter therefore summarizes the observationally determined properties of the solar magnetic field before discussing the variations in solar spectral radiance and the coupling of the solar coronal field to the heliosphere. These descriptions are based on the historical records of this activity that extend over only a few decades for high-energy particle and electromagnetic radiation, over about a century for its magnetic field, and four centuries for sunspot records.