The Active and Quiet Sun

Sunspot maxima correspond generally to periods of high solar activity. This activity includes increased solar wind and phenomena like auroras and magnetic storms that are correlated with the solar wind, increased eruptions from the solar surface (see the later discussion of flares and prominences), and increased nonthermal radio and X-ray emission. Conversely, near sunspot minima the Sun is much quieter with respect to these phenomena. These observations suggest that there is a set of related solar properties that increase and decrease their activity periodically. As we shall see, all of these indicators of solar activity are related in some rather direct way to the Sun's magnetic field.

Changes in the Corona

There are significant differences in the nature of the corona during periods of active and quiet Suns, as illustrated in the following images.

The left image, corresponding to a year of high solar activity (1980), has a much more symmetric corona than the right image, which is for a year with low solar activity (1988).

The Maunder Minimum

Historical records suggest that from 1645 to 1715 there were very few sunspots and little mention of auroras or the solar corona in eclipses:

This period is called the Maunder Minimum in sunspots. Although one must be careful with arguments based on the absence of data, the telescope had been invented and there was interest in observing the sky in this period. This suggests that the Maunder Mininum was real and that the Sun was much less active than usual in this period (see the right panel).