The Quiet vs. Active Sun

The principal manifestation of the Solar Activity Cycle is the changing number of sunpots visible on the surface of the Sun. As discovered in 1843 by Heinrich Schwabe, a German apothecary, the number of Sunspots varies roughly periodically with a period of 11 years (plot of sunspot number). (The cycle based on other markers suggests that the period is actually 22 years.) In addition to the variation in the number of sunspots, their location on the surface of the Sun changes over the sunspot cycle. As the activity level of the Sun increases the sunspots appear closer to the equator of the Sun, producing the so-called butterfly diagram. The activity of the Sun varies with the number of visible Sunspots.

Based on the historical record, it has been shown that the Sunspot Cycle can shut down, that is, there are stretches where there are no visible Sunspots. There have been several such intervals in recent history. The most prominent one is referred to as the Maunder Minimum which occurred in the 1600's.

QUIET SUN The Sun generates energy via nuclear reactions in its core. The energy is initially carried outward by the radiation (radiation transport). However, about 70 % of the way through the Sun, the energy transport shifts to convection. Convection carries the energy to the surface of the Sun where it is radiated from the photosphere. Above the photosphere, sits the upper two layers of the Solar atmosphere, the Chromosphere and the Corona. The motions in the convective layer leads to granulation

ACTIVE REGIONS Sunspots and Sunspot and granulation Flares and Flare Event. SOHO MPEG video of a Solar Flare. SOHO animated gifs of X-class flares, X-type, X8-type. Prominences Loops and Magnets Eruptive Loops, more eruptive loops, yet more eruptive loops (animated gif 1, animated gif 2) Corona Streamers. SOHO lasco MPEG video of a Coronal Mass Ejection (CME). SOHO Images of a double CME, and a large CME. X-rays, X-rays, and more X-rays. YOHKOH soft x-ray MPEG video of Sun. X-rays and Streamers.

The luminosity of the Sun is apparently larger at the time of maximum activity but the increase in luminosity is small, less than 1 %. For a review of the current state of understanding on the connections between Solar luminosity variations and the climate of the Earth, see Variations in Solar Luminosity and Their Effect on the Earth's Climate, Foukal et al. 2006, Nature, 443, 161.