The Quiet vs. Active Sun

The principal manifestation of the Solar Activity Cycle is the changing number (and location) of Sunspots visible on the surface of the Sun.

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Sunspots A sunspot is a region which is relatively cooler, T ~ 4,500 K, than the surrounding region on the Sun, T ~ 5,800 K. They are characterized by magnetic fields roughly a few thousand times stronger (0.35 Tesla) than the average Solar magnetic field of a few hundred micro-Teslas. For comparison, the magnetic field of the Earth is around 30 to 60 micro-Teslas and that of a clinical MRI machine is 0.1-3 Tesla. The strong magnetic field may inhibit the heat flow to the Sunspot which causes their lowered temperatures. Sunpots are around the size of the Earth and have lifetimes of 7-10 days. Sunspots generally appear in groups; groups can have lifetimes of several Solar rotation periods (an average rotation period is around a month). The dark region of the Sunspot is its Umbra; the lighter region is the Penumbra.

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As discovered in 1843 by Heinrich Schwabe, a German apothecary, the number of Sunspots varies roughly periodically with an average period of 11 years (plot of sunspot number). (The cycle based on the polarity of the Sun's magnetic field 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 when 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 During the times when there are no sunspots, we refer to the Sun as the 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. The granulation are the tops of convective cells. The whitish areas are the hotter upwelling gas and the darker areas are the sinking cooler gas. Granulation cells are ~1,000 km in size and last for 5-10 minutes.

Photosphere, Chromosphere, Transition Region, and Corona The visible surface of the Sun is the photosphere. Above the photosphere sits the chromosphere, and the corona. Interestingly, the temperature falls through the chromosphere (as is expected), however, after the temperature reaches ~4,500 Kelvin, it starts to rise rapidly through the transition region reaching ~1-3 million Kelvin in the corona. It is believed that the corona is heated by waves generated in the turbulent convection zone of the Sun which break in the corona.

The hot corona boils-off the Sun and flows outward in what is called the Solar Wind. The Solar Wind flows through regions known as Coronal Holes ( x-ray image, solar wind) and from around coronal streamers. The Solar Wind has two components, the slow solar wind and the fast solar wind. The slow solar wind moves at 400 km/s with T a few 10⁵ K with chemical composition like the corona. The fast solar wind moves at 750 km/s with T nearly one million K and chemical composition like the Sun's photosphere. The overall Solar Wind is weak; the Sun has lost only ~ 0.1 % of its mass over its lifetime of 4.6 billion years. If the Sun had a more intense wind in the past, then it could have lost more mass than suggested above. In this case, it was suggested that we could avoid the Faint Young Sun Paradox. There is no reason to believe, however, that in the past the wind from the Sun was as strong as needed to resolve the Faint Young Sun Paradox. Stars with more massive winds can lose significant amounts of their masses in several million years. The corona, in a sense, extends to the outer edge of the Solar System so that we can reasonably argue that we are immersed in the outer layers of the Sun.

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ACTIVE REGIONS Sunspots, Sunpots groups, and Plages Flares Coronal Zoo Prominence and Filaments Magnets, Buried Magnets, Prominences and Arcades Loop , Eruptive Prominence Streamers X-rays X-rays and Streamers Coronal Variation and the Sunspot Cycle The thread which links the disparate phenomena is apparently the magnetic field; the enhanced magnetic fields of sunspots, the arcade shapes of prominences, and the stored energy tapped by Solar flares.

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Solar Activity The Sun rotates differentially, that is, different parts of the surface of the Sun rotate at different rates; the rotation period ranges from 27 days near its equator to more than 30 days near its poles. This uneven spin winds up the magnetic field lines of the Sun (as shown to the left). This has the same effect as one winds up a rubber band. As the winding becomes tighter, the field (rubber band) is stressed. This stress leads to the formation of Sunspots, prominences, and an active corona. When the field snaps (locally) to release the tension, flares can result. Eventually after ~ 11 years, the stress causes a global relaxation of the field which also leads to a polarity reversal. The winding process then starts over leading to another relaxation after 11 years with another polarity reversal to return to the Sun to its original state. Sunspots vary on the relaxation time scale while the total cycle seems to be twice this relaxation time.

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The luminosity of the Sun is apparently larger at the time of maximum activity but the increase in luminosity is small, less than 1 %:

The luminosity of the Sun varies with the Solar Activity Cycle in that it is slightly brighter when the Sun is the most active (marked by the time when there are the most Sunspots visible on the Sun's surface). Apparently, although Sunpots are fainter than the quiet Sun, because they are relatively cooler, other features of the Solar Activity Cycle compensate for this and the overall luminosity is larger.