Solar sunspot cycle

A sunspot is an area of the Sun's photosphere, appearing darker to the eye than surrounding areas of the Sun . Although very hot by any terrestrial standard, sunspot regions are cooler that surrounding solar surface. Sunspots occur in cycles and are associated with a strong solar magnetic field . Variations in the solar magnetic field impact the space environment of Earth (sometime termed "space weather" and therefore have at least a correlated effect on Earth's weather and climatic conditions.

Large sunspots, visible to the naked eye, were noted by the ancient Chinese. The first specific mention of sunspots in modern scientific literature was by Italian astronomer and physicist Galileo Galilei (1564–1642) in his Starry Messenger published in 1610. Sunspot occurrence has been carefully noted by astronomers ever since. Astronomers have found that the frequency of sunspots varies with a period of between 11 and 13 years. This corresponds to the period of solar activity cycle involving solar flares, prominences and other phenomena associated with the outer layers of the Sun.

At the beginning of the solar cycle a few sunspots appear at the higher latitudes on the Sun near the poles. The sunspots then appear to move across the face of the Sun due to its rotation . Large spots may last for several rotation periods of about a month in length. As the cycle progresses the number of spots increases and they tend to be formed at lower latitudes toward the equator. The end of the cycle is marked by a marked drop in the number of low-latitude sunspots, which is followed immediately by the beginning of the next cycle, as small numbers of spots begin to appear at high latitudes.

Astronomers now know that sunspots are essentially magnetic storms on the surface of the Sun. The spots usually occur in pairs. Just as a bar-magnet placed under a sheet of paper will show a characteristic looping magnetic field when iron filings are scattered over the paper, so the sunspots making up the pair appear to be connected by a similar field. The ends of the bar magnet are characterized as being north or south magnetic poles depending on how a magnetic compass is affected by the poles. Similarly, each of the members of a sunspot pair will have the characteristics of either a north or south magnetic pole. During a particular cycle, the leading spot of the pair will always have the same polarity for spots formed in a particular hemisphere. The order of polarity is reversed for sunspot-pairs formed in the opposite hemisphere. Thus, if spots-pairs formed in the northern hemisphere of the Sun have the lead spot behaving as a south magnetic pole, the leading spot of a pair formed in the southern hemisphere will have north magnetic pole. This order of polar progression for the leading and trailing spots is preserved throughout the entire 11–13 year cycle. However, during the following sunspot cycle the order will be reversed in both hemispheres. This has led most astronomers to feel that the proper sunspot cycle should be reckoned as consisting of two 11–13 year cycles, since two cycles must pass before conditions are duplicated and the full pattern can repeat.

Although there have been attempts to link the solar cycle to changes on Earth, most are still characterized as correlations (i.e., the events are associated but there is no established cause and effect relationship. An exception may lie in the earth's weather. There have been some strongly suggestive correlations between solar activity and global temperature as well as rainfall variations. Analysis of tree-ring data spanning many centuries clearly shows the presence of a 11–13 year cycle. There is even compelling evidence from ancient rock layers that the solar cycle has been present since Precambrian times. Those involved in the launching and maintenance of Earth satellites are acutely aware that the upper layers of the earth's atmosphere respond to solar activity by expanding thereby increasing the atmospheric drag on satellites in low Earth orbit. Finally, there is a curious period of about 75 years shortly after Galileo's discovery when few sunspots were observed. This era is called the Maunder Minimum after the astronomer who first noted its existence. Other phenomena such as the Aurora Borealis (Northern Lights) that are associated with solar activity are also missing

from European records during this period. The interval is also associated with a time of unusually severe winters in both Europe and North America and is often called the "little ice age." In this century, Jack Eddy has found evidence that there may have been similar periods of cold associated with earlier interruptions of the solar cycle. Unfortunately, our understanding of the solar cycle is sufficiently crude that we have no explanation of what might cause these interruptions. Indeed, our understanding of the basis for the cycle itself is still largely in a phenomenological phase.

The very strong magnetic fields (i.e. several thousand times the general field of the earth) in a sunspot account for the dark appearance of the spot. The hot atmosphere of the Sun contains a significant number of atoms having a net positive charge resulting from collisions between them (i.e., they are ionized, having lost one or more electrons). Charged particles may move along magnetic field lines, but not across them. Thus, a magnetic field exerts a kind of pressure on the gas and helps to support it against the gravitational force of the Sun itself. This force is usually balanced only by the pressure of the hot gas surrounding a sunspot. With part of the pressure being supplied by the magnetic field, the gas will cool to a lower temperature. Because it is not as hot, it appears dark compared to the bright surrounding region called the photosphere. While they appear dark by comparison, sunspots are still hotter than any blast furnace on the earth and are only dark by contrast with the brilliant solar surface. A close inspection of a sunspot shows it to have a dark central region called an umbra surrounded by a lighter radial structured region called a penumbra. These regions can be understood in terms of the spreading and weakening magnetic field emanating from the core of the sunspot.

In the second half of this century, Eugene Parker suggested a mechanism that accounts for much of the descriptive behavior of sunspots. The Sun does not rotate as a rigid body and the polar regions rotate somewhat more slowly than the equator. Because of the charged nature of the solar material, the Sun's general magnetic field is dragged along with the solar rotation. However, it will be dragged faster and further at the equator than at the rotational poles. Although the general field of the Sun is quite weak (i.e., similar to that of the earth), the differential rotation strengthens and distorts the field over time. One can imagine the faster-rotating regions of the equator dragging the local magnetic field so that the field lines are drawn out into long thin tubes. The more these tubes are stretched, the stronger the field becomes. The magnetic pressure exerted on the surrounding gas causes the material within the tube to become "buoyant" compared to the surrounding material and rise toward the surface. As the magnetic tube breaks the surface of the Sun, it forms two spot-like cross-sections where it clears the lower solar atmosphere. As the field direction is out of the solar surface at one spot and into it at the other, one of these spots will appear to have one kind of magnetic pole and the other spot will appear to have the other. The global nature of the general solar field will guarantee that the stretched magnetic tubes will yield leading spots with opposite polarities in opposite hemispheres. A reversal of the Sun's general field between 11–13 year cycles would account for the reversal of this order. However, there is no compelling explanation of why the general field should reverse after each 11–13 year solar cycle

See also Aurora Borealis and Aurora Australialis; Coronal ejections and magnetic storms; Electricity and magnetism