The relationship between sunspot activity and the reversal of the sun's magnetic polarity
What are sunspots?
Sunspots are areas on the photosphere (the visible surface of the sun), which appear darker than the surrounding surface of the sun by virtue of their relatively lower temperature. But in absolute terms, they are actually very bright. Sunspots can last from hours to months, which are carried around the solar surface by the sun's rotation. Sunspots often appear in pairs or clusters. The formation of sunspots is associated with solar magnetic disturbances.
What is a sunspot cycle?
The number of sunspots changes periodically. In the early stage of a sunspot cycle, the number of sunspots increases when the solar state changes from less active to more active. The period or year with the most sunspots is called "solar maximum", it is also the time when the solar activity is most active and solar magnetic storms often occur. When the solar state becomes less active, the number of sunspots decreases in the later stage of the sunspot cycle. The period (in year) with the least sunspots is called "solar minimum", during which the solar activity is most quiescent. The period of a sunspot cycle usually refers to the duration starting at its solar minimum and ending at the next solar minimum. The whole period of sunspot cycle lasts about 11 years.
What is the reversal cycle of the sun's magnetic polarity (the solar magnetic activity cycle)?
The magnetic north and south poles of the sun reverse once in about 11 years. The magnetic north pole at the beginning will become the magnetic south pole later. Similarly, the magnetic south pole at the beginning will become the magnetic north pole later. The sun's magnetic north and south poles will reverse twice and resume to their original positions in about 22 years. So, the reversal period of the sun's magnetic poles or magnetic field is about 22 years.
Why does the sun's magnetic field change?
When the sun spins, the lines of magnetic force beneath the sun's surface will also rotate. The sun is a fluid. Unlike the earth having a crust, the sun does not have a hard shell. The sun's spinning speed decreases as its latitude increases (i.e., differential rotation). The sun rotates relatively slowly near the poles. The sun's rotational period at the equator is about 25 days. The sun's rotational period at latitude 60 degrees is about 29.3 days, which is relatively long. Because the sun has differential rotational speeds, the embedded lines of magnetic force beneath sun's surface near the equator rotate fastest. The early longitudinal lines of magnetic force in the sun will become curly and twist gradually. Hence, the magnetic fields gradually change.
What are the development characteristics of sunspot activity?
Sunspots often appear near 30-35 degrees North and South of the sun's hemisphere at the beginning of the sunspot cycle. The sunspots tend to appear in lower latitudes later, and can appear near 5 degrees North and South of the sun's hemisphere near the end of the sunspot cycle.
The sunspots usually appear in a pair or a group on sun's surface inside a bipolar magnetic region (BMR) with strong magnetic field, where the lines of magnetic force are twisted. The pair of sunspots shows opposite magnetic polarity. If one of the two sunspots is a magnetic south pole, the other sunspot will be a magnetic north pole. Annular curved lines of strong magnetic force connect the two sunspots. At the early stage, the axis joining the two sunspots with "preceding" (p) and "following" (f) members is roughly parallel to the equator. The preceding (relative to the direction of the sun's rotation) sunspot is usually larger and more obvious with relatively longer lifetime. In the sun's northern hemisphere, the preceding sunspot will tend to be closer to the solar equator. The following sunspot will tend to be closer to the sun's north magnetic pole. Similarly, in the sun's southern hemisphere, the preceding sunspot will tend to be closer to the solar equator. The following sunspot will tend to be closer to the sun's south magnetic pole. The number of sunspots will increase as the sun activity becomes active.
Joy's law and Hale's law
Sunspot pairs or groups are tilted with the leading sunspots closer to the equator than the following sunspots. The tilting angle increases with latitude. The tilting phenomenon of sunspot pair is known as Joy's law.
The sunspots of a new sunspot cycle after a minimum of solar activity, appearing in high latitudes, are of opposite magnetic polarity in the northern and southern hemispheres. As the cycle progresses, the mean latitude of sunspots in each hemisphere steadily decreases and the sunspots will appear closer to the equator without change in their magnetic polarities. However, the magnetic polarities of sunspots forming in the next sunspot cycle reverse. This phenomenon of sunspot polarity reversal was discovered by the American astronomer, G.E. Hale, and is known as Hale's law.
Fig.1 The Babcock's model of the Sun's magnetic cycle
What is the relationship between sunspot activity and the reversal of the sun's magnetic polarity?
The relationship is explained by the Babcock's model of the Sun's magnetic cycle (Fig.1). During the period of peak solar activity (solar maximum), the outward projected annular lines of magnetic force between two sunspots will grow up, which will later rupture and separate from the sunspots. The detached annular lines of magnetic force will then get in touch and join with the nearby lines of magnetic force outside the sun, forming an independent loop of lines of magnetic force outside the sun. On the other hand, some lines of magnetic force in the sun will neutralize the lines of magnetic force from some groups of sunspots, weaken, re-connect and organize into a new solar magnetic field with reversed magnetic north and south poles gradually. So, sunspot activity and the reversal of the sun's magnetic polarity are related. If the change of solar magnetic field is also taken into consideration, the period of a sunspot cycle should be about 22 years.
- "The Topology of the Sun's Magnetic Field and the 22-Year Cycle", by H. W. Babcock, Astrophysical Journal, vol. 133, p.572, 03/1961.
- "The Law of Sun-Spot Polarity", by George E. Hale, Proceedings of the National Academy of Sciences of the United States of America, Volume 10(1): pp53!V55. January 1924.
- "Sunspot group tilt angles and the strength of the solar cycle", by M. Dasi-Espuig, S. K. Solanki, N. A. Krivova, R. H. Cameron, and T. Penuela, Astronomy & Astrophysics, May 12, 2010.
- "The Solar Dynamo", The National Aeronautics and Space Administration (NASA).
- "How Sunspots are Thought to Develop", Windows to the Universe, at the website of University Corporation for Atmospheric Research (UCAR).