Sunspots

This week we have been seeing the largest sunspot/active region in years. In fact, the entire group is larger than Jupiter. So let's take a look at the history of Sunspots and also see how big this sunspot group labeled AR1339 (for Active Region) really is. 

This image is from November 7, 2011 and it shows the large active region 1339. On the lower right corner you have a size comparison between Earth, Jupiter and the AR 1339

This is AR1339 again on November 8, 2011. Look at this amazing image! 

What were sunspots? Galileo had guessed they were clouds floating in the Sun's atmosphere, obscuring some of its light. Their true nature only emerged in 1908 when George Elery Hale, leader among US astronomers, showed that they were intensely magnetic. Their magnetic field was as strong as that of a small iron magnet, some 3000 times stronger than the field near the surface of the Earth--yet those fields often extended over areas larger than the entire surface of the Earth. Apparently the magnetic field somehow slowed down the flow of heat from the Sun's interior, causing the sunspots to be slightly darker than the rest of the Sun.


The evidence for sunspot magnetism was their emitted light. Glowing gases emit light in narrowly defined wavelengths (i.e. colors), a different set for each substance. In 1897, however, Pieter Zeeman found that when such light was emitted from the region of a strong magnetic field, the emission split into slightly different wavelengths, with a separation that increased with the strength of the field. The colors of the light emitted from sunspots were "split up" in just this way.

The method was later improved by Babcock and others, allowing astronomers to observe not only the magnetic field of sunspots but also the weak fields near the Sun's poles. It turned out that the Sun has a polar field somewhat like the Earth's, but that it reverses its polarity during each 11-year cycle.

Sunspots have also led us to a better understanding of the Earth's own magnetic field. The face of the Sun consists of ionized hot gas ("plasma"), hot enough to conduct electricity. Sunspot fields were evidently produced by electric currents, and it was well known that such currents could be generated by a "dynamo process," by the motion of an electric conductor (e.g. the flow of solar plasma) through a magnetic field.

In 1919 Sir Joseph Larmor proposed that the fields of sunspots were due to such dynamo currents. He suggested that a closed chain of cause-and-effect existed, in which the field created by these currents was also the field which made them possible, the field in which the plasma's motion generated the required currents. Many features of sunspots remain a mystery, but Larmor's idea opened an era of new understanding of magnetic processes in the Earth's core.

Sunspots are caused by the uneven rotation of the Sun, the equator rotating faster than the polar regions. That stretches out magnetic field lines, crowding them together and making their magnetic field stronger. Strong magnetic field (under the surface) pushes away the solar gas, which therefore gets less dense, so that regions of strong field tend to float up to the top, the way oil floats to the surface of water. Where it breaks the surface, sunspots occur. 

The solar surface and interior rotation rate, where red regions represent areas of slightly faster than average rotation while areas in blue show slower rotational rates. Credit: NSO

But we still do not understand a lot--why exactly the Sun rotates unevenly, why the north-south magnetic polarity reverses every 11-year cycle, how sunspots slow down the flow of solar heat (which makes them dark). 


Credit: NASA SDO / GSFC & NSO