On a recent trip to Florida, my wife and I spent a couple of days at Jack Newton's imaging center. The objective was to image Jupiter but bad seeing prevented doing anything useful at night. However, Jack had a hydrogen alpha filter available for solar viewing and the very narrow bandwidth of this filter makes it much less sensitive to seeing. Jack set this up during the day and the views of the sun though this device are truly magnificent. Never allowing a photo opportunity to slip by, I bandaided the video camera to the telescope and got some fair images of prominances and sunspots.

We begin our tour of the Sun with a group of sunspots, the best known solar features, taken with the black and white video camera. This group was easily visable to the naked eye and required nothing but a very dark neutral density filter to see.

As a point of reference, the overall size of this group is about the size of the United States. Although dark or nearly black, sunspots are actually about 3800F and only look dark relative to the much hotter (5800F) background photosphere. Viewed against a dark background, sunspots would be a brilliant white hot.

Sunspots are extremely powerful magnetic storms, so powerful in fact, that they prevent energy from rising to the surface, and hence, are darker than the surrounding photosphere.

Sunspots are easily seen on the surface of the sun but the most dramatic aspect of them can only be seen when they are near the edges and viewed through an exotic filter known as a hydrogen alpha filter . This is a complicated and expensive apparatus that passes only a very narrow band of light centered around 6563 angstroms, the hydrogen alpha line. This allows the much dimmer prominences, as they are called, to be seen against the much brighter photosphere. Unfortunately, red is the weak link in color video and much better images could have been obtained with the monochrome camera if other problems did not prevent it's use at the time.

This giant prominence has cleared the northwest limb and will probably never fall back. It has risen to a height of about 80,000 miles.

This pair of prominences on the southeastern limb, is probably the interaction of two adjacent sunspots. The highest point is about 50,000 miles from the surface.

As we near the peak of the current 11 year sunspot cycle, a great deal of activitiy is taking place on the surface of the Sun. Many sunspots are large enough to be easily visible to the "naked" eye with a proper solar filter. These large disturbances are frequently associated with large magnetic storms, causing aurora dispays and communications problems.

In the case of the group in this week's image, my local weather radio channel was picking up the Chicago channel for about 3 days. A good aurora display was reported mid-week but I seem to have missed it.


The Sun rotates on it's axis in about 25 days so it takes about half that length of time for a sunspot to pass across the face of the Sun. These two images were taken one day apart and the spot on the Western limb of the first image has passed around behind on the second image.

This even larger group was taken on
Mar 27, 2001

So.... what are sunspots?

Contrary to what our eyes may be telling us, these "dark" spots are intensely brilliant. They are actually about 4800 degrees which is roughly 1000 degrees hotter and brighter than the white-hot filament in an incandescent light bulb. They just happen to be about a thousand degrees cooler than the surrounding surface of the sun and appear dark by comparison.

In another frame of reference, if a large sunspot appeared by itself in the night sky, it would appear as a bright red star about 10 times brighter than the full moon.

We call machines that can keep the temperature below that of the surroundings, refrigerators. Sunspots are therefore natural refrigerators on a tremendous scale. Kind of a solar ice berg; the only reason we see them is because of the temperature difference.

All this is very interesting but does not really answer the question and as near as I can tell, there is no simple answer. Sunspots are clearly the product of extrememly strong magnetic fields and as they usually appear in pairs, one can easily visualize that they are the ends of giant horseshoe magnets. It turns out that each of the two spots of a pair is oppositely polarized so the horseshoe magnet is not all that far fetched. The magnet field is so strong that it prevents energy from flowing horizontally and greatly reduces the flow of energy upward and makes it much cooler than the surrounding region that is free of magnetic fields.

So, where does the magnetic field come from? I mentioned above that the sun rotates in about 25 days just to keep things simple. However, this rate applies to the equator but near the poles, the rate is as long as 36 days. This so-called differential rotation would tear apart a solid body like the Earth but the gaseous structure of the Sun just takes it in stride. One of the sunspot theories relies on this differential rotation to produce a giant dynamo as layers of hot gasses pass each other. It is not hard to see how this dynamo effect could produce strong magnetic fields and even horseshoe magnets for Mr Wizzard in the sky.

These images were taken with a PC-27C color video camera using a reflective type solar filter. They were taken with two different scopes, hence the difference in image quality. The first was taken with an Orion Short Tube 4.5" Newt and the second with my home made 5" Newt described elsewhere on this site.

All photos taken with the...

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