Visual Solar Phenomena

When looking at the Moon,we learned the importance of looking for the fine detail in craters and other features that stand out very plainly in sight. The Sun's outer gas layers are arrayed in three layers. The Sun's outermost visible gas layer, which we see when we view it through a telescope, is called the photosphere. The other two layers, the chromosphere and the corona are invisible under normal circumstances. Looking at the photosphere requires attention to detail because at first glance, it looks homogenous. But on close inspection, the Sun begins to reveal a

Figure 6.2. Close up of sunspot group on Dec. 27, 2002. Celestron C8 and 35mm SLR with eyepiece projection. Photo by author.

lot of detail. Its face is not really smooth as you might think at first but rather looks more like a plate of white rice (or yellow or blue or whatever color your filter renders the Sun). The Sun's face is completely covered in these so-called faculae. The faculae are areas of upwelling gas in areas of rising convection. The gas rises to the surface, cools and falls back into the Sun. The Sun's entire surface is covered in these surging pockets of hot gas causing its surface to perpetually rise and fall in mountains and valleys of 10,000 degree Celsius gas. Through any telescope with a proper solar filter the mottled surface of the Sun easily reveals itself. Remember that when you look at these seemingly tiny pockets of gas, each little cell you see can easily swallow Earth whole. Another amazing fact about the gases is their astonishing age. Each time a pocket of gas wells to the surface it represents the culmination of a journey of some fifty thousand years from the time it began to rise up from the center of the Sun.

The Sun is powerfully magnetic and currents of magnetic activity are perpetually coursing through the photosphere. At times, many magnetic currents become bundled together in close proximity. This causes the local magnetic field to increase in strength by thousands of times what is normal for the solar surface. The strong magnetic field inhibits the upward movement of gas creating a "cool" area on the surface of the Sun. The blocking action of the magnetic field creates a depression on the surface of the Sun that can be several hundred kilometers deep. The average temperature in the depression is approximately 4,200 degrees Kelvin17, about 1,500 degrees cooler than the average temperature in the photosphere. Because the area is cooler, it does not glow as brightly as the surrounding surface area and in fact the contrast is so great that the depressed area appears black. This is an optical illusion. If viewed in isolation, the surface area of a Sunspot is actually extremely bright. The actual area of depression in the photosphere is the darkest area of a sunspot and is called the umbra. A lighter colored outer area called the penumbra surrounds the umbra. The lighter color is caused by faculae

17 The Kelvin scale is preferred for use in most physics applications. One degree Kelvin equals one degree Celsius. The zero point of the Kelvin scale is set at absolute zero, the theoretical point where all atomic motion stops. Zero K equals -273° C.

encroaching upon and overriding the outer edge of the sunspot, which is still visible below the overriding gasses. The average sunspot is approximately twice the span of Earth. Spots can form by themselves on the Sun but often appear in pairs. When two sunspots appear in close proximity they are usually areas of opposite magnetic polarity, like the two ends of a magnet. Powerful magnetic currents can be conducted between the spots and this in turn can cause powerful solar eruptions called solar flares. The Sun can throw off massive amounts of hot gasses into space along with enormous streams of electrically charged particles. When these particles interact with Earth's magnetic field, they cause the beautiful high-altitude light displays called aurora borealis or "northern lights" (in the southern hemisphere, this is called aurora australis or "southern lights"). sunspots can also occur in large groups of 100 or more in a long string that can persist across the Sun for many weeks. Often times such a group will form and rotate out of view around the Sun's far side and then reappear about two weeks later. Sunspots also only appear between a range of latitudes between about 40 degrees and 7 degrees north and south of the Sun's equator. Sunspots can never be seen in the polar regions. The reason for this is that the Sun does not rotate as a solid body. The Sun rotates at the equator once each every 25 days, but requires 29 days for a rotation at the poles. This amplifies the Sun's magnetic field at the equatorial latitudes.

Sunspot activity is cyclical, peaking on an eleven-year cycle that is extremely regular. From the peak,sunspots will fall off to a minimum over a three-year period then activity will gradually build again to a new peak over the next eight years. At peak times of activity, such as occurred in 2002, sunspots can number as many as 150 per day. When the sunspot cycle ebbs to a minimum, the Sun can be completely devoid of any spots for many weeks at a time. As activity begins to increase again, sunspots will first appear in the mid latitudes at about 35-40 degrees from the equator and then as activity increases, moves towards the equatorial regions. As each cycle is completed, the Sun completely reverses its magnetic field. At the end of the next eleven-year cycle, the field reverses again. This twenty-two-year pattern is called the "Hale cycle."

As the Moon travels around Earth, at least twice each year it will block at least part of the Sun creating a solar eclipse. If any part of the photosphere is left exposed then a partial eclipse will be the result. Even if the Moon passes centrally across the Sun's face, it may not completely cover the Sun. It is one of the most amazing coincidences in nature that the Sun is simultaneously 400 times larger than the Moon and 400 times farther away. When the Moon is near the perigee point of its orbit, the Moon appears slightly larger than the Sun and a total eclipse occurs. If the Moon is farther away from Earth near the apogee point of its orbit, then an annular eclipse occurs with the Moon leaving a ring of photosphere visible behind it. Other than the unusual "ring of fire" effect, there is no significant difference between a partial eclipse and an annular eclipse. It is the total eclipse that is most special. When the brilliant photosphere is completely hidden, the beautiful outer atmosphere of the Sun comes into view. The rim of the Moon is surrounded by the pinkish chromosphere. The name literally means,"sphere of color." After gases cool during the climb through the photosphere, they begin to heat up again during the ascent through the chromosphere. Temperatures will rise from about 5,700K in the photosphere back up to around 10,000K in the chromosphere. As the chromosphere becomes visible, one might also see the appearance of solar prominences peaking out of the pink gases. Prominences may or may not be associated with sunspot activity. Those that are associated with sunspots are called "active" and will persist for several hours. Active prominences can be among the most spectacular features on the Sun and sometimes will form dramatic arcs of gas between sunspots. Prominences that form away from sunspot activity are called "quiescent." These prominences may extend for tens of thousands of kilometers into space and persist for months. Gases erupting from a quiescent prominence will eventually peak out in altitude and fall back into the photosphere and can be seen as a sort of gaseous "rain" falling back out of the outer atmosphere. The Sun's outer atmosphere is called the corona and is the one of the great signature sights of astronomy. The sight of the corona during a total eclipse is one of those visions that turn people on to a lifetime of astronomy. The corona is divided into two major components. The high-energy "K" corona consists of high-energy electrons streaming from the Sun in various streamers and plumes. As the photosphere generates active regions beneath it, the corona changes in shape and texture to the eye as various plumes and streamers grow and shrink. The high-energy gasses in the corona can reach temperatures of nearly one million degrees K. The outer part of the corona, called the "F" corona, glows in a softer and more consistent white light that is scattered by slow moving dust particles around the Sun. The beauty of the corona is the thrill that brings amateur and professional astronomers alike from around the world to stand in the narrow shadow of the Moon.

Solar eclipses occur in saros series just as those of the Moon do. Currently there are 39 solar saros series in progress. It is ironic how the laws of probability work and don't work with eclipse tracks. During the last ten years, the same town in west Africa experienced two total solar eclipses within 18 (June, 2001 and December, 2002) months but the entire mainland United States is in an eclipse drought that began with the total eclipse of 1979 and will not end until 2024 (not counting annular eclipses of 1984,1994 and 2017). Solar saros series are numbered just as lunar series are, though the odd-even node relationship is reversed from lunar eclipses. Odd numbers are reserved for ascending node events while descending node events get the even numbers. A solar saros progresses much the same a lunar one does. For example, the youngest currently running series is Saros 155, which began its run of 71 events with the first of eight partial eclipses on June 17,1928. The series will then go on to produce 56 central eclipses, of which 33 will be total, 20 will be annular and three others will be a mix of both. The series then ends with seven partial eclipses, the last of which will occur on July 24,3190 ending a run of 1262.1 years!

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment