Solar Activity

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The Sun is a dynamic place! The chromosphere covered by a writing carpet of spicules. At times the Sun's appearance is mottled by numerous dark spots and occasionally the Sun will undergo sudden, explosive events that eject large amounts of high energy protons into the solar system. Understanding the active Sun is our goal in this section.

Observing the Sun - Safely

Galileo was one of the first persons to observe the Sun telescopically. Should you attempt to repeat Galileo's observation you must exercise great caution. Never look directly at the Sun!

By far the preferred way to observe the Sun (and the method Galileo used) is to project the image of the sun onto a white screen or sheet of paper. Figure 7.19 shows you how to do this with a simple pair of binoculars. If you have binoculars or a small telescope you are encouraged to try this technique. Just focus the telescope or binoculars on a distant object and then, by using projection point the telescope toward the Sun. By observing the shadow cast you will find it quite easy to centre the Sun. Project an image of the sun onto a white card. When you do this even a small telescope or pair of binoculars will reveal a wealth of detail. Limb darkening will be readily apparent and if there are sunspots they will show up clearly. With very large sunspots you may even notice a distinct brightening around the periphery of the spots. Over several day you will notice that the sunspots move in an organized way that tells you that the Sun is rotating.

Figure 7.19 A rollover image showing how to make a simple baffle that will allow you to use a simple pair of binoculars to observe the Sun safely.

Sunspots - A Measure of Solar Activity

Figure 7.20 SunSpots is an interactive graph that allows you to inspect the monthly sunspot numbers from 1749 to 2009.

A striking pattern appears in Figure 7.20. This pattern was studied in detail by the astronomer Rudolf Wolf who initiated an international project of daily sunspot counts that is still in operation today.   Wolf discovered an 11.1 year period of sunspot activity. He and others also discovered from historical records an interesting anomaly. From about 1645 - 1715 sunspots were virtually absent from the solar face. This is a tantalizing fact - called the Maunder Minimum. Tantalizing because this same time period corresponds to a time of unusually cold winters in Europe and the New World. Sometimes called the "mini ice-age" this has fueled speculation concerning a correlation between solar activity and climate. The astronomer Sir William Herschel announced in 1801 that, on the basis of anticipated sunspot activity, the price of wheat would drop! He was wrong! Still today we find newspaper articles prematurely claiming correlation of weather or the length of peoples noses and sunspot activity!

Sunspot activity does influence the brightness of the sun. Paradoxically, the sun seems to be brightest when it has the most sunspots! This effect is very small (less than 0.5%) but measurable. While recent findings demonstrate a link between air temperature and sunspot activity it is also becoming clear that global warming effects are now dominated by increasing atmospheric concentrations greenhouse gases than by variations in solar activity.

Solar Rotation

Figure 7.21 1 month of solar rotation

What is a Sunspot?

If the location latitude at which sunspots form on the solar surface is plotted as a function of time a new "wrinkle" on the 11-year solar cycle emerges. Figure 7.22 shows the resulting pattern - called the Maunder Butterfly Pattern after the person who first constructed such a graph and because it looks like a series of butterfly wings. In Figure 7.22 the green vertical lines mark

Figure 7.22 The Maunder Butterfly Pattern

the transition from solar maximum to solar minimum and captures an interesting fact: As the new solar cycle begins the sunspots form at around 30 N or 30 S latitude on the Sun's surface. A the cycle advances towards solar maximum the number of sunspots increases and they form a lower latitudes with the peak occurring around 5 N or 5 S. This curious behave is, we think, related to the differential rotation of the Sun.

In 1908 the American astronomer George Ellery Hale (Mount Palomar telescope founder) showed that sunspots are regions of high magnetic field strength and that sunspots often occur in pairs of opposite polarity as shown in Figures 7.23 and 7.24. Hale's work was the first astronomical application of an important piece of physics discovered a decade earlier by the Dutch physicist Peter Zeeman. Zeeman showed that spectral lines are split into pairs of lines in the presence of magnetic fields. In Figure 7.23 the black vertical line across the penumbra of the sunspot shows how the slit of the spectrograoph was oriented. On the right is the resulting spectrum clearly showing the spitting in the lines. Because there is a precise mathematical relationship between field strength and splitting it is possible to use this method to accurately measure the magnetic field strength around a sunspot.

Eventually the astronomer Horace Babcock constructed a theory that most likely is correct in its essential details. We can explain this with the following diagram and with a crucial piece of observational evidence - the sun rotates faster at the equator than at the poles. Field lines generated within the sun by the constant convective tumbling of ionized gases is called the dynamo effect. These filed lines become increasingly entangled as the sun rotates differentially.

Figure 7.23 Zeeman splitting of spectral lines produced in the region of sunspot. (Image courtesy NASA)	Figure 7.24 Rollover image showing the connection between pairs of sunspots.
	Eventually the field lines concentrate and "pop" through the photosphere. The magnetic field suppresses hot gases from the lower photosphere from rising further which explains their darker (cooler) appearance. The field lines are carried along in the photosphere and the differential rotation causes the field lines to stretch and tangle. All of this occurs over time and it takes about 11 years for the complete cycle of tangling field lines to develop. As field lines meet and cross bursts of magnetic field energy can be released explosively. Figure 7.25 illustrates the Babcock model.
Figure 7.25 The Babcock model predicts that differential rotation will cause a tangling in the Sun's magnetic field lines.

Example 7.8 How does the Babcock model explain the Maunder Butterfly Pattern?

Solution: As the field lines tangle they will becomes increasingly concentrated in the equatorial region since this rotates faster than the polar regions. This implies that the concentration of field lines and hence sunspots will increase and "migrate" toward the equatorial region.

Solar Flares, Prominences and Coronal Mass Ejections

While engaged in the forenoon of Thursday, September 1 (1859), in taking my customary observation of the forms and positions of the solar spots, an appearance was witnessed which I believe to be exceedingly rare. The image of the sun's disk was, as usual with me, projected on to a plate of glass coated with distemper of a pale straw color, and at a distance and under a power which presented a picture of about 11 inches diameter. I had secured diagrams of all the groups and detached spots, and was engaged at the time in counting from the chronometer and recording the contacts of the spots with the cross-wires used in the observation, when within the area of the great north group (the size of which had previously excited great remark), two patches of intensely bright and white light broke out ...

(Richard C. Carrington, Monthly Notices of the Royal Astronomical Society, vol. 20, 13-15, 1860.)

In 1859 the British astronomer Richard Carrington may have been the first person to observe a Solar Flare. The Sun's magnetic field represents a reservoir of energy that is replenished by the rotation of the Sun and the convective motion in the region beneath the photosphere.

As a solar cycle progresses and the magnetic field lines become increasingly tangled other, dramatic events play out on the Sun's surface. When the field lines cross and through processes not fully understood, the magnetic energy can be released in the explosive way described by Carrington. During a solar flare, which may occur over several hours, an enormous amount of energy can be released. (Estimates are that a large solar flare will emit more than 1 million times as much energy as volcanoes on Earth and temperatures in the flare are typically 10's of millions K). During a solar flare energetic particles can be released into the solar system as a strong "gust" in the solar wind. As we will discuss in the next section this can lead to brilliant aurora and geomagnetic "storms". Figure 7.26 shows a video clip of a solar flare observed at the Big Bear Solar Observatory on November 3, 2004.

Figure 7.26 Video of solar flare on solar limb (Courtesy of Big Bear Solar Observatory.)

In addition to solar flares is a "gentler" phenomenon occasionally visible on the solar limb - the solar prominence. Prominences are streams of cooler plasma threaded along magnetic field lines and arching upward from the Sun's surface. Prominences are relatively stable and may persist for many days. Figure 7.27 shows and example of a solar prominence and the inset image shows how the size of Earth compares to the size of a prominence.

Figure 7.27 A solar prominence (Image courtesy of The King's University Observatory)

Example 7.9 Estimate the size of a solar prominence.

Solution: Use Figure7.27. The inset inset image shows Earth in relation to an "average" prominence. In this case the prominence is about 6 "Earth-diameters" across. Since the Earth is roughly 13 000 km across this means that a typical prominence can reach up on the order of 80 000 km (or more) above the Sun's surface.

Summary of Features of the Active Sun

Table 7.2 provides you with a summary of some of the terms encountered in this section.

Practice

1. A solar flare can release 10²⁵ J of energy. North America's annual "energy bill" is about 3 x 10¹⁹ J. If we could harness the energy of a single solar flare how long could we meet North America's current energy needs?
2. Would you expect a solar flare to produce x-rays? Explain your reasoning.
3. If a coronal mass ejection leaves the Sun with a speed of 1000 km/s estimate the time it would take to cross Earth's orbit.
4. How do we know that sunspots are regions of high magnetic field strength?
5. Conduct a web search or use other resources to determine the magnitude of the magnetic fields found in regions around sunspots. How does this compare to the Sun's average magnetic field and the Earth's average magnetic field. (Note: the unit for magnetic field strength is commonly either the Tesla (T) or the Gauss (G). The tesla is a much larger unit; 1 T = 10⁴ G).

To understand what solar activity is and how it can be observed.

Chp 8.3

Like an erratic teenager our Sun still goes through "phases"!