General Astronomy/The Sun

The Sun is the star at the center of the solar system, around which Earth, the seven other planets, and numerous other bodies revolve. It is sometimes called Sol (hence the term "solar system"). The Sun has a mean diameter of 1.392 million kilometers, which is 109.1 times the diameter of Earth and 9.7 times the diameter of the largest planet, Jupiter. The Sun's rotation period relative to the distant stars is 25.05 Earth-days at its equator and 34.3 Earth-days at the poles.

Solar Structure and Composition

The Sun

The sun is composed of extremely hot, gaseous material. Because of the high temperatures, this material exists in a state called plasma, in which electrons have been stripped from their parent nuclei. The Sun's composition is roughly 90% hydrogen and 10% helium, by total numbers of nuclei. Considering mass, about 71 percent of the Sun is hydrogen, and 27 percent helium. The difference is due to the fact that helium nuclei have about 4 times the mass of hydrogen nuclei (protons). There are small percentages of other elements mixed in.

The Sun is made up of a series of layers, which can be thought of as a set of concentric "shells" from the core outward.

At the center of the Sun is its core, a region of some 15 million kelvin (About 27 million degrees F). The core is where the nuclear fusion reactions that power the Sun take place. Primarily, this is the fusing of hydrogen into helium.

Outside the core is a radiative zone, where energy produced in the core travels toward the surface. However, this energy can only travel outward by radiation for a certain distance, which limits its depth. Temperatures here run from about 7 to 2 megakelvins. An interface region above this is thought to be the source of the solar dynamo which powers the Sun's magnetic field.

Outside the radiation zone and interface region is a convection zone. This zone sits between the heated radiation zone and the cooler outer layers of the Sun. The result of this is a series of motions known as convection. Hot material, being less dense, rises upwards toward the surface; material near the surface, once cooled, becomes denser and sinks downwards again. Through this process, heat is gradually transported upward to the solar surface. The base of the convection zone is at about 2 megakelvins, while material at the very top is at roughly 6000 kelvins.

Atop the convection zone lies the photosphere, the visual "surface" of the Sun from our point of view. Material here is at approximately 6000 K. As a result of the convection going on below, polygonal shapes define the walls of the convection cells, called granules appear all across the surface.

Above the photosphere is the chromosphere, a very diffuse, hot layer. Material here rises in temperature from the 6000 K of the photosphere to about 20,000 K. A transition region above the coronasphere has an even more dramatic rise in temperature, up to a megakelvin.

Finally, outside the chromosphere and transition region is the corona, an extremely tenuous (very low-density) gas over a megakelvin. It is thought that the corona is primarily heated by the magnetic processes of the solar cycle.

Solar Cycles

Twisting of the magnetic field lines is caused by the effects of the Sun's rotation.

The Sun goes through regular cycles related to the formation and destruction of its magnetic field every 11 years. This process affects features on the surface of the Sun; it is by observing those features that our knowledge of solar cycles has developed over time.

The Sun's magnetic field is created naturally by the movement of the solar plasma. Because this plasma is made up of charged particles, its movement creates electric fields. Where electric fields are generated, magnetic fields are generated perpendicular to them.

The Sun's rotation, however, is not rigid like the Earth's, because the gaseous material is so much more fluid than the Earth's crust. Material at the equator moves more quickly than does material at the poles; this is called differential rotation. Furthermore, the magnetic field lines are embedded within the plasma, and tend to move with its motions. As a result, the magnetic field lines which initially run from pole to pole become more stretched in the middle, with the center of the line outrunning the pole-anchored ends. Eventually, these lines will wrap around the Sun many times.

As magnetic field lines cannot cross one another, they tend to protrude upwards when they come close to one another, forming "loops" that project out of the solar photosphere. It is along these loops that solar prominences are displayed. The more these lines are wrapped around the Sun, the more "tangled" and disrupted the magnetic field becomes. Eventually, the field is so disrupted that it breaks up completely. After this, of course, a new magnetic field starts to form, and the cycle begins again. The complete process takes about 11 years.

The presence of magnetic fields has an effect on the Sun's photosphere. In discussing solar structure and composition, it was mentioned that the process of convection brings heat from the inner regions of the Sun up to the photosphere. This convection tends to occur in fairly localized zones of rising hot and falling cool material known as convection cells. However, if a magnetic loop is present, it will disrupt one of these convection cells, trapping cool material at the Sun's surface. While these regions (about 4500 K) are only cool compared to the photosphere (at about 6000 K), the overall effect is that they appear dark against the solar "surface". As a result, these are known as sunspots.

The presence of sunspots reflects the cycle of the magnetic fields. In fact, it was the progression of sunspot number and position that gave the first evidence of cycle of magnetic field reversals. At the beginning of the magnetic cycle, when the magnetic field is newly forming, only a few sunspots will appear; this is called a solar minimum. Those sunspots that do appear will tend to be at high solar latitudes - that is, closer to the poles and further from the solar equator. As the cycle continues, sunspots will gradually appear in greater numbers and tend to be closer to the solar equator. The peak of this cycle is known as the solar maximum. After the sunspot numbers peak, they will continue to appear with less frequency, and be still closer to the solar equator. As this cycle trails off, the first few sunspots for the next cycle will start to appear at high solar latitudes, and so forth.

Because they are formed by magnetic field lines, sunspots occur in pairs; at the base of either end of the magnetic loop that formed them. Likewise, the sunspots will have opposite magnetic polarity from one another. There are distinct patterns in the polarities of each pair which can be observed during a cycle. For convenience, we will call the spot that appears to be further along in the direction of the Sun's rotation the "leading" spot, and its partner the "trailing" spot.

Sunspot pairs in the northern solar hemisphere will have opposite polarities from those in the southern solar hemisphere. That is, if the leading spot is polarized one way (call it +) and the trailing spot the other (−) in the northern hemisphere, then in the southern hemisphere the leading spot will be (−) and the trailing spot (+). Sunspot pairs in a given hemisphere will tend to all appear with the same polarities during a solar cycle. However, at the end of the cycle, the polarities will "flip": new sunspots in that hemisphere will have polarities opposite from the previous cycle.

The table below shows the sunspot pattern over four 11-year cycles, for pairs of (leading and trailing) sunspots.

Cycle	N. pair polarities	S. pair polarities
1	+ -	- +
2	- +	+ -
3	+ -	- +
4	- +	+ -

Connection between Sunspot Activity and Aurora on Earth

Aurora on Earth.

Aurorae are visible effects of the Sun's activity on the Earth's atmosphere. It results from the interaction of the solar wind, a continuous flow of electrically charged particles—a stream of electrons and protons coming from the Sun—with gases in the upper atmosphere of the earth at altitudes above 50 miles. When these charged particles reach the Earth's magnetic field, they become trapped in it. Bouncing back and forth many of these particles travel toward the Earth's magnetic poles. When the charged particles strike atoms and molecules in the atmosphere, the atmospheric atoms become excited or ionized causing them to emit photons. These photons cause the luminous auroras. The aurora seen in the Northern Hemisphere is called Aurora Borealis or the northern lights whereas the aurora seen in the Southern Hemisphere is called the Aurora Australis.

The collisions of the solar wind with Earth’s upper atmosphere produce electrical discharges which energize atoms of oxygen and nitrogen which subsequently release various colors of light. Mostly aurorae are seen green and red as it is emitted from atomic oxygen. Similarly molecular nitrogen and nitrogen ions produce low red and very high blue/violet aurorae. Whereas ionic nitrogen produces blue and green color aurora and neutral nitrogen produces the red and purple color with the rippled edges. Most auroras occur at about 60 to 620 miles above the Earth. Some extend lengthwise across the sky for thousands of miles or kilometers. A diffuse red aurora occurs above 150 miles. An aurora having a pinkish edge will have an altitude of around 50 to 60 miles.

Aurora.

Auroral activity occurs mostly in a belt called auroral zones, around the geomagnetic poles, between 65 and 70 degrees of geomagnetic latitude. An auroral zone is a ring-shaped region of approximately radius 2500 km around either magnetic pole of the Earth. It was hardly ever seen near that pole itself. At any time of day, the distribution of aurora is slightly different. It's center is offset from the magnetic pole by 3-5 degree night-ward of the magnetic pole. As a result, the auroral arcs reach furthest equator-ward around midnight during a major geomagnetic storm. Auroral zones migrate as close to the equator as 45 to 50 degrees latitude. At higher latitudes in spring and fall, a maximum of aurora activity is noticed as the Earth is at at that time is farthest north or south of the Sun’s equator due to which there is greater chance of Earth intercepting enhancements of the solar wind that emanate from the vicinity of sunspots.

Variation of heights of Aurorae with latitude:-

h’χ – h’ο = H logn sec χ.

where χ = earth-magnetic inclination

h’ο = height of luminosity maximum corresponding to χ = 0

H = (kT)/mg

K= Boltzmann’s constant

T= Absolute temperature

m = mass of gas

g = acceleration of gravity

Aurora occur mostly intensely during the peak phase of the 11-year Sunspot Cycle. Auroral activity also peaks near the maximum of the Sunspot Cycle and for the couple subsequent years. Most violent eruptions on the surface of Sun, called Solar Flares, originate in magnetically active regions around visible Sunspot groupings. Solar Flares release Electrons and Protons which increase the number of Solar particles that interact with the Earth's atmosphere, producing extremely bright Auroras. Sharp variations in the Earth’s magnetic field are called Magnetic Storms and are also the result of Sunspots.

Earth's magnetic field.

The Earth is surrounded by a magnetic field, called the Magnetosphere that forms a barrier to the solar wind. The solar wind pressure strongly compresses the magnetosphere on the sun-side and draws it out into an extremely long tail on the side opposite. Since the charged particles of the solar wind cannot cross the Earth's magnetic field lines, flow around it . This forms a standing shock wave in space upstream of the Earth, called the Bow Shock. Electrons of the solar wind diffuse into Magnetospheric tail and form a reservoir called the Plasma Sheet. The Magnetosphere and the solar wind form an enormous electrical dynamo where large and complicated electrical currents flow. One component of these currents is carried by the Electrons in the Plasma Sheet, which descends down from 300 km to 100 km following spiral paths along magnetic field lines. These particles then collide with the atmospheric gas causing it to glow, which we call Aurora.