STRUCTURE of the SUN
The Solar Surface
The visible surface of the sun is not solid but gaseous, being in continuous motion. This seething, boiling caldron is some 200 kilometres thick, and is commonly referred as the photosphere. Such turbulent features give the Sun its mottled appearance or the so-called granulation. All granules are some 200 to 300 kilometres across and last for several minutes before being replaced by newer ones. Often, dark spots appear on the solar disk from time to time on the solar disk as sunspots. Sunspots numbers vary significantly depending on stage in the cycle of the Sun’s activity. They are cooler regions on the Sun, and slowly increase in size over several days to weeks, reach a maximum, to then fade away. Some have been observed to last for two or three rotations before disappearing altogether. Sunspots mainly appear either as pairs or groups and are termed active areas. Single sunspots are, on the other hand, fairly rare.
Other features such as faculae appear as tiny strands of white light, 1 to 2 arcsec in length. On the solar surface, they can be thought of as hotter areas, opposite to the sunspots, and are about 800K to 1,000K than the surrounding photosphere. They are normally always associated with sunspots, and it is far more usually to see faculae near to the limb of the Sun.
The Solar Atmosphere
Above the photosphere, is the true atmosphere of the Sun called the chromosphere, and normally cannot be seen except during total solar eclipses just moments before and after totality. This hot 4,000K region appears as bright pink in colour seen against the lunar limb, with the observed thickness is between ten and twenty thousand kilometres. Its composition being mainly hydrogen. Beyond the chromosphere is the corona, whose true extent remains uncertain, but can be seen out anywhere between 3R⊙ to 10R⊙ in solar radii. The inner corona extends from 0.5R⊙ to about 1.0⊙ solar radii, while the outer corona extends from one to three solar radii. Made primarily of hydrogen and helium, the corona exists as tenuous gas whose temperature exceeds several million degrees. It emits strongly in both X-ray and ultra-violet light, but the cause of the temperature rise from the chromosphere to the corona is not fully understood. As most particles in the corona are highly ionised, the behaviour of the gas is affected by the prominences, flares and the solar magnetic field.
During any total solar eclipse, the corona can be seen by the naked-eye throughout all of totality. It can also be observed using special designed instruments called the coronascope, where the solar disk is completely obscured and a special filter is applied. To the naked-eye the corona appears as pearly white and is about as bright as the Full Moon. This is enhanced by radiating rays roughly perpendicular to the bright solar atmospheric surface. This is often seen over the general pearly white background. These rays are distributed in various places around the limb, but tend to avoid the Sun’s magnetic poles. Often during solar minimum the outline of the corona look very much like the outline of the magnetic field – like when seen metal fillings are placed on a piece of paper and influenced by a bar magnet. A telescopic view of the corona reveals tiny or little broken strands of light seemingly like filamentary structures that may extend between one to three solar radii. This does vary significantly from eclipse to eclipse. Observed complexity of the corona depends very much on the time of the event in the current eleven year solar cycle.
Interior of the Sun
Our knowledge of the Sun’s interior is fairly poor. Recent observations of solar seismic activity has allowed us to have a better understanding of its structure. Known as helioseismology, the method is much akin to using seismology to find out about the nature of the interior of the Earth. It is believed that below the photosphere lies the convection zone, the region that transports the hot pressurised gases from the Sun’s interior to the solar surface. We know of this kind of mixing must occur because the elements manufactured in the core can be seen spectroscopically on the surface. Further down, perhaps third to half a solar radius, is the radiative zone. Here the generated energy is passed into the upper atmosphere is very similar to bar radiators used to heating the surrounding air at home. At the very centre is the solar core. The core, the powerhouse of the Sun, is estimated to be about the size of Jupiter and is composed of very hot plasma, whose temperature is present estimated between three and five million degrees. Energy is created here, and under tremendously high pressures, where atoms are literally squeezed together to make the heavier elements.
Light once it is produced in the solar core may take about one million years to eventually radiate from the surface before it is seen on Earth. This is because the photons of light bounce continuously of surrounding atoms like some old pinball machine, whose sheer numbers make it almost impossible for the light to escape the core. It is quite astounding to think that the warm sunlight that we feel on our skin has taken about one million years since its creation for us to experience it. Yet once the light has been freed, it continues to travel through the universe, and unless it is absorbed or sactually hits some physical object, will show the Sun’s light as some distant star well after the ray of sunlight is finally extinguished billions of years from now. It is only the starlight eminating from distant suns that makes the whole universe visible to all of us. Aged light indeed!
Very strong electrical flows or currents also exist within the solar interior and are thought to be the mechanism that produces the observed strong magnetic fields. These fields may split into various localised areas that can create either enormous flares or prominences. They are commonly associated with sunspots and corona holes, but also interact with the corona itself.
Solar Activity and Sunspots
Our Sun is not a passive static body. During the times of high solar activity, streams of atomic particles and most of the electromagnetic radiation are produced, which are closely associated with increased prominence activity. Prominences are extremely violent events and appear like tongue of red gas leaping away from the photosphere and are commonly associated with the plage areas.
During total solar eclipses prominences appear like reddish-pink loops or spikes around the solar disk. A special solar filter, known as a Hydrogen-alpha filter, can be used to look at these prominences and flares at any time during the day. Many prominences may last several hours with the more violent ones extending millions of kilometres into interplanetary space. These can strongly interact with the Earth’s upper atmosphere, damaging circuitry in spacecraft and may cause serious problems with radiation sickness for astronauts.
Flares appear to emanate from the active regions of the photosphere. These suddenly brighten up and last from a few seconds to several minutes. Some of these events are violent and can appear as very intense in Radio, X-Rays and ultra-violet emissions. Prodigious charged particles are also emitted from these events often diminishing the strength of the protective ionosphere, and in some instances the most powerful of blasts may cause fade-outs, power and communication blackouts. These then absorb the radio waves that are normally reflected by the ionosphere. As streams of particles, they comprise several types of charged particles including, proton, neutrons and helium nuclei. Velocities of these streams of charged particles, called the solar wind, and travel at about six-hundred kilometres per second, or just 0.2% of the speed of light. These streams take about a day or so to travel the gulf between the Sun and Earth.
White Flares rarely appear on the Sun. They are really the brightest of the red-flares and can produce thousands of times more energy than average flares. White flares can be visually seen during optical observations of the Sun and may last several minutes.
In all, observations of flares can be for the observer of an interesting pastime as the changes that do occur happen, especially interesting as they can seen over a few minutes to several hours.
!!! W A R N I N G !!!
Using any telescope, the Sun should ONLY be observed by projecting the image onto a white screen or card. (Even so, this should be for just short periods.)
Direct viewing of the Sun, by either eye or any other optical equipment, is VERY DANGEROUS without proper eye protection. Otherwise, TOTAL BLINDNESS WILL RESULT, and even glancing will blind you in less than a ten-thousandth of a second.
If your telescope has something called a SUN FILTER — NEVER USE IT !! If this filter were to crack while you are observing the Sun, INSTANT blindness is the only possible outcome.
!!! W A R N I N G !!!
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Last Update : 23rd July 2018
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