Introduction

The nearest star, the source of energy, the radiant ‘effluent being’ - The Sun. In a Galaxy of about hundred billion stars, our Sun is a mediocre member in every physical aspect. It affects the motion of every major and minor planet in our solar system, asteroids, and the comets as well. So why study Sun? The Sun is the only star that shows a disc the different parts of which can be studied in isolation, unlike any other star which looks just like a point source. Using the fairly accurate model of the interior and atmospheric structures of the Sun, we can construct models for distant average stars and can thus understand the physical nature of those remote objects for which even a good spectrum can hardly be obtained with the most precision instruments ever made.

There are also some challenges in studying the Sun. The main body of the solar interior cannot be directly observed. We have to extend knowledge to the interior by applying the laws of physics governing the equilibrium of a radiating gaseous sphere, in combination with the observed results of the upper layers.

This article duology focuses on the most basic aspects of Solar Physics, defining the interior layers, its great prominences, the strange 11-year Solar Cycle, and Solar Flares, with the mathematical minimum.

Where does this energy come from?

The Sun’s radiant energy is generated by the thermonuclear transmutation of hydrogen into helium at a central temperature of about 16 million degrees Kelvin. It has been long believed that the reactions in the core occur due to proton-proton interactions. But a theory by A.B. Severny and his group says that if proton-proton reactions were operative in the core of the Sun then a certain amount of neutrino flux should emerge and a reasonable amount should be capable of being detected by a favorable set of experiments. But experiments accounted for a far less value than expected. Although there might be significant progress in understanding solar phenomena, this one is still is a mystery.

The Photosphere

The visible disc of the Sun which we recognize as the solar surface may be considered as the base of the photosphere. This is the innermost layer of the solar atmosphere to which our eyes can penetrate through the superficial transparent layers. Our view is obstructed beyond this layer by the rapidly increasing opacity of the denser layers of the gas inwards. 

The thickness of the entire photospheric layer which runs from completely transparent to perfectly opaque layers of gas is quite small, only of the order of 200-300 km. The pressure and density in the photosphere are much lower in comparison to those in the terrestrial atmosphere at sea level. When we see the disc near the limb our view penetrates through cooler superficial layers of the photosphere. This causes the well-known limb darkening effect in solar observation. Light from near the edge of the disc comes from upper, cooler, and more tenuous layers of the photosphere than that from near the center. So we see the limb of the solar disk redder and dimmer as compared to the bright and hot central part.

The decrease of the brightness of the solar disk as we go across from center to the edge, points to the fact that there exists a temperature gradient across the photospheric layers. The temperature decreases as we move from lower to upper photospheric layers. The temperature again rises rapidly in the chromosphere. But the gas density in these upper layers is so low that it is capable of absorbing or emitting a very small amount of radiation. The opacity thus being very low, these layers are almost transparent compared to the photosphere. This is why the disk appears to terminate abruptly at the end of the photosphere.

The process is repeated incessantly. These rising and falling convective cells appear in the photographs of the solar disk as bright granules of various sizes bordered by darker regions. One can study the nature of convection which gives rise to granulation by applying Reynolds or Rayleigh numbers. For granulation these numbers are high, indicating that turbulence beneath the photosphere is probably the basic cause of the entire manifestation.

They exhibit a peculiar temperature distribution, cooler below and hotter above than the surrounding gases, which can be maintained only by some kind of forced supply of energy. Faculae usually precede sunspot groups and outlive their several cycles. These and other considerations indicate that both faculae and sunspot activity are manifestations of some third agent. The subphotospheric magnetic field is believed to be a likely candidate.

The Chromosphere

Above the photosphere lies the second major layer of the solar atmosphere known as the chromosphere. The nomenclature is derived from its display of reddish color when the photosphere is eclipsed by the moon. The layer extends to nearly 20,000 km above the photosphere with gas density gradually thinning out but the temperature rising rapidly upwards. The chromosphere exhibits some phenomena like spicules, plages, and filaments. 

During a total solar eclipse or with the use of a coronagraph “spiky” structures of hot tenuous gas are seen to rise from the lower chromosphere, these projected chromospheric streamers are called spicules. When viewed near the solar limb, such a large number of these is seen together in projection as to give the appearance of a “forest of spikes”.

Large bright and dark clouds are revealed in the chromosphere when spectroheliograms in the monochromatic light of Ha and Ca II K are examined. These are respectively called Plages and Filaments. Plages are alternatively called Flocculi.

The inner corona which is also sometimes called the real corona extends between 1.03 Ro < R < 2.5 Ro. This part of the corona, known as the K-corona, imitates the continuous spectrum of the photosphere but the Fraunhofer lines are absent. The F-corona which lies at R > 2.5 Ro displays the solar spectrum with Fraunhofer lines superimposed on the continuum. This is sometimes called the “false” part of the corona and the prefix F stands for Fraunhofer. The F-corona together with the K-corona constitutes what is called the white corona which merges into interplanetary space with decreasing brightness.

In the optical range of the coronal spectrum, about two dozen emission lines are found to be superimposed on the continuous background. The total light of these emission lines formed by highly ionized atoms in the extremely hot inner part of the corona constitutes what may be called the E-corona or emission corona. The total radiation in these lines is, however, quite small, which is less than even 1% of the total coronal radiation.

The Solar Prominences

During a total solar eclipse or while photographing the Sun with the help of a coronagraph the solar disk exhibits the well-known red flames protruding from the chromosphere through the corona. These flame-like structures are called prominences which are sometimes seen to rise to a height of more than a million kilometers above the solar surface. The prominences generally are 30,000 km in height, 200,000 km in length, and 5,000 km in thickness which sums up to a humongous total volume of 3 x 1028 cm3. Observations show that the gaseous material in the prominences move downward along curved paths called arches as if the coronal materials are being continually poured into the chromosphere.

Sunspot Type Prominences

These prominences generally appear in regions above sunspot activity in the form of curved arches or loops having mean projected lengths of the order of 60,000 km. They consist of condensed coronal material moving downward along trajectories just above spot groups. In sunspot prominences, the magnetic energy density is much larger than the energy density of thermal or random motions.

These sunspot type prominences are further categorised into Condensation or Knot type prominences, Surges and Tornado type prominences. 

Various other types of prominences have been proposed by many researchers on the basis of their structure and other physical characteristics. But discussing about these will be beyond the scope of this article.

Till now we have covered the most basic phenomena of Solar Physics, and a yet larger part remains which will be covered in the next part of this article. The 2nd part will entail some still-in-development notions like the 11-year Solar Cycle, Theory of Sunspots, Solar Neutrino Puzzle and the Standard Solar Model. Stay tuned!

Note: Several results from spectroscopic studies which gave justification of some of the above-mentioned phenomena were skipped to make this article suitable for a general audience and for science communication. If you wish to get a further and detailed insight of the concepts please checkout the books and links mentioned in the references section.

References -

[1] Sagan, Carl, The New Solar System, Cambridge University Press, Cambridge, 1990.

[2] Phillips, Kenneth, J.H., Guide to the Sun, Cambridge University Press, Cambridge, 1992.

[3] NASA -  Marshall Space Flight Center