The Corona

Above the chromosphere lies the corona (Latin word for crown). Like the photosphere, the corona is also self-luminous, but the intensity of its light is so feeble that it cannot be seen with the naked eye except at the time of total solar eclipse when the dazzling light of the sun's face is blocked by the moon. However, it can be routinely observed by coronagraphs carried on board space satellites. The hot plasma of the corona emits most of its energy at ultraviolet, extreme ultraviolet and X-ray wavelengths. Ultraviolet radiation also comes out of the chromosphere and the transition region at its base. Radiation at these wavelengths is almost totally absorbed by the earth's atmosphere, so it must be observed through telescopes in space. Observations reveal that there is a sharp rise in temperature in a transition layer at the base of the corona where it jumps from about 10,000 K to 1,000,000 K or even more through a height of less than 100 km. The density of the gaseous plasma decreases sharply as the temperature shoots up. This is evident from Fig. 7.4, which shows the radial distribution of temperature and density in the solar atmosphere.

What causes this phenomenal rise in temperature in the solar corona and how heat can travel from a cooler to a hotter layer, without violating the second law of ther-

Fig. 7.4 Schematic showing radial distribution of

temperature (T) and density (p) in the solar atmosphere

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modynamics, remains a mystery. However, scientists feel that one of the plausible heating mechanisms may be the release of tremendous amount of heat energy in the gaseous plasma when oppositely-directed magnetic fields associated with magnetic loops interact by coming together in the great turmoil of the solar surface. The interaction would produce powerful electrical and magnetic 'short circuits' to heat up the coronal plasma. The gigantic flashes produced during these short circuits may also explain the origin of 'flares' on the solar surface.

The corona extends deep into space. During a total solar eclipse, it appears as a ghost-like white halo round the darkened face of the sun. However, the appearance of the corona changes with solar activity in an 11-year cycle, very much like the sunspots. During peak activity, it looks spiky and jagged with streaks of light flying off in all possible directions, while in quiet periods, it looks more even and rounded. On closer scrutiny, the space satellite Ulysses found that the observed jaggedness at maximum solar activity was due to luminous gases erupting out of the chromosphere in what are known as 'prominences'. The space satellite SOHO on 15 January 1996 observed magnetically-energized violent eruptions from the sun for a period of 8 h. Such Coronal Mass Ejections (CME) expand at high speed as they propagate outward from the sun. Like sunspots, the intensities of CMEs, flares and prominences also vary with the sun's 11-year cycle of magnetic activity.

The extreme ultraviolet and X-ray images of the sun taken by Yohkoh and SOHO have revealed the presence of large dark areas near its poles where there appears to be little hot material. These areas are known as 'coronal holes'. Their presence has been confirmed by the space satellite Ulysses which flew over both the poles of the sun in 1994-5.

The corona is for ever expanding into space and pouring out high-energy electrically-charged particles, such as electrons and protons as well as heavier ions and atomic nuclei in streams varying in speed from 300 to 1000 km s_1. These streams constitute what is known as the solar wind. The faster particles are generated during

7.6 The Solar Wind particularly energetic bursts of solar activity and ejected from solar latitudes higher than about 20°N and S.

The solar wind being under the control of the magnetic field of the sun which rotates with a mean period of about 27 days is ejected from the sun in a direction somewhat similar to that of water being ejected from a rotary garden sprinkler. Being constrained to move along the magnetic lines of force, the solar wind has no fixed direction. It rushes through space in a most complex and unpredictable manner filling up the whole interplanetary space with the solar radiation. As radiant energy gushes out from the outer layers of the sun's atmosphere as the solar wind, new radiation wells up from the interior of the sun to take its place. Ulysses in the mid-1990s found that the fast component of the solar wind pours out from the polar coronal holes where there are no magnetic field loops to prevent the escape of the charged particles from the sun's strong gravitational and magnetic pulls. The slow component emerged from equatorial corona.

Recent theoretical studies have suggested that although the solar wind is composed of charged particles of both positive and negative signs, it must travel through interplanetary space as an electrically neutral beam consisting of equal number of positive- and negatively charged particles. It is only on entering the earth's magnetic field that the charges get separated and follow the magnetic lines of force to enter the earth's atmosphere around the magnetic poles. In their downward passage, they pass through several layers of trapped radiation, such as the well-known van Allen radiation belts at altitude of about 5 earth radii, and then further down through the different ionization layers of the earth's upper atmosphere.

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