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Solar wind

QUESTION: What is solar wind?

T. Manjula, Tiruvarur, T.N.

ANSWER: A Comet starts developing its tail, only when it nears the orbital path of Mars (i.e. about 220 million kms. away from the Sun). This tail always points away from the Sun. For quite a long time, it was widely believed that the pressure of the Sun's radiation was blowing away the gaseous matter of the Coma (of the Comet) in the form of a tail. However, in course of time, it was realised that this pressure was not sufficient enough to account for the observed effect.

In 1950, a German scientist, L. F. Biermann (who was actively engaged in the study of Comets) postulated that the Sun was continuously emitting a stream of high speed particles (protons and electrons) and that this stream was responsible for the long- flowing tail of the Comet.

His studies also revealed that this emission from the Sun was not sporadic but a continuous one. It increased during periods of high solar activity i.e. when the Sunspot number was large.

The visible surface of the Sun is known as ``Photosphere''. This is surrounded by the ``Chromosphere'' and beyond the Chromosphere lies a hot gaseous atmosphere called the ``Corona''. The chromosphere as well as the corona would be visible, only during a total solar eclipse. The corona is somewhat irregular in form. Its shape varies with the period in the Sunspot cycle.

Around minimum sunspot activity, broad plumes in its equatorial region and shorter plumes in the poles are the common feature. During epochs of maximum sunspot activity, the Corona appears to have a fairly symmetrical appearance, with streamers all around.

Very high temperatures prevail in the Corona. The postulation of Biermann was confirmed in 1958 by American phycist Eugene Normal Parker, who reported results of his investigations on the solar corona. According to Parker, the thermal conductivity of the Corona (i.e. the capacity to transfer heat from one point to another) is quite large; that temperatures of the order of 1,000,000o K. (or more) must be extending out for quite large distances into space; that the Coronal expansion increases with distance; and that it attains unbelievable velocities at distances of the order of 20 million kms.

This persistent stream of gas blowing out of the corona and sweeping to a distance of about 40 astronomical units - the very limits of the planetary orbits - at supersonic speed is known as ``Solar Wind''. Its velocity has been estimated to be of the order of 350 to 700 kms per second. Owing to the rotation of the Sun, the solar wind tends to travel in spirals.

It is this solar wind that blows away the evaporated gases from the Coma of the comet and in that process, causes the Comet's tail to develop and extend for millions of kms. The solar wind disturbs and distorts the shape of the Magnetosphere. (This sphere extends to a distance of about 64,000 kms in the Earth-Sun direction). Since the solar wind travels with a very high velocity, it is natural that the region where it encounters the Magnetopause (the outer boundary of the Magnetosphere) should be highly disturbed.

The Magnetosphere acts as a shield against this continuously blowing Solar Wind. Hence, on the sunlit side of the earth, the solar wind sweeps and slides along the Magnetopause. Nevertheless, particles of solar wind do sometimes pierce the magnetic shield and enter the Earth's upper atmosphere, where they cause Auroral displays. Satellite studies indicate that the solar wind comprises 1 to 5 protons per cubic centimetre.

K. Ramamurthy, Bangalore

ANSWER 2: Solar wind is a combination of electrically charged stream of atomic particles - electrons, protons, with small proportions of heavier nuclei and the entrained solar magnetic field emitted from the sun's atmosphere.

E. N. Parker in 1958 showed that due to the very high temperature of the corona the pressure exerted results in an outflow of material into the interplanetary space in all directions in a phenomenon he called ``solar wind''. Solar wind is an extremely rarefied hot plasma at roughly 100,000 degree kelvin flowing radially outward at a speed of 500 kilometres per second which is variable in response to the varying solar activity.

The particles normally takes about 4 to 5 days to reach earth. The radial flow of the solar wind and the rotation of the sun, wind the solar magnetic field into a spiral which makes an angle of 45 degrees at the earth's orbit. The study of the solar wind becomes important as it contributes to the phenomenon such as aurorae and magnetic storms.

Jayant Kumar, Hyderabad

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