The Convective Layer

By the time, the outgoing radiation reaches the top of the radiative layer,the solar atmosphere becomes somewhat opaque to the outgoing radiation with the consequence that the heat energy piles up in a narrow transition zone at the top of the radiative layer causing the material below to be extremely hot as compared to that above. The pent-up energy of the transition zone then bursts into violent convection which rises to great heights, delivers the energy to the solar surface and then sinks. Across the convective layer, the temperature drops enormously from a value of about 2 million K at the bottom to about 6,000 K at the top. Helioseismology

Helioseismology which may be described as a new branch of solar physics, has thrown considerable light on what all goes on in the sun's convective layer. In fact, it has brought out that the root causes of several observed features of the sun and its atmosphere, such as sunspots, flares, prominences, coronal discharges, etc., may be traced to the happenings in this layer, particularly the periodic upheavals and constant churnings and turbulent motions of the plasma in this layer which produce intense and far-reaching electric and magnetic fields. While vertical oscillations generate pressure waves, differential rotation between the equator and the high-latitude belts produce complex circulations in this layer. Unlike seismic waves inside the earth, the pressure waves in the sun's interior do not travel in straight lines. They form a kind of standing waves between the surface of the sun and the lower surfaces of the convective layer, but how deep they penetrate or how far they travel depend on the wave-length of the waves.

Helioseismological investigations further inform us that the deeper part of the sun's interior from the core center to the upper boundary of the radiative layer may be in solid- body rotation, but the convective layer and the photosphere above have a strong latitudinal shear between the equator and the higher latitudes, which, through increase of vorticity, may give rise to a meridional component of the horizontal velocity. This may mean that combined with vertical motions, the horizontal motion may actually cause a mean meridional circulation of the kind found on the earth, with rising motion near the solar equator, poleward motion above, sinking motion in high latitudes and a possible return equatorward flow at the bottom of the layer. A sketch of this likely circulation is shown in Fig. 7.2. However, the formation of such a circulation is only an inference at present. Further studies will be required to prove or disprove its existence.

Fig. 7.2 Schematic showing likely mean meridional-vertical circulation in the Sun's interior (After SOHO/SOI/MDI Consortium) (Reproduced from Lang, 1999, in The New Solar System, 4th edn, edited by Beatty, Petersen and Chaikin, with permission of Cambridge University Press)

7.4 The Photosphere

At the top of the convective layer lies the 500km-thick surface layer of the sun, called the photosphere (photos - the Greek word for light), where the temperature is about 5760 K. It is from here that electromagnetic radiation in the form of heat and light as we know them on earth along with the very high-frequency radiation that wells up from the interior move out into space through the solar atmosphere. The photosphere constitutes the visible surface of the sun and the base of the solar atmosphere. Normally, its dazzling brightness prevents any direct eye observations. However, its surface can be seen, though for a few moments only, at the time of solar eclipse or using a coronagragh at any time.

Ever since the time of Galileo, scientists have studied the sun using several direct and indirect methods of observation, such as powerful telescopes, spectrometers, photometers, polarimeters, etc. A lot of details about the conditions of the solar surface and its interior have been gathered using space-borne probes designed specifically for solar studies, such as the multinational Solar and Heliospheric Observatory(SOHO), soft X-ray telescopes carried onboard the Japanese satellite 'Yohkoh'(sunbeam) or the NASA satellite Ulysses in the 1990s.

Prominent amongst the findings by these probes are: the great unsteadiness of the sun with periodic rises and falls of its surface, regular occurrences of sunspots with an 11-year cycle in regions of strong magnetic fields and frequent solar flares and prominences which often rise to great heights in the sun's atmosphere. Helioseismic soundings suggest that the vertical oscillations of the solar surface may be due to the standing sound waves in the convection layer below in which the upward-moving wave is reflected downward by the lower surface of the photosphere and the downward moving wave is refracted upward by the steep rise of temperature at the base of the convection layer. The observed period of these waves is found to be between 3 and 6 min and their wave lengths range from a few thousand kilometers to almost the full circumference of the sun.



Fig. 7.3 The 11-Year sunspot cycle showing migration of the sunspots (After David Hathaway, NASA/Marshall) (Reproduced from Lang, 1999, in The New Solar System, 4th edn, edited by Beatty, Petersen and Chaikin,with kind permission of Cambridge University Press)



Fig. 7.3 The 11-Year sunspot cycle showing migration of the sunspots (After David Hathaway, NASA/Marshall) (Reproduced from Lang, 1999, in The New Solar System, 4th edn, edited by Beatty, Petersen and Chaikin,with kind permission of Cambridge University Press)

7.4.1 Sunspots

These are relatively dark areas on the visible surface of the sun, as against the general brightness of the photosphere. The intense magnetic field within a sunspot acts as some kind of a filter or control valve, which does not let any heat or energy from the interior to flow outward. This keeps the sunspots a few thousand degrees cooler than their surroundings.

At the center of a sunspot the strength of the magnetic field may be a few thousand Gauss. Sunspots occur in pairs of opposite magnetic polarity, which are joined by loops of magnetic field which often rise high into the solar atmosphere. These loops appear very bright in X-ray images of the sun and often appear as flares on the visible surface of the sun. They constitute a kind of hot spots on the solar surface in which the temperature can shoot up to 1,000,000 K or more, as against about 3,000-4,000 K in the sunspots.

The number of sunspots varies from a maximum to a minimum in an approximately 11-year cycle (Fig. 7.3). They first appear in high latitudes around 35°-40° and then migrate equatorward to about 5° before a new cycle begins in high latitudes.

It is observed that the brightness or luminosity of the sun varies with the sunspot cycle, being maximum at the sunspot maximum and minimum at the sunspot minimum. This appears to be somewhat paradoxical, since sunspots are relatively cooler areas of the solar surface and one would expect that the intensity of solar emission would be minimum at the sunspot maximum. But what really happens is that the coolness of the sunspot regions is offset by the extreme heat released by the large number of flares that accompany the sunspots over the remaining surface of the sun. The net result, therefore, is an increase (decrease) in luminosity at the sunspot maximum (minimum).

7.5 The Solar Atmosphere

The sun has an extensive atmosphere with several layers the boundaries of which are not clearly defined in every case, especially in the case of the corona which is the outermost layer and which extends into interplanetary space.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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