Лекция: Solar radiation

The prime source of the energy injected into our atmosphere is the sun, which is continually shedding part of its mass by radiating waves of electromag­netic energy and high-energy particles into space. This constant emission is important because it repre­sents in the long run almost all the energy available to the earth (except for a small amount emanating from the radioactive decay of earth minerals). The amount of energy received by the earth, assuming for the moment that there is no interference from the atmosphere, is affected by four factors: solar output, the sun-earth distance, the altitude of the sun, and day length.

1 Solar output

Solar energy, which originates from nuclear reactions within the sun's hot core (16 * 106 K), is transmitted to the sun's surface by radiation and hydrogen convection. Visible solar radiation (light) comes from a 'cool' (~6,000 K) outer surface layer called the photosphere. Temperatures rise again in the outer chromosphere (10,000 K) and corona (106 K), which is continually expanding into space. The outflowing hot gases (plasma) from the sun, referred to as the solar wind (with a speed of 1.5 * 106 km hr-1), interact with the earth's magnetic field and upper atmosphere. The earth intercepts both the normal electromagnetic radiation and energetic particles emitted by the sun during solar flares.

The sun behaves virtually as a black body, meaning that it both absorbs all energy received and in turn radiates energy at the maximum rate possible for a given temperature. The energy emitted at a particular wavelength by a perfect radiator of given temperature is described by
a relationship due to Max Planck. The total energy emitted by a black body is found by integration of Planck's equation, known as Stefan's Law:

F = бT4

where б = 5.67 * 10–8 W m-2 K-2 (the Stefan-Boltzmann constant), i.e. the energy emitted (F) is proportional to the fourth power of the absolute temperature of the body (T).

The total solar output to space, assuming a tem­perature of 5,760 K for the sun, is 3.84 * 1026 W, but only a tiny fraction of this is intercepted by the earth, because the energy received is inversely pro­portional to the square of the solar distance (150 million km).

The energy received at the top of the atmosphere on a surface perpendicular to the solar beam for mean solar distance is termed the solar constant. The most recent satellite measurements indicate a value of about 1,368 Wm-2. For solar radi­ation, 8 per cent is ultraviolet and shorter wave­length emission, 39 per cent visible light (0.4–0.7 µm) and 53 per cent near-infrared (>0.7 µm). The mean temperature of the earth's surface is about 288 K (15 °C) and of the atmosphere about 250 K (-23 °C). Gases do not behave as black bodies, the absorption bands in the atmosphere cause its emission to be much less than that from an equiva­lent black body. The wavelength of maximum emis­sion varies inversely with the absolute temperature of the radiating body.

Thus solar radiation is very intense and is mainly short-wave between about 0.2 and 4.0 μm, with a maximum (per unit wavelength) at 0.5 µm, whereas the much weaker 'terrestrial radiation has a peak intensity at about 10 µm and a range of about 4 to 100 µm (1 µm = 1 micrometre = 10-6 m).

Satellite data show that the solar constant under­goes small periodic variations of about 0.1 per cent, related to sunspot activity. Sunspots are dark (i.e. cooler) areas visible on the sun's surface. Their number and positions change in a regular manner, known as the sunspot cycles. These cycles have wavelengths averaging 11 years (varying in length between 8 and 13 years), the 22-year (Hale) magnetic cycle, much less importantly 37.2 years (18.6 years – the luni-solar oscillation) and possibly 80–90 years. Between the thirteenth and eighteenth centuries, sunspot activity was gener­ally low, except for the periods AD 1350–1400, and 1600–1645. Output within the ultraviolet part of the spectrum shows considerable variability, with up to twenty times more ultraviolet radiation emitted at certain wavelengths during a sunspot maximum than during a sunspot minimum. The relation between sunspot activity and terrestrial temperatures is a matter of some dispute. However, some authorities believe that prolonged time- spans of sunspot minima (e.g. AD 1645-1705, the Maunder Minimum) and maxima (e.g. 1895–1940 and post 1970) can produce significant global cooling and warming, respectively.

Shorter-term relationships are more difficult to support, but mean annual temperatures have been correlated with the combined 10-11 and 18.6-year solar cycles. Satellite measurements during the 1980s, the latest solar cycle, show a small decrease in solar output as sunspot number approaches its minimum, and a subsequent recovery. Although sunspot areas are cool spots, they are surrounded by bright areas of activity known as faculae, which have higher temperatures; the net effect is for solar output to vary in parallel with the number of sunspots. Thus, the solar 'irradiance' decreases by about 1.5 Wm-2 from sunspot maximum to mini­mum. In the long term, assuming that the earth behaves as a black body, a long-continued differ­ence of 2 per cent in the solar constant could change the effective mean temperature of the earth's surface by as much as 1.2 °C; however, the observed fluc­tuations of about 0.1 per cent would change the mean global temperature by ≤0.06 °C, based on calculations of radiative equilibrium.

Упражнение 2.

Прочитайте следующие слова и определите их соответствия
в русском языке:

Radiation, mass, electromagnetic, moment, solar, photosphere, chromosphere, corona, plasma, fraction, proportional, perpendicular, constant, ultraviolet, infrared, peak, intensity, periodical, regular, manner.

Упражнение 3.

Найдите в тексте из упражнения 1 слова с суффиксом ~ly. Определите, какими частями речи они являются.

Упражнение 4.

В правой колонке найдите русские эквиваленты следующих английских словосочетаний:

Упражнение 5.

Заполните таблицу, вставив недостающие части речи.

Упражнение 6.

Вставьте в пропуски в тексте соответствующие предлоги: at, with, of, to, by, throughout, between, in, per, through