What Is the Solar Constant?

The solar constant refers to the solar radiation per second per unit area of the top of the atmosphere perpendicular to the sun's rays at the average distance between the sun and the earth (D = 1.496x10 ^ 8km). ^[1]

Under the condition of the average distance between the sun and the earth,

Because the earth orbits the sun in an elliptical orbit, the distance between the sun and the earth is not a constant, and

The radiation from the sun is vital to all life on Earth. The weather and climate on the earth are completely restricted by its incident quantity and its interaction with the earth's atmosphere, oceans and land. Even if the solar energy received by the earth changes by only one thousandth, as long as it is continuous, it will have an important impact on weather and climate. Because of this, in the meteorology, the determination of the solar constant has always received widespread attention. . In the field of meteorology.

On the other hand, the changes in the solar spectrum can be used to understand the changes occurring in the solar atmosphere. Understanding the short-term changes in the solar constant can help infer the internal mechanism of the sun itself. Therefore, the measurement of the solar constant is also a subject of great concern to the solar physics community.

Solar constant measurement error

Limited by the accuracy of the radiation measurement. Comparing the measurement errors of several basic units in the International System of Units, the measurement error of time is the smallest and has reached

Such an order of magnitude. Followed by length, quality, etc., and the measurement error of spectral radiance is the largest. After years of unremitting efforts by scientific and technological workers, the highest level in the world is only

. However, the solar radiation measurement is more accurate because it is more radiant and the measurement environment is more difficult to control.

Solar constant measurement history

In 1837-1838, French physicist Claude Pouillet (1790-1868) and British astronomer John Herschel (1792-1871) first attempted to directly determine the solar constant. The two independently designed different measuring devices, but the principle is the same. on

Solar constant

It is to use water of known quality for a certain period of time under the sunlight, and measure the temperature rise process with a thermometer. Since the specific heat of the water is known, the light intensity can be calculated. Since the absorption of light by the earth's atmosphere has not been taken into account, their estimated value is "about half of the value 1367 (± 4) W / m ² currently used".

In 1875, French physicist Jules Violle (Jules Louis Gabriel Violle) became famous for the first high altitude measurement of solar constants in the Mont Blanc Alps, on the border of France and Switzerland.

In 1884, Samuel Penlepont Langley made his first attempt to measure the solar constant at Mount Whitney, California, and tried to eliminate the effects of Earth's atmospheric absorption by measuring at different days and times. But the value he got was not correct, it was 2903W / m ² , maybe it was caused by a mathematical error.

The values measured by Charles Abbot and others in 1902 and 1957 were 1322 W / m ² and 1465 W / m ^{2, respectively} .

According to the continuous observation results of the observation platforms on 6 satellites from 1978 to 1998, the solar constant value obtained was 1366.1 W / m ² , the standard deviation was 425 ppm, and the fluctuation range was 0.37% (1363-1368 W / m ² ) (Lean and Rind, 1998). 20 years of observations have shown that the solar constant fluctuates on different time scales.

1902-1957, scientist CG Abbot (Charles Greeley Abbot) of the Smithsonian Institute and others based on the observations of high-altitude areas for many years, and the value determined based on the ground-based method was 1322-1465 W / m ² . In recent years, through various advanced methods, the standard value of the solar constant measured based on the ground-based method is 1353 W / m ² .

In 1976, NASA issued a solar constant value of 1353 (± 21) W / m ² (TheKaekara, 1976) based on the observation results of high-altitude platforms; according to 1978-1998, the observation platforms on 6 satellites have been continuous for 20 years Through constant observations, the obtained solar constant value is 1366.1 W / m2, the standard deviation is 425ppm, and the fluctuation range is 0.37% (1363-1368 W / m ² ) (Lean and Rind, 1998). Twenty years of satellite data also revealed that the solar constant also fluctuates on different time scales.

In 1981, the value of the solar constant published by the World Meteorological Organization (WMO) was 1367 ± 7W / m ² . ^[2]