On the plane where the upper boundary of the atmosphere is perpendicular to the light, the solar irradiance is basically a constant, but on the surface of the earth, the solar irradiance is constantly changing. This is mainly caused by the transparency of the atmosphere. The degree of atmospheric transparency is a parameter that characterizes the degree to which the atmosphere transmits through the sun’s rays. On clear and cloudless weather, the transparency of the atmosphere is the best, and the solar radiant energy reaching the ground is more; when the sky is cloudy or dusty, the transparency of the atmosphere is low, and the solar radiant energy reaching the ground is lower.
Sunshine time is also an important factor affecting the solar irradiance on the ground. If there are 14 hours during the day in a certain area, if the cloudy time is ≥ 6 hours, and the time of the sun is ≤ 8 hours, then it can be said that the day’s sunshine time in the area is 8 hours. The longer the sunshine time, the more total solar radiation that the ground receives.
In addition, the higher the altitude, the better the transparency of the atmosphere and the higher the direct radiation of the sun. In the Qinghai-Tibet Plateau of China, due to the average altitude above 4000m, clean atmosphere, dry air, and low latitude, the total solar radiation is mostly between 6000~8000MJ/m2, and the direct radiation ratio is large. In addition, the distance between the sun and the earth, topography, and topography also have a certain influence on solar irradiance. At the same latitude, the temperature in the basin is higher than that of Pingchuan, and the temperature on the sunny slope is higher than that on the shady slope.
Law of Sun-Earth Movement: The earth’s orbit around the sun is an elliptical orbit, and the sun is at one of the two focal points of its elliptical orbit. This elliptical orbit is called the ecliptic in astronomy. On the ecliptic plane, the distance between the sun and the earth is not a fixed value. The closest distance between the sun and the earth (1.47×108km) is the perihelion; the farthest distance between the sun and the earth (1.52×108km) is the aphelion. The difference between the two is 5×106km, which is about 1/30 of the average distance between the sun and the earth.
The intensity of the sun’s illumination on the earth depends on the following four aspects: the distance between the sun and the earth, the relative position of the sun to a certain place on the earth at a certain moment, the attenuation of solar radiation into the atmosphere, and the azimuth and inclination of the solar receiving surface. Since the position of the earth in the orbit is different, the position of the sun is different based on the observer’s ground plane on the earth, as shown in Figure 1. The specific situation is related to geographical latitude, but the position of the sun at the zenith can be seen at noon in different seasons.
Solar activity is closely related to some phenomena on the earth. Now, people have found that solar activity has a significant impact on the earth in the following aspects.
The solar flares and sunspots have significant geophysical effects on the ionosphere, magnetic field, and polar regions of the earth, which affect the short-wave radio communication on the ground and even cause a short interruption. This is called “sudden ionospheric disturbance.” These reflections appeared almost simultaneously with the outbreak of the large flare. The magnetic field descends along the lines of magnetic force and collides with the gas of the chromosphere layer, causing the heel parts of the magnetic lines of force on both sides of the neutral line to glow and become a flare that people see. The flares themselves are the result of unstable magnetic fields. It is precisely because of the non-equilibrium state of the magnetic field that the flare erupts to achieve a new balance of the magnetic field. The eruption process of the flare is also a process of releasing a large amount of energy. Larger flare explosions are not only due to the thermal motion of hydrogen atoms, the temperature can reach tens of millions of degrees or even hundreds of millions of degrees, but also strong radiation, ultraviolet rays and high-energy protons are emitted. These strong radiation rays increase the pressure of hydrogen atoms, causing hydrogen atoms, ions and other particles to be ejected at a speed of more than 1000km/s and become the sun’s particle radiation. The phenomenon of “magnetic storm” shows that the entire earth is a large magnetic field, and the surroundings of the earth are full of magnetic field lines. When a flare appears, high-energy particles are emitted from its vicinity, and a magnetic field is generated when the charged particle moves. When it reaches the earth, it disturbs the original magnetic field and causes changes in the geomagnetism. When a magnetic storm occurs, the intensity of the magnetic field changes greatly, which will have a great impact on human activities, especially work related to geomagnetism.
Another phenomenon that the sun affects the earth is the auroral phenomenon: in the north and south poles of the earth, at night or even in the daytime, light green, red, pink bands or arcs of light can often be seen in the sky, called aurora. This is because when the stream of charged high-energy particles from solar activity reaches the earth, they rush towards the polar region under the action of a magnetic field, which excites or ionizes molecules or atoms in the upper atmosphere of the polar region to produce light. The sun’s far ultraviolet and solar wind will affect the density of the atmosphere.
The change cycle of atmospheric density is 11 years, which is obviously related to solar activity. Solar activity may also affect atmospheric temperature and the ozone layer, which in turn affects the yield of crops and the balance of natural ecosystems. Because solar activity has an impact on humans, especially on aerospace, radio communications, meteorology, etc., studying solar activities, especially the law of solar flares, and trying to make predictions, is of great value to the use of solar energy.
The statistical data of solar energy resources are also constantly changing. Research in recent years has found that with the increase of air pollution, the amount of solar radiation in various places is showing a downward trend.