The sun is the main source of light and heat on Earth. The sun does not stop for a moment to send its enormous energy continuously to the earth. How is it delivered?

As we all know, there are three forms of heat transmission: conduction, convection and radiation. The sun mainly transmits its heat and particles to the vast expanse of the universe in the form of radiation. This process of propagation is called solar radiation. Solar radiation is not only the fundamental way for the earth to obtain heat, but also the most important factor affecting the survival activities of human beings and all other creatures and the change of the earth’s climate.

There are two types of solar radiation. One is the light radiation emitted from the surface of the photosphere, because it propagates light and heat in the form of electromagnetic waves, so it is also called electromagnetic wave radiation. This radiation consists of visible light and invisible light that is invisible to the human eye. The other is particle radiation, which is a stream of particles composed of positively charged protons and roughly equal numbers of negatively charged electrons and other particles. Particle radiation is usually weak and its energy is unstable. It is most intense during the period of maximum solar activity, which has a certain impact on human beings and the upper atmosphere of the earth. But generally, before it radiates to the surface of the earth, it gradually disappears on the long journey of the sun and the earth. So it will not send any heat to the earth. Therefore, the solar radiation introduced below mainly refers to light radiation.

When the solar radiation is sent to the earth, it not only has to go through a long journey, but also encounters various obstacles and various influences. As we all know, the surface of the earth where we live and live is tightly surrounded by three layers of atmosphere: the troposphere, the stratosphere and the ionosphere, with a total thickness of more than 1200km. The layer of atmosphere from the ground to within 10~12km is called the troposphere. The layer of atmosphere from above the troposphere to within 50km is called the stratosphere. The layer of atmosphere from above the stratosphere to about 950km is called the ionosphere. When the sun radiates its light, heat and particle flow to the earth at a speed of 300,000 kilometers per second from a distance of 150 million kilometers, it will be disturbed and blocked by the earth’s atmosphere and cannot be projected onto the earth’s surface unimpeded.

The earth is a big magnet, and a large magnetic field is formed around it. The vast area of more than 1,000 kilometers, up to tens of thousands of kilometers, or even as high as hundreds of thousands of kilometers, controlled by the magnetic field is called the magnetosphere of the earth. When solar particle radiation hits the ground, the magnetosphere acts like a thick wall blocking it from reaching the ground. Even if a few particles break in, they are often “captured” on the spot by the magnetic field inside the magnetosphere. This can be said to be the Earth’s “first line of defense” against solar radiation

In Earth’s atmosphere below Earth’s magnetosphere, the troposphere, stratosphere and ionosphere all absorb, reflect and scatter solar radiation. The ionosphere can not only absorb or reflect the radio waves in the solar radiation, but also block the harmful ultraviolet rays and rays from reaching the ground. This is the “second line of defense”.

The “third line of defense” is about 24km high in the stratosphere, and there is a layer that is particularly rich in ozone, called the ozone layer. It has a great effect and can absorb most of the ultraviolet rays that enter here.

Since the earth has set up such “three lines of defense”, the harmful part of the solar radiation is eliminated, so that human beings and various creatures are protected and can survive safely on the earth.

The next article will look at the effects of various substances in Earth’s atmosphere on solar radiation.