The solar radiation cycle and solar irradiance at the Earth's surface vary due to several factors, including the Earth's tilt, its rotation, and the presence of an atmosphere.

The Solar Radiation Cycle

The solar radiation cycle is an approximately 11-year cycle of variation in the Sun's activity. During periods of high solar activity, the Sun emits more energy, including more ultraviolet (UV) radiation. This can lead to increases in the Earth's ozone layer, which can have both positive and negative effects on human health and the environment.

The solar radiation cycle is not uniform across the Earth's surface. The equator receives more solar radiation than the poles, and this difference is exacerbated during periods of high solar activity. This is because the Earth is tilted on its axis, and the equator is always tilted more directly towards the Sun than the poles.

Solar Irradiance at the Earth's Surface

Solar irradiance is the amount of solar radiation that reaches the Earth's surface. The amount of solar irradiance that reaches the Earth's surface is lower than the amount of solar radiation that reaches the top of the atmosphere due to several factors, including:

• Atmospheric absorption: The Earth's atmosphere absorbs some of the solar radiation that reaches it, including UV radiation.
• Scattering: Some of the solar radiation that reaches the Earth's atmosphere is scattered by clouds and other particles. This scattered radiation does not reach the Earth's surface.
• Reflection: Some of the solar radiation that reaches the Earth's surface is reflected back into space.

As a result of these factors, the amount of solar irradiance that reaches the Earth's surface varies depending on a number of factors, including the time of day, the season, and the latitude. The amount of solar irradiance that reaches the Earth's surface is also affected by clouds, aerosols, and other atmospheric conditions.

Dr Murtadha Shukur thank you for your contribution to the discussion

Average annual solar radiation arriving at the top of the Earth's atmosphere is roughly 1361 W/m2. The Sun's rays are attenuated as they pass through the atmosphere, leaving maximum normal surface irradiance at approximately 1000 W/m2 at sea level on a clear day. Because Earth is a sphere, not all part of the Earth receives the same amount of solar radiation. More solar radiation is received and absorbed near the equator than at the poles. Near the equator, the Sun's rays strike the Earth most directly, while at the poles the rays strike at a steep angle.When the sun's rays strike Earth's surface near the equator, the incoming solar radiation is more direct (nearly perpendicular or closer to a 90˚ angle). Therefore, the solar radiation is concentrated over a smaller surface area, causing warmer temperatures. Areas around the equator absorbed about 200 watts per square meter more on average (orange and red) than they reflected or radiated. Areas near the poles reflected and/or radiated about 200 more watts per square meter (green and blue) than they absorbed. Mid-latitudes were roughly in balance. At the equator the sun is perpendicular to the surface, allowing maximum solar radiation to be distributed over a small surface area. Closer to the poles, the incoming solar radiation is the same but the light is spread over a larger surface area so the intensity is lower at a particular location. On the daylight side, only the point directly under the Sun receives full-intensity solar radiation. From the equator to the poles, the Sun' rays meet Earth at smaller and smaller angles, and the light gets spread over larger and larger surface areas. The average solar flux received in LEO is approximately the solar constant, or 1,361 W/m2 at 1 astronomical unit (AU) from the Sun, though it can vary a bit since the solar irradiance received by an object is really function of distance from the Sun and not constant at all, since the Earth's distance isn't fixed at 1. The solar radiation (or irradiance) at the top of the earth's atmosphere is called extraterrestrial radiation and the current accepted value for this “solar constant” (now referred to as the total sky irradiance) is 1366 W/m2. When the sun's rays strike Earth's surface near the equator, the incoming solar radiation is more direct (nearly perpendicular or closer to a 90˚ angle). Therefore, the solar radiation is concentrated over a smaller surface area, causing warmer temperatures.