The total solar irradiance (TSI) is a measure of the Sun’s energy output. It fluctuates by 0.2 Wm-2 over 11 year and is therefore a reliable indicator for atmospheric temperature and circulation, particularly in the upper atmosphere. According to Abbot, the downward trend of TSI was correlated with the cooling of the climate.
Solar radiation can be used to generate energy in a continuous way. The temperature of the Sun determines its emission rate and wavelength range. This radiation has a high density and is abundant. It is an excellent source to energy for a wide variety of applications. The Sun’s radiation emits approximately 3.8×1011 Watt of energy per second. Only about two-thirds of this radiation reaches Earth, and the rest goes into space. In addition to warming the Earth, solar radiation also has the potential to warm other planets in our solar system.
Six experiments are used to monitor the Sun’s radiation. The TSI is measured using NASA’s ACRIM3 satellite and NOAA’s Nimbus7 earth radiation budget. These experiments have been measuring the TSI for the last thirty years.
The total solar radiation emitted is almost constant over time. However, fluctuations in the sun’s magnetic activity can be observed. 2001 saw the Sun experience its highest level of magnetic activity. However, it then dropped during the longest-ever solar minimum in more than 80 years. This solar minimum ended in 2009 and is currently in a decreasing phase. This pattern is expected continue until at most 2020.
Changes in absorbing, scattering and total losses
Three processes modify the radiation from the sun that travels through the Earth’s surface. These processes occur when radiation interacts and is suspended with gases. First, radiation undergoes absorption as small particles diffuse the light. This reduces the amount sunlight reaching the Earth’s surfaces. The wavelength and size of the particles can influence the amount of insolation.
The atmosphere also reduces the power from solar radiation that falls on the Earth. The amount and quality of the atmosphere’s dust and air molecules will determine the scattered light. If there was no atmosphere, then the sky would be blank and the sun would only be visible as a circle. Finally, the atmospheric composition changes can cause local variations of the incident solar energy. For example, cloud cover that is heavier than usual reduces incident solar power.
Both the Earth’s and vegetation’s surface temperatures can be affected if there is an increase in solar radiation. This is particularly true in the northern part of the hemisphere, where slopes face south. The slope has a greater absorbing capacity than a slope facing north. Tillage implements can also be used to modify the slope of a field. A tillage furrow directed east-west will create an uneven surface which will lead to higher soil surface temperatures.
Changes to the amount of energy absorbed into the atmosphere are related to changes in absorbing, scattering and total losses caused by increased solar radiation. The Earth’s atmosphere absorbs around 19 units solar radiation and 111 units forfrared radiation. 51 units of energy are absorbed by the surface.
TSI’s effect on solar radiation
Significant variations in solar radiation from TSI have occurred over the past 9000 years. The quiet solar luminosity, for example, has seen a millennial-scale variation during Maunder minimal. Significant variation in heliospheric activities is also evident during this period of almost no sunspots. TSI proxy simulations reproduce large TSI variation during this time period. They range from 3 to 6. W/m2.
The positive correlations found between TSI and solar activities cycles in combined TSI measurements taken by the ERBS experiment as well as the ACRIM1 experiment are clear. Despite the fact the solar activity has declined by a factor 2 over the last ten years, the TSI-trend has remained within +0.05 Wm-2 over the past 21 year. Although this is a slight decrease in the 21 year average, it is still significant due to the reductions in the TSI amplitude (from the maximum to the lowest of cycle 24).
TSI sensors have a two-ppm accuracy and can detect changes to total solar irradiance. They are also capable of detecting variations induced by the most powerful X-class solar flares. To further analyze TSI measurement, a robust algorithm was developed to calculate radiative output. This algorithm can be used to account for both impulsive TSI measurements. We used data from four solar flares for comparison, including the 2006 December 6th flare.
The solar variability is a significant component of the energy conservation equation for the Earth system. It does have an effect on the climate, but it is unlikely that it will directly affect global temperature. It also influences TSI indirectly, amplifying the effects of direct solar forcing.