Long-term Solar Irradiance Variability and Implications for Sun-Climate and Space-Climate Studies

The solar irradiance is the dominant energy input for Earth’s environment. So the accurate knowledge of the solar variability over decades to centuries is critical for Sun-climate studies, such as for trending changes in stratospheric ozone and surface temperatures, and for space-climate, such as for estimating thermospheric density changes that impact satellite lifetimes. The satellite era of observing the solar irradiance has provided almost-daily measurements of the total solar irradiance (TSI) and solar spectral irradiance (SSI) in ultraviolet wavelengths for more than four decades, thus observing over Solar Cycles 21-25. Composites of these observations and solar variability models based on those observations indicate the importance of the solar rotational variability (with approximately 27-day period) and the solar activity cycle (with about 11-year period) for the changing amount of energy into the environments at Earth and other planets. Some of the solar variability models also extend backwards in time using solar proxies, such as with the sunspot record dating back into the 1600s. The more elusive secular changes of the solar irradiance variability is sometimes estimated by examining the solar irradiance changes at solar cycle minimum times. Recent improvements to the composites and models indicate that there are small, if any, changes of the irradiance levels at solar cycle minimum times. More importantly, they clarify the differences in the magnitude of the solar maxima during different solar cycles. The solar maximum levels depend largely on the amount of solar magnetic energy available during a solar cycle and also on the timing of when the solar activity in the northern and southern hemispheres each have their own peak in activity. Nonetheless, the solar maximum variability has large differences for different cycles, while the solar minimum variability is small between different minima. Consequently, the magnitude of the solar maxima dominates the solar influence on Sun-climate and space-climate effects. This conclusion is particularly enlightening considering that Solar Cycle 24 maximum was the lowest in the past 100 years and that the current Solar Cycle 25 may also have a similarly low maximum.