Research Page
Sun and Climate
The Sun is a variable star, and Earth is a sun-powered planet. The Climate and Radiation Lab (CRL) plays a critical role in developing and operating NASA’s solar radiation missions, which provide fundamental solar irradiance measurements for Sun-Climate research. Satellite observations have revolutionized our view of the Sun, providing the most accurate measurements to study both active and quiet Sun. A challenging question that remains is whether the quiet Sun is a time-invariant base level or a secular change in the Sun’s radiative output exists. Continuing the legacy of the NASA’s Solar Radiation and Climate Experiment (SORCE), the Total and Spectral Solar Irradiance Sensor -1 (TSIS-1) carries the state-of-the-art instruments enabling the most accurate total and spectral solar irradiance measurements and has been taking solar observations from the International Space Station (ISS) since 2018. NASA is planning to launch TSIS-2 in 2025 on a free flyer to continue these solar irradiance measurements.
NASA’s observation of solar radiation
While the Sun’s variable features have been observed since ancient times, consistent observations of solar radiation started only in 1978 via satellite measurements. The total and spectral solar irradiance have been observed from space with unprecedented accuracy and stability for the last four solar cycles. These observations have provided new insights in solar variability and its influence on the terrestrial atmosphere.
The total solar irradiance (TSI), also called sometimes (misleadingly since TSI varies) the “solar constant”, is the integrated solar energy arriving at Earth. TSI changes by ~0.1% in an 11-year solar cycle. Prior to the measurements obtained by the SORCE mission, TSI was estimated at 1366 Wm-2. One of the major SORCE contributions was to establish a more accurate value of 1361 Wm-2, which leads to 340 W m-2 for the globally averaged solar input to Earth. The most current TSI value from TSIS-1 is 1361.6 ± 0.3 Wm-2 for the 2019 solar minimum.
The 96% of spectral solar irradiance (SSI), encompassing ultraviolet, visible, and near-infrared wavelengths, has been measured by SORCE since 2003. Significant progress has been made in developing the reference spectrum and understanding spectral variability. It remains a great challenge to quantify solar spectral variability at the detailed wavelength level over the 11-year cycle, as it requires good instrument stability and reliable degradation correction methods. The TSIS-1 SSI measurements, which rely on a 3-channel degradation correction technique, can achieve accuracy of 0.2% at wavelengths greater than 400 nm. The TSIS-1 SSI measurement is closely monitoring the phase and magnitude of solar spectral variability as solar activity begins to increase in solar cycle 25.
Sun-Climate connection
Solar-Terrestrial coupling is an interdisciplinary research field involving atmospheric physics and chemistry, climate change, and heliophysics. Solar radiation is the ultimate energy source for Earth’s climate system and drives daily weather and climate by forcing oceanic and atmospheric circulations and hydrologic cycle including clouds and precipitation. One of the most intriguing aspects of Solar-Terrestrial science is to understand the effects of the solar variability on the Earth’s climate. For example, the Maunder minimum appears to be associated with unusually cold European winters when no sunspots were reported for several decades. The sunspot number is one of the major proxy indicators of the effect of solar radiation on past climates.
Unlike proxy data, total and spectral solar irradiances have been monitored from space continuously for more than four decades. The high-quality solar irradiance data of the space era provide the key record to study the Sun-Climate connection and its underlying coupling processes. The availability of these accurate TSI measurements revealed that the recent warming on Earth’s surfaces was not caused by solar variability. Still, scientists are still perplexed by the bottom-up and top-down coupling processes in the Sun-Climate connection and the interplay between natural and anthropogenic variabilities over longer periods.
In addition to its role in Earth’s radiation budget, solar radiation has a direct impact on human health. Excessive exposure to the solar ultraviolet (UV) radiation may increase the risk of various skin cancers. The atmospheric ozone layer shields us from excessive exposure to the harmful UV from the Sun. While UNEP sets a high priority to monitor the atmospheric total ozone worldwide, the natural variability induced by 11-year solar cycles has a significant contribution to the ozone loss and recovery processes. CRL research activities have focused on understanding spectral variability over the solar cycles, the dominant processes and pathways of the UV forcing in the middle and upper atmosphere, and the tracer responses of the atmospheric dynamics and chemistry. Both satellite observations and Earth system model simulations are employed to better characterize the underlying thermal, dynamical, and chemical mechanisms of the Solar-Terrestrial links.
At even shorter wavelengths, the effect of X-rays from the solar corona give rise to effects on Earth’s upper atmosphere by modulating highly variable atmospheric species such as nitric oxide radicals (NOx), which trigger direct and indirect effects on ozone loss. In addition, the solar forcing in the E/D-region of the ionosphere can modulate the atmosphere-ionosphere electrical coupling through the framework of global electric circuit (GEC), which correlates with global thunderstorm activity. Solar X-rays also play an important role on space weather in the ionosphere.
More topics of Sun-Climate research can be found at Sun Climate and the Sun-Climate Symposium web sites.
Contact: Dong L Wu, Jae N. Lee
NASA’s observation of solar radiation
While the Sun’s variable features have been observed since ancient times, consistent observations of solar radiation started only in 1978 via satellite measurements. The total and spectral solar irradiance have been observed from space with unprecedented accuracy and stability for the last four solar cycles. These observations have provided new insights in solar variability and its influence on the terrestrial atmosphere.
The total solar irradiance (TSI), also called sometimes (misleadingly since TSI varies) the “solar constant”, is the integrated solar energy arriving at Earth. TSI changes by ~0.1% in an 11-year solar cycle. Prior to the measurements obtained by the SORCE mission, TSI was estimated at 1366 Wm-2. One of the major SORCE contributions was to establish a more accurate value of 1361 Wm-2, which leads to 340 W m-2 for the globally averaged solar input to Earth. The most current TSI value from TSIS-1 is 1361.6 ± 0.3 Wm-2 for the 2019 solar minimum.
The 96% of spectral solar irradiance (SSI), encompassing ultraviolet, visible, and near-infrared wavelengths, has been measured by SORCE since 2003. Significant progress has been made in developing the reference spectrum and understanding spectral variability. It remains a great challenge to quantify solar spectral variability at the detailed wavelength level over the 11-year cycle, as it requires good instrument stability and reliable degradation correction methods. The TSIS-1 SSI measurements, which rely on a 3-channel degradation correction technique, can achieve accuracy of 0.2% at wavelengths greater than 400 nm. The TSIS-1 SSI measurement is closely monitoring the phase and magnitude of solar spectral variability as solar activity begins to increase in solar cycle 25.
Sun-Climate connection
Solar-Terrestrial coupling is an interdisciplinary research field involving atmospheric physics and chemistry, climate change, and heliophysics. Solar radiation is the ultimate energy source for Earth’s climate system and drives daily weather and climate by forcing oceanic and atmospheric circulations and hydrologic cycle including clouds and precipitation. One of the most intriguing aspects of Solar-Terrestrial science is to understand the effects of the solar variability on the Earth’s climate. For example, the Maunder minimum appears to be associated with unusually cold European winters when no sunspots were reported for several decades. The sunspot number is one of the major proxy indicators of the effect of solar radiation on past climates.
Unlike proxy data, total and spectral solar irradiances have been monitored from space continuously for more than four decades. The high-quality solar irradiance data of the space era provide the key record to study the Sun-Climate connection and its underlying coupling processes. The availability of these accurate TSI measurements revealed that the recent warming on Earth’s surfaces was not caused by solar variability. Still, scientists are still perplexed by the bottom-up and top-down coupling processes in the Sun-Climate connection and the interplay between natural and anthropogenic variabilities over longer periods.
In addition to its role in Earth’s radiation budget, solar radiation has a direct impact on human health. Excessive exposure to the solar ultraviolet (UV) radiation may increase the risk of various skin cancers. The atmospheric ozone layer shields us from excessive exposure to the harmful UV from the Sun. While UNEP sets a high priority to monitor the atmospheric total ozone worldwide, the natural variability induced by 11-year solar cycles has a significant contribution to the ozone loss and recovery processes. CRL research activities have focused on understanding spectral variability over the solar cycles, the dominant processes and pathways of the UV forcing in the middle and upper atmosphere, and the tracer responses of the atmospheric dynamics and chemistry. Both satellite observations and Earth system model simulations are employed to better characterize the underlying thermal, dynamical, and chemical mechanisms of the Solar-Terrestrial links.
At even shorter wavelengths, the effect of X-rays from the solar corona give rise to effects on Earth’s upper atmosphere by modulating highly variable atmospheric species such as nitric oxide radicals (NOx), which trigger direct and indirect effects on ozone loss. In addition, the solar forcing in the E/D-region of the ionosphere can modulate the atmosphere-ionosphere electrical coupling through the framework of global electric circuit (GEC), which correlates with global thunderstorm activity. Solar X-rays also play an important role on space weather in the ionosphere.
More topics of Sun-Climate research can be found at Sun Climate and the Sun-Climate Symposium web sites.
Contact: Dong L Wu, Jae N. Lee
