Solar Irradiance

The primary source of energy to the Earth is radiant energy from the Sun. This radiant energy is measured and reported as the solar irradiance. When all of the radiation is measured it is called the Total Solar Irradiance (TSI); when measured as a function of wavelength it is the spectral irradiance. Light of different wavelengths reaches different parts of the Earth's atmosphere. Visible light and infrared radiation reach the surface, warming the surface to livable conditions. Ultraviolet radiation in the UV-A, B, and C wavelengths is absorbed at higher and higher altitudes. Extreme ultraviolet and soft X-ray radiation (wavelengths less than 120 nm, EUV) is absorbed by the atmosphere above 100 km (60 miles). Although it is completely absorbed by our atmosphere, EUV radiation is quite dangerous to people and electronics in space.

Each part of the solar irradiance changes over the 11-year solar cycle, becoming brighter than average at solar maximum and dimmer at solar minimum. Each wavelength also changes as the Sun rotates and during solar flares. Changes seen at the Earth are not the same as those at the other planets.

To understand the mechanisms that cause EUV irradiance variations, two sets of measurements are required. Measurements of the EUV spectral irradiance over a wide range of wavelengths at a rapid cadence. Next, images of the Sun at the same wavelengths to identify the sources of the radiation. The EVE and AIA instruments will provide these complementary measurements.

Our Sun, a Weather Maker

Energy leaves the Sun as photons, energetic particles, and magnetic fields. Each has a measurable impact on Earth or near-Earth space. Photons move in a straight line from the Sun to the Earth, while other types of energy must follow a tortuous path through the Sun's atmosphere, the heliosphere, and the Earth's magnetosphere to reach the Earth's atmosphere. We will concentrate on the photons that move energy from the Sun to the Earth.

The Sun is a variable magnetic star. An astronomer would say the Sun's variations are typical for a Sun-like star. However, to a planet that seems to grow smaller each year, and a human population that is becoming increasingly reliant on technology, the Sun's variations are significant and dramatic. Changes in the Sun's magnetic field cause all of the variations that impact our life and technology here on Earth.

The science of SDO and LWS involves a complete understanding of the nature and manifestations of the Sun's energy. SDO's investigations are designed to understand the source of the Sun's energy, and how that energy is stored and released by the layers of the Sun's atmosphere. One part of that research is how the changes in the solar irradiance, or photon output, are produced.

Even subtle changes in irradiance can have a dramatic impact on the Earth's climate, atmosphere and ionosphere. Images at different wavelengths can be used to understand changes in the irradiance, but accurate measurements of the solar irradiance at a very rapid cadence are needed to truly understand the energetic input to Earth. Solar Irradiance on SDO

Extreme Ultraviolet Variability Experiment (EVE) investigation will measure the solar spectral irradiance at Extreme Ultraviolet (EUV) wavelengths from 1 to 1050 Angstrom (0.1 to 105 nm) plus the important hydrogen emission line at 1215 Angstrom. The EUV irradiance is absorbed by the atmosphere at altitudes above 100 km. This means changes in the EUV irradiance affect the thermosphere, ionosphere, and near-Earth space.

At left is a spectrum from the SEE instrument on the TIMED spacecraft, modified to show the EVE wavelength sampling. Data from SEE has shown that a daily value of the EUV irradiance is not enough to model the thermosphere and ionosphere. EVE will provide the solar data needed to drive the next generation of thermosphere/ionosphere models.

Measuring the EUV irradiance can be as easy as measuring the photocurrent from a piece of metal exposed to the Sun in space. This photoelectric effect won Einstein the Nobel Prize in 1921. EVE will use gratings to disperse the spectrum onto specially prepared CCDs to measure the irradiance with an accuracy of 5%.