Solar radiation

The total solar radiation that falls on Earth is the primary factor in determining Earth’s weather and climate. Even the smallest variation in the solar irradiance, if sustained, could alter the terrestrial environment drastically.

Overview

Solar radiation spans the entire electromagnetic spectrum, from very short-wavelength, high-energy gamma rays given off by some solar flares to extremely long-wavelength, low-energy radio waves given off by magnetic disturbances associated with and other kinds of solar activity. Between these two extremes, there are X-rays, ultraviolet light, visible light, and infrared radiation, all of which provide clues to the processes occurring in and on the Sun. Astronomers study solar radiation and the solar with the use of ground-based telescopes, high-flying aircraft, balloons, and spacecraft. The study of solar radiation addresses one of the most important problems in solar physics: Does the Sun change its radiation output over time, or is it constant?

The Sun’s is the total electromagnetic energy emitted by the Sun per unit time into space, or the solar radiative power. Solar luminosity is approximately 3.8 10²⁶ watts, meaning it radiates 3.8 10²⁶ joules of electromagnetic energy into space every second. The solar radiation per unit time per unit area impinging upon the top of Earth’s atmosphere is known as the total solar irradiance, also called the “solar constant.” This value is 1,368 ± 7 watts per square meter. Whether the Sun’s luminosity is really constant, however, is in question. A decrease of as little as one-half of 1 percent over one century could send the entire Earth into an ice age. An increase of the same could produce first a global tropical rain forest and eventually scorched desert continents.

Most of what is known about the Sun’s radiation is derived from an analysis of its electromagnetic spectrum. The Sun’s visible spectrum, like that of most other stars, consists mainly of a smooth distribution of intensity of emitted light known as the continuum (or the continuous spectrum), with narrow dips in brightness, termed dark absorption lines. A simplified explanation is that the continuum is emitted from the Sun’s photosphere, an extremely thin shell of gas giving off the visible light that can be seen with the human eye (although damage to the retina is severe if the Sun is observed directly). Also known as , solar absorption lines are produced when electrons in atoms in the Sun’s atmosphere absorb photons with specific energies or wavelengths of light, causing the electrons to jump to higher energy levels in the atoms.

The visible colors are only a narrow band of wavelengths within the entire electromagnetic spectrum. The wavelengths of visible light range from just under 400 nanometers in the violet to more than 700 nanometers in the far red. Approximate boundaries for the other regions of the outside the visible range include the near ultraviolet, from about 120 nanometers up to just below 400 nanometers, where visible violet begins; extreme ultraviolet, wavelengths between about 10 and 120 nanometers; soft X-ray wavelengths, between about 0.1 and 10 nanometers; hard X-rays, between about 0.001 and 0.1 nanometers; and gamma rays, shorter than about 0.001 nanometers. At the other end of the spectrum, emissions range from more than 700 nanometers (where visible red ends) up to about 1,000 nanometers; far infrared refers to wavelengths between about 1,000 and 1 million nanometers (about 1 millimeter, or 1,000 microns); and radio waves are longer than 1 millimeter and can be many kilometers in length. All these regions of the spectrum provide information about different layers of the solar atmosphere as well as about aspects of solar activity associated with sunspots, prominences, and flares.

The spectral distribution of the Sun’s approximates that of a blackbody, which is a hypothetical object that is opaque to all the radiation that falls upon it. It is a highly useful concept for describing the way stars radiate their continuous spectra. Blackbodies of higher temperatures emit much more electromagnetic energy at all wavelengths than those of lower temperatures, and the peak of the radiation distribution (spectrum) is at progressively shorter wavelengths for increasing blackbody temperature. The Sun’s continuum radiation distribution is similar to that of a blackbody whose temperature is about 5,800 kelvins.

Changes in the Sun’s output of electromagnetic energy and hence the solar “constant” could profoundly affect the climate on Earth. There is some evidence that short-term climatic variations, measured in terms of decades to millennia, may be brought on by changes in the Sun’s luminosity and the solar constant. Astronomers have wondered whether even shorter-term climatic changes, such as extensive droughts, might be caused by changes in the Sun’s energy output. On the other hand, long-term variations, such as the advance and retreat of continental glacial ice sheets over periods of tens of thousands to hundreds of thousands of years, are best explained by seasonal and latitudinal variations of the solar energy input. These are caused by changes in the geometry of Earth’s orbit and axial tilt. Even longer intervals of hundreds of millions of years between major continental glaciations are thought to result from changes in ocean and continent geometries caused by continental drift.

Claude Pouillet, who introduced the concept of the in 1837, tried to measure the solar constant by monitoring the temperature of a blackened box filled with water. The temperature increase of the water per unit of time would reflect the energy gained via sunlight. Samuel Pierpont Langley and Charles Greeley Abbot were later pioneers in measuring and monitoring the solar constant. In 1878, Langley invented a device that he named the bolometer. The bolometer measures the energy of incoming radiation, regardless of its wavelength. Langley discovered far-infrared light while using his bolometer to study solar radiation. He used the bolometer at high altitudes to try to measure the solar constant, making mathematical corrections for Earth’s atmospheric extinction. His values of the solar constant are considered fairly accurate even by today’s standards.

Nonetheless, modern solar astronomers question the reality of the small variations Langley and Abbot detected in the solar constant and solar luminosity. Such doubts have arisen because of the lack of precision of nineteenth-century equipment and the inability to estimate accurate experimental errors and uncertainties. It can be said that, if Langley and Abbot’s study suggests anything, it suggests that the solar constant is indeed constant. Any small variations simply escaped the capabilities of their measurements, even though they were convinced that they detected small changes of about 0.5 percent or less.

Certain types of variations, however, have since been demonstrated, principally variations associated with the solar cycle. John Eddy conducted an exhaustive historical study, looking back many years for evidence of past solar cycles. Much of this work concentrated on eras before the invention of the telescope and thus necessitated gathering descriptions of the appearance of the solar during solar eclipses, accounts of sunspot observations, and descriptions of the borealis.

Regular magnetic changes occur in the solar atmosphere over a period of about 11.2 years, the period of the solar cycle. Sunspots are observed to be at a maximum each time the cycle reaches its highest magnetic strength. The years 1969 and 1980 were such peak years; during the latter, the Solar Maximum Mission (SMM) was launched. The is driven by the Sun’s interior magnetism. Magnetic fields are wrapped by the rotation of the Sun. The Sun is a differential rotator; that is, its rotation rate depends on latitude; its equator rotates with a period of about twenty-five days, but at latitudes of 75° north and south, the rotation period can be thirty days or longer. lines between the Sun’s magnetic poles are dragged by the differentially rotating gas and coiled more and more tightly around the Sun. As the magnetic field lines become twisted and snarled, they arch into the upper layers of the solar atmosphere and produce a variety of solar magnetic activity, including sunspots, (arching spires of gas), and flares (caused by magnetic trapping of thermal energy, which, at its breaking point, suddenly triggers a release of immense energy).

These cycles vary in intensity and are irregular in occurrence. The intensity of a cycle is measured most directly by the number of sunspots observed during the cycle. These numbers vary noticeably from one cycle to another. Several cycles have been observed to be deficient in sunspot production, and in a few cases, almost no spots are produced. These “weak” cycles might be associated with a smaller luminosity and solar constant. Some solar physicists believe that the rate of solar rotation may vary from one cycle to another and that this rate may affect the solar constant. The magnetic centers of activity on the Sun are thought to interfere with the normal outflow of radiation; they can act to obstruct or reduce the flow outward and thus slightly modify the solar constant.

Peter Foukal and Jorge Vernazza found a possible correlation between solar rotation and the solar constant variation by examining the data from the experiments by Langley and Abbot. Their statistical analysis found a change of 0.07 percent every twenty-seven days in the observations. This tiny variation, occurring suspiciously in phase with the solar rotation, is accepted by some solar physicists and doubted by others. Some hold that such a small change is difficult to verify.

The period of solar rotation seems to have suffered “glitches” in the past. There is even modern evidence for small changes in solar rotation on a very short timescale, weeks or even days long. A known, but presumably very small, connection between solar rotation and the solar constant is the fact that as solar rotation carries sunspots around the Sun, their appearance on or disappearance from the earthward side of the Sun alters the solar constant very slightly, because less light is emitted by the sunspot than from a comparable area of the photosphere.

Variable solar rotation, if real on both long and short timescales, may also be an indication of other types of changes in the Sun’s deep interior that could lead to changes in the solar constant. A. Keith Pierce and James C. LoPresto, using the McMath solar telescope at Kitt Peak, Arizona, reported in 1984 that the Sun’s rotation does quickly change; it speeds up and slows down in a large cap around the polar regions within periods as short as a day or two.

Many astronomers and solar physicists have not been particularly interested in measurements of the solar constant over periods of time long enough to include one or more solar cycles. They contend that such experiments are difficult and expensive and that it is unlikely that significant changes in the solar constant could be documented. However, in response to the need for good solar irradiation measurements, it was decided to include an experiment for measuring the solar constant on board the Solar Maximum Mission (SMM) satellite, launched in 1980. Although the primary mission of the satellite was to study the Sun’s behavior during the peak (or “maximum”) activity cycle, it was also decided that monitoring any changes in the solar constant during the maximum was of utmost importance. Richard C. Willson thus devised an instrument dubbed the active cavity radiometer irradiance monitor (ACRIM) that was carried onboard SMM. Willson’s measurements with ACRIM from SMM gave a value for the solar constant of 1,368 ± 7 watts per square meter. Furthermore, it has been established that large sunspots decrease the solar irradiance by a few watts per square meter for very short periods (a week or so).

Knowledge Gained

Despite all the studies of the solar constant, attempts to verify its constancy or variability were inconclusive until the late twentieth century. Richard C. Willson measured the solar constant from high-altitude balloons in 1969 and again from an Aerobee rocket in 1976. The solar constant during that period remained unchanged to within 0.75 percent. This period included the last half of a solar cycle. In 1967, 1968, and 1978, David A. Murcray of the University of Denver measured the solar constant from high-altitude balloons. He found no change from 1967 to 1968, but he detected a 0.4 percent change between 1968 and 1978.

The SMM spacecraft, launched into Earth orbit by the space shuttle in 1980, carried a diverse, sophisticated payload to study solar activity. Among the instruments on board was the ACRIM, which was designed by Willson to detect changes in the solar constant of as little as 0.1 percent. ACRIM measured the solar input during half of each 131.072-second measurement interval and a known radiation source onboard the during the other half of each measurement interval, during which time a shutter blocked solar radiation. The solar irradiance received by Earth is proportional to the difference between the heating rates with the shutter open and with it closed. The ACRIM results indicate that the average value of the solar constant is 1,368 ± 7 watts per square meter. It decreased slightly but steadily between 1980 and 1986, leveling off in 1987 and 1988. There are indications of a possible slow period variation connected with the solar cycle of about 2 watts per square meter. It was also determined that sunspots cause very short-term variations, for periods equal to the life of the spot.

Solar astronomers working at the National Solar Observatory, Kitt Peak, Arizona, continue to monitor certain absorption lines in the solar spectrum that are known to be sensitive to stellar luminosity. They have found that the strengths of these lines closely correspond to the variability first detected by ACRIM, both in magnitude and in the temporal variation associated with the sunspot cycle. Monitoring these changes and similar solar radiation events is critical for managing Earth's increasing temperatures. Solar radiation modification (SRM) is a geoengineering technique designed to reflect or reduce the amount of solar radiation reaching Earth to counteract climate change. It does not reduce greenhouse gas emissions but instead aims to temporarily cool the planet by controlling how much heat the atmosphere absorbs. Some SRM methods include stratospheric aerosol injection, marine cloud brightening, and surface albedo enhancement. However, advisors to the European Commission have emphasized that while solar radiation modification (SRM) technologies might offer temporary climate relief, they cannot fully address the root causes of climate change. They advocate for responsible research into SRM's potential impacts and caution against viewing it as a standalone solution.

Context

The goal of solar spectral irradiance measurements is to measure solar absolute brightness and how much and how rapidly it changes. The dominates the energy emission, and both it and the infrared seem to be relatively constant. In contrast, radio, ultraviolet, and X-rays show large fluctuations associated with solar magnetic activity during the solar cycle.

Before the space age, measurements of solar irradiance had a very incomplete time record, even over one solar cycle. Many astronomers and solar physicists were not particularly interested in measuring the solar constant over long periods of time, contending that such experiments were difficult and expensive and it was unlikely that significant changes in the solar constant could be documented. Since the solar spectrum covers an enormous wavelength range, from radio waves to X-rays, taking measurements of absolute brightness was, and remains, very challenging, since different detectors and different techniques are required in each wavelength band. Over the electromagnetic range, at least five different experimental techniques are required for imagery and dispersion of the solar spectrum.

Another difficult problem in attempts to measure the solar constant was the attempt to determine how much energy Earth’s atmosphere blocked before the radiation reached detectors on the ground. Earth’s atmosphere filters out radiation according to wavelength; its blocking of many parts of the electromagnetic spectrum is very effective. Ozone in the upper atmosphere completely absorbs all radiation shorter than the very longest near-ultraviolet waves, including gamma rays, X-rays, and most ultraviolet rays. At the lower end of the spectrum, water vapor and strongly absorb much of the infrared, and water vapor and oxygen absorb radio waves shorter than about 1 centimeter. The ionosphere, a thin layer of charged particles at an of about 100 kilometers, reflects radio waves longer than about 10 meters. Even those wavelengths that are not completely blocked, still may suffer some absorption and scattering as they pass through our atmosphere.

The solution to this problem appeared when it became possible to make observations above Earth’s atmosphere, and just about the entire solar spectrum has been observed using the Orbiting Solar Observatories (OSOs), Skylab, the Solar Maximum Mission (SMM), and the Solar and Heliospheric Observatory (SOHO).

In 2023, researchers noted an unusual event: the Sun was detected emitting an extraordinary amount of gamma rays. These emissions were so extensive, they formed the largest emissions of radiation from the sun ever recorded. Although the Earth’s atmosphere prevents this radiation from reaching the surface, the event demonstrated a production of higher energy light from the Sun than was previously believed possible. The increased radiation was first detected by the High-Altitude Water Cherenkov Observatory (HAWC) near Puebla, Mexico.

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