Solar infrared emissions

Most solar infrared and far-infrared radiation is emitted from the coolest layers in the solar atmosphere, which are found in the upper portion of the photosphere. Analysis of this radiation not only allows scientists to understand these important layers of the solar atmosphere but also provides observational confirmation of the simplest known interaction between matter and radiation: local thermodynamic equilibrium.

Overview

Infrared (or IR) radiation is a form of electromagnetic radiation between visible light and microwaves (short-wavelength radio waves). Gases in Earth’s atmosphere (especially water vapor) absorb much of the IR radiation—particularly the “far infrared,” the longer-wavelength IR—before it reaches the ground. Effective study of IR radiation from astronomical sources must be done using telescopes at dry, high-altitude mountaintop observatories above the densest part of Earth’s atmosphere, or preferably using space telescopes completely outside our atmosphere.

The IR radiation emitted by the Sun represents only a tiny fraction of the total solar radiative energy. The rate at which the Sun emits IR energy is only about 0.058 percent of the total solar luminosity, which is the rate at which the Sun radiates energy over the entire electromagnetic spectrum. The solar constant is the average total solar irradiance (electromagnetic energy per unit time per unit area) impinging on the top of Earth’s atmosphere. Its value is 1,368 ± 7 watts per square meter, as measured by the Solar Maximum Mission (SMM). Of this, only 0.802 ± 0.026 watts per square meter is in the IR.

The IR is the simplest part of the solar spectrum. The radiation is almost continuous over the entire IR spectral range, with very few dark absorption lines. The continuous background spectrum is thermal radiation, such as a blackbody (a perfect thermal radiator) emits. Absorption lines (called Fraunhofer lines in the solar spectrum) are formed when photons with specific energies are absorbed by electrons jumping to higher energy levels in atoms or molecules or by molecules going to higher-energy rotational or vibrational states. Consequently, less energy remains in the spectrum at the wavelengths corresponding to the absorbed photon energies, so the spectrum looks darker at those wavelengths. The absorption lines that are present serve as a chemical fingerprint, revealing that specific elements and compounds are present in the source. However, the absence of absorption lines in some part of the spectrum—for example, in the IR—does not necessarily indicate certain elements or compounds are not present, just that the physical conditions are not right for them to absorb in that spectral range if they are present in the source.

IR radiation arises primarily in the upper photosphere (the visible surface of the Sun) and the lower chromosphere (the layer of the Sun’s atmosphere, a few thousand kilometers thick, immediately above the photosphere). These levels of the solar atmosphere consist of homogeneous strata. From place to place and layer to layer, the gas and its behavior show a remarkable similarity. This is where the solar atmospheric temperature falls to its minimum value and begins to rise to the higher temperatures found in the chromosphere and corona, the extensive outer layers of the solar atmosphere above the photosphere.

The photosphere consists of a series of layers from which most of the Sun’s visible light is emitted into space. These layers make up a very thin shell, only a few hundred kilometers thick. The opacity of the photosphere increases rapidly with depth; thus, the intensity of emitted radiation drops off rapidly with depth into the photosphere. A photon emitted outward from the lower photosphere has a large probability of being absorbed or scattered by the atoms and free electrons within the photosphere. Because of this, photons are likely to escape into space only from the photosphere’s uppermost layers. For this reason, the edge (or limb) of the Sun is sharply defined, and the Sun appears to have a definite surface.

The photosphere produces the continuous radiation observed across the entire solar spectrum. The intensity peaks in the yellow-green part of the visible spectrum and falls off toward both longer and shorter wavelengths. That is why the IR contributes such a small percentage of the Sun’s total electromagnetic radiation. This spectral distribution of intensity is similar to that of a blackbody (an ideal thermal radiator) with a temperature of about 5,800 kelvins. In reality, each layer making up the photosphere emits its own blackbody spectrum, which, in turn, depends on the temperature of that layer. The sum total of all the emissions from all the layers is similar to one imaginary layer emitting at approximately 5,800 kelvins; this is referred to as the Sun’s “effective temperature.” Those layers that are deeper in the photosphere emit at higher temperatures, and those that are higher in the photosphere emit at lower temperatures, as low as 4,400 kelvins.

The blackbody spectral distribution results from the high opacity of the layers. (Perfect blackbody radiators are perfectly opaque.) The source of this high opacity is the presence of negative hydrogen ions, first proposed by Rupert Wilt in the late 1940’s. The negative hydrogen ion is a hydrogen atom with an additional (second) electron weakly attached. It easily absorbs radiation in the visible spectrum and especially in the infrared. The second electron is bound to the hydrogen atom very weakly—with a bond about 3.5 percent as strong as that of the first electron. Since the bond of the second electron is so weak and since the temperature drops off rapidly toward the top of the photosphere, the electron density also diminishes rapidly in the photosphere’s upper reaches. In other words, the number of free electrons and negative hydrogen ions per unit volume—known as their “number densities”—are very sensitive to temperature and thus height in the photosphere.

The phenomenon of limb darkening can be observed on any good white-light solar image (an image of the Sun obtained over the entire range of visible wavelengths, the combined colors from violet to red giving a white image with a slight yellow tinge). The limb of the Sun is noticeably darker than the center of the solar disk because of the temperature gradient in the photosphere. As an observer’s line of sight moves toward the limb, it passes through only the upper, cooler layers, whereas deeper, hotter layers are seen near the disk center. Thus, the intensity of light decreases toward the limb due to the drop in temperature with height. (In contrast, the Sun’s limb appears brighter than the disk center in observations of the chromosphere and lower corona because of the reversal of the temperature gradient in those layers.)

Since the intensity of radiation can be measured as a function of both distance from the disk center and wavelength, much can be learned about the way in which physical properties change with depth once the processes of photon emission, absorption, and scattering within the atmosphere are understood. These processes control the flow of electromagnetic radiation through the solar atmosphere by radiative transfer. Using the processes of radiative transfer and observed details of the solar spectrum and limb darkening, astronomers can calculate mathematical models of the solar atmosphere. These models are tabulations of height versus variables of interest, such as temperature, density, and pressure.

Knowledge Gained

The entire solar photosphere emits radiation like a stacked system of glowing shells, each shining at its characteristic temperature. The net effect of all these glowing shells is similar to that of one thin shell emitting at a temperature of 5,800 kelvins, the “effective temperature” of the photosphere. The infrared and far-infrared spectra are emitted from the coolest layers (4,400 kelvins), located at the top of the photosphere. Most of the solar IR spectrum is in a continuous form, with few dark absorption lines. IR radiation originates mainly in the uppermost photosphere. At this level, the solar atmosphere is at its coolest temperature: about 4,400 kelvins.

Context

The study of the IR spectrum is important because these observations, along with limb-darkening measurements, are needed in modeling the solar atmosphere. These layers of gas are essentially in local thermodynamic equilibrium (LTE). Radiation is transferred in LTE by well-understood processes: photon emission, absorption, and scattering. The primary contributor to the opacity of all layers of the photosphere is the negative hydrogen ion. Sources of opacity and transport of energy by known nonthermal processes—such as mechanical mechanisms, sound waves, shock waves, and magnetic energy—are very small in the photosphere.

Furthermore, spectral studies of solar IR have illuminated details of the atomic and molecular composition of Earth’s atmosphere. These studies, in turn, help scientists to understand the interaction of other bands of electromagnetic radiation, such as ultraviolet, with the terrestrial atmospheric constituents detected in the IR.

Bibliography

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“Solar Radiation Basics.” Department of Energy, www.energy.gov/eere/solar/solar-radiation-basics. Accessed 18 Sept. 2023.