Solar ultraviolet emissions

The Sun’s upper chromosphere, visible as a reddish ring around the Moon during a solar eclipse, has been found to be the principal source of solar ultraviolet radiation.

Overview

Ultraviolet (or UV) radiation is a form of electromagnetic radiation between visible light and X-rays. Most UV radiation is absorbed by ozone in the Earth’s upper atmosphere; only the “near ultraviolet” (UV with wavelengths not too much shorter than visible violet light) can penetrate to the ground. Solar near-UV is what produces suntans and sunburns. Since the Earth’s atmosphere acts as an efficient UV filter, astronomical observations in most of the UV range are best conducted by instruments on board a spacecraft, beyond the atmosphere of the Earth.

Observations of both UV and extreme ultraviolet radiation (EUV radiation, with short wavelengths approaching “soft X-rays”) are important because of the effects these emissions have on the Earth’s upper atmosphere and also because they can be used to model empirically the layers of the solar atmosphere known as the chromosphere and lower corona. The UV spectrum is a good spectral diagnostic for inferring physical conditions in these layers, with the brightness and variations of the spectrum providing clues about the solar atmosphere.

Before the Orbiting Solar Observatories and Skylab were launched, observations of the Sun’s ultraviolet region had been made by brief sounding-rocket observations, lasting for several minutes at best. The results of these observations made it clear that the study of the Sun’s atmosphere and its UV radiation would be crucial to an understanding of the Sun. Observations of total solar eclipses made solar astronomers somewhat aware that the chromosphere and corona, invisible to the human eye outside those eclipses, emit much of their light in UV and shorter wavelengths. It became apparent with the eight Orbiting Solar Observatory satellites that wavelength regions other than the visible had to be observed and mapped in detail if these layers were to be understood. It was also clear that solar EUV radiation over time underwent significant variations in brightness that could perhaps be attributed to changing solar magnetic activity.

The solar atmosphere has a thin but significant layer known as the transition region, located between the upper chromosphere and corona. For a variety of reasons, the temperature, which steadily falls as the distance from the Sun’s center increases outward, reverses in the upper chromosphere and spikes upward in the transition region to very high temperatures in the corona, near 2 million kelvins. Solar astronomers had long wanted to use ultraviolet images to study the upper chromosphere and transition region. The complete study of the chemical composition, temperatures, densities, and physical processes within these regions would help astronomers to understand the complex mechanisms involved in the structure and dynamics of the solar atmosphere. How is heat transferred to the Corona through the transition region? How are the transition region and chromosphere related to the corona above and Photosphere below?

Skylab provided the first detailed pictures of the transition region, using the ultraviolet Spectrograph associated with the Apollo Telescope Mount. At UV wavelengths, the edge, or limb, of the Sun appears brighter than the disk because of the increase of temperature with increasing height above the surface. This thin region has a temperature of about 150,000 kelvins in its lower layers and 300,000 kelvins at the halfway point between the surface and the coronal layers. This behavior is opposite to that observed in ordinary visible light, where limb darkening occurs in the visible image because of a decrease in temperature with height in the photosphere. The duration in space and the large size of the Apollo Telescope Mount observatory telescopes enabled the astronauts to obtain sensational images that would never be possible on the ground because of interference from the Earth’s atmosphere. Previously, these images had not been obtainable from space either, since the weight of instrumental packages on Spacecraft was highly restricted.

Observations of the UV chromosphere give insight into the way the Sun’s atmosphere changes with distance from the surface, as well as details of how changes come about as a result of solar magnetic activity. The chromosphere is covered by a mesh of bright spike-like jets of gas, shooting from the chromosphere into the low corona, called spicules. When viewed near the limb of the Sun, they resemble slanted spikes arranged in uniformly spaced circles, separated by distances slightly larger than the diameters of the circles, so that they do not intermesh. This arrangement is referred to as the “spicule forest.” Many solar astronomers believe that spicules play a crucial role in transferring nonradiative energy (energy produced by mechanisms other than electromagnetic radiation) into the layers of the solar atmosphere at and above the transition region.

UV images show fluffy loops that join and link active regions together in a lacelike pattern. This pattern shows up in the images as a mottled network that is no doubt directly related to the spicule network. In higher layers of the chromosphere near the transition region, this mottled network takes on a more uniform distribution.

The Skylab full-disk pictures provided a better understanding of spicules and the chromospheric network and also revealed larger spicule-like features found at the Sun’s poles. These observations provided new insights into the way energy is transmitted upward to heat the outer layers of the solar atmosphere. The network fades and becomes nearly indistinguishable at the poles, because the larger spicules protrude to roughen the appearance of the solar limb. The overall chromospheric network is also distorted and made into a nonuniform pattern near centers of activity in middle solar latitudes.

Fine brushes of lines sprout from bright UV regions of the chromosphere, revealing lower legs of magnetic loops that stand in magnetically active regions. The burning-bush appearance of these curving lines is formed by the roots of the magnetic loops that correspond to concentrated magnetic areas in the photosphere where sunspots and other aspects of solar activity are observed. Above the photosphere, in the chromosphere, corresponding to higher arch structures in these same loops, other forms of activity are observed, such as prominences (large flamelike tongues of gas that appear to be extensions of the chromosphere into lower regions of the corona) and flares (sudden, triggerlike releases of tremendous amounts of thermal energy trapped by these magnetic arched loops). They appear to occur in tubelike sections of these arched loops from the corona through the chromosphere and on occasion into the photosphere. The flares are believed to occur where two or more of the arches magnetically join one another and violently rearrange their magnetic fields. This entire chromospheric ultraviolet network fades in the lower corona and gives way to a looser, less uniform distribution of material.

Almost all the light emitted by hot coronal gas comes from loops that trace out and mark patterns of magnetic lines of force rooted in bright, active regions in the chromosphere. The entire corona appears to be made up of intertwined arrangements of many of these arched loops. Snarled and twisted magnetic fields show up as particular patterns in the UV chromospheric network where magnetic activity is located; these UV patterns also signal the beginning of energetic events associated with this activity.

In addition to revealing the overall networklike structure of the chromospheric layers and the location of solar activity, the UV images from Skylab provided great insight into the structure and physical properties of various forms of transient solar activity such as prominences, surges (sudden eruptions of gas from deeper layers), and flares. Skylab obtained many detailed pictures of prominences, showing their temperature and three-dimensional structure. The ghostly prominences are shaped and supported by magnetic forces and are made up of chromospheric material elevated and immersed in the hotter coronal gas. Detailed UV images of prominences show coronal tunnel-shaped refrigerators maintained by magnetic tubes, whose outside diameter is surrounded by the hot corona. The tube has a temperature that is lowest in a line running down its center. The temperatures of these structures are inferred from the brightness of the UV image. The brightness-temperature correlation is scientifically defined in terms of a quantity called “emission measure,” but simply put, the basic principle is the hotter the gas, the brighter the glow.

The ultraviolet spectrograph on board the Solar Maximum Mission (SMM) Satellite gave solar astronomers the ability to observe prominences at wavelengths, and hence temperatures, not seen from the Earth. These observations established the existence of downflows at the footpoints (regions of strongest photospheric magnetic fields) of the arches of the prominences. After sufficient loss of mass to lower layers, the prominence can rise into the corona and bow up into an archlike shape. A prominence can be thought of as a long magnetic tube whose length twists and turns much like a rope. A tight wrapping of the ropelike structure can give rise to various magnetic activities associated with phenomena such as flares or eruptive prominences.

Observations of sunspots made with the ultraviolet spectrograph provided information on the mass flow in the Umbra (darkened central area) of Sunspots and measures of the emission of radiation from them. With the ultraviolet spectrograph, scientists detected upward-propagating acoustic waves above the umbra, created by either a subphotospheric cavity trapping plasma waves or the resonant transmission of magnetoacoustic waves by a chromospheric cavity.

Knowledge Gained

In the near UV, radiation with a wavelength of about 300 nanometers originates in the photosphere, and at progressively shorter wavelengths the radiation originates at higher and higher layers throughout the chromosphere. Shorter than 140 nanometers, the spectrum changes from a bright continuum with dark Fraunhofer lines to a faint continuum with emission lines. At wavelengths near 140 nanometers, the solar material is very opaque to radiation from the lower levels. As a result, it becomes possible to see emission lines from the chromosphere, since the continuum becomes so faint.

Many new emission lines were discovered by the Skylab and SMM spectrographs. The strongest emission line, the Lyman alpha line of hydrogen at 121.6 nanometers, is a very good solar activity indicator much like the line of ionized calcium at 392 nanometers. Images at the wavelengths of both lines show a mottled structure that fills in and becomes patchier and more irregular near centers of magnetic activity. Study of the Sun in various emission lines allows scientists to study higher and higher layers of the chromosphere and transition region. The Lyman alpha emission line, the strongest line in the entire solar spectrum, emits more energy than the entire solar spectrum between 0 and 120 nanometers.

The total solar UV irradiance is very low, accounting for about 1 percent of total solar electromagnetic emission. There is some evidence that the entire UV irradiance varies with the solar cycle. This variation has been established for EUV but not for all of the UV range. EUV irradiance is very sensitive to solar activity.

Radiation wavelengths of 100 nanometers or less have important terrestrial effects. This radiation originates in the chromosphere, transition region, and lower corona. The elaborate network structure of the chromosphere was verified and understood in much more detail than had previously been known from ground-based observations.

The transition region is striking in UV images. Since the temperature rises with height quickly in the transition region, the images taken in the EUV band show a pronounced limb brightening. The edges of these images are similar to a lightened ring surrounding the disk.

UV and EUV images of prominences show them to be coronal refrigerators. They are essentially tunnel-shaped magnetic tubes twisted like a rope, often into an arch rising above the chromosphere into the corona. The motion of material in the tube can be very complex. Motion occurs predominantly downward near the footpoints of the arches, whereas material near the top of the arch appears to have a buoyant quality. The arches are often seen to rise and stretch simultaneously until dissipating into the corona.

The ultraviolet spectrograph on the SMM verified that material motion, which had been detected in ground-based observations, occurs in the umbrae of sunspots. Ground-based observations suggest a motion starting at the center of the spot and radiating outward into a circle. In some cases, the reverse of that phenomenon—radiating motion inward to the center of the circle—was observed. The motions observed by the ultraviolet spectrograph seemed to be essentially vertical, rising outward from the spot. The motions also seemed to have a wavelike, pulsing character, suggesting a form of solar oscillations.

Context

Solar UV striking the upper atmosphere dissociates diatomic oxygen molecules (O₂) into separate oxygen atoms. Between 15 and 50 kilometers above the Earth’s surface, ozone (O₃) is formed by a combination of single oxygen atoms and diatomic oxygen molecules. Ozone is destroyed by UV radiation, particularly between 210- and 310-nanometer wavelengths. This radiation is harmful to living tissue. Equilibrium is normally maintained between the creation and the destruction of ozone. the Earth’s ozone concentration peaks at an Altitude of about 20 kilometers, the “ozone layer,” where its concentration is about ten parts per million. Nevertheless, this thin concentration is essential for the preservation of human life.

Knowledge of the UV and the EUV has allowed astronomers and solar physicists to study the elusive transition region between the chromosphere and corona. The transition layers glow mostly in the UV and show a pronounced increase in temperature and decrease in density with height in the solar atmosphere. Within these layers, various waves (both mechanically and magnetically generated) dissipate energy, mostly in the form of mechanical heat (the vibration of atoms and particles) to the layers above. Thus, they account for the steep increase in temperature with height in the chromosphere and corona above the photosphere. The pronounced decrease in density within these layers is partly responsible for allowing only certain types of wave energy to move outward, retaining most of the thermal energy. Various heat mechanisms, such as sound waves, induce vibrations among the particles, which in turn introduce heat and other forms of energy to their surroundings.

Most solar physicists still consider the increase in temperature of the chromosphere and corona with height above the photosphere to be one of the most difficult problems in solar physics. The foregoing explanations account for some of the mechanisms responsible, but not all. UV and EUV observations show fine structural details that may also be observable in X-ray wavelengths. These details may reveal phenomena that might contribute to this heating and also provide clues as to the physical nature of solar flares. Most notably, these details reveal the interaction of many small magnetic loops giving rise to new local magnetic geometries having the ability to accelerate charged particles and waves into the chromosphere and corona. Solar oscillations of various modes and amplitudes, the “ringing” of the Sun three-dimensionally, no doubt send energy to the chromosphere and corona.

The National Aeronautics and Space Administration (NASA)’s Hubble Space Telescope, launched in 1990, is adept at the detection of ultraviolet wavelengths in the galaxy. Hubble is assisted in that it is situated 340 miles above the Earth. In 2020, Hubble undertook ULLYSES, a project to observe the ultraviolet light from more than 300 low-mass, newly formed stars. Its objective was to create a library of data specific to these types of celestial objects. Scientists hope to gain more complete data on how stars originate, which itself should assist in the understanding of the evolution of galaxies.

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