16-7 The corona ejects mass into space to form the solar wind

Figure 16-14: R I V U X G
The Solar Corona This striking photograph of the corona was taken during the total solar eclipse of July 11, 1991. Numerous streamers extend for millions of kilometers above the solar surface. The unearthly light of the corona is one of the most extraordinary aspects of experiencing a solar eclipse.

(Courtesy of R. Christen and M. Christen, Astro-Physics, Inc.)

The corona, or outermost region of the Sun’s atmosphere, begins at the top of the chromosphere. It extends out to a distance of several million kilometers. Despite its tremendous extent, the corona is only about one-millionth (10⁻⁶) as bright as the photosphere—no brighter than the full moon. Hence, the corona can be viewed only when the light from the photosphere is blocked out, either by use of a specially designed telescope or during a total solar eclipse.

Figure 16-14 is an exceptionally detailed photograph of the Sun’s corona taken during a solar eclipse. It shows that the corona is not merely a spherical shell of gas surrounding the Sun. Rather, numerous streamers extend in different directions far above the solar surface. The shapes of these streamers vary on timescales of days or weeks. (For another view of the corona during a solar eclipse, see Figure 3-10b.)

Comparing the Corona, Chromosphere, and Photosphere

Like the chromosphere that lies below it, the corona has an emission line spectrum characteristic of a hot, thin gas. When the spectrum of the corona was first measured in the nineteenth century, astronomers found a number of emission lines at wavelengths that had never been seen in the laboratory. Their explanation was that the corona contained elements that had not yet been detected on Earth. However, laboratory experiments in the 1930s revealed that these unusual emission lines were in fact caused by the same atoms found elsewhere in the universe—but in highly ionized states. For example, a prominent green line at 530.3 nm is caused by highly ionized iron atoms, each of which has been stripped of 13 of its 26 electrons. In order to strip that many electrons from atoms, temperatures in the corona must reach 2 million kelvins (2 × 10⁶ K) or even higher—far greater than the temperatures in the chromosphere. Figure 16-15 shows how temperature varies in the chromosphere and corona.

Figure 16-15: RIVUXG
Temperatures in the Sun’s Upper Atmosphere (a) This graph shows how temperature varies with altitude in the Sun’s chromosphere and corona and in the narrow transition region between them. In order to show a large range of values, both the vertical and horizontal scales are nonlinear. (b) This composite portrait shows the different features that appear as wavelengths corresponding to higher temperatures are imaged. At visible wavelengths, the 5800 K photosphere is visible, while ultraviolet light images features at millions of degrees. On the far right, modeling has been used to draw lines of the Sun’s magnetic field in the corona.

(a: Adapted from A. Gabriel; b: NASA)

The low density of the corona explains why it is so dim compared with the photosphere. In general, the higher the temperature of a gas, the brighter it glows. But because there are so few atoms in the corona, the net amount of light that it emits is very feeble compared with the light from the much cooler, but also much denser, photosphere.

The Solar Wind and Coronal Holes

Figure 16-16: R I V U X G
The Ultraviolet Corona The SOHO spacecraft recorded this false-color ultraviolet view of the solar corona. The dark feature running across the Sun’s disk from the top is a coronal hole, a region where the coronal gases are thinner than elsewhere. Such holes are often the source of strong gusts in the solar wind.

(SOHO/EIT/ESA/NASA)

Earth’s gravity keeps our atmosphere from escaping into space. In the same way, the Sun’s powerful gravitational attraction keeps most of the gases of the photosphere, chromosphere, and corona from escaping. But the corona’s high temperature means that its atoms and ions are moving at very high speeds, around a million km per hour. As a result, some of the coronal gas can and does escape. This outflow of gas, which we first encountered in Section 8-5, is called the solar wind.

Each second the Sun ejects about a million tons (10⁹ kg) of material into the solar wind. But the Sun is so massive that, even over its entire lifetime, it will eject only a few tenths of a percent of its total mass. The solar wind is composed almost entirely of electrons and nuclei of hydrogen and helium. About 0.1% of the solar wind is made up of ions of more massive atoms, such as silicon, sulfur, calcium, chromium, nickel, iron, and argon. The aurorae seen at far northern or southern latitudes on Earth are produced when electrons and ions from the solar wind enter our upper atmosphere.

Special telescopes enable astronomers to see the origin of the solar wind. To appreciate what sort of telescopes are needed, note that because the temperature of the coronal gas is so high, ions in the corona are moving very fast (see Box 7-2). When ions collide, the energy of the impact is so great that the ion’s electrons are boosted to very high energy levels. As the electrons fall back to lower levels, they emit high-energy photons in the ultraviolet and X-ray portions of the spectrum—wavelengths at which the photosphere and chromosphere are relatively dim. Hence, telescopes sensitive to these short wavelengths are ideal for studying the corona and the flow of the solar wind.

Earth’s atmosphere absorbs most ultraviolet light and X-rays, so telescopes for these wavelengths must be placed above the atmosphere on spacecraft (see Section 6-7, especially Figure 6-25). Figure 16-16 shows an ultraviolet view of the corona from the SOHO spacecraft (Solar and Heliospheric Observatory), a joint project of the European Space Agency (ESA) and NASA.

Figure 16-16 reveals that the corona is not uniform in temperature or density. The densest, highest-temperature regions appear bright, while the thinner, lower-temperature regions are dark. Note the large dark area, called a coronal hole because it is almost devoid of luminous gas. Particles streaming away from the Sun can most easily flow outward through these particularly thin regions. Therefore, it is thought that coronal holes are the main corridors through which particles of the solar wind escape from the Sun.

Evidence in favor of this picture has come from the Ulysses spacecraft, another joint ESA/NASA mission. In 1994 and 1995, Ulysses became the first spacecraft to fly over the Sun’s north and south poles, where there are apparently permanent coronal holes. The spacecraft indeed measured a stronger solar wind emanating from these holes.

The temperatures in the corona and the chromosphere are not at all what we would expect. Just as you feel warm if you stand close to a campfire but cold if you move away, we would expect that the temperature in the corona and chromosphere would decrease with increasing altitude and, hence, increasing distance from the warmth of the Sun’s photosphere. Why, then, does the temperature in these regions increase with increasing altitude? This question has been one of the major unsolved mysteries in astronomy for the past half-century.

In 2011, the Solar Dynamics Observatory found evidence that spicules (Section 16-6) play a role in heating the corona. Some spicules can get quite large—the width of a typical state in the United States and as tall as Earth—and shoot gas at about 150,000 miles/hour. Observations show that some of the gas ejected by spicules is heated to millions of degrees as it is shot into the corona. With so many spicules present at any given time (about 300,000), estimates indicate this ejected gas makes a significant contribution to coronal heating.

However, spicules are only one contribution to coronal heating, and as astronomers have tried to resolve this dilemma, they have found important clues in one of the Sun’s most familiar features—sunspots.