Coronal Holes and Coronal Mass Ejections

The corona is the outermost layer of the Sun. It is extremely hot but so tenuous that it is visible only when a solar eclipse blocks the brighter photosphere. Coronal holes are less dense regions of the corona where coronal matter streams outward into interplanetary space. Coronal mass ejections occur when magnetic field lines in the solar corona snap and eject large clumps of solar material into interplanetary space.

Overview

The outermost layer in the Sun’s atmosphere is the corona (which means “crown”). Gas in the corona can reach temperatures of a few million kelvins. This gas is very thin, however, with a density on the order of 10^-12 kilograms/meter³. Thus, the corona is faint, so faint that it cannot normally be seen because its feeble light is overwhelmed by the much brighter photosphere. The corona must be observed optically either during a total solar eclipse or by using a coronagraph. The latter is a disk, blocking the photosphere, in the focal plane of the telescope.

The chromosphere is the layer of the sun’s atmosphere between the photosphere and corona. The chromosphere is only a few thousand kilometers thick. The temperature of the gas in the chromosphere is slightly higher than that in the photosphere. In the approximately 100-kilometer-thick transitional region between the chromosphere and corona, the temperature rapidly increases from about 6,000 kelvins to a few hundred thousand kelvins.

The Sun is a ball of gas without anything that could be considered a solid surface. The sun’s photosphere, however, is the closest the sun has to a surface. The photosphere is relatively opaque and blocks our view of the solar interior, so most photographs or observations of the solar disk show the sun’s photosphere. It is also the coolest layer of the sun. The bottom layer of the photosphere is at a temperature of 5,800 kelvins. The photospheric temperature drops with increasing height to a temperature of about 4,500 kelvins at the top of the photosphere, begins to increase in the chromosphere, and is extremely high in the corona.

Relatively cool stars are reddish in color, while hot stars are bluish. The sun’s corona, at a few million kelvins, is much hotter than most stars, so it emits most of its energy in the extreme ultraviolet (the shortest ultraviolet wavelengths) to X-ray region of the electromagnetic spectrum. The photosphere is not hot enough to emit significant amounts of energy in this spectral region. Fortunately for human beings, Earth’s atmosphere blocks most extreme ultraviolet and X radiation, so astronomers study the sun at these wavelengths from satellites.

Extreme ultraviolet and X-ray pictures of the sun show a bright corona and dark photosphere, which is the reverse of optical pictures showing a bright photosphere and much fainter corona. Solar astronomers therefore study the solar corona using extreme ultraviolet or X-ray images. At these wavelengths the corona shows structures that are not visible at optical wavelengths.

One common structure in the X-ray corona is the coronal hole. On X-ray images of the sun’s corona, coronal holes show up as dark areas because they are regions where the corona is much more tenuous than normal. In the coronal holes, the corona does not show up in X rays and the photosphere below is very dark at X-ray wavelengths. Gas density in the coronal holes is typically about one-tenth the density of the normal portions of the corona. Near the north and south poles of the sun, coronal holes tend to be relatively stable. Near the equatorial and midlatitude regions of the sun, coronal holes are less stable. Coronal hole activity varies with the sun’s magnetic activity cycle. Coronal holes are therefore in some way related to the sun’s magnetic field. The largest coronal holes, which are a few hundred thousand kilometers in diameter, can last for months. More typical coronal holes are tens of thousands kilometers in diameter. These smaller coronal holes typically last only for hours rather than months.

Most of the corona contains coronal loops, which are magnetic field structures. The solar magnetic field lines come up from the lower layers of the sun, loop into the corona, then flow back down into the solar interior. Charged particles, such as protons and electrons, in strong magnetic fields generally travel in spiral paths around the magnetic field lines. The magnetic forces do not allow these particles to travel across the magnetic field lines. Solar material is a plasma in which electrons are separated from the atomic nuclei; it is composed of charged particles and flows along these coronal loops.

Coronal loops do not exist in coronal holes. In coronal holes the magnetic field lines from the sun’s interior do not loop back into the sun. They extend outward into interplanetary space. In the coronal holes, solar material moves upward from the sun’s lower layers along these magnetic field lines. Rather than falling back down into the sun, this material—which is mostly protons (hydrogen nuclei) and electrons with occasional heavier, ionized atoms—streams out into interplanetary space and leaves a low-density coronal hole. Coronal holes therefore contain solar matter flowing into interplanetary space and are a major source of the solar wind.

The solar wind consists of charged particles from the sun flowing outward into space. Were it not continually replenished from the lower layers of the sun, the solar wind would evaporate the corona in about a day. A few billion kilograms of solar material flow outward in the solar wind every second. The sun permanently loses this mass. It would seem that the sun might quickly evaporate from the cumulative effect of the coronal holes and solar wind, but the sun is very much more massive than Earth. Hence, in the nearly five billion years of its existence, the sun has lost less than one-tenth of 1 percent of its mass to the outflow of the solar wind.

Coronal mass ejections occur when coronal loops break. Normally solar material flows along the coronal loops from the solar interior, into the corona, and back into the interior. However, occasionally the magnetic field lines in a coronal loop break. When this happens, the loop more or less explodes. The solar material in the loop is no longer confined by the magnetic field; it shoots outward into space. These events are called coronal mass ejections, or CMEs.

A typical CME flings about 10¹² to 10¹³ kilograms of solar material into space. Typically a CME releases 10²⁴ to 10²⁵ joules of energy. Because CMEs are related to the sun’s magnetic field, they occur more frequently during the maxima of the solar activity (sunspot) cycle. CMEs happen as often as a few times a day. When a CME is pointed toward Earth, the resulting magnetic storm can severely disrupt long-distance communications on Earth and cause increased auroral activity.

Knowledge Gained

In contrast to the sun’s photosphere, the spectrum of the sun’s corona contains emission lines, which form from a hot, thin gas. Many of the emission lines in the sun’s coronal spectrum are lines not visible from Earth. Astronomers originally thought these lines might be a new element, but they turned out to be what scientists call forbidden lines. The existence of these forbidden lines was an early clue to the extremely low density of the corona.

Although the corona can be studied optically from the ground during eclipses, much of our knowledge of the corona, coronal holes, and coronal mass ejections comes from satellites, particularly those equipped to observe the sun at extreme ultraviolet and X-ray wavelengths as well as optically. Such studies started in earnest in the 1970’s using X-ray telescopes on the Skylab mission. During this crewed mission, solar astronomers first noticed the connection between coronal holes and the solar wind.

Other satellites have been important to understanding coronal phenomena. The Japanese Yohkoh Satellite was launched in 1991 and fell back to Earth in 2005. Yohkoh for the first time allowed daily images of the corona allowing solar astronomers to study rapid changes in the coronal structure. The joint European and National Aeronautics and Space Administration (NASA) Solar and Heliospheric Observatory, SOHO, mission launched in 1995, and the NASA Transition Region and Coronal Explorer (TRACE) mission, launched in 1998, also made many contributions to our understanding of coronal and other solar phenomena. SOHO discovered a magnetic carpet on the sun’s surface that plays a major role in providing the energy needed to heat the corona. TRACE takes very high-resolution, extreme ultraviolet images of the corona. Although TRACE could image only a small region of the corona at one time, it did allow very detailed studies of coronal phenomena. TRACE was retired in 2010. In 2006, NASA launched the Hinode Mission to study the sun's magnetic field. As of 2022, the spacecraft was still in orbit. IIn 2018, NASA launched the Parker Solar Probe, which became the first spacecraft to “touch” the sun by flying into its corona. By 2021, the craft had made fourteen of a planned twenty-four obits of the sun, sending back valuable information about the solar wind.

Context

Coronal mass ejections as well as variations in the solar wind can affect Earth, causing geomagnetic storms. These geomagnetic storms are often referred to as space weather. The aurora borealis and aurora australis, also known as the northern and southern lights, are caused by these geomagnetic storms. Hence they are more likely to be visible when a coronal mass ejection reaches Earth. Other geomagnetic effects are less benign. Earth’s upper atmosphere expands a little and disrupts long-distance radio communications that depend on radio waves reflecting off the ionosphere or communication satellites. The expanded upper atmosphere can cause some friction on low-Earth-orbit satellites leading to orbital decay and eventually falling back to Earth. Geomagnetic storms caused by coronal mass ejections can also disrupt the electrical power grid.

Coronal holes and coronal mass ejections are part of the complex magnetic phenomena of the sun’s corona. Coronal holes seem to play a still poorly understood role in the sun’s magnetic activity cycle, though they occur during periods of lower activity in the sun's eleven-year cycle. These phenomena do not exist in isolation. The corona and its magnetic fields interact with the sun’s chromosphere, photosphere, interior, and their magnetic fields. Via the solar wind, originating in coronal holes, and coronal mass ejections, the sun’s corona also interacts with Earth. To understand any facet of this complex sun-earth system completely, astronomers need to understand all the other facets.

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