Solar corona

The solar corona is the outermost layer of the Sun's atmosphere. It is a region filled with hot, thin plasma shaped by the solar magnetic field. The corona sits above the photosphere and chromosphere, stretching out until it mixes with the solar wind.

During a total solar eclipse or when using a special tool called a coronagraph, the corona looks like a glowing light around the Sun. Scientists have learned that the corona is much hotter than the Sun’s surface, with temperatures above one million kelvins.

The corona has many interesting shapes, such as bright prominences, curved coronal loops, and large helmet streamers.

During a total solar eclipse, the Sun's corona and prominences are visible to the naked eye.

Observational history

For broader coverage of this topic, see Solar observation.

Corona sketched by José Joaquín de Ferrer during the solar eclipse of June 16, 1806 in Kinderhook, New York.

In 1724, an astronomer saw that the bright light during a solar eclipse comes from the Sun, not the Moon. In 1809, this light was named the "corona." In 1930, a special tool called a coronagraph was made to see this light without an eclipse.

Later, scientists found that the corona’s light comes from very hot iron atoms. This helped explain some of its strange features.

General characteristics

Temperatures in the solar atmosphere rise precipitously with height in the transition region between the chromosphere and corona.

Energy from nuclear fusion in the Sun's core heats the layers above it. The temperature gets cooler as you move away from the core, reaching its lowest point at the Sun's surface, called the photosphere. But above this surface, the temperature begins to rise again.

The solar corona is the outermost layer of the Sun's atmosphere. It is very hot but very thin, with temperatures ranging from about 1,000,000 K to 2,000,000 K. The solar magnetic field shapes the corona, guiding the movement of charged particles within it.

Structure

The shape of the solar corona changes all the time because of the Sun's magnetic field. When the Sun is quiet, the corona is mostly near the middle part of the Sun, with special areas called coronal holes near the poles. But when the Sun is more active, the corona spreads out more, especially where there are sunspots.

Configuration of solar magnetic flux during the solar cycle

The Sun goes through a cycle about every 11 years. During this time, the Sun can be calmer or more active. When it's more active, the magnetic field gets twistier, and we see more sunspots and other interesting things on the Sun. Sunspots are dark spots on the Sun because the magnetic field pushes aside the hot surface, letting us see cooler stuff underneath.

Coronal loops are important parts of the corona. They are like loops of magnetic field that fill up with very hot solar material. This material can move along the loops in different ways.

Coronal loops rooted in an active region. Imaged in 171 Å by TRACE.

These loops can last for very short times, or for much longer periods. They help scientists study how the corona gets heated up, but it's still a mystery. Special tools like the Parker Solar Probe are getting closer to the Sun to help solve this puzzle.

Active regions are groups of these loop structures that connect areas with opposite magnetic fields on the Sun's surface. They can be very hot and dense and are linked to many solar activities like sunspots, bright areas, and big eruptions from the Sun.

Solar prominences and sunspots

Helmet streamers are big, hat-like shapes in the corona that often sit over sunspots. They are thought to be sources of the slower part of the solar wind.

Bright points are tiny active areas on the Sun that can be seen in X-ray images. They change with the solar cycle and are linked to small magnetic fields.

Coronal holes are dark areas in X-ray images where the magnetic field stretches out into space. The fast solar wind comes mostly from these areas.

The quiet Sun is the part of the Sun that isn't in active regions or coronal holes. It changes size depending on the solar cycle, usually being larger when the Sun is quieter.

The Alfvén surface is where the corona meets the solar wind. It was hard to find, but in 2021, the Parker Solar Probe finally reached it during one of its close passes by the Sun.

Variability

The solar corona changes in many ways. Studying these changes can be hard because different parts change at different speeds — from seconds to months. The sizes of areas where these changes happen also vary a lot.

Flares

On August 31, 2012, a long filament of solar material that had been hovering in the corona erupted

Main article: Solar flares

Solar flares happen in active areas and cause a quick burst of energy from small parts of the corona. These bursts are complicated and can be seen in different kinds of light, including special ultraviolet and X-ray light. Most flares last about 15 minutes, but big ones can go on for several hours. During a flare, the area gets much hotter and denser.

Flares can look different depending on how we observe them, but there are two main types: small flares that don’t change shape much, and longer-lasting flares that can cause big changes in the Sun’s magnetic field.

Filament erupting during a solar flare, seen at EUV wavelengths (TRACE)

Coronal mass ejections

Main article: Coronal mass ejection

Big solar flares and other solar activities often lead to coronal mass ejections (CME). These are huge bursts of material and magnetic field that shoot away from the Sun at very high speeds. Some large CMEs can send massive amounts of material into space at speeds up to millions of miles per hour.

Coronal event	Typical time-scale	Typical length-scale (Mm)
Active region flare	10 to 10000seconds	10–100
X-ray bright point	minutes	1–10
Transient in large-scale structures	from minutes to hours	~100
Transient in interconnecting arcs	from minutes to hours	~100
Quiet Sun	from hours to months	100–1000
Coronal hole	several rotations	100–1000

Physics

Thermal conduction

Heat moves from the Sun’s outer layers to its inner layers in the corona. Electrons, which are lighter and move faster than other particles, help carry this heat.

A mosaic of the extreme ultraviolet images taken from STEREO on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C (195Å—green), 60000–80000°C (304 Å—red), and 2.5 million °C (286 Å—yellow).

When magnetic fields are present, heat moves more easily along the direction of these fields. Particles moving across the fields are pushed and bend along the field lines, which helps heat travel in one direction more than others.

Coronal seismology

Main article: Coronal seismology

Coronal seismology is a way to study the Sun’s corona by looking at special waves in the plasma. These waves help scientists learn about the Sun’s magnetic fields, density, and structure. This method is similar to how we study waves inside Earth or in laboratory devices.

Coronal heating problem

Why is the Sun's outer layer, called the corona, so much hotter than the Sun's surface? This is a big mystery in solar science.

The Sun's corona is much hotter than its surface, but we do not fully understand why. Some theories suggest that waves or magnetic activities might carry energy up to heat the corona. Scientists are still studying this to learn more about our Sun.

Main article: Magnetic reconnection