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The Heliopedia

An encyclopedia of terms encountered in heliophysics science

Encyclopedia
Updated Jul 9, 2026
A circular NASA logo representing heliophysics. It features a stylized sun with radiating golden rays, a blue arc resembling Earth's magnetosphere, and a depiction of Earth inside a large eye-like shape. The word "HELIOPHYSICS" is written at the top, with "NASA" at the bottom.

Contents

Overview

Heliophysics — the study of the Sun and how it influences space — is full of exotic phenomena that shape space from the Sun to the outer edges of the solar system, billions of miles away. Some of them are well understood by scientists, while others remain under study. The Heliopedia is a collection of “living” definitions of some of those phenomena, adapted and expanded upon as we learn more.

Entries are listed alphabetically. Click on a letter below or use the index at left to navigate the Heliopedia.

Did You Know?

The term heliophysics was first used in a scientific newsletter in 1910 — that's 48 years before NASA was created!

The Sun against a black background. The Sun appears mostly orange and fades to a darker red on the edges. Toward the middle and slightly to the left on the solar surface a few dark splotches.
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A transparent image

A, B, C, D

Alfvén surface/Alfvén zone

The Alfvén surface (or zone) is the point past which material leaving the Sun is moving too quickly for it to propagate back to the Sun. As material leaves the Sun — in the form of solar wind or explosive clouds called coronal mass ejections — it accelerates, eventually moving too fast to return. The Alfvén surface marks the transition point between the Sun’s outer atmosphere (the corona) and the solar wind.

In this animated illustration, the Sun appears at left. Wrapped around the top and right side of the Sun is an orange, wispy, wrinkled, wave-like feature. A curved green line passes through this structure, and a spacecraft moves along the curved line.
The boundary that marks the edge of the corona is the Alfvén surface. Inside that surface, plasma is connected to the Sun by waves that travel back and forth to the surface. Beyond it, the Sun’s magnetic fields and gravity are too weak to contain the plasma and it becomes the solar wind, racing across the solar system so fast that waves within the wind cannot ever travel fast enough to make it back to the Sun.
NASA/Johns Hopkins APL/Ben Smith

Annular solar eclipse

An annular solar eclipse happens when the Moon passes in front of the Sun but the Moon is near its farthest distance from Earth in its orbit, so its apparent size in the sky is too small to completely block the Sun’s bright disk. As the Moon moves in front of the Sun, its slightly smaller apparent size means that the Moon is completely contained within the solar disk and leaves a bright ring of the Sun visible around the edges. This ring around the edge of the Moon is too bright to look at safely with the unaided eye and is often described as a “ring of fire.” The path of the Moon’s shadow cast during an annular eclipse is called the path of annularity.

A thin orange ring appears against a black background
An annular solar eclipse was photographed from Albuquerque, New Mexico, in 2023.
NASA/Jim Spann

Aurora

An aurora is a brilliant display of light in the night sky. The aurora borealis and aurora australis — also known as the northern and southern lights — occur mainly near Earth’s poles. When the solar wind reaches Earth’s magnetosphere, it can send charged particles trapped in Earth’s magnetic field raining down toward Earth’s poles, driven by a powerful process called magnetic reconnection.

Along the way, particles can collide with atoms and molecules in Earth’s upper atmosphere, providing the atoms with extra energy that they release as a burst of light. These interactions continue at lower and lower altitudes until all the excess energy is lost. Glowing auroras are the result of millions of individual particle collisions, lighting up Earth’s magnetic field lines. Studying auroras offers insights on how our magnetosphere reacts to near-Earth space weather.

See cusp aurora, diffuse aurora, discrete aurora, pulsating aurora

An animated GIF shows a space-based view of auroras appearing like moving ribbons of green light snaking across Earth’s atmosphere at night.
This animated view of auroras was captured from the International Space Station.
Earth Science and Remote Sensing Unit, NASA Johnson Space Center

Baily’s Beads

During a total solar eclipse, as the Moon moves across the Sun, the light forming the “diamond” in the “diamond ring” stage may break up into several points of light that shine around the Moon’s edge. Known as Baily’s Beads, these are light rays from the Sun streaming through low-lying valleys along the edge of the Moon. Baily’s Beads are very short-lived and may not last long enough to be noticeable to all observers of the total solar eclipse. Baily’s Beads can also be visible just after totality.

The black circular disk of the Moon blocks out the Sun at the center of this photo, creating a total solar eclipse. All around the dark lunar disk is a thin, faint, gray, fuzzy glow from the Sun's corona. A few bright, white bursts of light appear along the lower left edge of the Moon. The eclipse is set against a black background.
Baily’s Beads appear as the Moon makes its final move over the Sun during the total solar eclipse on Aug. 21, 2017, above Madras, Oregon.
NASA/Aubrey Gemignani

Chromosphere

The chromosphere is a layer of the Sun that lies just above the photosphere and is about 1,050 miles thick on average. The temperature in the chromosphere rises from about 10,000 degrees Fahrenheit to about 36,000 degrees Fahrenheit, hotter than the photosphere but nowhere near as hot as the Sun’s multimillion-degree upper atmosphere, known as the corona. Named for the bright reddish color it gives off, the chromosphere is notoriously tricky to study, because it’s where the physical laws affecting the motion of solar material begin to change. In the lower chromosphere, solar material moves as a typical gas or fluid; in the upper chromosphere and above, magnetic forces dominate the motion.

A small portion of the Sun appears as a patchy glow of red, orange, and yellow at the bottom of the image. Rising above the Sun, a tangle of thin, red, wispy structures form a slanted triangular formation against the black background of space.
The Hinode mission captured this image of the Sun’s chromosphere on Jan. 12, 2007.
NASA/JAXA

Convection zone

The convection zone is the outermost layer of the solar interior and makes up about two-thirds of the Sun’s volume. At the base of the convection zone, the temperature is about 3.5 million degrees Fahrenheit. The convection zone is much less dense than the radiative zone, with about the same density as the air 50 miles above Earth’s surface. The hot material there rises to the surface of the star, carrying (or convecting) heat with it. Once the material cools by giving off sunlight, it sinks down, where it picks up more heat. The convective motions themselves are visible at the Sun’s surface as features called granules and supergranules.

An illustration shows a star with its upper left section cut out to show its interior. Yellow loops appear in the Sun's interior with an arrow pointing outward on one side of each loop and an arrow pointing inward on the other side of each loop.
An artist’s illustration depicts the interior of a low-mass star, like our Sun. The yellow loops with arrows represent the circular motion of material outward from the core and then back down toward the core.
NASA/Chandra X-ray Center/M. Weiss

Core (Sun)

More than 27 million degrees Fahrenheit and 10 times denser than lead, the solar core is the very center of our Sun. Here, the intense pressure from surrounding layers compresses the center to a dense ball — about 172,000 miles across — where hydrogen atoms are squeezed together to form helium, releasing energy and light in the process. This reaction, known as nuclear fusion, has powered our Sun for over 4 billion years and will continue for an estimated 5 billion more.

A cutaway illustration shows layers of the Sun from the atmosphere to the core. Each layer appears in a different color and is labeled. The core is shown as a small blue region at the center of the Sun, surrounded by the radiative zone in orange, then the convection zone in yellow.
The Sun and its atmosphere consist of several zones or layers. The core is at the very center of the Sun.
NASA

Corona

The Sun’s dynamic upper atmosphere is called the corona. It is filled with plasma, whose movements are governed by the tangle of magnetic fields surrounding the Sun. Temperatures in the corona can reach millions of degrees. The corona is the source of the solar wind as well as solar flares and coronal mass ejections — the energetic solar eruptions that create the strongest space weather.

An illustration depicts the Sun's corona and solar wind. The Sun's surface is shown as a white disk with white lines representing magnetic field lines extending into its atmosphere, labeled “Corona” and “Solar Wind.” A blue background represents outer space.
An artist’s concept shows the solar corona as it transitions into the solar wind.
NASA

Coronal hole

A coronal hole is a patch of the Sun’s atmosphere with much lower density than elsewhere. In ultraviolet views of the Sun, coronal holes appear as dark splotches. These are regions where the Sun’s magnetic field lines are connected directly to interplanetary space, allowing solar material to escape out in a high-speed stream of solar wind, leaving a dark “hole” near the surface of the Sun. Coronal holes appear throughout the solar cycle, but can last for much longer during solar minimums, when the Sun is less active.

The Sun appears in patchy shades of gold with some brighter and darker regions, set against a black background. In the upper part of the Sun is a large, irregularly shaped dark area known as a coronal hole.
The dark area across the top of the Sun in this image is a coronal hole. This image was captured on Oct. 10, 2015, by NASA’s Solar Dynamics Observatory.
NASA/SDO

Coronal (plasma) rain

Coronal rain, also known as plasma rain, is made of giant globs of plasma that drip from the Sun’s outer atmosphere back to its surface. It occurs when particular conditions, such as magnetic field line configurations and local heating events in the corona, cause the plasma globs there to become cooler and denser than their surroundings, making them rain down.

A small portion of the Sun appears as a red, patchy semicircle at the bottom of the frame. Rising above it, against the black background of space, is a large, yellow, glowing arch containing material that appears to be raining down on to the Sun. A much smaller image of the Earth, about one-tenth the diameter of the arch, is shown for scale.
NASA’s Solar Dynamics Observatory captures a plasma downpour on the Sun.
NASA/SDO/Scientific Visualization Studio/Tom Bridgman

Coronal mass ejection (CME)

A coronal mass ejection, or CME, is a large cloud of solar plasma and embedded magnetic fields released into space after a solar eruption. A CME expands as it sweeps through space, often measuring millions of miles across, and can collide with the magnetic fields of planets. When directed at Earth, a CME can produce geomagnetic disturbances that ignite bright auroras, short-circuit satellites and power grids on Earth, or endanger astronauts in orbit.

A blue image shows the Sun's atmosphere. In the middle is a dark blue disk covering the Sun, with a small white circle on it showing the Sun’s size and location. Surrounding the disk are faint white streams of light. A large burst of light shoots out from the center in an expanding circle moving in all directions.
The NASA/ESA Solar and Heliospheric Observatory (SOHO) captured this video of a coronal mass ejection on March 13, 2023.
NASA/European Space Agency/SOHO

Cosmic rays

Cosmic rays are not a form of light as their name suggests, but instead are high-energy pieces of atoms that move at nearly the speed of light. When produced in or near the Sun, they are known as solar energetic particles, but in high-energy environments across the galaxy, such as supernovae and black holes, they are called galactic cosmic rays. To understand more about high-energy environments where cosmic rays are made, scientists study these particles with special detectors in space.

On Earth, scientists commonly measure cosmic rays indirectly after they run into particles in our atmosphere. The collision of the particles and cosmic rays creates a shower of smaller particles, which are easily picked up by special detectors on the ground.

In this animation, Earth is surrounded by a light-blue sphere embedded within a darker blue, teardrop-shaped bubble. A flurry of particles travel toward the bubble and strike it, causing it to shrink and move closer to Earth. As it shrinks down, the inner sphere around Earth turns from blue to red. As fewer particles hit it, the bubble expands outward again.
This animation shows cosmic rays bombarding the heliosphere.
NASA’s Goddard Space Flight Center/Conceptual Image Lab/Walt Feimer

Cusp aurora

Earth’s magnetosphere has two cusps: regions in the magnetosphere where Earth’s magnetic field lines funnel solar wind directly to the upper atmosphere. A unique kind of aurora occurs at the cusp, marking where auroras can be seen during the daytime. They are unique not only for where they are found, but also how they form. Unlike other auroras, they are sparked directly by solar wind particles.

An animated GIF shows Earth surrounded by a transparent, bubble-shaped shield against a star-filled black background. White horizontal streaks, symbolizing solar wind particles, stream toward Earth from the left and flow around Earth along the boundaries of the bubble. Two funnel-like features connect the outer edge of the bubble to Earth's poles, and some particles flow toward Earth along the funnels. The animation then zooms in to Earth's North Pole, which is surrounded by a ring of green and red auroras.
In this animation, Earth’s northern and southern polar cusps appear as two funnels, where the solar wind can collide with Earth’s atmosphere. The collisions create the cusp aurora and hot fountains of outflowing oxygen.
NASA’s Goddard Space Flight Center/Conceptual Image Lab/Josh Masters

Diamond ring effect

As the Moon moves in front of the Sun during a total solar eclipse, there is a moment when a single bright spot is left — a bright spot that, in combination with the atmosphere of the Sun visible around the Moon, looks like a giant diamond on a ring. This “diamond ring” effect is created by sunlight streaming through valleys on the Moon’s limb — the “diamond” — along with the Sun’s outer atmosphere, the corona, forming a “ring” around the Moon. The diamond ring effect also occurs shortly after totality.

Set against a black background, a radiant gray ring of light appears around the black disk of the Moon during a total solar eclipse. On the right side of the ring, a small, bright, burst of light appears like a diamond.
A “diamond ring” of sunlight is seen in the moments after totality during the total solar eclipse on Aug. 21, 2017.
NASA/Rami Daud

Diffuse aurora

Diffuse auroras are dim, often motionless auroras. They can be green, whitish, or red and occur over a wide area, typically closer to the equator than discrete auroras. They might be confused for clouds.

A photograph shows a green-colored sky with dozens of stars above a dark landscape with several tall, skinny trees and snow in the foreground. Some clouds can also be seen along the horizon.
This diffuse aurora was observed above Poker Flat, Alaska.
NASA/Robert Michell
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Discrete aurora

Discrete auroras are bright, thin bands with a definite lower border. They can stretch high into the sky and take on curtain-like shapes when viewed from the side. They can wave slowly or rapidly across the sky.

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