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Encyclopedia
Updated Jul 1, 2025
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.

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.

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.

Alfvén surface/Alfvén zone

The Alfvén surface (or zone) is the point past which material leaving the Sun is moving too quickly for its influence to propagate back to the Sun itself. The Alfvén surface marks the transition point between the Sun’s outer atmosphere (the corona) and the solar wind. As material leaves the Sun — in the form of solar wind or explosive clouds called coronal mass ejections — it accelerates, eventually pushing its speed past the maximum speed of an Alfvén wave, a type of plasma wave. If the material is moving away from the Sun faster than this top plasma wave speed, any disturbances produced in the material itself can no longer travel backward toward the Sun. Some evidence suggests that material leaving the Sun reaches the Alfvén speed not at a singular Alfvén surface, but at various points within an Alfvén zone, where co-mingled streams of material reach the Alfvén speed at different points.

Animated visualization showing the Parker Solar Probe crossing the Alfvén surface, the boundary in the Sun's atmosphere where the solar wind transitions from sub-Alfvénic to super-Alfvénic speeds. Inside this boundary, magnetic forces dominate and solar material is still gravitationally bound to the Sun. Beyond it, the solar wind escapes freely into space. The animation highlights this crossing with dynamic motion and glowing visual effects to indicate energy and particle flow.
The boundary that marks the edge of the corona is the Alfvén critical surface. Inside that surface (circle at left), plasma is connected to the Sun by waves that travel back and forth to the surface. Beyond it (circle at right), 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. The results suggest that the Alfvén critical surface has a wrinkled structure that is connected to giant plumes of solar material called coronal streamers.
NASA/Johns Hopkins APL/Ben Smith

Annular solar eclipse

Annular solar eclipses happen when the Moon is near its farthest point from Earth, so its apparent size in the sky is too small to completely block the Sun’s bright disk. As the Moon moves across the Sun, its slightly smaller apparent size means that it 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 shadow cast during an annular eclipse is called the path of annularity.

A map of the United States has a narrow, diagonal band labeled "Annular Eclipse" running across the West, beginning in the Pacific Northwest and continuing to the Texas Gulf Coast. Additional thin lines spread outward from this band toward the northeast and southwest. These thin lines are labeled with percentages from 80% to 10%, with the lower numbers being farthest from the annular eclipse path.
A map of the United States has a narrow, diagonal band labeled “Annular Eclipse” running across the West, beginning in the Pacific Northwest and continuing to the Texas Gulf Coast. Additional thin lines spread outward from this band toward the northeast and southwest. These thin lines are labeled with percentages from 80% to 10%, with the lower numbers being farthest from the annular eclipse path.
NASA/JPL-Caltech

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.

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.
Image courtesy of the 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 stage known as a “diamond ring” may break up into several points of light that shine around the Moon’s edges. Known as Baily’s Beads, these are light rays from the Sun streaming through low-lying valleys along the Moon’s horizon. 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.
The Baily’s Beads effect is seen as the Moon makes its final move over the Sun during the total solar eclipse of Aug. 21, 2017, above Madras, Oregon.
NASA/Aubrey Gemignani

Chromosphere

The chromosphere is a layer of the Sun. The chromosphere lies just atop 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.
Hinode, a collaborative mission of the space agencies of Japan, the United States, United Kingdom, and Europe, captured this dynamic picture of our Sun’s chromosphere on Jan. 12, 2007.
NASA/JAXA

Core

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.
The Sun and its atmosphere consist of several zones or layers.
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 up to 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

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 of a star, with a cutout showing its interior.
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 gas outward from the core and then back down toward the core.
NASA/CXC/M. Weiss

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 at their worst, even 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/ESA/SOHO

Cosmic ray

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. WhenThey can be produced in or near the Sun, they — which are known as solar energetic particles, but — and in high-energy environments across the galaxy, such as supernovae and black holes, theywhich 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 raysthem 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/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 rare places where dayside auroras are visible to people. 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 showing an illustration of Earth protected by a transparent bubble-shaped shield with white streaks symbolizing solar wind being deflected in space, with stars in the background.
Illustration of the Earth’s magnetosphere, polar cusps and the solar wind. The 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 CILab/Josh Masters

Diamond ring effect

As the Moon moves in front of the Sun during a total solar eclipse, there comes a time when there is 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 is also occursvisible 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; if you can see stars through the glow, then it is likely an aurora.

Blue glowing lines encircle one of the poles of Jupiter.
The Hubble Space Telescope captured this close-up view of an electric-blue aurora eerily glowing one-half billion miles away on the giant planet Jupiter. Auroras are curtains of light resulting from high-energy electrons racing along the planet’s magnetic field into the upper atmosphere. The electrons excite atmospheric gases, causing them to glow. The image shows the main oval of the aurora, which is centered on the magnetic north pole, plus more diffuse emissions inside the polar cap. Though the aurora resembles the same phenomenon that crowns Earth’s polar regions, the Hubble image shows unique emissions from the magnetic “footprints” of three of Jupiter’s largest moons. (These points are reached by following Jupiter’s magnetic field from each moon down to the planet). Auroral footprints can be seen in this image from Io (along the left limb), Ganymede (near the center), and Europa (just below and to the right of Ganymede’s auroral footprint). These emissions, produced by electric currents generated by the moons, flow along Jupiter’s magnetic field, bouncing in and out of the upper atmosphere. They are unlike anything seen on Earth. For more information, visit: hubblesite.org/news_release/news/2000-38
NASA/ESA/University of Michigan/John Clarke

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. Discrete auroras occur closer to the magnetic poles. They can wave slowly or rapidly across the sky.

Aurora in Ny-Ålesund, Svalbard, on December 6, 2018. A GIF optimized for Twitter.
Discrete aurora in Ny-Ålesund, Svalbard, on December 6, 2018.
NASA’s Goddard Space Flight Center/Joy Ng

Energetic neutral atom (ENA)

An energetic neutral atom (ENA) is a type of uncharged, or neutral, particle. An ENA forms when an energetic ion — a negatively charged particle — runs into a slow-moving neutral atom. The ion picks up an extra electron in the collision, making it neutral — hence the name energetic neutral atom. Since ENAs aren’t charged, they don’t interact with the magnetic fields that permeate space. This means they travel in a straight line, which allows scientists to track their originswhere they came from and study distant plasma-filled regions of space, such as the boundary of the heliosphere.

Against a black background, a gridded oval contains patches of colors including dark blue, light blue, green, yellow, and red. The outer parts of the oval are mostly dark and light blue. Running from the upper left to the lower right is a roughly L-shaped band of green with small patches of yellow and red within it.
This all-sky map shows the distribution of energetic neutral atoms (ENAs) across space, with color-coded intensity levels. The colors range from dark blue (low intensity) to red (high intensity), forming a ribbon-like structure arcing diagonally across the map. The background is a star field, indicating that the map represents space observations.
NASA’s Goddard Space Flight Center/Scientific Visualization Studio

Filament eruption

Filaments are strands of solar material, cooler and denser than their surroundings, suspended above the Sun by magnetic forces. They appear as dark lines when seen against the bright Sun. When solar filaments become unstable, they can either fall back onto the Sun or erupt into space, sending a coronal mass ejection away from the Sun. (When a solar filament is seen at the edge of the Sun, against the blackness of space, it is called a “prominence” instead.) Space weather forecasters keep an eye on filaments, sinceas they can be important contributors to space weather.

A close-up, animated view of the Sun's surface shows a massive, glowing, swirling eruption of solar plasma. A bright, fiery filament arcs upward from the surface, showcasing the dynamic, intense energy of solar activity.
A large snaking magnetic filament erupted during the early hours of Feb. 24, 2012, launching a coronal mass ejection in Earth’s direction.
NASA/SDO

Flux rope

A flux rope is kind of a magnetic structure that is thought to be at the heart of many of the Sun’s eruptions. Flux ropes form in plasmas, such as the Sun’s corona, when loops of magnetic field lines connect with each other. The resulting flux ropes are formed from bundles of magnetic fields that have a magnetic field wrapped around them, like the stripes on a candy cane. These twisted structures extend in a series of loops from the Sun’s surface, and can be carried away from the Sun by a coronal mass ejection.

Side-by-side images show a large magnetic loop erupting from the Sun’s surface. The left image, in teal hues, shows looping arcs of plasma extending into space. The right image, with enhanced contrast and false colors, highlights the same prominence with glowing red and teal outlines, emphasizing the magnetic field lines and intense energy of the eruption.
The image on the left shows a series of magnetic loops on the Sun, as captured by NASA’s Solar Dynamics Observatory on July 18, 2012. The image on the right has been processed to highlight the edges of each loop and make the series of loops clearer.
NASA/SDO

Geospace

Geospace is the broad term used to refer to the region of space surrounding Earth. It is, usually used to reference the area from the mesosphere through the magnetosphere.

A visualization of Earth shows curved orange lines looping out of one pole and back into the other pole. Perpendicular gray lines form a grid pattern against a black background behind Earth. The timestamp “2017 Dec 20 09:20:00.000 (UTC)” is displayed in the lower left corner of the image.
A visualization shows Earth and surrounding geospace filled with Earth’s magnetic field lines. The magnetic field structure is represented by orange lines. The semitransparent gray mesh in the distance represents the boundary of the magnetosphere.
NASA’s Goddard Space Flight Center/Scientific Visualization Studio

Heliosphere

The Sun’s constantly outflowing material, the solar wind, inflates a bubble in space called the heliosphere. The heliosphere encloses all of the planets in the solar system and is filled by the Sun’s plasma and magnetic field. In interstellar space outside the heliosphere, the interstellar medium and the galactic magnetic field are dominant. The heliosphere acts as a shield for our solar system, blocking many of the high-energy galactic cosmic rays from elsewhere in our galaxy. Of the spacecraft sent from Earth, only the twin Voyager spacecraft — initially launched intraveling since 1977 — have crossedbeen confirmed to have made it beyond the boundaries of the heliosphere.

At the center of this illustration, a yellow dot represents the Sun. Around the Sun is a small blue sphere. The sphere appears at the left end of a long light-blue oval region labeled "Heliosphere," which extends off the right edge of the view. The Voyager 1 and Voyager 2 spacecraft are shown near the left edge of the heliosphere, with Voyager 1 just outside the edge and Voyager 2 just inside the edge.
This graphic shows the position of NASA’s Voyager 1 and Voyager 2 probes, relative to the heliosphere — a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in 2012. Voyager 2 is still in the heliosheath, or the outermost part of the heliosphere.
NASA/JPL-Caltech

Hybrid solar eclipse

Hybrid solar eclipses happen when an eclipse transitions from total to annular (or vice versa) during the eclipse.

Three side-by-side images show (from left to right) a total solar eclipse, an annular solar eclipse, and a partial solar eclipse.
From left to right, these images show a total solar eclipse, annular solar eclipse, and partial solar eclipse. A hybrid eclipse appears as either a total or an annular eclipse (the left and middle images), depending on the observer’s location.
Total eclipse (left): NASA/MSFC/Joseph Matus; annular eclipse (center): NASA/Bill Dunford; partial eclipse (right): NASA/Bill Ingalls

Ionosphere

The ionosphere is the dynamic, electrically charged region of Earth’s atmosphere, located between 50 and 400 miles above our planet’s surface. The ionosphere overlaps with both the lower reaches of space and many of the upper layers of Earth’s atmosphere, like parts of the thermosphere and mesosphere. The ionosphere is electrically charged because the Sun’s radiation hits particles and gases there, electrifying (or ionizing) them. As such, tThe ionosphere is created by the Sun’s energy.

As night falls on one side of the Earth, the ionosphere diminishes there and grows on the daytime side. The ionosphere carries enormous electrical currents that link it to the magnetosphere, and which heat the upper atmosphere and can even drive damaging currents in power lines on the ground.

A diagram shows Earth's atmospheric layers in different shades of blue. The layers are labeled from bottom to top: troposphere (0-10 miles), stratosphere (10-31 miles), mesosphere (31-53 miles), thermosphere (53-375 miles), and exosphere. The ionosphere, which overlaps the exosphere, thermosphere, and mesosphere, is also marked.
A population of electrically charged particles in the ionosphere exists in the same space as the Neutral upper atmosphere.
NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith

Lagrange point

Lagrange points are positions in space near two massive bodies (a star and a planet, or a planet and its moon, for example) where objects tend to stay put. At Lagrange points, the gravitational pull of the two large masses equalize,combine to keeping a smaller object stable. Spacecraft can be placed at Lagrange points to reduce the amount of fuel needed to remain in position.

In any two-body system there are five Lagrange points labeled L1 through L5. In the Sun-Earth system, L1, located between the Sun and Earth, is the current home of many spacecraft studying the Sun and solar wind. L2, about a million miles away on the opposite side of Earth, is the home of NASA’s James Webb Space Telescope. L3, L4, and L5 are not home to any current or planned spacecraft.

In this diagram of the five Earth-Sun Lagrange points, the Sun appears at the center, with Earth and the Moon orbiting to the right. Green lines and labeled points indicate the positions of the Lagrange points: L1 is between Earth and the Sun; L2 is beyond Earth, on the opposite side; L3 is directly opposite Earth on the far side of the Sun; L4 and L5 form equilateral triangles with the Sun and Earth, located ahead of and behind Earth in its orbit. A spacecraft is shown orbiting around L2.
An illustration shows the five Lagrange points (L1 to L5) in the Sun-Earth system, with a spacecraft at L2.
NASA/WMAP Science Team

Magnetic reconnection

When magnetic field lines become mixed, they can explosively snap and realign, flinging away nearby particles at high speeds in a process called magnetic reconnection. This process occurs across the universe, including on the Sun, near black holes, and around Earth. Particles launched by magnetic reconnection near Earth can travel down along magnetic field lines into the atmosphere, where they can spark auroras.

Animated visualization of magnetic reconnection in Earth's magnetosphere. Earth's magnetic field lines, shown in blue, interact with incoming solar wind magnetic field lines, shown in red or orange. As the opposing field lines meet on the dayside and nightside of Earth, they break and reconnect, releasing energy and redirecting charged particles along the newly formed magnetic paths toward the poles.
As magnetic field lines explosively connect and realign, particles are flung towards Earth and into space.
NASA’s Goddard Space Flight Center/Conceptual Image Laboratory

Magnetosphere

A magnetosphere is the region around a planet dominated by the planet’s magnetic field. In our solar system, several planets, including Earth, and even one of Jupiter’s moons have magnetospheres. Magnetospheres of planets have a “teardrop” or ice cream cone shape, with a rounded, shorter end created as the Sun’s material pushes against the magnetic field and a long tail trailing away on the other side. Earth’s magnetosphere has played a crucial role in our planet’s habitability as it shields our home planet from solar and cosmic particle radiation, as well as erosion of the atmosphere by the solar wind.

An illustration shows Earth’s magnetosphere interacting with the solar wind. Earth appears at the center of the imageto the right of center with symmetrical blue magnetic field lines extending fromcoming out of its poles and looping around the planet. The solar wind, represented by orange streaks from the left, flows toward Earth but is deflected by the magnetosphere, forming a teardrop-shape region that extends into space toward the right.
The solar wind flows around Earth’s protective magnetic field, the magnetosphere.
NASA’s Goddard Space Flight Center/Conceptual Image Laboratory

Mesosphere

The mesosphere is located in the middle of Earth’s upper atmosphere, sandwiched between the stratosphere and thermosphere aroundbetween 31 toand 53 miles above Earth’s surface. The mesosphere is the atmospheric layer where meteors burn up, creating what we call shooting stars. The mesosphereIt is also the coldest layer of Earth’s atmosphere.; Aat its upper reaches, the temperature of the mesosphere averages about minus 120 degrees Fahrenheit (minus 85 degrees Celsius).

An illustration shows the layers of Earth's atmosphere. From lowest to highest, it labels the troposphere, stratosphere, mesosphere, thermosphere, exosphere, and interplanetary space.
The mesosphere is one of the middle layer of Earth’s atmosphere. It is usually recognized as the lowest layer of the upper atmosphere.
NASA GSFC/Mary Pat Hrybyk-Keith

Nanojet/nanoflare

Nanojets are bright, thin tendrils of plasma that travel perpendicular to magnetic structures in the outer solar atmosphere, reaching lengths of thousands of miles. They are spawned by nanoflares, tiny explosions on the Sun caused by a process known as magnetic reconnection, which occurs in tangled magnetic field lines.

An image showing a solar prominence, where bright, fiery arcs of plasma rise and loop outwards from the sun's surface against the dark background of space. The foreground features an intense, bright light marking the sun's edge.
These images showing nanojets on the Sun were captured by NASA’s IRIS mission on Apr. 3, 2014.

Noctilucent cloud (polar mesospheric cloud)

Noctilucent, or night-shining, clouds are clouds of ice that reflect sunlight and shine in electric-blue. They drift about 50 miles overhead. They are also known as polar mesospheric clouds, since they tend to huddle over Earth’s poles and form in the mesosphere. Noctilucent clouds appear in the summer, when this part of the upper atmosphere has all three ingredients the clouds need to form: dust from burned-up meteors, extremely cold temperatures, and water vapor. They form when water molecules freeze around the dust. The clouds and their variations season to season help scientists better understand the mesosphere and its connections to the rest of the atmosphere, weather, and climate.

Two side-by-side animations show wispy, blue-white clouds moving across a dark blue sky.
Cameras aboard a NASA balloon mission captured these images of noctilucent clouds that reveal underlying turbulence — chaotic movements in the atmosphere that can influence weather and climate.
NASA/PMC Turbo

Parker spiral

The Sun’s magnetic field envelops all the planets of our solar system and beyond, carving out a vast space we call the heliosphere. As the Sun rotates and the magnetized solar wind blows outwards, the magnetic field forms a spiral, known as the Parker spiral, that looks like the water coming out of a rotating sprinkler. Around the spiral, there is a surface where the Sun’s magnetic field changes polarity, forming a wavy spiral like a ballerina’s skirt called the heliospheric current sheet.

Illustration of the Parker Spiral, a model of the solar magnetic field as it extends through the solar system. The Sun is at the center, emitting a spiraling stream of solar wind that twists into a spiral shape due to the Sun’s rotation. The pink spiral structure represents the interplanetary magnetic field carried by the solar wind. Several planets, including Earth and Mars, are shown orbiting the Sun within this magnetic structure.
An artist’s illustration of the heliospheric current sheet, which marks where the Sun’s magnetic field changes polarity. The Parker spiral refers to the spiral shape of the underlying magnetic field that creates this “ballerina skirt”. The rotating Sun is located in the center.
NASA’s Goddard Space Flight Center

Partial solar eclipse

A partial solar eclipse happens when the Moon passes between the Sun and Earth but the Sun, Moon, and Earth are not perfectly lined up. Only a part of the Sun will appear to be covered, giving it a crescent shape. During a total or annular solar eclipse, people outside the area covered by the Moon’s inner shadow see a partial solar eclipse.

A crescent Sun against a dark orange and red background. It's behind the top of the U.S. Capitol Building, shown in silhouette.
A partial solar eclipse is seen as the Sun rises behind the United States Capitol Building, Thursday, June 10, 2021, as seen from Arlington, Virginia. The annular or “ring of fire” solar eclipse is only visible to some people in Greenland, Northern Russia, and Canada.
NASA/Bill Ingalls

Penumbra

The penumbra is the outer part of the Moon’s shadow. During a solar eclipse, observers in the penumbra will see a partial eclipse, and the Sun will not be completely blocked by the Moon.

A diagram shows the Sun on the left, Earth on the right, and the Moon in between. The Moon blocks some sunlight from reaching Earth, casting a smaller shadow cone, labeled the umbra, and a larger shadow cone, labeled the penumbra. One arrow labeled “total eclipse” points to the small area on Earth touched by the umbra. A second arrow labeled “partial eclipse” points to the larger area on Earth touched by the penumbra. A dashed circle around Earth is labeled “Moon's orbit,” and a dashed arc passing through Earth is labeled “Earth's orbit.” At the top are the words “total solar eclipse,” and at the bottom are the words “not to scale.”
When the Moon eclipses the Sun, it produces two types of shadows on Earth. The dark central shadow, or umbra, is relatively small when it reaches Earth, and is where an observer would see a total eclipse. The penumbral shadow covers a much larger area of Earth’s surface, and is where an observer would see a partial eclipse.
NASA’s Goddard Space Flight Center

Photosphere

Often called the “surface” of the Sun, the photosphere is actually the first layer of the solar atmosphere – and it is far less dense than Earth’s air at sea level. About 250 miles thick and averaging about 10,000 degrees Fahrenheit, this layer emits the white light we can see with our eyes. (The Sun appears yellow from the surface of Earth because the blue light is scattered out by the particles in our atmosphere, which also makes the sky appear blue.)

An image shows an orange, spherical Sun with a large cluster of black splotches just below center. A few other black spots dot the Sun on the left and right.
Sunspots: They look like dark holes in the Sun, but they are actually areas that are slightly cooler than the surrounding photosphere. Sunspots are created where bits of the Sun’s magnetic field poke out from the interior into the Sun’s atmosphere. Lasting from days to months, sunspots range in size from 1,000 to 100,000 miles (1,600 to 160,900 kilometers).
NASA’s Scientific Visualization Studio/SDO

Plasma

Plasma is a state of matter distinct from solids, liquids and gases. Though rare on Earth, plasma makes up over 99% of the observable (i.e., not dark) matter in the universe, including every star and much of the material between them. On Earth, plasma is found in fluorescent lights, torches used for metalworking, and lightning strikes.

Plasma forms when the atoms in a gas become ionized, meaning electrons separate from the atom and move around independently. This makes plasmas electrically charged and they can interact with external electric and magnetic fields.  They can also create their own electric and magnetic fields. Plasmas also undergo an explosive process called magnetic reconnection. Magnetic reconnection is a rapid transfer of magnetic energy into motion that powers solar flares and coronal mass ejections.

Animated visualization of Earth's magnetosheath—the region between the bow shock and magnetopause—showing the flow of solar wind plasma. The animation illustrates turbulent, compressed magnetic field lines and energetic particle motion as the solar wind slows and deflects around Earth's magnetic shield. The bow shock appears as a curved boundary ahead of Earth, with the magnetopause forming the inner edge of the magnetosheath.
Plasma is everywhere in the Sun-Earth system. The Sun itself is made of plasma, and so is the solar wind. The boundary of Earth’s magnetic influence or magnetopause, shown here as a yellow line, and the turbulent swirling material inside it, known as the heliosheath, are all made of plasma.
NASA’s Goddard/Space Flight Center/Mary Pat Hrybyk-Keith/Conceptual Image Lab/Josh Masters

Plasma wave

Plasma waves move energy through a plasma. They are also an important source of heat: As waves break down, the leftover random particle motion becomes heat.

Plasmas generate both electric and magnetic fields, and some plasma waves disturb the magnetic field (electromagnetic waves) while others do not (electrostatic waves). Some kinds of plasma waves only affect the electrons in a plasma (Langmuir waves; Whistler mode waves), while others only affect the ions (ion cyclotron waves; Alfven waves). Heliophysicists study plasma waves throughout the Sun-Earth system to better understand how the Sun influences the rest of the solar system and how plasma behaves throughout the universe.

Animated 3D visualization of a turbulent mixing surface, illustrating fluid dynamics and chaotic motion. Colorful streaks and swirling patterns represent the interaction and blending of different fluid regions, showing complex structures that evolve over time. This type of simulation is often used to study turbulence in atmospheric, oceanic, or astrophysical environments.
This simulation shows an example of plasma wave dissipation. An area of low density plasma, shown by blue, mixes with areas of higher density plasma, red, first forming Kelvin-Helmholtz waves that dissipate into turbulent tornadoes of plasma.
IWF/Takuma Nakamura

Plume/plumelet

Plumes are streamers of solar material that stretch out from coronal holes — dark patches of open magnetic field — on the Sun. They appear bright in extreme ultraviolet views of the Sun, and are made up of many smaller streamers, called plumelets. Plumes play a role in creating the high-speed solar wind.

Two side-by-side images of the Sun's corona, derived from NASA data, showcase intricate solar structures. The left image reveals a detailed, glowing region in brown hues. The right image, processed in black and white, highlights sharp, radiating plasma structures. Both images are zoomed in from a full solar view.
Scientists used image processing on high-resolution images of the Sun to reveal distinct “plumelets” within structures on the Sun called solar plumes.
NASA/SDO/Uritsky, et al

Prominence

A prominence is a snakelike structure made of cool, dense solar material that is suspended above the Sun’s surface by a strong local magnetic field. (When they are viewed against the solar disk, head-on rather than off the visible edge, they are called filaments.) Prominences can erupt when the magnetic structure becomes unstable, flinging the plasma outward in a blast called a coronal mass ejection.

High-resolution image of the Sun captured in extreme ultraviolet light, showing a detailed view of the solar surface and atmosphere. Bright, active regions indicate intense magnetic activity, including sunspots, solar flares, and prominences. A large solar prominence is visible on the left edge, arching out into space.
A solar prominence (upper left side of Sun’s disk, about 10 o’clock) observed by Solar Dynamics Observatory on Dec. 31, 2012. The prominence lasted on the Sun for about three hours before collapsing.
NASA’s Goddard Space Flight Center/SDO

Radiative zone

The radiative zone is the layer just outside the Sun’s core. This region varies in density, from denser than gold to less dense than water. The radiative zone gets its name from how light is transferred from the core below to the zone above. Here, the light is passed from atom to atom, instead of circulating as it does in the less dense convection zone.

An infographic illustrating the different regions of the Sun, including the convection zone, radiative zone, corona, coronal streamers, Sun’s core, and chromosphere. Each section is labeled with descriptive text and depicted with corresponding visuals.

Solar energetic particle (SEP)

Solar energetic particles, or SEPs, are high-energy charged particles — mostly electrons and protons — accelerated by activity on the Sun. SEPs can be accelerated on the Sun in conjunction with solar flares, or they can be accelerated in space — for example, at the forefront of a fast-moving coronal mass ejection. Because SEPs are charged particles, their movement is guided by magnetic fields as they move away from the Sun and travel through space. SEPs can pose a radiation hazard to astronauts and electronics in space, and their effects on Earth’s atmosphere can hinder high-frequency radio communications like GPS.

Animated visualization of solar energetic particles (SEPs) being emitted from the Sun during a solar flare or coronal mass ejection. Bright bursts erupt from the solar surface, sending fast-moving particles outward through space. These high-energy particles travel along magnetic field lines and can pose radiation hazards to spacecraft, astronauts, and electronics in space.
This animation compares two models for particle distribution over the course of three hours after an SEP event. The white line represents a magnetic field line, the general path that the SEPs follow. The line starts at an SEP event at the sun, and leads the particles in a spiral around the sun. The animation of the updated model, on the right, depicts a static field line, but as the SEPs travel farther in space, turbulent solar material causes wandering field lines. In turn, wandering field lines cause the particles to spread much more efficiently than the traditional model, on the left, predicted.
NASA’s Goddard Space Flight Center/UCLan/Stanford/ULB/Joy Ng

Solar flare

Solar flares are energetic bursts of light and particles triggered by the release of magnetic energy on the Sun. Flares are by far the most powerful explosions in the solar system, with energy releases comparable to billions of hydrogen bombs. The energetic particles accelerated by flares travel nearly at the speed of light, and can travel the 93 million miles between the Sun and Earth in less than 20 minutes. Some solar flares have an associated coronal mass ejection.

A portion of the Sun fills the center, most of the top, and right side of the image. The left of the image is black. The Sun is dark orange with bright yellow areas. From a very bright yellow area, solar material bursts off the Sun, into the area of black space.
NASA’s Solar Dynamics Observatory captured this image of a solar flare seen as the bright flash on the left on Oct. 23, 2024. The image shows a subset of extreme ultraviolet light that highlights the extremely hot material in flares and which is colorized in orange.

Solar layer

The Sun can be divided into distinct layers, including (from innermost to outermost) the solar core; the radiative zone; the convection zone; the photosphere; the chromosphere; the transition region; and the corona.

This infographic labels the parts of the Sun (from most inward to outward): Solar Core, Radiative Zone, Convection Zone, Photosphere, Chromosphere, Transition Zone, and Corona.It explains that the Sun's outermost layer is hotter than the layers immediately below that. This is a major unsolved puzzle in heliophysics.
The Sun’s layers from the core to the corona. The coronal heating problem is one of several mind-melting facts about the Sun.
NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith/Miles Hatfield

Solar wind

The solar wind is a gusty stream of material that flows from the Sun in all directions, all the time, carrying the Sun’s magnetic field out into space.  While it is much less dense than wind on Earth, it is much faster, typically blowing at speeds of one to two million miles per hour. The solar wind is made of charged particles — electrons and ionized atoms — that interact with each other and the Sun’s magnetic field. The extent of the solar wind creates the heliosphere, the Sun’s region of influence within interstellar space.

A data visualization shows columns of glowing, golden-brown clouds streaming across the screen. The motion creates the illusion of flying through a tunnel of swirling plasma, with brighter, denser regions pulsing and twisting to suggest turbulence and varying intensity. Warm tones of amber and bronze contrast against a deep black background, enhancing the sense of depth and motion as the clouds flow dynamically from right to left, capturing the energetic and storm-like behavior of the Sun’s outflowing atmosphere.
Computer-processed data of the solar wind from NASA’s STEREO spacecraft.
NASA/SwRI/Craig DeForest

Sounding rocket

Sounding rockets are suborbital rockets that launch scientific instruments into space, flying between 30 and 800 miles (48 to 1287 kilometers) high before falling back to Earth. Typical flights last between 5 to 20 minutes. 

Sounding rockets were NASA’s first space vehicles and remain valuable tools for research today. Sounding rockets can study everything from the atmosphere immediately surrounding the rocket to distant galaxies. Their instruments can often be recovered and re-used across several flights, significantly lowering development costs. They are also quicker to develop than satellite missions — as little as a year from idea to launch — so scientists often use them to test new technologies and explore cutting-edge scientific ideas. 

A rocket launches into the blue sky from a snow-covered launch range, leaving a bright cloud of rocket exhaust in its wake.
The Endurance rocket ship launched from Ny Ålesund, Svalbard, on May 11, 2022.
Andøya Space/Leif Jonny Eilertsen

Space dust

Space dust is made up of fine particles that float around in between the stars, planets, and galaxies. This material is composed of tiny clumps of molecules and compounds leftover from the formation of objects in space or shed by comets and asteroids. In fact, when you see the fuzzy silhouette of a comet, you’re looking at gas and dust from the comet as it disintegrates. Unlike most other individual particles that scientists study in space, space dust is composed of multiple atoms — though most are still much smaller than a grain of sand. While the dust you encounter here on Earth is made up of mostly organic materials — such as dirt and skin cells — space dust is largely rocky or carbon-rich grains.

Illustrated view of the inner solar system showing the Sun and the orbits of Mercury, Venus, and Earth. Each planet is labeled, and the image shows dust and debris particles swirling in rings around the Sun, representing the zodiacal cloud. The background is a dark, star-speckled space.
This illustration shows the dust rings around the inner planets.
NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith

Space weather

Space weather refers to conditions in space produced by the Sun’s activity. The Sun affects the space around us through a constant stream of plasma known as the solar wind, with occasional bursts from solar flares and coronal mass ejections. These solar discharges carry their own magnetic field, so when they collide with Earth’s magnetic field, the two magnetic fields can repel or attract each other like two magnets. This repulsion and attraction creates geomagnetic disturbances. Space weather events produce the beautiful glow of the northern and southern lights, but they can also endanger astronauts, disrupt radio communications, and even cause large electrical blackouts.

Every planet in the solar system experiences its own space weather as the solar wind interacts with the planet’s own magnetic field (or lack thereof).

An illustration of the Sun and the effects of space weather on Earth
Space weather can have a variety of effects on Earth and our technology.
European Space Agency

Spicule

At any given moment, as many as 10 million wild jets of solar material burst up from the Sun’s surface. Known as spicules, these grass-like tendrils of plasma erupt as fast as 60 miles per second and can reach lengths of 6,000 miles before collapsing.

Animated visualization of solar spicules—thin, jet-like structures on the Sun’s surface. The animation shows these dynamic plasma jets rapidly rising and falling from the solar chromosphere into the outer atmosphere. The spicules appear as bright, flickering filaments against the backdrop of the Sun’s surface, demonstrating the Sun’s highly active and constantly changing nature.
Observations of spicules from NASA’s Solar Dynamics Observatory, or SDO. Over a few hours observation of the northern pole area of the Sun in extreme ultraviolet light (Aug. 3, 2010), we can see a continual frenzy of spicules.
NASA’s Goddard Space Flight Center/SDO

STEVE

The Strong Thermal Emission Velocity Enhancement, or STEVE, is an aurora-like phenomenon that looks like a purple ribbon in the sky, accompanied by a green picket fence-like structure. It runs east to west and appears closer to the equator than typical aurora. The phenomenon has been observed for centuries by skywatchers and citizen scientists—including, more recently, those participating in the Aurorasaurus project, whose observations were instrumental to the first modern STEVE scientific paper published in 2018. STEVE appears to result from processes different from other auroras. Scientists don’t yet fully understand the mechanisms that cause STEVE to appear.

In the foreground is a flat body of water. Green aurora and stars are reflected on the water's surface. In the sky is a purple band of aurora traveling diagonally across the image from lower left to upper right. Parallel and below to it are vertical stripes of green.
This photo captures the purple arc of STEVE and green “picket-fence” aurora.
Neil Zeller

Substorm

Substorms can be thought of as the magnetic, near-space counterpart to severe weather storms on Earth. Under certain conditions, Earth’s magnetosphere captures and gradually stores a small fraction of material and energy from the solar wind. This energy is released over the course of several minutes about once every 3 hours, creating an event known as a substorm.  

Substorms initially start small but spread across vast regions of the magnetosphere within minutes and typically initiate the most intense space weather effects — those that create dramatic auroral displays, disrupt communications, cause power line transmission failures, and produce the most penetrating radiation. During a substorm, auroras brighten and expand poleward.

Substorms differ from geomagnetic storms, because geomagnetic storms last from 1 to 3 days and are generally caused by a single large solar wind event arriving at Earth that affects the entire magnetosphere and energizes the radiation belts.

Animated GIF of nightside reconnection forming STEVE. Credit: NASA GSFC/CIL/Krystofer Kim
This animation shows a magnetospheric substorm, in which reconnection ultimately energizes particles and sends them streaming down into the nightside atmosphere, where they cause aurora to brighten.
NASA’s Goddard Space Flight Center/Conceptual Image Lab

Sunquake

Sunquakes are seismic-like activity on the Sun that ripple across the visible surface, not unlike earthquakes. They are known to accompany some solar flares, but scientists are uncertain how exactly they are triggered.

Two square panels show solar observations. The left panel, in yellow, displays sunspots. The right panel, in red, is a corresponding image without visible sunspots. A scale bar beneath indicates a spatial scale of 50,000 km and a temporal scale up to 42 minutes.
Movie of a sunquake – the earthquake-like waves that ripple through our star. Left frame shows the active region in visible light (amber) and extreme ultraviolet (red) on July 30, 2011. Right frame shows the ripples on Sun’s outlying surface up to 42 minutes after the onset of the flare, which is marked by the label “IP” for impulsive flare.
NASA/SDO

Sunspot

Occasionally, dark spots freckle the face of the Sun. These are sunspots, cooler regions on the Sun’s visible surface caused by a concentration of magnetic field lines. Sunspots are the visible component of active regions, areas of intense and complex magnetic fields on the Sun that are the source of solar eruptions. Lasting from days to months, sunspots typically stretch 1,000 to 100,000 miles across. The number of sunspots goes up and down as the Sun goes through its natural 11-year cycle. Scientists use sunspots to help them track this cycle.

A portion of the yellow Sun fills the image. Rotating from left to right, a dark splotch — the sunspot — rotates into view. Below it is a much smaller blue dot, added onto the image, that is labeled "Approximate Size of Earth"
A sunspot rotates into view in this video captured by NASA’s Solar Dynamics Observatory between July 5-11, 2017. Like freckles on the face of the Sun, sunspots appear to be small features, but size is relative: The dark core of this sunspot is actually larger than Earth.
NASA’s Goddard Space Flight Center

Supergranule

Supergranules are networks of cells covering the Sun’s visible surface that stretch some 18,000 miles across — more than large enough to frame two Earths side by side. They are caused by the convection of material in the Sun. Supergranules sometimes appear to move faster than the rotation of the Sun, but scientists have confirmed that this apparent motion is of the cells themselves is actually wave-like motion propagated across them.

An artist’s concept of switchbacks emerging from between supergranules on the Sun. A closeup of the solar surface showing yellow funnels emerging from between darker orange parts of the solar surface representing supergranules. White squiggling lines stream out from the funnels, generating switchbacks in the solar wind.
Data from Parker Solar Probe has traced the origin of switchbacks’ magnetic zig-zag structures in the solar wind back to the solar surface. At the surface, magnetic funnels emerge from the photosphere between convection cell structures called supergranules. Switchbacks form inside the funnels and rise into the corona and are pushed out on the solar wind.
NASA GSFC/CIL/Jonathan North

Thermosphere

Above the mesosphere is the thermosphere, which begins about 53 miles above Earth and reaches 372 miles high. The name thermosphere comes from the region’s extremely high temperature, up to 1,500C (2,700F). However, the low density in this region means it doesn’t contain much heat. The thermosphere is a busy place — this is where you can find the International Space Station, many satellites, and the auroras. Like the mesosphere, the thermosphere experiences its own active “weather,” with high and low pressure zones, winds up to 1600 miles per hour, and complex chemical disturbances.

graphic showing atmospheric layers on Earth including the heights at which different kinds of airglow appear.
The thermosphere is the highest and hottest atmospheric layer, where the ISS flies and the aurora and airglow can be observed.
NASA’s Goddard Space Flight Center/Genna Duberstein

Totality

Totality is the time when the Moon is completely blocking the Sun’s bright face. During totality, the Sun’s relatively faint and wispy outer atmosphere, the corona, is visible to observers – the only time the corona is visible to the unaided eye. Totality only happens within the Moon’s inner shadow, the umbra. 

During totality – and ONLY during totality – it is safe to watch the eclipse directly without solar viewing glasses or solar filters. Observers must resume using an indirect viewing method or a solar filter before the end of totality. The reappearance of Baily’s Beads or the diamond ring indicate that totality is over.   

A black disk appears in the middle of the photo with white rays radiating out from the circle, gently fading into the black background.
NASA photographer Keegan Barber captured this image of totality from Dallas, Texas, during the total solar eclipse on April 8, 2024. Because of how the image was processed, the rays are shorter than in the prediction and don’t extend out as far, however much of the structure matches what can be seen in the prediction image.
NASA/Keegan Barber

Total solar eclipse

A total solar eclipse happens when the Moon passes between the Sun and Earth, completely blocking the face of the Sun. People located in the center of the Moon’s shadow when it hits Earth will experience a total eclipse. The sky will darken, as if it were dawn or dusk. Weather permitting, people in the path of a total solar eclipse can see the Sun’s corona, the outer atmosphere, which is otherwise usually obscured by the bright face of the Sun. A total solar eclipse is the only type of solar eclipse where viewers can momentarily remove their eclipse glasses (which are not the same as regular sunglasses) for the brief period of time when the Moon is completely blocking the Sun.

A square image with a black background features nine stages of a total solar eclipse arranged in a nine-by-nine grid. With the exception of the stage in the exact middle of the image, the Sun is seen as a yellow-orange crescent in varying states of thinness. The middle stage shows the eclipse at totality, where the full black silhouette of the moon is outlined by the wispy white rays of the Sun's atmosphere, the corona.
On April 8, 2024, the moon’s shadow swept across North America. This collage shows different stages of a total solar eclipse, seen from the ground, as the moon slowly slips between Earth and the Sun.
NASA/Keegan Barber

Transit

A transit happens when one celestial body crosses in front of another from a specific point of view. Eclipses are a type of transit. On Earth, we most often see Mercury transit the Sun, or – even more rarely – Venus.  

Transits are also one of the primary ways scientists look for evidence of exoplanets, planets beyond our solar system. As exoplanets pass between their host star and Earth, the light we measure from the host star decreases slightly, giving scientists clues about the planet or planets that may be orbiting that star.

A sequence of black silhouettes of a planet transiting across the bright, glowing yellow surface of the Sun, showing different stages of the transit against the Sun’s outer atmosphere and swirling surface.
A composite view from NASA’s Solar Dynamics Observatory satellite shows Venus transiting the Sun on June 5, 2012.

Transition region

The transition region is where the chromosphere becomes the corona, and the temperature rapidly rises from thousands to millions of degrees. Estimated to be about 60 miles thick, its exact height and position is not well defined. Instead, the “transition region” forms a kind of halo around the shifting, churning features of the chromosphere.

This image, taken on Dec. 31, 2013 by the AIA instrument on NASA’s Solar Dynamics Observatory at 171 Angstrom, shows the current conditions of the quiet corona and upper transition region of the Sun.
This image, taken on Dec. 31, 2013 by the AIA instrument on NASA’s Solar Dynamics Observatory at 171 Angstrom, shows the current conditions of the quiet corona and upper transition region of the Sun.
NASA/SDO

Umbra

The umbra is the inner part of the Moon’s shadow. During a total solar eclipse, the umbra’s path across Earth’s surface is what creates the path of totality. Observers inside the path of totality can see the Moon completely block the Sun’s face.

The Earth, seen from above. A dark splotch of shadow is covering much of North America.
The above image was acquired during the eclipse by NASA’s EPIC (Earth Polychromatic Imaging Camera) imager aboard DSCVR (Deep Space Climate Observatory), a joint NASA, NOAA, and U.S. Air Force satellite. In this view, acquired at 16:58 Universal Time (11:58 a.m. Central Daylight Time), the shadow, or umbra, from the Moon can be seen falling across the southeastern coast of Texas, near Corpus Christi.
NASA/DSCOVR

Upper atmosphere (Earth)

Earth’s atmosphere is traditionally divided into layers that differ in their temperature, density, and composition: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.

Heliophysicists study the upper layers of Earth’s atmosphere, including the mesosphere, thermosphere and exosphere. They also study the ionosphere, a layer overlapping with the upper portions of the mesosphere and through the thermosphere and exosphere, identified by its population of ionized, or electrically charged, particles.

See the individual entries for mesosphere, thermosphere, and ionosphere to learn more.

Thermal and compositional structure of the atmosphere.
Thermal and compositional structure of the atmosphere. The upper atmosphere, comprising the mesosphere, thermosphere, and embedded ionosphere, absorbs all incident solar radiation at wavelengths less than 200 nanometers (nm). Most of that absorbed radiation is ultimately returned to space via infrared emissionsfrom carbon dioxide (CO2) and nitric oxide (NO) molecules. The stratospheric ozone layer absorbs radiation between 200 and 300 nm.

The plot on the left shows the typical global-average thermal structure of the atmosphere when the flux of solar radiation is at the minimum and maximum values of its 11-year cycle. The plot on the right shows the density of nitrogen (N2), oxygen (O2), and atomic oxygen (O), the three major neutral species in the upper atmosphere, along with the free electron (e−) density, which is equal to the combined density of the various ion species. The F, E, and D regions of the ionosphere are also indicated, as is the troposphere, the atmosphere’s lowest region.
Naval Research Laboratory/J. Emmert

Van Allen radiation belts

Named for their discoverer, James Van Allen, these concentric, doughnut-shaped rings encircle Earth and are filled with high-energy particles trapped by Earth’s magnetic field to create the radiation belts. The particles – electrons and ions – gyrate, bounce, and drift through the region, sometimes shooting down into Earth’s atmosphere, sometimes escaping out into space. The radiation belts swell and shrink during 1-3 day long geomagnetic storms as part of a much larger space weather system driven by energy and material that erupts off the Sun and fills the entire solar system. The Van Allen Belts are an important component of the Earth’s magnetosphere.

In a visualization, Earth is shown against the blackness of space. White lines emanate out from the poles showing the planet’s magnetic field lines. Concentric rainbow-colored semicircles on either side of Earth visualize belts of trapped electrons. The second belt from Earth is colored purple to indicate it’s composed of protons as well as electrons. The third belt from Earth (rainbow color) represents the new electron belt.
The May 2024 solar storm created two extra radiation belts, sandwiched between the two permanent Van Allen Belts. One of the new belts, shown in purple, included a population of protons, giving it a unique composition that hadn’t been seen before.
NASA/Goddard Space Flight Center/Kristen Perrin