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Welcome to the STEREO Learning Center

Important STEREO Science Concepts

If you are reading through the STEREO web pages you may come across a number of terms that are not exactly household words, but are central to a real understanding STEREO science.

Here are brief discussions of a few of important STEREO Science Concepts:


Total Solar Eclipse The beautiful corona during a 1980 total solar eclipse. Image courtesy of the High Altitude Observatory and Rhodes College.

The corona is the Sun's hot, thin outer atmosphere. From Earth it is most easily seen during a total solar eclipse in which the Sun's bright disk is covered by the Moon, revealing the much fainter corona.

The highly structured corona is shaped by the Sun's complex magnetic field, and is very active, exhibiting coronal mass ejections and flares among other solar magnetic phenomena. The corona is so hot at over a million degrees celcius that it produces ultraviolet light and X-rays.

STEREO's SECCHI imaging suite shows us the corona in two ways. Its coronagraphs imitate a solar eclipse in space by covering the disk of the Sun with an occulting disk, so that we can see scattered light from the corona. The SECCHI Extreme-Ultraviolet Imager (EUVI) lets us observe the ultraviolet light produced by the corona.

For further reading:


CME Artist's rendition of the relationship between the Sun, the solar wind and Earth's magnetic field.

The Sun's super hot atmosphere expands out into the solar system flowing past Earth and the other planets. In fact, you could say that Earth is inside the outermost layer of the Sun.

This flow is called the Solar Wind and it consists mostly of hydrogen, with some helium and small amounts of heavier elements. It is exceedingly thin - just a few particles per cubic centimeter, and moves at velocities from 200 to 800 km/s, or even faster if you count the fastest CMEs which can move outwards at over 1000 km/s (two million mph). However, this is still far slower than the speeds of solar energetic particles which can shoot out at over 100,000 km/s, nearly the speed of light.

The solar wind varies in time and location, with higher speed, lower density streams flowing out of areas known as coronal holes, where the Sun's magnetic field opens out into the solar system. The solar wind carries the Sun's magnetic field outwards to form the interplanetary magnetic field (or IMF). The area which is affected by the solar wind and IMF extends beyond the orbit of Pluto and is know as the heliosphere.

The STEREO PLASTIC and IMPACT instruments sample the solar wind as it passes by the two spacecraft.

For further reading:


CME An image showing a CME observed by the STEREO's SECCHI/Cor2 coronagraph.

A billion tons of matter traveling at a million miles an hour, these giant magnetic structures blast off the Sun into the solar system and can create major disturbances in Earth's magnetic field, resulting in the beautiful aurora but also problems with spacecraft and power systems. They are also a source of Solar Energetic Particles (SEPs), which are a hazard to astronauts.

Although CMEs are huge and powerful, they are very thin and spread out, with just a few particles per cubic centimeter. Much of the power they have to affect us comes from their magnetic fields, which are part of the interplanetary magnetic field, and can disturb the magnetic field of Earth. It usually takes a CME two to four days to reach Earth, although extremely fast ones have been known to reach here in just over a day.

Understanding what causes CMEs and how they move through the solar system is one of the chief goals of the STEREO mission. The CMEs are imaged by the different telescopes in the SECCHI instrument suite. The actual material in CMEs is measured as they pass the spacecraft using the IMPACT and PLASTIC instruments, and IMPACT measures their magnetic fields. The SWAVES instruments observe radio signals produced by shock waves formed as the CMEs plow through the solar wind.

For further reading:


CME One of the brightest solar flares on record on the right (western) limb of the Sun imaged in ultraviolet light by SOHO/EIT. The flare was so bright the detectors could not handle all the photons, resulting in a series of horizontal streaks.

Solar flares are bright, explosive events that take place in the Sun's lower corona. They can be associated with CMEs, but are not the same thing. Scientists will use the SECCHI imaging instruments aboard STEREO to improve our understanding of how flares are related to CMEs.

Although most of what is called a solar flare occurs relatively low in the Sun's atmosphere, flares do release charged particles which travel along the magnetic field lines of the interplanetary magnetic field (IMF). Electrons emitted in this way by flares produce radio waves detected by the SWAVES instruments and allow researchers to map the IMF.

Sometimes these charged particles may be high enough in energy to qualify as solar energetic particles (SEPs). SEPs along with the X-rays and gamma-rays produced by flares can be harmful to astronauts.

For further reading:


IMF A depiction the interplanetary magnetic field. The crosses show the path of energetic particles emitted by flares near the Sun's surface and tracked by the Ulysses spacecraft.

The Sun's magnetic field is pulled out into space by the charged particles of the Solar Wind. Because the Sun is rotating, structures in the solar wind tend to form a spiral pattern, like water shooting out of a rotating sprinkler head. As with the water drops coming out of the rotating sprinkler, the solar wind particles themselves move straight outwards. However, structures extending across the solar wind flow form a spiral pattern extending down to the point on the Sun from which they originally came.

Super-fast charged particles called solar energetic particles (SEPs) move much faster then the solar wind and because they are charged move along rather than across magnetic field lines. As a result they trace out the spiral pattern of the IMF.

One of the goals of STEREO is to better map the structure of the interplanetary magnetic field. The STEREO/SWAVES instruments maps that structure using radio signals given off by energetic charged particles moving along the magnetic field lines. The IMPACT magnetometers measure the magnetic field at the location of the spacecraft.

For further reading:


April 21, 2002 Flare
April 21, 2002 CMESOHO/LASCO A flare and CME that occurred on 21 April 2002 on the western limb of the Sun resulted in SEPs measured at Earth, 90 degrees away in longitude.

Particles, especially protons, moving at super high speeds can be a deadly hazard to astronauts and spacecraft. These are frequently emitted during solar activity and are thought to be caused by CMEs and/or solar flares. Here on Earth's surface, we are shielded from their damaging effects by Earth's magnetic field.

Most SEPs are protons and electrons, but there are also other heavier particles all of which are measured by STEREO's IMPACT and PLASTIC instruments. The electrons give off radio signals which are measured by the SWAVES instruments.

SEPs move at relativistic speeds (30-90% of the speed of light), far faster than the regular solar wind or coronal mass ejections. Since they are charged particles they move along the interplanetary magnetic field's spiraling magnetic field lines. Because of this SEPs are often detected far eastward of wherethey are thought to originate.

SEPs usually arrive at Earth from 20 minutes to several hours after the start of a flare or CME. The time it takes for the particles to get here depends on how fast they are going, where and when the protons are accelerated, and the path they take.

It is still not entirely clear if SEPs are emitted by CMEs as they plow through the solar wind, by solar flares low in the Sun's atmosphere, or both. The instruments aboard STEREO are being used to better understand their origins.

For further reading:


IMF The aurora borealis, or northern lights, photographed near Fairbanks, Alaska. These lights in the sky are due to disturbances in the Earth's magnetic field caused by CMEs and other features in the solar wind. Image courtesy of Jan Curtis.

Space Weather describes changes in the solar system environment caused by variations in the Sun and Solar Wind. These include, coronal mass ejections and solar flares, and changes in the interplanetary magnetic field due to solar surface features like coronal holes. Space weather phenomena cause the beautiful aurora (northern and southern lights) and can also affect communications, power systems, aviation, and spacecraft. Some space weather occurrences, such as solar energetic particles can present grave dangers to astronauts.

Like Earth weather, space weather varies substantially in space and time. A CME headed towards Earth may completely miss Mars or Venus and vice versa. Spacecraft deployed around the solar system, like STEREO, will give us a better, solar system wide view of what is happening.

For further reading:

Last Revised: Tuesday, 08-Feb-2022 10:28:22 EST
Responsible NASA Official: [email address: therese.a.kucera<at>nasa<dot>gov]
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