First 360° image of the Sun

For the first time, it is possible to view the whole Sun at once. Obtaining a simultaneous 360° view of the Sun has become possible thanks to the twin spacecraft of NASA’s STEREO (1) mission. French researchers from CNRS, the Paris Observatory, and Paris-Sud 11, Pierre et Marie Curie, Paris Diderot and Toulouse 3 universities (3) are participating, with backing from CNES, in three of the four instruments on board the spacecraft. The scientists will be able to follow the evolution of solar structures across the lifespan of the spacecraft. They hope in particular to better understand coronal mass ejections and their impact on the Earth.

An artist’s concept of STEREO surrounding the sun. Credit: NASA

The Sun constantly emits flows of particles (electrons, protons and ions) into interplanetary space: this is known as the solar wind. Solar flares, on the other hand, are phenomena of much higher energy with a much greater outflow of material. Several billion tons of material can be ejected at speeds exceeding 400 km/s. Perturbations in the Earth’s magnetosphere, caused in particular by these flows of particles, result in a wide range of phenomena, such as aurorae (Northern or Southern lights), disruption of radio communication and power supplies, and exposure of astronauts to dangerous levels of radiation. The physical processes that trigger such ejections are little known.

The STEREO mission aims to analyze these flares and study their impact on the terrestrial environment. One of its main goals is to significantly improve prediction of these solar storms, something that is still not familiar. Solar flares have already been observed by satellites such as SOHO, located between the Sun and the Earth. However, these instruments are almost completely blinded when the ejections are moving towards the Earth. This is why it is so useful for scientists to have two spacecraft positioned on opposite sides of the Sun. This is the case for the twin STEREO (3) spacecraft launched in November 2006 by NASA. They follow two different orbits around the Sun: one trails behind the Earth, while the other is ahead. Today they are 180° apart, in other words on opposite sides of the Sun. Each spacecraft can thus observe one half of the Sun, which for the first time makes it possible to obtain a 360° view of the entire Sun at exactly the same instant.

Remarkably, scientists can now access the side of the Sun facing away from the Earth, which cannot be seen when observing the Sun from our planet. This will therefore enable them to monitor the evolution of solar structures such as sunspots and filaments throughout their lifetime. Until now, such structures were no longer visible once they had moved beyond the western edge of the Sun. From now on, a new phase in the observation of the Sun begins. Researchers are also hoping to anticipate solar flares that appear on the side of the Sun invisible from the Earth, and which could nonetheless have an impact on our planet. By closely watching active regions—where flares are generated—they hope to better understand the mechanisms that trigger such phenomena. The Sun will continue to be observed not only with the two STEREO spacecraft but also through the development, around 2018-2020, of new missions that will make an extremely close approach to the Sun. These will look at regions that are still unexplored, relying on innovative instruments that will make it possible to probe the solar environment in ways that have never been done before.

Notes :

(1) Solar TErrestrial RElations Observatory
(2) En France, les laboratoires impliqués sont : le Laboratoire d’études spatiales et d’instrumentation en astrophysique (Observatoire de Paris/CNRS/UPMC/Université Paris Diderot), l’Institut d’astrophysique spatiale (CNRS/Université Paris- Sud 11), le Laboratoire Charles Fabry de l’Institut d’optique (CNRS/Université Paris-Sud 11/IOGS) et l’Institut de recherche en astrophysique et planétologie (CNRS/Université de Toulouse 3).
(3) Voir les communiqués de presse : « Les éruptions solaires observées par STEREO » – octobre 2006 (, « Voir le Soleil en 3D avec STEREO » – avril 2007 ( sse/communique/1083.htm)

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