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Astronomy

Solar shocks

Study details the relation between explosions in the Sun and magnetic storms on Earth

STEELE HILL / NASAThe Sun and the Earth: a bombardment of energySTEELE HILL / NASA

Looking like a huge ball of fire in the sky, the Sun is indeed far from being a mild celestial body. Explosions take place – one a week in calmer periods and two or three a day when it is more active – that shoot extremely hot gases and particles far away at speeds of as much as 2,500 kilometers a second and disturb the solar wind. Just like a stone cast into the water creates concentric waves, so these explosions eject material and give rise to shock waves that can reach the Earth. An impressive phenomenon, it is dazzling when captured in pictures. However, in this field of astronomy, it is surprising how little is known. Reducing this ignorance, describing the consequences of these explosions and assessing how they affect our planet is the occupation of the special geophysicist Cristiane Loesch, from the National Institute for Space Research (Inpe). Understanding solar activity is an increasingly crucial objective, as Sir John Beddington reminded us in the interview in this issue (see text here).

The material ejected by the Sun during these explosions has a magnetic field that changes the Earth’s magnetic field when it comes close to it, causing the so-called magnetic storms. The phenomenon can cause problems for navigation, aviation, astronauts working in space and can even interfere with power grids, causing blackouts such as that which left part of Canada in the dark in 1989. One of the problems to describe the phenomenon precisely is that it is not enough to point a telescope at the Sun, as its brightness obfuscates what is happening around it. The Inpe researcher, therefore, resorts to simulations based on models that describe the effects of these solar gas explosions, known as CMEs (coronal mass ejections). “Nobody knows yet how their eruption in the Sun works exactly,” she explains. With this theoretical resource, she is looking at the part of the solar atmosphere that is closest to the Sun, known as low solar corona, a zone seldom explored to date.

During her doctorate, under the guidance of Maria Virginia Alves, also from Inpe, and in collaboration with Merav Opher, a Brazilian astrophysicist established in the United States, Cristiane compared the forecasts of two such theoretical models in order to study, in that region, the signatures of two CMEs with different configurations. She observed that the magnetic energy of the CME is converted into thermal and kinetic energy as it draws away from its source and that the initial magnetic characteristics are not very important for the shock speeds that follow. Additionally, the two models turned out to be quite similar regarding the consequences of CME’s that are close to the Sun, at a distance of two to six times its radius, as shown in an article published in April of this year in Journal of Geophysical Research. “There, the wind still has a very solar structure, with the typical characteristics of its immediate surroundings,” she says, to explain her choice, “and closer to the surface lots of things happen that we don’t understand.” To have an idea of the scale, the distance between the Earth and the Sun is about 212 times the sun’s radius.

In theory
The similarity between the results obtained with the two models was a surprise, because they start with assumptions that one would expect to generate different interactions between the CME and solar wind. However, in both cases, the CMEs generate a shock wave that spreads faster than the explosion itself and that moves toward the Earth, while pushing in front a zone of disturbed solar wind, known as sheath. This widens as it moves away from the Sun and, as Cristiane tells us, it can increase the entry of energy into the magnetosphere by as much as 29%. This contributes to the magnetic storms on Earth.

Cristiane found that the magnitude of the sheath is different in the two models and she observed in them a second shock wave. However, it is still necessary to develop a better understanding of why this is the case. To investigate what generates this posterior shock, which appears at a little less than 2.5 solar radiuses, Merav suggested to Indajit Das, who at the time was his doctoral student, that he examine the CMEs as a whole and to analyze what might generate compression following the shock. The compression is especially high in the low solar corona, where the density of the solar wind is higher, according to the work of Das, published in March in Astrophysical Journal. Co-authored by Cristiane, the article shows that when the CME moves away from the sun, the magnetic field in front of it becomes compressed and the plasma between the field lines moves sideways, creating a region that is less dense in the sheath. “It’s like a boat cutting through water,” the Inpe researcher compares, “the water passes along the sides.” The study also shows that the CME can give rise to the posterior shock when it pushes the plasma from the sheath, accumulating mass.

A lot must still be done for it to become possible to describe in detail how the phenomena behave and why. Still, it seems certain that, up to three radiuses distance from the star that illuminates the Earth, the shocks caused by CMEs are tied to the acceleration of particles. Now, Cristiane is trying to understand what happens in the rest of the space that separates the Sun from the Earth. She wants to monitor the disturbance caused by the coronal mass ejection sheaths up to this planet to see what fluctuations they cause in the Earth’s magnetic field and how this might be related to what happens in the Sun. It is a long road.

Scientific articles
LOESCH, M. et al. Signatures of two distinct driving mechanisms in the evolution of coronal mass ejections in the lower corona. Journal of Geophysical Research. v. 116. Apr. 2011.
DAS, I. et al. Evolution of piled-up compressions in modeled coronal mass ejection sheaths and the resulting sheath structures. The Astrophysical Journal. v. 729, n. 112. Mar. 2010.

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