Onset and propagation of shock waves created by ejections of solar matter

Researchers correlated two different types of electromagnetic emission associated with the phenomenon – radio bursts and extreme ultraviolet waves. Coronal mass ejections can disrupt telecommunications.

By José Tadeu Arantes

Agência FAPESP – The structure and complex dynamics of the Sun’s atmosphere are relatively well known. However, many aspects have not yet been completely established, requiring new research. One such aspect is the ejection of solar matter into interplanetary space. This phenomenon concerns mankind directly because part of the ejected matter can fall to Earth and interfere with terrestrial processes, especially telecommunications.

A study by researchers at Paraíba Valley University (UNIVAP) in Brazil investigated the relationship between coronal mass ejections (CMEs) and the production of shock waves that propagate through the Sun’s atmosphere.

A paper describing their findings was published in the journal Astronomy & Astrophysics by Rafael Douglas Cunha da Silva, Francisco Carlos Rocha Fernandes and Caius Lucius Selhorst. The study arose from Cunha da Silva’s PhD thesis, which was supervised by Fernandes and Selhorst and supported by FAPESP.

“CMEs produce shock waves that propagate through the solar atmosphere at between 200 and 2,000 kilometers per second,” Fernandes told Agência FAPESP. “The atmospheric disturbances triggered by these shock waves generate electromagnetic emissions in several different frequency bands. These emissions are the shock waves’ signatures, as it were. Our study set out to correlate two different types of electromagnetic emission – radio bursts and extreme ultraviolet waves.”

Fernandes is the head of UNIVAP’s PhD course in physics and astronomy and principal investigator for the Thematic Project entitled “Development of Brazilian Decimetric Array (Phase II)”, supported by FAPESP.

“We set out to determine the heights in the solar atmosphere at which shock waves are produced and how they propagate,” Selhorst said. “The solar atmosphere’s density declines with height, and emission frequency depends on local plasma density. So, by measuring frequency, you can calculate density and, by extension, height.”

Selhorst is a professor at UNIVAP and a principal investigator in another research project supported by FAPESP: “Study of the changes in the solar magnetic field based on radio observations”.

CMEs release huge amounts of hot matter into interplanetary space; this matter is mainly composed of electrons and protons but also contains a small percentage of ions from heavier elements, such as helium, oxygen and even iron. This material, together with the solar wind, propagates to the extreme outer bounds of the heliosphere, far beyond Pluto’s orbit and approximately 100 times the distance between the Earth and the Sun.

Magnetic field rearrangement

CMEs appear to be associated with sudden releases of energy deriving from magnetic field rearrangements in the solar atmosphere. “They’re recurring phenomena,” Selhorst said. “During periods of maximum solar activity, they happen two or three times a day on average. During periods of low activity, they happen once a week.”

Electromagnetic emissions in the form of radio bursts are not produced directly by CMEs but rather by the shock waves they generate as they hurtle through the Sun’s atmosphere. “These shock waves can be detected by satellites in the ultraviolet band. What we found in our study was a significant temporal correlation between shock wave expansion detected in the extreme ultraviolet band and radio bursts,” Cunha da Silva said.

This association is important because shock wave production and propagation cannot be observed accurately in ultraviolet alone: the equipment used for this purpose, such as the twin satellites of NASA's Solar TErrestrial RElations Observatory (STEREO), has a time resolution on the order of five minutes, compared with milliseconds for radio data.

“The new generation of satellite instruments has greatly improved the temporal resolution for detection in the extreme ultraviolet spectrum. The AIA (Atmospheric Imaging Assembly) on board the SDO (Solar Dynamics Observatory) satellite, which was launched in 2010, captures images of the entire Sun every 12 seconds. This greatly facilitates event identification, but there remains the problem that the images are two-dimensional projections of three-dimensional events,” Selhorst said.

“This is why the use of radio spectra is still one of the main methods used for indirect observation of the formation of coronal shock waves, especially those produced by CMEs in their initial burst. Analysis of these spectra enables us to estimate the altitude range in the solar atmosphere at which radio emissions occur, as well as the direction of the source, which may be radial or oblique.”

Transition region

Most CMEs originate relatively close to the Sun’s “surface”, as it were – in fact, what we call the “surface” of the Sun is a region in which visible light becomes opaque, so that the star’s internal structure cannot be observed. Above this opaque region is the atmosphere proper, with three distinct layers called the photosphere, chromosphere and corona. Between the latter two is a narrow transition region in which plasma temperature and density change drastically.

The corona is so rarefied that it cannot be observed with the naked eye except during a total eclipse of the Sun. To study the phenomena that occur in the corona, scientists use a coronagraph to simulate an eclipse. A solar coronagraph is a telescopic attachment that blocks light from the lower layers of the Sun’s atmosphere to reveal the faint corona.

“When we study the Sun using a more conventional instrument such as a coronagraph, we can’t detect the point at which shock waves are created because the coronagraph hides part of the Sun’s atmosphere as well as the solar disk itself. In the case of extreme ultraviolet and radio, this obstruction doesn’t happen, so we can observe the onset of shock wave propagation in regions very close to the surface,” Selhorst said.

Animation showing coronal mass ejection with Sun’s disk covered by coronagraph (researchers' archive)