Study proposes new interpretation for solar emissions

A study conducted by researchers from Brazil and the United States explains radiation at frequencies close to far-infrared produced by eruptions of the Sun.

By José Tadeu Arantes

Agência FAPESP – An explanation for the high-frequency radiation produced by solar emissions has just been proposed based on a physical process observed in laboratory particle accelerators. The article detailing the study “The contribution of microbunching instability to solar flare emission in the GHz to THz range of frequencies,” by John Michael Klopf of the College of William and Mary (Williamsburg, Virginia, United States) and Pierre Kaufmann, Jean-Pierre Raulin and Sérgio Szpiegel from Mackenzie Presbyterian University, was published August 10, 2014 in The Astrophysical Journal.

The study was conducted under the FAPESP-funded thematic project “Solar activity emissions from sub-millimeter waves to infrared (SIRA).”

The proposed explanation was substantiated through a computer simulation that was based on the physical processes that take place in laboratory particle accelerators. The simulation investigated how these processes might occur in a context governed by solar parameters. “The results proved to be quite convincing. It was one of those rare occasions in which laboratory simulation of space plasma was successful,” Kaufmann told Agência FAPESP.

The physical process in question is that of coherent synchrotron radiation, which can occur when electron beans are accelerated until they reach speeds close to the speed of light. This radiation is produced at the same time as the well-known incoherent synchrotron radiation, generated by the same beams, and it depends upon the interaction of the electrons in the magnetic fields.

If the waves that describe the electrons remain in phase coherence, upon releasing energy, all the electrons would do so at the same time. This is what constitutes coherent radiation, characterized by the emission of high-intensity energy pulsations.

“In the accelerators, the particles are artificially accelerated through magnetic fields. On the Sun, the process is associated with sunspots, which are magnetic poles,” the Mackenzie researcher explained.

“Particles are confined largely to the region situated above the sunspot in the solar atmosphere. Typically, this confinement tends to disappear in two Earth months, which is how long sunspots last, supported by the period of the Sun’s rotation,” Kaufmann said. “Because of some yet-unknown mechanism, it could be that instead of becoming undone, the particle plasma is accelerated and expelled from the Sun as an eruption called a flare because the region emits large quantities of radiation in a very short period.”

To have an idea of the magnitude of the phenomenon, just consider that the number of particles involved in a solar eruption is estimated at 10 to the 30^th power (the numeral 1 followed by 30 zeros, i.e., one nonillion, or one thousand billions of billions of billions). The energy released is approximately 100 billion times more than that of the atomic bomb the United States dropped on the Japanese city of Hiroshima during World War II.

The electromagnetic radiation generated by the solar eruptions has been studied since the 1950s. Scholars, however, had identified only that radiation in the radio and microwave frequency bands. Nearly a decade ago, however, thanks to a Brazilian radio telescope installed in the Argentine Andes, it was discovered that the eruptions also emitted much higher frequencies, close to far-infrared, also known as the terahertz band.

“This introduces a huge problem in terms of interpretation,” Kaufmann commented. “To explain this type of emission, simultaneous with the microwave band of emissions, we’re now proposing the analogy with coherent synchrotron radiation, previously observed in particle accelerators.”

The explanation recognizes the possibility that electrons group together in similar states of energy and phase, which may occur, for example, when electron beams propagate through regions affected by irregular magnetic fields. “The agglomerations of electrons may then suddenly emit synchronic radiation together, coherently. The phenomenon is known as microbunching,” the researcher reported.

According to Kaufmann, physicist John Michael Klopf, first author of the article, is a specialist in accelerator physics and has worked for many years at the Thomas Jefferson National Accelerator Facility, commonly known as the Jefferson Lab, a U.S. national laboratory, one of the first to detect the phenomenon of microbunching.

“The magnetic fields in the solar regions where the eruptions occur are very complex and could easily give rise to bunches of electrons similar to those detected in the laboratories,” the researcher said.