Brazilian experiment will investigate mysterious solar radiation

Equipment focused on detecting emissions with frequencies over 1 trillion Hertz will be placed 40 kilometers from the Earth’s surface using stratospheric balloons.

By José Tadeu Arantes

Agência FAPESP – Brazilian equipment to measure solar radiation in the range of 1 terahertz ( 1 trillion Hertz, or 1012 Hz), corresponding to wavelengths below 1 millimeter, will soon be sent to altitudes of 40 kilometers from the Earth’s surface in long flights on stratospheric balloons. 

The experiment, named Solar-T, is focused on exploring one of the lesser known and more enigmatic aspects of the Sun’s activity. In the study of solar emissions, the terahertz (THz) range of the electromagnetic spectrum, between microwaves and near-infrared waves, has been almost ignored until recently.

“It was thought to be of little importance, including only radiation from thermal phenomena. But relatively recent discoveries at the frequencies of 0.2 THz and 0.4 THz changed this concept,” said the coordinator of the experiment, Pierre Kaufmann, of the Center for Radioastronomy and Astrophysics (Craam) at Universidade Presbiteriana Mackenzie, in an interview with Agência FAPESP. 

Terahertz emissions associated with solar explosions were detected by solar radiotelescopy for sub-millimeter waves by an instrument located at El Leoncito, in the Argentinean Andes. This landmark discovery left researchers perplexed and excited.

“It began a decade of enormous theoretical and experimental efforts focused on understanding the phenomenon. That’s why we dedicated eight to nine years to conceiving and building the Solar-T in collaboration with the Center for Semiconductor Components at Universidade Estadual de Campinas (Unicamp) and the Bernard Lyot Solar Observatory in Campinas, São Paulo,” commented Kaufmann.

Terahertz emissions, sometimes called “T-rays”, are thought to result from (previously unsuspected) particle acceleration mechanisms at high energy levels. 

One of the hypotheses is that the emissions are produced by ultrarelativistic electrons (accelerated by electromagnetic fields at speeds close to that of light). “Others relate the origin to the decay of pions, producing high-energy positrons,” said Kaufmann.

The Solar-T, which could help to clarify this mystery, is basically a system of photometers, instruments used to gauge the intensity of photons. Specifically, the device includes two photometers; collectors; filters to block undesirable radiation levels (near-infrared and visible light) that could mask the phenomenon; a source feed; and a telemetry system to submit information to Earth by radiowaves, using the Iridium satellite network. 

“The Solar-T is a quasi-radio, quasi-optical telescope. It does not form images like optical telescopes do but detects and measures radiation between the frequencies of the upper limit of radio (microwaves) and the lower limit of visible light (infrared),” explained Kaufmann.

The equipment had to be launched in the stratosphere because the atmosphere blocks almost all terahertz radiation and prevents it from reaching the Earth’s surface. “The interpretation of the production mechanism of T-radiation is dependent on obtaining more information about this range of the spectrum. And the Earth’s atmosphere is highly opaque,” he said. 

“At El Leoncito, we managed to explore two small windows at the frequencies of 0.2 THz and 0.4 THz. But we need to investigate higher frequencies. The Solar-T will operate at 3 THz and THz and observe the entire solar disk, detecting any small variations resulting from explosions at specific points,” explained Kaufmann. The study coordinated by Kaufmann is funded by FAPESP through its Thematic Project and Regular Research grant program.

Transportation via balloons

One alternative to using stratospheric balloons is to place the Solar-T onboard satellites. If this happened, however, what would be stratospheric is the cost of the experiment.

A second option is to transport another version of the Solar-T to very dry and cold high-altitude areas, such as the High Plains of Atacama, to observe radiation in atmospheric windows of terahertz frequencies. This option has not been excluded, but it would require major infrastructure. 

According to Kaufmann, transportation by balloon will have a cost of practically zero for the Brazilian experiment. Because of the high impact of articles that have been published in scientific journals and the success of presentations at conferences, the Brazilian researchers have received the cost of transportation as part of a collaboration offer.

“We have accepted two invitations: one for a 7- to 10-day flight over Russia, in collaboration with the Lebedev Physics Institute in Moscow, and another for a two-week flight over Antarctica, in cooperation with the University of California at Berkeley,” he said.

Because these gigantic balloons carry several types of equipment on board, with a total cargo of around 8 to 12 tonnne, and the Brazilian apparatus weighs only 60 kilograms, the dates of the launch may change so that they are compatible with the two different experiments. The mission over Russia is slated for July or August 2014. The mission over Antarctica is scheduled for the summer of 2015-2016 in the Southern Hemisphere, preceded by a 1 day test flight over Texas during the previous year.

In the flight over Antarctica, the Solar-T will be installed along with a gamma-ray experiment, GRIPS, from the University of California at Berkeley, which has its own automatic detection and tracking system. The balloon will be launched and recovered at the McMurdo base on Ross Island, near the Antarctic Coast.

According to Kaufmann, the flight over Russia, to be launched from Kamchatka, on the extreme eastern edge of Siberia, and recovered in Volgograd, will require an automatic tracking cabin, developed and constructed in collaboration with the University of California at Santa Barbara.

“The mission of the Solar-T in stratospheric balloons will be conducted in the short-term to take advantage of the cyclical phase of intensification of solar activity over the next few years, when explosions will become more frequent,” said Kaufmann.