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Physics

At a new frequency

Telescope developed in São Paulo will detect solar flares in the terahertz range from a stratospheric balloon

[Photo caption, p. 61] Solar-T telescope undergoes last adjustments at the company Propertech, in Jacareí (SP), before being sent to the United States

Léo RamosSolar-T telescope undergoes last adjustments at the company Propertech, in Jacareí (SP), before being sent to the United StatesLéo Ramos

After 10 years of work, the first Brazilian device for measuring and studying one of the least known and most mysterious aspects of the Sun’s activity—the radiation emitted by the explosions that occur on the star in the far-infrared range, also known as the terahertz (THz) range—is ready. It is called the Solar-T, and is a telescope that does not form images like its optical counterparts. It identifies and measures the radiation emitted by the objects observed. It works like a photometer to measure the intensity of photons, which are the particles associated with electromagnetic waves such as light. The device is expected to make its first flight over Antarctica aboard a stratospheric balloon at an altitude of 40 kilometers in conjunction with an experiment being carried out by the University of California, Berkeley, in the summer of 2015, on a mission lasting two weeks.

The device was developed by Mackenzie Presbyterian University, São Paulo, with funding of R$590,000 from FAPESP, in collaboration with the Center for Semiconductor Components of the University of Campinas (Unicamp). The radiation emitted by the Sun will be measured in the terahertz range of the electromagnetic spectrum, which includes radio waves, infrared and visible light. “There is no equipment like it in the world operating at THz frequencies to study solar flares,” says the researcher Rogério Marcon, of the X-Ray Diffraction Laboratory of the Unicamp Institute of Physics and founder of the Bernard Lyot Solar Observatory, a private institution in Campinas, which also contributed to the Solar-T project. “The THz range is used in medicine and security, but in solar astrophysics it is unprecedented.”

According to Marcon, the work being done by the groups at Mackenzie and Unicamp is putting Brazil at the forefront of research in THz detectors and on the nature of solar flares. “Everything is new,” he says. The same team is developing the Hats (High Altitude Terahertz Solar Telescope) project, a ground telescope with objectives similar to those of the Solar-T, but with technological and operational differences. This new equipment should be ready by the end of 2014, and will probably be installed in the Atacama Astronomical Park, 5,100 meters above sea level in the Chilean Andes. “So far we have obtained funding of R$300,000 from CNPq [National Council for Scientific and Technological Development] and Mackenzie,” says Pierre Kaufmann, of the Mackenzie Center for Radio Astronomy and Astrophysics (CRAAM), and the coordinator of both projects.

The work that culminated in these two pieces of equipment began in 1984 when Kaufmann detected the first signs that solar flares might emit radiation in the terahertz range, also called T-rays. According to Kaufmann, until the 1970s it was believed that the explosions emitted radiation with frequencies up to those of microwaves, at a maximum, and then decayed. After that, some researchers, such as the Englishman David Croom and the American Fred Shimabukuro, showed that the radiation of solar flares had increasing intensity, currently believed to potentially reach terahertz frequencies. However, due to the limitations and the low sensitivity of their telescopes, they were unable to determine what maximum frequency this radiation reached.

060-063_RadiacaoSolar_219In 1984, a discovery by Kaufmann and his team increased the knowledge in this area of research. “With a telescope with greater sensitivity, at the Itapetinga Radio Observatory in Atibaia [upstate São Paulo], we detected a solar flare with an increasing intensity of radiation up to 100 gigahertz [GHz],” he says. “At the time, we published a scientific article in 1985 in the journal Nature, in which we proposed the existence of solar radiation with frequencies over 100 GHz. The discovery had a tremendous impact. Based on this work, corroborated by other authors, we started trying to detect radiation in the higher ranges.”

Kaufmann recounts that, in 1997, FAPESP approved a proposal for research on solar radiation at frequencies of 200 and 400 GHz (0.2 and 0.4 THz) to that end. The funding enabled them to build the Solar Telescope for Submillimeter Waves (SST) that was installed at the El Leoncito Astronomical Complex (Casleo) located in the Argentinean Andes at an altitude of 2,600 meters. “In November 2003 we detected this radiation at two frequencies: 212 GHz and increasing to 405 GHz,” says the researcher from Mackenzie. Until then, the highest frequency measured anywhere was 100 GHz “With that telescope, we detected the existence of two components of radiation in solar flares, one in the microwave range, which was well known, and another, simultaneous element in the THz range, which had never before been seen. However, due to the limitations of the ground observations, we could not determine how high the radiation frequencies could become. Even at high altitudes the atmosphere is opaque to almost the entire range of the THz spectrum.”

Now, with Solar-T and the ground telescope, the researcher intends to go further. The first device is subdivided into two units, one to detect 3 THz radiation and the other to detect 7 THz radiation. Each consists of two parts: the first is the collecting system, or the telescope proper, to capture solar radiation, and the second is the sensor system. Each telescope has a Cassegrain-type optical configuration with two mirrors, the main mirror being concave with a diameter of 7.6 cm, and the other convex, and smaller, plus special filters to block unwanted radiation, such as electromagnetic waves in the near-infrared and visible ranges that could overheat and even cause the equipment to catch fire, in addition to masking the phenomenon sought in THz frequencies. Other filters and wire mesh delimit the frequency to be detected, in this case 3 THz and 7 THz. Although they do not form images, mirrors are needed to capture and concentrate the electromagnetic radiation.

The second part of the Solar-T is the sensor system, which is composed of a Golay cell, a device manufactured by Tydex, in St. Petersburg, Russia. It is an optical-acoustic detector that records the variations in radiation intensity. The Solar-T also has a system for acquisition, storage, transmission and reception of data, produced by the Brazilian companies Propertech Tecnologia, in Jacareí, and Neuron, in São José dos Campos, both in the state of São Paulo. The former is also responsible for integrating all components and for final assembly of the equipment.

The configuration of the Solar-T will be partially applied to Hats, a ground telescope to be installed in the Chilean Andes

Léo RamosThe configuration of the Solar-T will be partially applied to Hats, a ground telescope to be installed in the Chilean AndesLéo Ramos

The two telescopes contain two innovations. The first is the larger mirror, which is rough. “The purpose of this roughness is to diffuse the infrared radiation,” says Kaufmann. “It can diffuse 80% of this light. The other 20% is suppressed by the filters, thus eliminating the infrared and visible light.” Another innovation, which was the subject of a patent application, is a device that captures any solar flare. The sensor surface must focus on an image of the entire surface of the sun in order for this to work. Data obtained from the Solar-T is stored and sent to the Iridium satellite network, which transmits it to a ground station and, from there, via the Internet to the researchers.

The goal of the ground telescope, Hats, is basically the same, but its size and configuration are different. It has a concave mirror with a diameter of 46 cm and a short focal length, and is based on the same optical concept used in the Solar-T in which solar radiation is reflected to the sensor. The goal is to detect radiation in “windows” of 850 gigahertz and 1.4 terahertz. “It is entirely robotic with its own system for tracking and maneuvers used to calibrate and determine the atmospheric opacity. It also has an automatic retractable dome commanded by a weather station to protect it during local storms,” he explains. “It will have its own power generating station, using solar panels, and facilities for remote data transmission.”

The technologies used in the two pieces of equipment will make important scientific advances possible, including knowledge of the mechanisms, principally the production of energy, which are behind solar flares. According to Kaufmann, there have been almost no conceptual advances in this area over the past 60 years. “We know as much today as when they were discovered,” he says. “There are several models that attempt to explain the phenomenon, but none has been confirmed.” Understanding the role of radiation in the terahertz range is no mere scientific curiosity. These phenomena, which repeat with greater intensity every 11 years or so, have direct implications on our day-to-day activities. In 1989, for example, when one of the strongest solar flares ever recorded occurred, power transmission fell in some countries, such as eastern Canada and the eastern United States, and in Sweden. It is now known that these events can affect satellites, navigation systems such as GPS, and telecommunications including mobile phones. As a result, damage to satellites can occur, leading to malfunctioning of the communication and navigation systems of aircraft and ships. Understanding the phenomenon is the best way to prevent all this.

The Solar-T will fly in a stratospheric balloon at an altitude of up to 40 km to avoid the opaque cloak over terahertz radiation in the atmosphere. Kaufmann’s team received two proposals to fly at almost no cost. At the University of California, the Solar-T is expected to fly with the Grips (Gamma-Ray Imager-Polarimeter for Solar Flares) experiment, which has an automatic system for pointing towards and tracking the Sun. First, a test flight of one day has been scheduled for Texas in September 2014 by a NASA balloon launch group (with 80% probability of confirmation). The other invitation is to take part in a mission lasting 7 to 10 days over Russia, in collaboration with the Lebedev Institute of Physics in Moscow. In the latter case, it would be necessary to develop a new Sun targeting system, which would require more resources.

Project
Solar flare THz measurements from space: phase I (2012-2013) (nº 2010/51861-8); Grant Mechanism Regular Line of Research Project Award; Principal investigator Pierre Kaufmann (Mackenzie Presbyterian University); Investment R$ 429,972.33 and $64,000.00 (FAPESP).

Scientific article
KAUFMANN, P. et alSolar burst with millimetre-wave emission at high frequency onlyNature. v. 313, p. 380. 1985.

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