Brazilian physicist Pierre Kaufmann is worried about an experiment that is scheduled to begin in the coming weeks. The US space agency (NASA) plans to launch a balloon from its base in Antarctica on December 1, 2015. It will rise 40 km above sea level, transporting two pieces of equipment to study the Sun. One of these devices is the Solar-T, a double photometric telescope designed and built by Kaufmann’s team in order to analyze a special band of solar radiation (see Pesquisa FAPESP Issue nº 219). If everything goes as planned, the Solar-T, which is part of an experiment being carried out at the University of California, Berkeley, should spend two to four uninterrupted weeks collecting the radiation emitted by the Sun, which never sets at the South Pole at this time of year.
The reason why the physicist is worried is because NASA intends to launch the Solar-T turned off, and only turn it on when the balloon reaches maximum altitude. “This strategy increases the risk of failure, which is inherent to any stratospheric balloon mission,” says Kaufmann, who monitored the telescope’s tests in the United States under conditions similar to those it will face in the skies of Antarctica. “The equipment worked very well on all occasions, but the evaluations were performed with it turned on,” recounts the physicist, coordinator of the Astronomy and Astrophysics Center (CRAAM) at Mackenzie Presbyterian University. “The problem with launching it turned off,” he explains, “is that, if something fails to work after it reaches altitude, it cannot be fixed.”
While it is hovering over the icy continent, the Solar-T will capture the energy emitted by solar flares in two specific frequencies: 3 and 7 terahertz (THz), which correspond to a fraction of distant infrared radiation. Located in the electromagnetic spectrum between visible light and radio waves, this radiation band allows us to more easily observe flares associated with magnetic fields in the Sun’s active regions, which often eject jets of particles with negative charge (electrons) in Earth’s direction, accelerated to high speeds. Near Earth, these particles affect the operation of telecommunications and GPS satellites, and generate the Aurora Australis and Aurora Borealis.
The radiation in this infrared band also allows us to investigate phenomena that transfer energy from the surface of the Sun, the photosphere, where the temperature reaches only 5,700 degrees Celsius, to the hotter, upper layers: the chromosphere, where temperatures reach 20,000 degrees Celsius, and the corona, which is at over 1 million degrees Celsius.
Despite allowing us to observe the Sun, terahertz radiation, which has been called T-Rays, has been little used. The reason was that there were — and still are — some challenges in detecting it. The first is that Earth’s atmosphere prevents most of this radiation from reaching ground-based telescopes. Additionally, not just any telescope can detect terahertz radiation. “In order to detect or produce an image of the Sun at these frequencies, we need to use a telescope made only of mirrors, since lenses made of glass or common optical materials absorb this frequency,” explains Matthew Penn, an associate astronomer at the National Solar Observatory (NSO) and the McMath-Pierce Solar Facility, both in Arizona.
Another complicating factor is that the detectors cannot be made of silicon, which is transparent to these energy frequencies, and must be refrigerated at very low temperatures. “Before Pierre Kaufmann began working in this area, few observations of the Sun were made at these frequencies because it was difficult to exploit the technology,” recounts astronomer Stephen White, at the Air Force Research Laboratory, in New Mexico.
Kaufmann hopes that the Solar-T data contribute to updating a figure that he has been helping develop for more than 30 years. This curve represents the profile of the energy emitted at the origin of the solar flares, generally seen in the region of the sunspots that sometimes discolor the surface of the star. It is a sort of energy signature of these solar flares that, in the opinion of physicists, astronomers and astrophysicists, could help explain the phenomena that lead to them.
The figure of the quantity of radiation ejected into space at each frequency began to take shape in the 1960s, based on observations of solar flares. For a long time, it recorded only radiation emitted in the low-energy portion of the radio-wave band — with frequencies from 30 megahertz (MHz) to 30 gigahertz (GHz). In 1972, John Castelli and Jules Aarons, of the Air Force Cambridge Research Laboratories (AFCRL) in the United States, produced an energy profile of solar flares, summarizing the data from 80 events. The figure had taken the approximate shape of the letter U and indicated that most of the energy released by these flares was in two low-frequency, low-energy radio-wave bands: part was at a frequency below 1 GHz, while part was in a frequency band ranging from 3 GHz to 30 GHz.
Shortly before, in 1968, researchers C.D. Clark and W.M. Park had obtained evidence that solar flares could produce radiation at a higher frequency, with greater energy. Using the telescope at Queen Mary University of London, they detected 250 GHz energy pulses. This frequency is about 30 times greater than that corresponding to microwaves, and unexpectedly very intense. Perhaps due to their sparseness, this and other data in the microwave region did not attract much attention. “Despite several suggestions, for a long time researchers in this area ignored this evidence,” says Kaufmann.
The belief that solar flares might release much more energy only reappeared two decades later, partly due to Kaufmann’s work. With the Itapetinga radiotelescope, located in Atibaia, São Paulo State, he observed a solar flare that took place on May 21, 1984. The data show that most of the energy was emitted in millimeter-length waves, at a frequency of 90 GHz, in the form of pulses lasting hundredths of a second. This was a new signal that there was more to be discovered about the flares. “At the time, we realized that there was a component of the flares that reached higher frequencies,” says the physicist.
Together with researchers at the University of Campinas (Unicamp), he developed equipment that was installed in different observatories to record energy at higher frequencies. In the early 2000s, Kaufmann and his team monitored solar flares with the Solar Telescope for Submillimeter Waves (SST) that was installed at the El Leoncito Astronomical Complex in the Argentinean Andes, and recorded a radiation flow that grew again at 0.2 THz and 0.4 THz. These results led Kaufmann and researcher Rogério Marcon, of the Unicamp Physics Institute, to develop equipment capable of detecting even higher frequencies, in the 30 THz region.
With a 30 THz telescope installed at El Leoncito and another on the roof of one of the Mackenzie buildings in downtown São Paulo, the Brazilian physicist’s group, which includes researchers from Argentina and the United States, has already recorded three solar flares — one on March 13, 2012, another on August 1, 2014, and a third on October 27, 2014 — that released a huge quantity of energy in this band of the electromagnetic spectrum. An analysis encompassing different regions of the spectrum revealed that, in truth, these events produce 10 to 100 times more energy in the far infrared (terahertz) band than in the microwave (gigahertz) band, according to an article published in June 2015 in the Journal of Geophysical Research – Space Physics.
In addition to the observations made by Kaufmann’s group, Matthew Penn and his team have recorded emissions at 30 THz and 60 THz. After updating the energy profile of the flares with the new data, the figure takes the form of the letter W, rather than the U as suggested by Castelli and Aarons in the 1970s. This signature suggests that the flares coincide with intense energy flows in two radiation bands: a lower energy flow of radio waves and higher-energy submillimeter radiation in the infrared range whose limit is still unknown.
One possible source of this energy is electrons accelerated to speeds near the speed of light in dense regions on the Sun’s surface that, when stopped by intense magnetic fields, could emit radiation in the infrared band. Another is that these accelerated particles could heat up the plasma in the chromosphere which, as a result, would respond by releasing radiation. “At the moment, no one can explain this double spectrum,” says Kaufmann who, in addition to funding from FAPESP, also receives support from the Ministry of Science and Technology, the Mackenzie Research Fund and the United States Air Force Science Office.
“We still don’t have enough examples of the terahertz events observed to explain how emissions can occur in such a broad range,” says Stephen White, of the Air Force Research Laboratory and a colleague of Kaufmann. “We think that this could reveal how the Sun accelerates high-energy particles.”
Although answers have not been found, Kaufmann is attempting to complete the curve with more information from more frequencies, in the hope that the data will help clarify the phenomena generating the flares. Recently, he and Marcon completed a new telescope, HATS (High Altitude Terahertz Solar Telescope), that will operate in the 0.85 THz and 1.4 THz bands in an observatory at an altitude of more than 5,000 meters in Famatina, in the Argentinean Andes. A more modern version of the detectors is also now ready, which should improve the observation capacity of the El Leoncito telescopes. For now, Kaufmann is anxiously awaiting the flight of the Solar-T. “We are relying on NASA,” he says. “But the Sun must also collaborate and produce flares during this period.”
Solar flare diagnostic in an unprecedented frequency range from microwaves to THz frequencies: challenges for interpretation (FLAT) (nº 2013/24155-3); Grant Mechanism Thematic Project; Principal investigator Pierre Kaufmann (UPM); Investment R$1,836,374.29.
KAUFMANN, P. et al. Bright 30 THz impulsive solar bursts. Journal of Geophysical Research – Space Physics. June 30, 2015.