EDUARDO CESARShortly after 4 o’clock in the afternoon on November 4 last year, one of the telescopes from the El Leoncito Astronomical Complex (Casleo), installed at an altitude of 3,000 meters in the Argentinean Andes, captured the signals emitted by the most intense solar explosion ever recorded. In one of the rooms of the observatory, protected from the 10º Celsius cold of the Andean summer, Brazilian physicist Pierre Kaufmann and Argentinean engineers Adolfo Marun and Pablo Pereyra kept their eyes stuck to the computer screen of the Solar Submm-wave Telescope (SST). At that moment, they were using the US$ 2.5 million telescope, built with funds from Brazil and Argentina, to observe a new kind of solar radiation. As soon as he saw the graph that formed on the monitor, Kaufmann imagined he had finally found T-rays, the form of radiation that he had started to look for 20 years before, as soon as he noted its first signals in 1984, at the Itapetinga Radio-Observatory, in Atibaia, in the state of São Paulo. Emitted in the solar explosions, these rays stand out for the frequency at which their waves vibrate. The rays captured by the SST’s 1.5-meter antenna exceeded the limit of 100 Gigahertz (GHz), until then the maximum frequency of energy in the radio band observed in solar explosions. During the solar explosion of the beginning of November, the team led by Kaufmann, from the Mackenzie Presbyterian University, of São Paulo, detected this radiation in two frequencies: 212 and 405z GHz, or 0.2 and 0.4 Terahertz (THz), the unit of measurement generally adopted, which explains the name of this radiation and locates it in the electromagnetic spectrum between radio waves and visible light. “The emission of this form of radiation is the phenomenon with the highest intensity, compared with that of other bands of energy released in solar explosions”, explains Kaufmann, the coordinator of the study that reports the identification of the T-rays in a solar explosion in the Astrophysical Journal Letters of March 10.
The quantity of energy produced by this radiation in a given space of time – that is, its intensity – is three to five times greater than radio emissions, the forms of radiation with a longer wavelength, and from 1,000 to 10,000 times greater than X- and gamma ray emissions, those with the shortest wavelength. This means that the radiation corresponding to the T-rays would be the brightest, should the human eye manage to capture all the bands of the electromagnetic spectrum with the same efficiency as it detects visible light. In spite of its intensity, one reason prevented physicists from observing the T-rays: the production of equipment capable of capturing radiation in Terahertz requires the mastery of technologies with restricted access. Another difficulty was that the majority of the theoretical models did not foresee the existence of radiation in this frequency band of the spectrum. “For these reasons, hardly anyone had tried to identify solar activity in the Terahertz band”, says the researcher from São Paulo, who in 1985 proposed the existence of solar radiation with frequencies higher than 100 GHz in an article in Nature.
Kaufmann noted that he was on the right track when he saw, four years ago, more consistent indications of radiation in the Terahertz band, captured at the start of the tests with the SST itself, the only telescope designed to observe the Sun in this radiation band, which emits waves with a length less than a millimeter (submillimetric). But there was a lack of continuous, ample and unequivocal signals, such as the ones that came last November, with the telescope now in regular activity. One hour before the signals appeared on the monitor at El Leoncito, the engineers had reformulated the computer program that converts the signals captured by the antenna, because, until then, it was not functioning as expected.
Better than X-rays
Two characteristic properties of T-rays – high intensity and low frequency – are turning this radiation into a candidate to be used in equipment intended for diagnosing diseases. As the T-rays vibrate much more slowly than the radiation from the other end of the spectrum, X-rays and gamma rays, they possibly produce less damage to the genetic material of living organisms, just as radio waves, emitted by nuclear magnetic resonance devices. There are now at least two companies created recently in Europe with the purpose of exploring the medical applications of T-rays, which besides being safer than X-rays for being charged with much less energy and not penetrating the body so rapidly, also offer better contrast between healthy cells and the sick ones. The physicists believe that the radiation identified by Kaufmann can be useful for detecting drugs and weapons, or even in fossil research, preventing the damage caused by excavating in the rocks.
Aside from its practical applications, T-rays should serve as a new indicator of the possible origins of the solar explosions, with as yet uncertain causes, and they may be able to contribute towards foreseeing the impact of these explosions on terrestrial telecommunications. Associated with these explosions – common at moments of intense solar activity, when the inversion of the Sun’s magnetic poles occurs – is the detachment of masses close to the surface of the Sun, which generates gigantic flames – associated with brightening or flares –, which launch into space clouds ten times the size of the Sun itself, made up of overheated and electrically charged particles, which hit our planet at a speed of up to 2,000 kilometers a second.
When there is an explosion on the Sun, you can expect problems down here. The explosions that occurred at the end of October and beginning of November 2003 – the most intense ever observed since physicists started to record these sudden releases of energy in the 1940s – switched off the electricity transmission network in Sweden, silenced cell phones in Argentina, damaged two Japanese satellites, and affected the functioning of the communications and navigation systems of aircraft and ships around the world.
On the basis of the intensity and on another trait of T-rays, the rapid pulses, which last from 100 to 300 milliseconds, Kaufmann believes that the radiation in the Terahertz band is produced by electrically charged atomic particles, accelerated to speeds close to the speed of light (300,000 kilometers per second). These particles are ultra-relativistic electrons, so called for showing energy of over 1 million electron volts, the unit of measurement of the energy of atomic particles. Accordingly, the T-rays, produced by electrons with hundreds of millions of electron volts, represent the energy released by the movement of these ultrarapid particles, interacting with the magnetic fields of the Sun. Oddly enough, this form of radiation also appears in experiments done in particle accelerators, equipment used in tests of atomic physics, capable of driving electrons to speeds close to the speed of light, making these particles produce electromagnetic emissions in the Terahertz band.
At the same time that they may explain the origin of T–rays, the ultra-relativistic electrons continue to intrigue physicists, since it is not known what kind of phenomenon could produce these particles in the Sun. Be that as it may, the hypothesis for forming particles with such a high speed, whether as a cause or as a consequence of the explosions, suggests some adjustments to the way of looking at this 5 billion year old star. It used to be believed that the energy of the particles resulting from solar explosions would not exceed 1 million electron volts. As a result of the movement of these electrons, called slightly relativistic, form the radio emissions.
Now, the discovery of the radiation that indicates the existence of particles with hundreds of millions of electron volts is creating an alternative for explaining the origin of X-rays and gamma rays. Regarded as the most energetic forms of electromagnetic radiation produced in solar explosions, X-rays and gamma rays may not be the result merely of the collision of clouds of high-energy electrons with the denser regions of the Sun. According to Kaufmann, these two kinds of radiation could also result, like T-rays, from the shock of the clouds of ultra-relativistic electrons with the radiation that the electrons themselves generate, a phenomenon known as the inverse Compton effect, used to interpret explosions on far larger scales, such as the active nuclei of galaxies. “The radiation in the Terahertz band was the link that was missing in order to restudy the origin of X-rays and gamma rays”, says he. Both in the Sun and in other stars – there are 100 billion of them just in our galaxy, the Milky Way – there is still a lot to explore. “I wouldn’t be surprised if many more solar explosions appear, with the continuous observation carried out in a higher band of T-rays, between 5 and 100 Terahertz”, comments Kaufmann, whose work has been limited to the frequencies of 0.2 and 0.4 Terahertz. But, according to the researcher, only satellites in space can capture solar emissions in frequencies higher than 1 Terahertz. “And there is still not one of them equipped for this task.”
Applications of the Solar Submm-Wave Telescope (SST) (nº 99/06126-7); Modality Thematic Project; Coordinator Pierre Kaufmann – Mackenzie Presbyterian University; Investment R$ 137,496.00 and US$ 83,061.06 (FAPESP), R$ 30,000.00 (CNPq), R$ 65,000.00 (MackPesquisa)