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astrophysics

Pulsating Sun

Group from the IAG discovers that variation in the diameter of the sun is ten times less than used to be believed

nasaSOHO satellite: complementary techniquesnasa

For almost four hundred years astronomers, observing the diameter of the Sun, have known that it may increase or diminish. What they were unable to establish for sure was the amplitude of this variation. Last year, after a long path, this doubt has started to be cleared up: a study carried out by researchers from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) of the University of São Paulo (USP) is providing data that is closer to the reality. Using more precise methods for observation and measurement, developed by themselves on the basis of satellite images, they found that the oscillation of the radius of the sun may be as much as 15 kilometers – far less than used to be imagined.

Before this, they worked with the idea that there was a 150 kilometer variation – for a total diameter of the sun of roughly 1,400.000 kilometers.The study also showed that the energy responsible for the variation in the diameter and luminosity of the Sun does not come from its core, as used to be thought, but for the more external layers – specifically, from the sunspots, which indicated how much the diameter of the sun may vary as a result of the star’s period of activity.

The discoveries come at the right moment. Between 2000 and 2002 is the solar maximum period, repeated every 11 years, which is characterized by a more intense solar activity, caused by the variation in the sun’s magnetic field. The results may help to understand the influence of the diameter of the sun on this peak of activity and on the climate on Earth: the increase or decrease in the luminosity of the sun can cause changes in temperature and periods of glaciation and drought, as well as interference in telecommunications in a general way.

The idea that led to these results arose when Professor Nelson Vani Leister and post-doctorate student Marcelo Emílio, under his supervision, realized that they had to leave behind techniques for measuring the diameter developed from observations from Earth. Working with this traditional method, Leister did the first historical series of measurements of the diameter of the Sun, within the course of twenty years, from 1974 to 1994. Done in partnership with French researchers, this survey pointed to an oscillation of around 150 kilometers in the Sun radius. It was a step forward, but the figure obtained was not very reliable: the suspicion was that refraction and turbulence in the atmosphere of the Earth could have interfered in the final results.

Then came the inspiration: why not try to observe the Sun from space? The desire met up with the possibility. In January 1999, Emílio, then studying for his doctorate, received an invitation to spend one year and three months at Hawaii University, United States. It is one of the centers where they work with images of the Sun captured directly by SOHO (the Solar and Heliospheric Observatory), launched in 1995 through a partnership between the European (ESA) and American (NASA) space agencies. The opportunity was decisive to arrive at more conclusive data on the variation of the diameter of the Sun.

In orbit roughly 1,5 million kilometers from the Sun, SOHO provides material for studies by around 500 researchers in 20 countries. It sends information on the phenomena that takes place in the Sun’s corona, eruptions, winds, and variations in its magnetic field. It was designed to follow up these activities, not the variations in the diameter of the sun, although this was feasible, provided that there were suitable computer programs for analyzing the images that arrived. The data was there: all that was needed was someone capable of deciphering it.

Interference
That was when Brazilian creativity came into action. In his baggage for Hawaii, Emílio took his computer along, with the programs he had developed and used in research with Leister, already imagining that they could be useful to compare with the results of observations from the Earth. He had the way of interpreting the images, but the work would not be easy. The fact that SOHO had not been designed to record directly the measurements of the diameter of the Sun caused some problems. From the start, the analysis of the images collected between July 1996 and July 1998 revealed that there really was some oscillation in the diameter of the Sun.

But there were three obstacles that could be masking the results: the image of the Sun was registered from varied distances, deliberate changes in the focus of the instrument would increase and diminish the diameter of the Sun artificially – which could cause false impressions – and the very temperature of the lens was not constant. These effects had to be eliminated from the observations. At this moment, Emílio turned to the programs for reading and analyzing images that he had brought from Brazil. It was as if the computer itself were following the movement of the Sun, marking the points of measurement and taking decisions for its own account. For safety, the researcher assessed, one by one, the images of around 2 million limbos (limit points) of the Sun.

SOHO generates 120 images a day, each one of them with 16 limbos, which corresponds to 1,920 points a day. In the three years he took to analyze them, Emílio observed roughly 2 million limbos. It was this work of comparison, based on mathematical calculations, that made it possible to separate the wheat from the chaff and to eliminate, in succession, the interference caused by the distance from the satellite to the Sun, by the focus variation, and by the effect of the variation in temperature to which the satellite is submitted.

What was left over was the oscillation in the diameter of the Sun. “We reached that point that we call the higher limit of variation, the maximum oscillation of the diameter of the Sun. It may be even smaller, if in future other unknown elements are identified, but it will never be bigger than this”, says Emílio.

Watershed
After running all this course, and now free from the interference caused by the Earth, the researchers managed to show that the variation in the diameter of the Sun is ten times less seen from space than when observed from the surface of the Earth. The results shook the prevailing suppositions. The difference is a significant one, which led the researchers to believe that what they were determining from Earth was not correct. “Between space and the surface, there are numerous effects, many things that act and modulate, and that we do not know properly yet”, explains Leister. His conclusion recalls the classic words of Hamlet: “There are more things between heaven and Earth than our vain philosophy supposes”.

In reality, Leister explains, the Earth’s atmosphere, which is not homogeneous, introduces effects that divert the beam of light and disturb the measurements. Climatic conditions – variations in temperature, humidity and pressure – also have an influence on refraction. The group’s findings represent a watershed in the studies on the variation of the diameter of the Sun. In November 2000, Emílio’s work appeared in the Astrophysical Journal, in an article signed by him, Jeffrey Kuhn, of Hawaii University, plus Rock Bush and Philip Scherrer, from Stanford University.

Shortly afterwards, in the March 15th 2001 issue of the Nature magazine, Douglas Gough, from the Cambridge Institute of Astronomy, recognized the importance of the work. “This observation is certainly not the first one to detect small variations in the radius of the Sun, but is should be the first to survive the test of time”. With all this repercussion, research carried out in the area may come to be divided between before and after SOHO. Leister has no pretensions for it to be like that, but admits that the changes have really had a great impact.

The assessments made from space have become a point of reference for new enterprises, and lay the foundations for research more directed, for example, towards ascertaining the interference from magnetic fields – which cause sunspots – in the variation of the diameter of the Sun. In this area, with similar research, there are researchers working at the National Observatory, in Rio de Janeiro, and at the Radioastronomy and Space Applications Center (CRAAE), of the Mackenzie Institute, in São Paulo. In the international scenario, in the front line are specialists from the Centre d’Etudes et de Recherches en Géodynamique et Astrométrie (Cerga), in France; from Yale University, United States; and from the Locardo Observatory, Switzerland.

Astrolabe and camera
“I call this whole process Brazilian-style discoveries”, says Leister, who started doing his measurements in the 70’s with a solar astrolabe. Made up of a horizontal telescope with a 1,2 meter focus and a 10 centimeter lens, the astrolabe was at the Abrahão de Moraes Observatory, in Valinhos (SP). The main technique was a simple one: the interval of time is measured that the Sun takes to pass along an imaginary line in the sky, defined by the instrument. The drawback is that the human eye, although it is an excellent instrument for observation, has its limitations, and variations may occur, for example, between one researcher and another.

In the first half of the 90’s, in an attempt to overcome this difficulty, the IAG/USP acquired a sophisticated CCD (Charge Coupled Device ) camera, which was coupled to the astrolabe, and at that time already transferred to the IAG in Água Funda, in the capital. The images captured by the camera were transmitted to a computer. The disappointment: even with all this apparatus, the results of the new observations showed that the gains in precision, in terms of the variation of the diameter of the Sun, had been minimal.

In compensation, it was this whole process of building up knowledge that allowed Emílio to develop his techniques and his image treatment programs. A little later, they were to be fundamental for the success of his doctorate, which lasted 50 months – a relatively short period in the career of a researcher.

The projects
1. Solar Astrolabe (nº 92/03762-0); Modality Regular research benefit line; Coordinator Nelson Vani Leister – IAG/USP; Investment R$ 5.918,32
2. Study of the Variability of the Diameter of the Sun (nº 97/07176-2); Modality Grant for doctorate; Coordinator Nelson Vani Leister – IAG/USP; Investment R$ 64.238,72

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