By Marián Moreno Otero

 “There are many reasons for studying the Sun. We can say that what occurs on Earth is often linked to the behaviour of the Sun”

 “Studying the Sun allows us to calibrate observations, models, and theories which can then be applied to other stars”

 “There are more Sunspots and solar storms during the maximum of the solar cycle, and they do significantly affect our planet”

Juan Manuel Borrero studied astrophysics at the University of La Laguna, and since he left for the Max Planck Institute for Solar System Research (Germany) in 2001 to work for his doctoral thesis his scientific activity has been oriented towards the study of sunspots. At the present time his research is centred on the development of a numerical method which can allow us to determine the magnetic field in the solar atmosphere in three dimensions. Although this study was started in the past, under the name of Inversion of the Equation of Radiative Transport for Polarized Light, the magnetic field which is predicted, is unfortunately inconsistent with a number of fundamental equations in physics, such as Maxwell’s equations and the equations of magnetohydrodynamics. So his objective is to correct these discrepancies. Borrero assured me that “The IAC is the best place to carry out this research” because within its solar physics group some of the scientists with the greatest experience and knowledge of the inversion of the equation of radiative transfer are working.  “They have been pioneers in this field, and their reputation at world level in incomparable”. Borrero points out that for him personally this is a magnificent opportunity to meet again and, after 15 years, to work with many of those who were his teachers during his period as a university student in astrophysics.

Question: Why is it important to study and understand the evolution of the Sun? What are the models which let us understand its physical properties?

Answer: There are many reasons to study the Sun. We could say that what occurs on Earth is often linked with the behaviour of our own star. On the sun there are physical conditions: pressure, density, temperature, and magnetic field, which cannot be achieved in the laboratory. So that we use our star to confirm, (or not) aspects of physics studied in the different fields, such as quantum mechanics, plasma physics, and hydrodynamics, which cannot be verified here on Earth. Finally the Sun allows us to calibrate observations, models and theories which can then be applied to other stars.

The determination of the physical properties of the Sun, or of any other star, is based on the light which they send us. The techniques most widely used are spectroscopy and spectropolarimetry, which help us to interpret the light they emit, and specifically how this varies as a function of wavelength.

The light we receive from the Sun comes only from its outer layers, including its atmosphere. Its interior is not directly accessible. However astrophysics has developed various methods (helioseismology, mean field dynamo theory, surface transport dynamo theory etc.) which allow us to derive the properties of the Sun from what we observe at its surface.

Q: How do  solar cycles work? What are the objectives of studying solar activity at different wavelengths? And what about solar polarization?

A: The most observable effect of solar cycles is the number of sunspots which appear on the solar surface, which reaches a maximum every 11 years. As the sunspots are produced by the effects of magnetic fields, this means that the magnetic field at the solar surface varies with a period of 11 years.  However until now nobody has been able to give an exact explanation of how the solar cycle is produced. It is a big unknown, and a very active field of research. At the present time the most widely accepted theory is that which explains the cycle in terms of models related to the observation that the Sun rotates more rapidly at the equator than at the poles

Observing the Sun at various wavelengths is important because by changing the wavelength (for example from the visible to the ultravioleta) we observe layers of the atmosphere at different depths. This way we can infer the physical parameters in three dimensions. In addition, many phenomena which occur on the Sun, such as the Coronal Mass Ejections (CME) emit at very specific wavelengths, principally in X-rays. So it is important to cover the full range of the electromagnetic spectrum.

The magnetic field at the solar surface, (which as we have said, varies with a periodo f 11 years) modifies th estate of the polarization of the light. Studying this polarized light, by spectropolarimetry, es a very important technique which is often applied and allows us to determine the strength of the magnetic field.

Q: What effects are produced by the activity, the sunspots, and the solar storms on the Sun itself? Do they affect our planet and present technology?

A: The effects of these phenomena, although real (for example after a solar storm the configuration of the magnetic field at the surface changes notably), are limited. However we use them as tools to determine and to understand what goes on in the interior, because they are consequences of internal processes.

But sunspots and solar storms are more plentiful during the maximum of a solar cycle. They do indeed affect our planet. Solar storms not only cause auroras, but they degrade our satellites, and therefore affect communications. They can cause overloading of the electrical system, producing blacouts and losses of power to factories, generators, and to transport etc. Although it is unusual there have been cases of aircraft flying near to the Arctic which have had to be deviated because the levels of radiation arriving at the Earth after a solar storm could be dangerous for the passengers. In general the effects grow in importance as our society advances technologically.

Q: Could use the Sun as a laboratory to copy its energy generating method to produce clear energy? If so how could we do this?

A: No method for generating energy can be considered completely clearn. Al of them contaminate in one way or another. The Sun produces its energy by nuclear fusion, while here on Earth we use fission. We need to note that if we could use fusion as a way to produce energy, we would also generate radioactive waste (although in much reduced proportions compared to fission). One advantage of fusion is that for fuel we would be using elements which are very abundant naturally, such as hydrogen or deuterium. On the contrary fission uses very scarce elements, such as uranium. Also fusion would be much safer.

In theory fusion is possible. The proof is that the Sun is using it all the time, burning hundreds of millions of tons of hydrogen per second to produce its energy, However we are still unable to generate energy this way due to the enormous technical difficulties involved. In spite of all the money invested up to now to develop fusion, it is expected that the first reactor of this type to generate energy continuously will not be on line until 2050, and in any case it will have to work in a commercial way at a competitive price.

Q: What will the construction of the European Solar Telescope (EST) imply compared to other telescopes of the same type? What is this project about?

A: The European Solar Telescope will have a primary mirror of 4m in diameter. If we consider that the largest present day solar telescope has a diameter of 1.6m the EST will enable us to make a qualitative leap forward in the quality of the data we obtain when observing the Sun, and will greatly improve our understanding. In particular this telescope will allow us to measure the physical properties of our star such as its magnetic field, obtaining higher precision, and greater resolution, so more details. Its construction implies a big economic investment, and entails such a major technical challenge that it needs the joint action of many European countries. Spain, and the IAC in particular, with its important and prestigious Solar Physics group, is one of the centres which has worked most strongly so that it could be installed in the Roque de los Muchachos Observatory (Garafía, La Palma) or in the Teide Observatory (Izaña, Tenerife)