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Henry M. Robert

How we study the Sun these days: what we know and what we do not...

As told by Corresponding Member of the National Academy of Sciences of Ukraine Natalia Shchukina
20 March, 2018 - 11:36

“What is the Sun?” I was puzzled to hear this question from Natalia Shchukina as we were beginning our conversation. I see it in the window right now. It is a star, a yellow dwarf. But when one communicates with a scientist of this high caliber, one is afraid to say a stupid thing.

Shchukina has worked at the Main Astronomical Observatory (MAO) of the National Academy of Sciences (NAS) of Ukraine for many years. Since 2002, she has been in charge of the MAO’s department of physics of the Sun. The astronomer is a corresponding member of the National Academy of Sciences of Ukraine, doctor of physical and mathematical sciences; she has participated in many international projects, cooperating, in particular, with the Institute of Astrophysics in the Canary Islands in Spain and the Astronomical Institute of the Utrecht University in the Netherlands.

Some time ago, Shchukina and her colleagues from the Netherlands and Norway proposed a mechanism that let them explain the reasons for the glow exhibited by a new class of spectral lines that had been discovered in the distant infrared spectrum of the Sun in the early 1980s. This has provided great opportunities for diagnosing solar magnetic fields with the help of those lines. Recently, she and her co-authors from the Institute of Astrophysics in the Canary Islands were able to obtain previously unknown data on the topology and energy of small-scale magnetic fields in the solar photosphere. And thanks to the efforts of Shchukina and her colleagues Serhii Osypov and Roman Kostyk, the solar telescope ATsU-5 has been reconstructed, making it a Top 4 facility globally in terms of spectral resolution.

We discussed with the scientist the achievements of Ukrainian solar researchers, what the study of our universe revolved around and why warnings about the magnetic storms alleged to happen, say, next month were nonsense. But still, we talked about what the Sun is first.


“The Sun is an ordinary star, a yellow dwarf,” Shchukina said to start the conversation. “It is yellow because if you look at the distribution of energy in the spectrum, the maximum energy output falls on the yellow color segment. It is a dwarf because it is small in size. The visible surface of the sun, called the photosphere or the sphere of light, has a temperature of about 5,700 degrees. The radius of the Sun is about 700,000 kilometers, the density of its core is about eight times that of gold, and the density of the outer envelope, that is, the photosphere and chromosphere, is 10,000 times less than the density of air on Earth.



“Our star is a self-regulating thermonuclear reactor that provides for long and steady energy production. The most important reaction, namely the transformation of hydrogen into helium in the core of the Sun, has lasted billions of years. In the core, these reactions form quanta, which after long wandering, so-called diffusion, in the radiative zone reach the surface of the Sun. On average, a quantum reaches it in about a million years. Sunlight which you are seeing is very old. You did not exist when it appeared, and intelligent life on Earth did not either. The core and radiative zone occupy two-thirds of the radius of the Sun.

“The next layer is the convection zone. Here energy is transferred not by radiation, but by convection. Huge streams of hot gas rise upwards, where they transfer their heat, and the cooled solar gas goes down. It seems that the solar matter is boiling and getting stirred like some viscous granular mass on the fire. The convection zone reaches the visible surface of the Sun itself, called the photosphere.

“Between the radiative and convection zones, there is a thin layer called the tachocline. It is there that a magnetic field is created that shapes the activity of the Sun.

“The outer layers are the photosphere, the chromosphere, and the corona. The photosphere is a very thin layer, about 500 kilometers deep. The temperature in its deeper layers is about 10,000 degrees, and approaching the upper limit, it drops to 4,500 degrees. The photosphere consists of granules and intergranular lanes.”

These granules and intergranular lanes are similar to boiling rice porridge.

“But the size of the seeds in this ‘porridge’ is about 700 kilometers each. The surface of the photosphere seethes, resembling a boiling liquid.

“Above the photosphere, there is the chromosphere. It is about 10,000 kilometers deep. This is what we see around the edges of the Sun during eclipses. In this layer, the temperature starts to rise again. It rises to almost 20,000 degrees. And in the next layer, the corona, which is about one solar radius deep, the temperature is even higher, reaching a million degrees. Why does in the lower layer of the Sun’s atmosphere, that is, in the photosphere, the temperature drop with the altitude, but above that layer, in the chromosphere and the corona, it is rapidly increasing? There is no definitive answer to this question yet.”

I saw it mentioned that the MAO’s studies helped to resolve the issue of chromospheric heating. Tell us more about Ukrainian scientists’ contributions to studying this problem.

“Without knowledge of the sources and mechanisms of heating in the outer layers of the Sun, we cannot understand the causes of the cyclic activity of the Sun, and, therefore, it is impossible to obtain reliable forecasts of the space weather, which affects everything that is happening on Earth.

“Studies conducted in the department of physics of the Sun of the MAO of the NAS of Ukraine in cooperation with the Spanish colleagues from the Institute of Astrophysics in the Canary Islands have brought us closer to understanding the problem of energy accumulation and transfer from the lower layers of the atmosphere of the Sun, the photosphere, to the upper layers, the chromosphere and the corona. The results of this study were published in Nature, which is one of the most prestigious scientific journals.

“We have shown for the first time that the energy of turbulent magnetic fields in a calm atmosphere of the Sun can be substantially larger than previously assumed. This energy is large enough to heat the chromosphere. The computer modeling performed by our former employee Olena Khomenko, who is currently working at the Institute of Astrophysics in the Canary Islands, has shown that magnetic energy can be transferred to the chromosphere by the common diffusion of electrons and ions.”


I read on the MAO’s website that the horizontal solar telescope ATsU-5 became one of the most powerful in the world in spectral resolution after its modernization. What research does it enable?

“Since 1966, the horizontal solar telescope ATsU-5 of the MAO of the NAS of Ukraine has been involved in the implementation of several observation projects, including international ones. When implementing these programs, a number of important scientific results have been obtained.

“Firstly, we have created a self-consistent system of forces [the oscillator’s force is the probability of absorbing electromagnetic radiation at transitions between the energy levels of an atom or molecule. – Author] which has come to be widely used in all branches of astrophysics where quantitative spectral analysis is carried out.

“We have also created a spectrophotometric model of solar radiation in absolute energy units, which is used in astrophysics, meteorology, geophysics, and aeronomy to solve a set of applied problems. For example, it is used when simulating the interaction of solar radiation and Earth’s atmosphere, or creating solar radiation simulators and spectrophotometric standards.

“Thirdly, with the help of observations on the ATsU-5 telescope, telescopes of the DIFOS series were set up for extra-atmospheric studies of global fluctuations in the brightness of the Sun. These fluctuations contain information about the inner structure of the Sun. The DIFOS telescopes were launched into the Earth orbit and successfully worked onboard the international space stations CORONAS-I in 1994 and CORONAS-F in 2001-05.

“The results obtained on the ATsU-5 telescope have been published in foreign journals with high impact factor: Nature, Astrophysical Journal, Astronomy and Astrophysics, Monthly Notices of the Royal Astronomical Society, Solar Physics.

“In 2011-12, we reconstructed the telescope ATsU-5, improved its hardware and software, and repaired the building which houses it. At present, the ATsU-5 telescope is a unique scientific facility that is best suited for monitoring the calm Sun. Among its special features, I would like to name the abovementioned high spectral resolution (R~430,000), which places it among the four best-performing telescopes in the world.

“Another feature of the ATsU-5 is long-term metrological stability. Today there are two monitoring programs for the Sun’s long-term variations. The first is the American SOLIS observation program for long-term synoptic optical studies of the Sun. They have been using the solar telescope of the Kitt Peak National Observatory in the US since 2006. The second one is the Ukrainian observation monitoring program, which has been implemented since 2012 with the Solar Horizontal Telescope ATsU-5 at the MAO of the NAS of Ukraine.

“The SOLIS aim is long-term monitoring of the Sun as a star. The aim of the program of the MAO of the NAS of Ukraine is to perform long-term monitoring of the calm component of the Sun’s atmosphere, variations of which are almost an order of magnitude smaller than the variations of the Sun as a star. Today, these variations are still almost unexplored.

“One of the important sources of data on the variations of the Sun is long-term, encompassing the 11-year cycle of solar activity, observation of changes in the parameters of the Fraunhofer lines [these absorption lines are visible against the background of a continuous spectrum of the Sun and stars. – Author] in the spectrum of calm areas of the solar surface.

“Monitoring performed during 2012-17 on the ATsU-5 telescope showed that the depth and half-width of spectral lines in calm areas of the Sun respond to the modulation of the general magnetic field caused by the 11-year cycle of solar activity. We explain the behavior of these parameters by variations in the temperature of the calm photosphere of the Sun during the 11-year cycle: the photosphere of the Sun becomes hotter at maximums of solar activity.”


In general, what do we need the monitoring of the Sun for?

“Firstly, we need it to understand how solar activity affects space weather, namely, the ionosphere, magnetosphere, radiation belts, and the ozone layer, as well as the Earth’s biosphere and social life on our planet. Information on this will help prevent the negative effects of solar activity phenomena on human health and society’s activities.

“Secondly, the results expected to be obtained during monitoring of variations in the physical parameters of the calm atmosphere of the Sun with an 11-year cycle are important for solving major problems of solar physics. Some of them are the problem of the internal structure and evolution of this star and its magnetic activity, the problem of energy interaction in the system ‘the photosphere – the chromosphere – the crown’ and the heating of the latter. It also allows us to study the mechanisms that cause eruptive phenomena in the Sun, the causes of solar activity cycles, and so on.”

When you completed the modernization of the telescope, in 2012, a new 11-year solar observation cycle was to begin. We have already entered the second half of this cycle. How would you characterize it?

“To be more precise, the current 24th cycle of solar activity began in 2009. We started our monitoring program shortly before the first maximum of this cycle in 2012. Since that year, observations have been performed from March to October every day, whenever the weather conditions allow. The total number of observation days from 2012 to 2017 exceeded 340 days.

“We are currently approaching the minimum of the 24th cycle. The level of activity of the Sun in this cycle is four times lower than the maximum values recorded over 260 years of continuous observations of the Sun. In other words, solar activity is approaching a minimum similar to the Dalton Minimum, which was observed from 1790 through 1830. As a reminder, the Dalton Minimum and the better known Maunder Minimum, which occurred from 1645 through 1715, coincided with global coolings of the climate in the 17th and 19th centuries.”


Recently, The Astrophysical Journal Letters published a study by researchers at the University of Chicago, according to which the Solar system could have formed in a shell, a kind of bubble, around a giant dead star. How do you feel about this hypothesis? What is still unclear about the origin of the Solar system?

“There are at least two theories dealing with how the Solar system could have formed. None of them can explain all the observed facts.

“According to the commonly accepted theory, our Solar system was formed about five billion years ago as a result of a supernova explosion. Due to this explosion, a gas and dust nebula appeared, and it was from it that our Sun was formed later.

“It is known that supernovas produce the same amount of isotopes called aluminum-26 and isotope iron-60. At the same time, in meteorites left over from the early Solar system, there is an excess of the isotope aluminum-26 and a shortage of the isotope iron-60. Scientists at the University of Chicago have shown that this fact can be explained by assuming that our system was not formed by the explosion of a supernova, but by the explosion of a Wolf-Rayet star, which was 40-50 times larger than the present Sun.

“It is believed that the Wolf-Rayet stars produce a variety of chemical elements that get blown away from their surfaces by stellar winds. Computer simulation has shown that as a result of this process, so-called gas bubbles with an increased content of the isotope aluminum-26 and a reduced content of the isotope iron-60 are formed over millions of years around the Wolf-Rayet stars. Shells of such bubbles and dust and gases that accumulate under them are the ideal environment for the production of new stars and the formation of planetary systems similar to our Solar system. Meanwhile, the Wolf-Rayet stars themselves end up either exploding as supernovas, or collapsing directly into black holes.

“Astronomers believe that approximately 1 to 16 percent of all Sun-like stars could have appeared as a result of such a scenario playing out.”

How does the study of the Sun help to study the formation of chemical elements after the Big Bang, the evolution of galaxies and stars in general?

“Employees of the department of physics of the Sun of the MAO of the NAS of Ukraine in cooperation with researchers from Spain, the Netherlands, Norway, the US, and Australia have conducted a series of studies to determine the chemical composition of the Sun and stars.

“Calculation of the content of chemical elements in the Sun, performed by a number of researchers in the early 2000s, showed an abnormally low metallicity of the Sun. This contradicted the data of helioseismology and the theory of the internal structure of the Sun. To solve this problem, my colleagues and I recalculated the content of carbon, nitrogen, silicon, and iron in the solar photosphere. The last two elements are used as the standard in determining the metallicity of the Sun and meteorite content. These studies have shown that changing the content of iron and silicon in the Sun can be avoided if one takes into account a number of physical effects that were not taken into account in previous studies.

“Another important achievement is the study of the chemical composition of stars that were formed at various stages of the evolution of the Universe. The employees of our department have calculated mathematical ratios for a large grid of stellar atmosphere models that allow us to estimate the content of lithium, oxygen, and iron depending on the effective temperature, gravity acceleration, and metallicity. These results are important in solving such fundamental questions of astrophysics as the origin of the Universe and its evolution, the nucleosynthesis of elements during the Big Bang, the evolution of galaxies and stars, the internal shape and structure of the atmospheres of stars and the Sun.”


From time to time, both quality and suspicious websites carry stories about abnormal solar activity which affects the lives of earthlings. How justified are claims about anomalies on the Sun impacting people’s lives?

“There is no abnormal solar activity and no anomalies on the Sun. We just have a well-known cycle of solar activity. On average, its duration from a minimum to a minimum is 11 years, but it can also be less than that at about 7 years, and more than that at up to 13 years.

“Through the interplanetary environment, solar activity affects the Earth, namely its ionosphere, magnetosphere, radiation belts, and the ozone layer. Its manifestations include ultraviolet and ionizing radiation of the Sun, sunshine, and emissions of matter during flares and coronal mass emissions.

“Ultraviolet and ionizing radiation, which reach Earth in eight minutes, ionize its atmosphere and destroy the ozone layer. High-energy charged particles ionize the upper atmosphere in about 100 minutes and change the geomagnetic field. Emissions of matter during solar flares and coronal mass emissions lead to geomagnetic storms after one-and-a-half or two days. Moderately strong storms occur approximately weekly, while the strongest storms occur much less frequently, that is, every two or three years.

“Extremely powerful magnetic storms can lead to the destruction of power systems and damage to transformers. For example, this was the case with the Canadian Quebec Blackout of 1989. They affect the spacecraft, their orientation, the links with them and their tracking systems. During the storms, astronauts and passengers on transcontinental flights can be exposed to dangerous radiation doses.

“A magnetic storm targets the lungs, the circulatory, cardiovascular, and autonomic nervous systems of a person. Most heart attacks and strokes occur precisely during magnetic storms. The main risk groups are patients with pathologies of the cardiovascular system, especially those who have suffered myocardial infarction, healthy people with functional strain, like astronauts, pilots of transcontinental flights, operators and dispatchers of power stations, airports, as well as children who still develop and have immature adaptation systems.

“Due to solar activity, computer systems as well as mobile and satellite communications can break down, radio waves get distorted and radio communications get interrupted, while airports have troubles functioning. There is a correlation between solar activity and a number of natural and social phenomena: changes in groundwater levels, recurrence of droughts and hurricanes, the number of earthquakes, epidemics, the increase of crime levels, etc.

“Since 1900, damages from disasters related to solar activity are estimated at trillions of dollars. Millions of people become victims of natural cataclysms every year. In order to predict in advance the influence of solar activity on the biosphere and the socio-economic system of Earth, it is necessary to simulate space weather, which is impossible without the long-term monitoring of the global changes in the Sun.”

Can we predict solar activity in advance?

“Today, the most reliable are probabilistic two-day and one-hour forecasts. Their reliability, respectively, is about 30-50 percent and about 95 percent. The two-day forecast is based on current observations of the Sun near the central meridian. The one-hour forecast uses direct measurements of plasma parameters and the magnetic field on the spacecraft.”


You recently held a public lecture in which you talked about the evolution of the Sun and research into it. Going by your observations, is the interest of Ukrainians in science increasing?

“My answer to your question depends on which categories of Ukrainians you meant. If we talk about ordinary Ukrainians and especially young people, then yes, their interest in science is growing. If we talk about the national leadership and all those working for the Ukrainian government, then my answer is ‘no.’

“You will not hear the word ‘science’ in the speeches of the national leadership. Probably because this category of Ukrainians is not interested in science at all. Moreover, the impression is that the government simply does not understand the decisive role of science in the successful development and prosperity of Ukraine. Hence the poor funding of research institutions, the lack of money for scientific equipment and heating, employees being forced into part-time work, and, consequently, the massive outflow of ‘scientific brains’ to foreign institutions, where science is a priority for society.”

Books by Stephen Hawking, translated into Ukrainian a couple of years ago, are quite popular in this country. They primarily deal with the evolution of the Universe, the emergence of black holes. Can you suggest us any popular science books about the Sun?

“Let me note that in addition to the books by Stephen Hawking, it is worth mentioning books by Carl Sagan and Iosif S. Shklovskii. Their books Cosmos and Intelligent Life in the Universe, published in 1980, have not lost their relevance since. We can learn from them about the evolution of the Universe, the formation of galaxies, the birth of life and intelligence, and the possibility of life existing in other planetary systems.

“As for the popular books about the Sun, a lot of interesting and beautifully illustrated books on astronomy for children were published in recent years. Information about them can be obtained, for example, on the Mamina Skazka website in the section Cosmos. I think these books are interesting for adults as well.

“Unfortunately, no really popular books discussing the Sun and targeting adult audience have been published over recent years. I can suggest, first of all, the book by the famous Ukrainian astronomer Ivan Klymyshyn Astronomy of Our Days, which was published in 1986. It describes the main concepts, notions, and laws on which observational and theoretical astronomy, astrophysics, and radio astronomy are based. The author describes virtually all known celestial objects: the Sun, the Moon, planets, stars, galaxies, pulsars, black holes, and quasars.”

In some way, the Sun is in full view of all earthlings, and you have studied it in detail for many years. What remains the biggest mystery about the Sun?

“There are no mysteries, there are scientific problems and tasks that are being solved or have yet to be solved. I will mention a few of them.

“Flares. We know a lot of details about this explosive process of energy emission in the Sun’s atmosphere, we understand the basic mechanisms of flares, but many details are still absent. For example, we cannot predict with high probability when and where the next flare will occur, or how strong it will be.

“The question of the causes of the high temperature of the corona, which reaches into millions of degrees, is still being debated. What heats it up? Can waves heat up the upper layers of the Sun’s atmosphere? If so, what type of waves is most effective for this?

“We are not yet able to predict solar activity in advance, not several hours or days, but a few years in advance.

“Also, we cannot predict in advance the influence of solar activity on the biosphere and the socio-economic system of Earth.”

By Maria PROKOPENKO, photos by Artem SLIPACHUK, The Day