According to the Agency of Scientific Information, the Institute of Physics named after P. Lebedev summed up the results of space experiments held in 1994-2009 within the framework of the Koronas program (complex orbital circumterrestrial solar activity observations) effected by the RAS that envisaged launching of 3 new sun-oriented satellites to the orbit.

Extraatmospheric studies of the nearest star has been carried out for 50 years. On the initiative of the well-known Soviet physicist Sergei Mandelshtam, Corresponding Member of the USSR AS from 1979, the second artificial satellite of the Earth* launched on November 3, 1957, was furnished with the device registering X-ray short wave radiation. The equipment designed at the Laboratory of X-Ray Astronomy of the Sun, FIAN, registered the first monochromatic pictures of the disc and the corona attached to it in the X-ray spectrum with high spatial and temporal resolution. As a result, scientists discovered a new class of hot plasma structures with sizes varying from tens to hundreds of thousands of kilometers and the temperature exceeding 5 mln пїЅC. They also ascertained some other scientific data: the temperature of the external boundary layer of the Sun called the photosphere is about 6,000 пїЅC; while moving away from the surface it drops to about 4,000 пїЅC, but then suddenly rises. In a thin, the so-called transitional layer (in a solar scale), where the plasma density reduces by some orders the temperature reaches 1.5 mln пїЅC.

"Why is the corona hot? We do not know,"--Alexander Urnov, a leading research assistant of the Spectroscropy Department, FIAN, tells us. "An enormous amount of energy is reserved in the magnetic field of various mag-netoplasma structures. Explanation of the mechanism

* See: Yu. Markov, "Breakthrough Into Cosmos--Our Glory and Pride", Science in Russia, No. 2, 2007.--Ed.

of its isolation and transformation into other forms (accelerated particles, plasma flows, heat, electromagnetic radiation)-is a fundamental task of astrophysics." In a word, answers to these and many other questions on the structure and "vital functions" of the Sun are still open. One of the reasons is lack of data on the physical characteristics of magnetoplasma formations (temperature composition, electron density, etc.). To understand heating mechanisms of the corona and phenomena of solar activity, it is necessary to increase significantly spatial and temporal resolution of the research equipment. Today, we can distinguish objects of about a second of the arc in the angle measure, which is about 1,000 km in linear dimensions. While we need to make out objects smaller by at least one order.

Research assistants of the RAS Institute of Physics have been studying the short-wave radiation of the corona since 1958. The "depicting spectroscopy" method applied to register images in narrow spectral intervals made through special filters has been used by scientists for more than 20 years. But this method has certain disadvantages that significantly complicate the process of determining temperature composition of the corona plasma. To overcome these difficulties, there has been designed a unique device called spectroheliograph that makes it possible to get monochromatic pictures of the whole Sun disk and its lower corona in the X-ray spectrum and 160 lines of vacuum ultraviolet. Such spectral information (in addition to spatial and temporal) is really important to develop plasma models of solar activity phenomena that help understand processes of their formation and development.

In 1994 and 2001, within the framework of the said Koronas program, two solar observatories were put into orbit. The first observatory was destined to carry out the Terek experiment: scientists observed the Sun in extreme ultraviolet and X-ray areas of the spectrum, carried out systematic studies of the solar plasma in the wave length areas of 132, 175 and 304A with the spatial resolution of some angle seconds, as well as got first monochromatic pictures of the Sun in the ranges of 8.41-8.43A and 180-209A.

The second satellite served as a base to run tests of the Spirit gear that turned out the most successful in the history of national science. During 4 years of observations, scientists obtained about 300,000 high-precision pictures in nine spectral channels that characterize the spatial structure and dynamics of plasma in the atmosphere

of the closest star to the Earth in the temperature range from 70,000 to 10 mln ºC, discovered new structures and phenomena occurring in hot plasma, assessed the role of plasma in the thermal energy budget of solar flares (up to 80 percent).

The third satellite Kownas-Photon* placed in orbit in 2009 within a year transferred really priceless information in different monochromatic beams with a high spatial and record-breaking temporal resolution. Scientists outlined the dynamics of plasma structures in an almost unknown area of the Sun-lower corona-in natural ultraviolet radiation, not in the diffused light. Pictures of those areas of the Sun where so-called coronal discharges of masses of an unknown origin are observed are regularly registered by the SOHO laboratory (joint project of the European and American Space Agencies) launched on December 2, 1995. To get these photos, scientists were forced to cover both the solar disk and the lower corona to protect the equipment from "going blind" from radiation, more powerful than the diffused light by tens of millions times, i.e. to observe only what is going on at a distance of two radiuses away from the Sun. Employees of the Institute of Physics designed a special telescope--a coronagraph, covering the solar disk, but at the same time perfectly distinguishing its rarified corona. Greatly dynamic in relation to measured range intensity, this device made it possible to study previously unexplored areas.

On the basis of data obtained by Koronas satellites, the institute developed new methods of quantitative diagnostics of plasma, determined spatial and temporal characteristics of hot structures and classified them. As a result, physicists could develop a model of the observed phenomenon--so called "spider" or a large-scale long-living formation identified for the first time on the monochromatic X-ray pictures in the course of the Spirit experiment. As a whole, space observatories furnished with hi-tech equipment clarified some fundamental problems of solar physics.

* See: "Solar Flares", Science in Russia, No. 2, 2009.--Ed.

According to the materials of the FIAN-Inform Scientific Information Agency, October 28, 2010 Photos from the web-site of the Koronas space program Prepared by Marina KHALIZEVA