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ISSN 1062 8738, Bulletin of the Russian Academy of Sciences: Physics, 2011, Vol. 75, No. 1, pp. 87­90. © Allerton Press, Inc., 2011. Original Russian Text © S.V. Kuzin, S.A. Bogachev, A.A. Pertsov, S.V. Shestov, A.A. Reva, A.S. Ulyanov, 2011, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2011, Vol. 75, No. 1, pp. 91­94.

EUV Observations of the Solar Corona with Superhigh Spatial Resolution in the ARCA Project
S. V. Kuzina, S. A. Bogacheva, A. A. Pertsova, S. V. Shestova, A. A. Revaa, b, and A. S. Ulyanova, b
a

Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia b Moscow Physicotechnical Institute, Moscow, 115409 Russia e mail: kuzin@lebedev.ru

Abstract--Observing the Sun's hot corona with sub second spatial resolution is important in solving a num ber of basic solar physics problems. The new ARCA satellite observatory under development at the Lebedev Physical Institute, Russian Academy of Sciences, will be first to provide images of the hot solar corona with a spa tial resolution of about 0.18 arcsec. Scientific and technical features of the observatory are discussed. DOI: 10.3103/S1062873811010163

INTRODUCTION The Lebedev Physical Institute, Russian Academy of Sciences, is currently working on a new project aimed at studying solar activity in the vacuum ultravi olet (VUV), ultraviolet (UV), and optical wavelength regions. The objective is to create and launch a new generation spaceborne solar observatory (tentatively called the ARCA) into an earth orbit, with the focus on basic geliophysical studies and the problems of moni toring active solar regions with super high spatial res olution. Due to its characteristics, the instrumentation can be installed and operate quite well on the KARAT unified satellite microplatform developed by the Lavoch kin Science and Production Association. The basic instruments of the observatory are two unique two mirror Ritchey­Chretien telescopes with a primary mirror 50 cm in diameter and an effective focal length of 15 m, allowing observation of Sun with a record high spatial resolution of 0.18 arcsec (130 km on the Sun's surface). This is an order of magnitude finer than the angular resolution of wide field of view solar observatories (e.g., STEREO and TESIS/CORONAS PHOTON) and finer by a factor of 6 than that of the TRACE dedicated high resolution solar observatory. Due to the large number of its covered spectral regions, this observatory alone and for the first time will allow the simultaneous observation of all layers of the solar atmosphere: the photosphere, the chromo sphere, the transition layer, and the hot corona, along with the plasma of solar flares. In its design, the instru ment will incorporate such new technologies as the creation of high precision multilayer coated aspheri cal mirrors over 30 cm in diameter, coatings of CCD arrays with multilayer filters, the creation of image sta bilization system, and so on. The creation of the observatory is the joint project of a corporation of
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leading scientific institutes of the Russian Academy of Sciences: the Lebedev Physical Institute, the Institute of Microstructure Physics, and the Space Research Institute. SCIENTIFIC TASKS AND SUBSTANTIATION OF THE CHOICE OF ARCA SPECTRAL CANNELS Our experiment was aimed at solving one of the key problems in the physics of solar and stellar coronas: the mechanisms of their heating. Most widespread theories of heating rest on the microprocesses of energy release (so called nanoflares) or the energy dis sipation of waves formed in the inner solar regions. To record nanoflares, it is necessary to perform observa tions of solar structures with temperatures of 100 000 to 2 million degrees with a spatial resolution no worse than 200 km. To analyze wave dissipation, we must synchronously observe all of the main layers of the solar atmosphere, from the chromosphere to the lower corona, with a time resolution of up to several seconds. Our method of study in the ARCA project is Sun imaging spectroscopy in VUV range, developed at the Physical Institute under the guidance of S.L. Man delshtam and I.A. Zhitnik. This method was first applied in the TEREK satellite experiment on the PHOBOS 1 interplanetary station (1988) [1] and performed successfully during the SPIRIT [2] and TESIS [3] experiments aboard the CORONAS F and CORONAS PHOTON satellites. Imaging spectroscopy is based on the acquisition of solar images in the nar rowest spectral intervals and, optimally, at the mono chromatic lines in the VUV and soft X ray spectral regions, precisely where the transition layer, the quiet and hot solar coronas, and the upper chromosphere primarily radiate. With so many imaging channels


88 Table 1. ARCA spectral recording channels Spectral range 132 å 171 å 195 å 304 å 211 å 284 å 1216 å Optical Spectral lines

KUZIN et al.

Target object Flare plasma Solar corona Solar Solar Solar Solar Solar Solar corona transition layer corona corona chromosphere photosphere

Temperature interval More than 10 million K 0.8­1.0 million K ~1.6 million K ~80 thou. ~2.0 million K ~2.2 million K ~30 thou. ~6 thou.

Fe XX Fe XXIII Fe IX Fe X Fe XII He II Fe XIV Fe XV HI 1216 å Continuous emission with a sensi tivity maximum of ~5000 å

operating simultaneously, the radiative intensity in dif ferent spectral channels can be directly compared, creating a firm practical basis for the multiwavelength and multi temperature analysis of solar plasma. In addition, the simultaneous scrutiny of a few solar lay ers (such as the active processes in the corona and the plasma dynamics in the chromosphere) is a powerful tool for studying the mechanisms and propagation pathways of energy in the solar atmosphere. The imagery spectral ranges of the ARCA observa tory were selected to be those that ensure the largest content of information from its observations (see Table 1). The transmission bands of the channels are centered with respect to the lines of the VUV and UV ranges. DESCRIPTION OF THE ARCA INSTRUMENTATION The instruments suggested as the basis for the observatory are two four channel Ritchey­Chretien
Telescope 1 132 , 171 , 195 , 304 å channels

2

3 4

5

6

Telescope 2 211 , 284 , 1216 , 5000 å channels.

1
Schematic diagram of ARCA telescopes: (1) protective cap; (2) panel of entrance windows with filters; (3) channel changer (for selecting the working quadrant of the mirror); (4) secondary mirror, equipped with a control and focusing system; (5) primary mirror; and (6) filter/detector unit.

telescopes, implemented in a single optical scheme but adapted to operate in different spectral regions (figure). The basic characteristics of the ARCA instru mentation are presented in Table 2. The telescopes operate independently of one another, so the Sun can be observed in two spectral channels simultaneously. Each telescope operates in several channels, thanks to individual multilayer coat ings of mirror quadrants. The working areas of a mirror and, correspondingly, the wavelength intervals in the observations are switched with the help of a shutter located behind the entrance window of the instru ment. This scheme was validated in the TESIS exper iment [4], which used a two wavelength (171 and 304 å) telescope equipped with a mechanic shutter. The effective focal length of the ARCA telescopes is 15 m, ensuring an angular resolution of 0.18 arcsec and a field of view of 0.8 â 0.8 solar radii. The visible and UV solar radiation is blocked by freely attached thin film aperture filters and specially coated filters on the surfaces of the CCD detectors. The intensity of the scattered light is damped through the tight sealing of each unit of the instrumentation. The use of telescopes with large focal distance in satellite experiments calls for the application of two mirror optical schemes, keeping the telescope sizes relatively compact (1.5­ 2.0 m in length). The possibility of increasing the spatial resolution during ARCA observations is due to progress in the following technologies: the creation of X ray multi layer coatings, the fabrication of large area mirrors, the elaboration of detectors based on CCD arrays with linear sizes of over 4000 pixels, and the creation of image stabilization systems. For the sectioning of narrow spectral intervals, the ARCA telescopes include multilayer normal inci dence optics with an effective reflection bandwidth of several angstroms and thin film filters. These optical systems have very high (tens of percent of the initial current) efficiency. During the TESIS experiment, e.g., the exposure time during the registering of the
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EUV OBSERVATIONS OF THE SOLAR CORONA Table 2. Technical characteristics of ARCA instrumentation Optical scheme Mirrors Primary mirror diameter Effective focal distance Mirror spacing Field of view Detectors CCD pixel size Effective angular resolution Total weight of scientific instrumentation Energy consumption Telemetry Overall size of each telescope (length â width â height) Ritchey­Chretien system Normal incidence, multilayer coated 500 mm 15 000 mm 2000 mm 12 â 12 arcmin (0.8 â 0.8 solar radii) 4096 â 4096­pixel back illuminated CCD array 13 µm 0.18 arcsec 90 kg 60 W 50 Gbytes per day 2200 â 520 â 520

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undisturbed corona was on the order of 0.5 s. Use of these optical elements will allow the ARCA observa tory to reach super high time resolution as well. The more than 80 fold reduction in the radiative flux per detector pixel (due to the high angular resolution and the two mirror optical scheme) can be compensated for by such factors as larger mirror area, higher detec tor sensitivity, and the greater brightness of sources on Sun during flares and other consequences of solar activity. The possibility of creating highly accurate large diameter aspherical mirrors is due to the progress in fabricating the elements of X ray optics for EUV lithography applications, which require 10 angstrom precision in shaping [5]. The Institute of Microstruc ture Physics has already created such mirrors and has developed and implemented methods for controlling precision in their manufacture [6]. Another necessary condition for achieving super high resolution is to reduce the effect of diffraction on the supporting lattices of the telescopes' aperture fil ters. To ensure an angular resolution as fine as 0.18 arcsec at a wavelength of 300 å, it is necessary that the supporting structure have a cell of no less than 18 â 18 mm. The Institute of Microstructure Physics has experience in fabricating of freely attached thin film filters more than 100 mm in size [7]. Previous sat ellite experiments used the filters on supporting lat tices with sizes on the order of 2 â 2 mm to keep the fil ters intact during the launch of the instrumentation into space. One way to circumvent this problem could be vacuumization of the telescope before launch. The manufacture of large format CCD arrays has recently evolved from the experimental stage to the stage of creating commercial specimens. The TESIS experiment included in flight tests of new type detec tors with 2048 â 2048 ­pixel CCD arrays, created at the Lebedev Physical Institute. These detectors use back illuminated arrays, allowing them to record VUV

and soft X ray radiation without resorting to conver sion systems. These arrays yielded an angular resolu tion of 1.5 arcsec in practice. The enlargement of CCD arrays through the use of 4096 â 4096 ­pixel detectors is envisaged as a resource for a further increase in resolution in the ARCA project. Super high spatial resolution imagery also requires the use of an image stabilization system. Foreign expe rience in creating this instrumentation (in the TRACE experiment) showed that image stabilization on the basis of a feedback system is most efficient. The device is stabilized by shifting the secondary (small) mirror of the two mirror optical system according to the control signal of the solar limb detector. The Lebedev Institute of Physics is now creating such an image stabilization system for the LYRA V experiment on board the International Space Station. The ARCA experiment will use a regime for the simultaneous control of all the scientific instrumenta tion on the observatory. Such control systems devel oped at the Lebedev Physical Institute [8] have proven invaluable during previous experiments. The SPIRIT experiment [2] used a regime of control over two data streams, while the TESIS experiment [3] was the first to use a regime of control over four stream processes; it became the first in the world to image the Sun simul taneously in four arbitrary recording channels. CONCLUSIONS The ARCA project will help solve problems associ ated with energy conversion in the physics of solar and stellar coronas. The project will substantially develop imaging spectroscopy at super high spatial resolu tions. The development and fabrication of the instru mentation will trigger new technologies in the fields of X ray optics, detectors in the VUV and UV ranges, image stabilization systems, onboard computers, and
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KUZIN et al. 2. Zhitnik, I.A., Kuzin, S.V., Sobel'man, I.I., et al., Astron. Vestn., 2005, vol. 39, pp. 495­507. 3. Kuzin, S.V., Bogachev, S.A., Zhitnik, I.A., et al., Izv. Akad. Nauk. Ser. Fiz., 2010, vol. 74, no. 1, pp. 39­43 [Bull. Russ. Acad. Sci. Phys. (Engl. Transl.), 2010, vol. 74, no. 1, p. 33]. 4. Zuev, S.Yu., Kuzin, S.V., Polkovnikov, V.N., and Salashchenko, N.N., Izv. Akad. Nauk. Ser. Fiz., 2010, vol. 74, no. 1, pp. 58­61 [Bull. Russ. Acad. Sci. Phys. (Engl. Transl.), 2010, vol. 74, no. 1, p. 50]. 5. Naulleau, P.P., Goldberg, K.A., Lee, S.H., et al., Appl. Opt., 1999, vol. 38, no. 35, pp. 7252­7263. 6. Klyuenkov, E.B., Pestov, A.E., Polkovnikov, V.N., et al., Ros. Nanotekhn., 2008, vol. 3, nos. 9­10, pp. 90­98. 7. Chkhalo, N.I., Gusev, S.A., Drozdov, M.N., et al., Proc. SPIE, 2010, vol. 751, p. 752105.

control systems for long term autonomous experi ments. ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research, project nos. 08 02 01301 a and 08 02 13633 ofi_ts; the Presidium of the Russian Academy of Sciences' Program of Basic Research no. 16, Part 3: the Department of Physical Sciences of the Russian Academy of Sciences' pro gram Plasma Processes in the Solar System; Project SOTERIA, grant no. 218816, www.soteria.eu; and the European Union's Seventh Framework Program (FP07/2007 2013). REFERENCES
1. Zhitnik, I.A., Tindo, I.A., Urnov, A.M., et al., Trudy FIAN, 1989, vol. 195, pp. 3­18.

8. Pertsov, A.A., Ignat'ev, A.P., Zhitnik, I.A., and Kuzin, S.V., Prib. Tekhn. Eksperim., 2008, no. 5, pp. 67­70.

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