We describe the RESIK (REntgenovsky Spektrometr s Izognutymi Kristalami) instrument, consisting of two double-channel X-ray spectrometers, designed to observe solar active region and flare plasmas. RESIK is one of the instruments making up the scientific payload of the Russian CORONAS-F solar mission. The uncollimated spectrometer uses two silicon and two quartz bent crystals observing flare, active region and coronal spectra in four wavelength bands with a resolving power (λ/ λ) of ∼1000. The wavelength coverage, 3.3 -6.1Å, includes emission lines of Si, S, Cl, Ar, and K and in the third diffraction order, the wavelength range includes He-like Fe lines (1.85Å) and Ni lines (1.55Å) with dielectronic satellites, emitted during intense, hot flares. The instrument is believed to be the best calibrated space-borne crystal spectrometer flown to date. The spectrometer dynamically adjusts the data gathering intervals from 1 s to 5 minutes, depending on the level of solar X-ray emission at the time of observation. The principal aims of RESIK are the measurements of relative and absolute element abundances in the emitting plasma and the temperature distribution of plasma (differential emission measure) over the temperature interval 3 and 50 MK. This paper summarizes the scientific objectives of RESIK and describes the design, characteristics, and performance of the instrument.

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High-resolution temporal series of monochromatic full-Sun images in the L Mg XII emission line at 8.42Å have been obtained by the SPectroheliographIc X-Ray Imaging Telescope (SPIRIT) spectroheliograph on the CORONAS-F satellite. For a six-month period since the launch on 2001 July 31 more than 10 000 Mg XII images were recorded. The images revealed plasma structures with the temperature in the range 3-20 MK characterized by specific shape and dynamics, different to that of relatively low temperature (1-2 MK) plasma. The main remarkable features of the phenomena first seen in the monochromatic images are as follows:(1) high spatial variability in brightness with the contrast of more than two orders of magnitude, (2) the presence of some compact sources with high altitude locations up to 0.3 solar radius and well-distinguished 'spider'-like forms, (3) substantially different variability in dynamical behaviour characterized by lifetimes from minutes to days. Sporadic condensations of coronal plasma with log N e ≈ 10 and peak temperature of about 10 MK were observed in the vicinity of active region systems.

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The SPIRIT complex onboard the CORONAS-F satellite has routinely imaged the Sun in the 171, 175, 195, 284, and 304 Å spectral bands since August 2001. The complex incorporates two telescopes. The Ritchey-Chretien telescope operates in the 171, 195, 284, and 304 Å bands and has an objective similar to that of the SOHO /EIT instrument. The Herschel telescope obtains solar images synchronously in the 175 and 304 Å bands with two multilayer-coated parabolic mirrors. The SPIRIT program includes synoptic observations, studies of the dynamics of various structures on the solar disk and in the corona up to 5 solar radii, and coordinated observations with other spaceborne and ground-based telescopes. In particular, in the period 2002-2003, synoptic observations with the SPIRIT Ritchey-Chretien telescope were coordinated with regular 6-hour SOHO /EIT observations. Since June 2003, when EIT data were temporarily absent ( SOHO keyholes), the SPIRIT telescope has performed synoptic observations at a wavelength of 175 A. These data were used by the Solar Influence Data Analysis Center (SIDC) at the Royal Observatory of Belgium for an early space weather forecast. We analyze the photometric and spectral parameters of the SPIRIT and EIT instruments and compare the integrated (over the solar disk) EUV fluxes using solar images obtained with these instruments during the CORONAS-

The DIOGENESS X-ray crystal spectrometer on the CORONAS-F spacecraft operated only for a single month (25 August to 17 September) in 2001, but in its short lifetime obtained one hundred and forty high-resolution spectra of eight solar flares with GOES importance ranging from C9 to X5. The instrument included four scanning flat crystals with wavelength ranges covering the regions of Si XIII (6.65 Å), S XV (5.04 Å), and Ca XIX (3.18 Å) X-ray lines and associated dielectronic satellites. Two crystals covering the Ca XIX lines were oriented in a "dopplerometer" manner, i.e. such that spatial and spectral displacements, both of which commonly occur in flares, can be separated. We describe the DIOGENESS spectrometer and the spectra obtained during flares that include lines not hitherto seen from spacecraft instruments. An instrument with a very similar concept is currently being built for the two Russian Interhelioprobe spacecraft that are scheduled for launch in 2020 and 2022 and will make a near-encounter (perihelion ∼ 0.3 AU) with the Sun in its orbit. We outline the results that are likely to be obtained.

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