In this work, we describe the design and flight performance of the REFOS instrument (from Russian REntgenovskiy FOto Spektrometr; in English “X-ray Photo Spectrometer”). REFOS is a soft X-ray spectrophotometer that registers full-disk integrated solar spectra (“Sun as a star”). It operates on board the Impulse-1 nanosatellite, which was launched on 24 June 2023. REFOS has a 1.2 – 30 keV spectral range, a 0.123 keV nominal full width at half-maximum (FWHM) resolution at 5.9 keV, and a cadence of 16 s. We illustrate the instrument flight performance using the spectra of the X5.0 flare that occurred on 31 December 2023 at 21:55 UT. For this flare, REFOS registered a meaningful signal in all of its energy bins. Based on a comparison between the GOES and REFOS fluxes, we corrected the REFOS spectral sensitivity. Additionally, we assessed the quality of the calibration based on the shape of the continuum. The observed continuum allows diagnosing the plasma temperature, and the observed spectral lines allow studying abundances of the Mg, Si, S, Ar, Ca, Fe, and Ni.

Based on data from the ‘‘TESIS’’ scientific instrumentation aboard the ‘‘CORONAS–Photon’’ satellite, collected during the solar activity minimum of 2009, the positions on the disk of 1014 solar flares were analyzed, with X-ray classes ranging from A0.003 to C2.7. A change in the spatial distribution was found in the region of class A events. As flare power decreases, a transition is observed from their localization within the activity belts to a more uniform latitudinal distribution. An estimate of the asymmetry in the positioning of the events on the solar disk was also obtained—0.009. Thus, it was established that the number of flares in the southern and northern hemispheres is almost identical.

We propose a method to minimize the amount of aspherization required to produce hyperboloid mirrors for the widely used Ritchey-Chrétien telescope design. The approach is based on fine-tuning the telescope's geometry to precisely match a discrete set of high-precision etalons, which are used to control the polishing of the spherical substrates. We support this optimization technique with an open-source implementation in Python and demonstrate its effectiveness through several real-world examples, achieving improvements of up to more than an order of magnitude, with the telescope parameters adjusted by only a fraction of a millimeter. We also show that this method can be extended to several other reflecting telescope designs.

The work is devoted to the analysis of the possibility of implementing measurements of the Earth's magnetic field on board small spacecraft of the CubeSat 1U format. In particular, the problem of ensuring magnetic purity for the operation of magnetometers as part of nanosatellites was solved. The required accuracy of the equipment was estimated and the recommended maximum level of the spacecraft's residual magnetic field was determined – 0.1 nT. Ensuring the required value of the background of the magnetic field is possible only if the platform and the magnetometer are spaced apart - due to a special boom. To estimate its minimum length, the simulation of the magnetic field of the nanosatellite was carried out. The resulting value was 1250 cm.

The association of coronal mass ejections (CMEs) with flares is related to the question of whether reconnection is necessary for the CME eruption. Indeed, if reconnection happens during a CME eruption, the plasma is heated, which can be observed as a flare. In this work, we study the CME-flare association using data obtained with the Mg xii spectroheliograph on board the Complex Orbital Observations Near-Earth of Activity on the Sun (CORONAS-F) satellite. This instrument is sensitive only to the emission of plasma with a temperature greater than 4 MK, which makes it a convenient tool for detection of flaring activity. During our analysis, we first searched for CMEs detected during the Mg xii observations by the Large Angle and Spectroscopic Coronagraph (LASCO). Then, we visually checked the Mg xii images for flaring activity. We found that during the Mg xii observations (2001 – 2003), 198 CMEs were detected by LASCO. One hundred sixty of them (81

Within the Universat program, a set of solar vacuum ultraviolet (VUV) telescopes has been developed for deployment on 6U nanosatellites. Telescopes are designed to get images of the solar corona. The spectral ranges of observations is considered, the characteristics of the nanosatellite from the point of view of the observations feasibility are opmized, the optical scheme of the telescope and VUV multilayer mirrors coatings and thin-film filters are modelled. Keywords: Nanosatellite, VUV, telescope, solar corona.

Aims. We aim to improve the existing techniques to probe the nanoflare hypothesis for the coronal heating problem. For this purpose, we propose using the solar extreme ultraviolet (EUV) emission variability registered with modern space-based imagers. Methods. We followed a novel model-based approach. As a starting point, we used the EBTEL 0d hydrodynamic model. We integrated the arising system of stochastic differential equations to calculate the covariance matrix for plasma parameters. We then employed a Taylor expansion technique to relate model parameters with observable EUV intensity variation statistics. Results. We found that in the high-frequency approximation, the variability of the EUV emission is defined by the dimensionless factor ϖ , which is inversely proportional to the frequency. We calculated the factor ϖ throughout the solar disk and found that it does not exceed 0.01, except for the finite number of compact regions. The distribution of ϖ follows the power law with an index of ≈ − 2.6. To validate our approach, we used it to probe the temperature of the coronal plasma. We show that the line-of-sight temperature distribution is close to homogeneous with a mode of ≈1.25 MK, which is in perfect agreement with the results of the spectroscopic diagnostics.

In this work, we study where heating takes place during coronal mass ejections (CMEs). For this purpose, we have used the data of the Mg xii spectroheliograph on board the Complex Orbital Observations Near-Earth of Activity on the Sun (CORONAS)-F satellite. This instrument obtained images of the solar corona in the Mg xii 8.42 Å line, which emits only at temperatures higher than 4 MK. After analyzing the Mg xii data archive from 2001 to 2003, we found ten high-temperature eruptive events. Each of them was associated with a CME and nine were associated with a flare. The eruptive structures had temperatures higher than 4 MK and a characteristic size of 100 – 200 Mm. The events were observed by the Mg xii spectroheliograph for 10 min to 3 h. In the Mg xii images, the peak intensity of the eruptive structures was 0.2 – 14.4

Solar extreme ultraviolet (EUV) imaging instruments usually have a channel centered at 304 Å to observe the strong He ii 303.8 Å line, which is valuable for studying the dynamics of chromospheric and transition-region structures. In off-limb regions where He ii is weak, however, the coronal Si xi 303.3 Å line becomes significant and provides a background haze that reduces the contrast of He ii structures such as jets and macrospicules, complicating the interpretation of the observations. Generally, the separation of this background would require spectroscopic observations. In this article, we take an alternate approach by reconstructing the differential emission measure (DEM) of the quiescent corona to obtain synthetic radial emission profiles in the Si xi 303.3 Å line and show that at altitudes above 20 Mm it makes the major contribution to the background. We also find the silicon abundance to be significantly, by around 80

The paper analyzes the latitudinal distribution of high-temperature plasma (T>4 MK) and microflares on the solar disk during low solar activity in 2009. The distribution of A0.1–A1.0 microflares contains belts typical of ordinary flares of B class and higher. In total, we have registered 526 flares, most of which, about 96 %, occurred at high latitudes. About 4 % of microflares were found near the solar equator. We believe that they were formed by the residual magnetic field of previous solar cycle 23. Ordinary flares were almost not observed near the equator during this period. The number of microflares in the southern hemisphere was slightly higher than in the northern one. This differs from the distribution of ordinary flares for which the northern hemisphere was previously reported to be dominant.

We have investigated 49 solar macrospicules observed in 2010 at the rising phase of the solar cycle at high (circumpolar) and low (near the solar equator) latitudes. We have found that, within the measurement accuracy, the macrospicule birth rate does not depend on the latitude and is $${\sim}0.1$$ deg $${}^{-2}$$ h $${}^{-1}$$ . Concurrently, we have established that the mean heights of macrospicules near the equator and the poles differ noticeably. After the elimination of projection effects, we have obtained $$31.7\pm 0.2$$ thousand km for macrospicules at low latitudes and $$39.1\pm 0.3$$ thousand km for high-latitude macrospicules.

In the solar corona, magnetic reconnection occurs due to the finite resistivity of the plasma. At the same time, this resistivity leads to ohmic heating. Therefore, the reconnecting current sheet should heat the surrounding plasma. This paper presents experimental evidence of such plasma heating being caused by magnetic reconnection. We observed the effect during a C1.4 solar flare on 2003 February 16 at the active region NOAA 10278, near the solar limb. Thanks to such a location, we successfully identified all the principal elements of the flare: the flare arcade, the flux rope, and, most importantly, the presumed position of the current sheet. By analyzing the monochromatic X-ray images of the Sun obtained by the CORONAS-F/SPIRIT instrument in the Mg xii 8.42 Å spectral line, we detected a high-temperature ( T ≥ 4 MK) emission at the predicted location of the current sheet. The high-temperature emission appeared during the CME’s impulsive acceleration phase. We believe that this additionally confirms that the plasma heating around the current sheet and the magnetic reconnection inside the current sheet are strongly connected.

Active regions (ARs) on the Sun are discussed as a possible source of the slow solar wind (SW) whose origin is still a subject of debates. In this paper we present experimental evidence for the possible influence of ARs on the SW speed and temperature near the maximum of solar cycle 23 (2000–2002). We have studied separately the characteristics of the SW formed in the periods when ARs were on the central meridian (CM) of the Sun ( $${\approx}40{\%}$$ of the entire observing time) and the characteristics of the SW formed in the absence of ARs on the CM ( $${\approx}60{\%}$$ of the observing time). The SW speed in the former case (in the presence of ARs), on average, have turned out to be lower than the speed of the SW formed in the absence of ARs approximately by 1 $${\%}$$ (434.06 versus 438.09 km s $${}^{-1}$$ with a measurement error $$\sigma\approx 0.37$$ km s $${}^{-1}$$ ). For the SW temperature the corresponding difference is about 6 $${\%}$$ ( $$94\,600$$ K versus $$100\,500$$ K with an error $$\sigma\approx 340$$ K). This result confirms, on average, a lower speed and temperature of the SW forming in ARs in comparison with the SW component forming in coronal holes.

Within the Universat program, a set of solar vacuum ultraviolet (VUV) telescopes has been developed for deployment on 6U nanosatellites. Telescopes are designed to get images of the solar corona. The spectral ranges of observations is considered, the characteristics of the nanosatellite from the point of view of the observations feasibility are opmized, the optical scheme of the telescope and VUV multilayer mirrors coatings and thin-film filters are modelled.

Investigations of solar activity require information about plasma in a wide range of temperatures. Generally, researchers require observations from telescopes producing monochromatic images of coronal plasma with cool, warm, and hot temperatures. Until now, monochromatic telescopic imaging has been made only in the Mg XII 8.42 Å line with the Mg XII spectroheliograph on board CORONAS-I, CORONAS-F, and CORONAS-PHOTON satellites. The Mg XII spectroheliograph used Bragg crystal optics. Its design is based on two main principles: (1) to select the working wavelength and the crystal in such a way that reflection occurs at small incident angles; (2) to use the aperture of the mirror as a spectral filter. We believe that these design principles can be applied to other spectral lines. In this article, we will review the design of the Mg XII spectroheliograph and present our thoughts on how to apply these principles to the Si XIV 6.18 Å and Si XIII 6.65 Å lines. A combination of the monochromatic Mg XII 8.42 Å, Si XIV 6.18 Å, and Si XIII 6.65 Å images will help us to study the dynamics of the hot plasma in the solar corona.

We present a description of the recent advances in the development of the KORTES assembly—the first solar oriented mission designed for the Russian segment of the International Space Station. KORTES consists of several imaging and spectroscopic instruments collectively covering a wide spectral range extending from extreme ultraviolet (EUV) wavelengths to X-rays. The EUV telescopes inside KORTES will trace the origin and dynamics of various solar phenomena, e.g., flares, CMEs, eruptions etc. EUV spectra provided by grazing-incidence spectroheliographs will enable precise DEM-diagnostics during these events. The monochromatic X-ray imager will observe the formation of hot plasma in active regions and outside them. The SolpeX module inside KORTES will offer an opportunity to measure fluxes, Doppler shifts and polarization of soft X-ray emission both in lines and continuum. SolpeX observations will contribute to studies of particle beams and chromospheric evaporation. The instrumentation of KORTES will employ a variety of novel multilayer and crystal optics. The deployment of KORTES is planned for 2024.

Due to the increase in the spatial and temporal resolution of observations of the solar atmosphere, which is mainly associated with progress in space research, we now understand that the Sun’s activity not only is associated with large centers, but also extends to significantly smaller scales. Each new advance in experimental technology over the past 60 years has led to the discovery of more and more numerous and small solar structures: X-ray active regions in the 1960s, hot X-ray points in the 1970s, solar microflares in the 1980s, and finally, from the end of the 20th century, solar nanoflares. At the same time, the total energy release, obtainable from observations, is still insufficient to ensure a balance between heating of the corona and its rapid radiative cooling. For the smallest-scale phenomena, nanoflares, it is still not possible to resolve their structure and mechanism, which raises the question of whether it is correct to classify them as flares. We present a review of the main results obtained so far in the field of small-scale solar activity, mainly microflares and nanoflares, and discuss the main issues that need to be solved in order to move forward.

В настоящее время достигнут значимый прогресс в многослойных зеркалах нормального падения ВУФ диапазона спектра. В первую очередь, это касается повышения коэффициента отражения на рабочей длине волны, уменьшения спектральной ширины кривой отражения, появления высокоэффективных МС покрытий для коротковолнового (3-9 nm) и длинноволнового (более 50 nm) участков спектра. Создание таких зеркал открывает новые возможности для проведения астрофизических исследований Солнца, так как узкие спектральные ширины зеркал в сочетании с высоким коэффициентом отражения позволяют регистрировать изображения солнечной короны в монохроматических линиях. Особенно перспективным является использование телескопов на базе этих зеркал для динамической спектральной диагностики полного диска Солнца методом изображающей спектроскопии. Метод основан на регистрации монохроматических изображений Солнца в линиях ВУФ диапазона спектра с высоким пространственным и временным разрешением. Обсуждается возможный прогресс в солнечных исследованиях с применением этого типа оптики. Ключевые слова: солнечная корона, изображающая спектроскопия.