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.

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 paper presents the results of a study of capabilities of the SITES algorithm for reconstructing the differential emission measure (DEM) of a source from its radiation in several parts of the electromagnetic spectrum in the context of observing solar nanoflares with the AIA/SDO instrument. The SITES method was implemented in the Python programming language and was first used to construct the DEM of nanoflares. For this purpose, we tested the efficiency of the algorithm on model single- and double-peak DEM at characteristic temperatures of solar nanoflares. The test results indicate that the SITES algorithm can be of limited applicability for studying the DEM of nanoflares in the single-peak approximation. The algorithm has a combination of high accuracy and high counting rate in the studied temperature range from 1 to 3 MK. The features of DEM nanoflares reconstructed by the SITES method were examined using our previously found sample of 58855 events observed in 2019 with the AIA/SDO instrument. The results confirm that the characteristic plasma temperature in nanoflares is 1–2 MK. The reconstructed DEM of nanoflares generally have one maximum within this range, but the temperature distribution we obtained for all flares forms two clusters with maxima at 1.2 and 1.7 MK. We interpret this as possible evidence for the existence of two types of solar nanoflares, but this result requires further confirmation.

Pin-hole camera as a soft X-ray solar telescope is presented. The instrument is aimed on imaging of flares in the solar corona with angular resolution up to 40''. It will allow to register temporal profiles of flares and determine spectra in soft X-ray. The telescope consist on pin hole 0.1 mm in diameter in tungsten disk, thin film Al/maylar filter and back illuminated CMOS based 2d detector. The telescope designed for 6 U cubesat. The instrument itself has dimension of about 0.5 U with extended tube from 20 to 50 cm length. Keywords: Pin-hole camera, solar corona, soft X-ray, cubesat.

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.

The frequency and rate of solar nanoflares (NF) were measured in 6 coronal spectral ranges (094, 131, 171, 193, 211, 335 Å) and one, related to the transition layer (304 Å). We used SDO/AIA data obtained at solar minimum in May 2019. We analyzed the same region of the Sun, covering \(360\text{\textquotedblleft} \times 720\text{\textquotedblleft}\) field-of-view, in all channels over the same time interval of 1 hour. In all the spectral bands, to search for NF we applied the same algorithm based on the amplitude analysis of fast brightenings in AIA images. The frequency and rate of NF, as can be expected, vary significantly in different wavelengths. For threshold \(5\sigma \), the highest NF frequency, 207 c–1, was measured in 171 Å. The next spectral ranges are 193 Å (85% of 171 channnel), 211 Å (74%), and 131 Å (63%). We have not been able to reliably measure the frequency in channels 094 Å, and 335 Å, but found that it is less than 15% of the frequency in channel 171 Å. In the 304 Å channel, we found a large number of brightenings that do not match any coronal events. However, about 40% of NF in corona have a counterpart in the 304 Å line with an amplitude higher than \(5\sigma \).

We propose a method to measure the energy distribution of low-energy flares (nanoflares) in the energy range below 1023 erg. As an example, we measured the spectrum of nanoflares in the 1021–1026 erg range for two Sun’s frames observed by the SDO/AIA telescope in the 171 Å channel. Nanoflares are shown to have the power law spectrum in the 1022–1026 erg range. The spectral index is approximately constant, i.e. energy-independent. For energies below 1022 erg, the spectrum begins to collapse. For lower energies, below 1021 erg, the method does not give statistically significant results due to major errors. The results of the study indicate that solar nanoflares can be detected up to 1021–1022 erg energies. Results have previously been reported only for 1023 erg and above. The total energy flux of nanoflares in the energy range above 1022 erg, according to our data, is P2104 erg cm–2 s–1, which is about 15 times less than heating losses of the solar corona.

In this paper, we theoretically study the linear polarization of radiation that arises in the lines of atomic helium in the solar chromosphere and in solar protuberances. In the visible region of the solar spectrum, several intense lines of the He I atom are observed, in which polarization can occur when the solar plasma is impacted by accelerated proton and electron flows. We present the results of calculations for two lines of the atom, namely the D3 5876 Å line, which is widely observed in the experiment, as well as the He I 5015 Å line, for which the degree of linear polarization turned out to be the highest among the lines studied by us in the helium spectrum (more than 30%). Our calculations point to good opportunities for experimental detection of polarization in both of these lines, both in the course of ground-based observations, for example, during solar eclipses, and during space experiments.

Solar nanoflares are small-scale events that can play a significant role in coronal heating. Because of the weak signal, searching for and investigating nanoflares is not an easy task that is handled differently by different authors. We present our own nanoflare detection method that, in our opinion, allows the occurrence rate of nanoflares and their spatial distribution in solar images in the vacuum ultraviolet spectral range to be efficiently measured. We have analyzed a series of 300 solar images obtained by the SDO/AIA telescope in the 171 Å channel in a period of low solar activity (from 12:00 UT to 13:00 UT on May 20, 2019), determined the occurrence rate of nanoflares, $$P\approx 4.23\times 10^{-21}$$ cm $${}^{-2}$$ s $${}^{-1}$$ , which has turned out to be in agreement with the results of other authors, and investigated the dependence of $$P$$ on heliographic latitude and brightness of the solar corona. According to our results, the occurrence rate of nanoflares does not depend on heliolatitude, which differs from the behavior of ordinary flares occurring in narrow activity belts. We have also found a correlation between $$P$$ [pix $${}^{-1}$$ h $${}^{-1}$$ ] and coronal intensity $$I$$ [counts]: $$\log P={-}2.27+0.00327I$$ . We assess our results as favorable for the theory of coronal heating by nanoflares.

We use data from the Atmospheric Imaging Assembly (AIA) telescope on the Solar Dynamics Observatory (SDO) spacecraft in the 171 Å channel to investigate the spatial distribution of low-energy flares (nanoflashes). We have studied two periods: 05/20/2019 from 1200 UT to 1300 UT and 05/10/2020 from 1200 UT to 1300 UT. In total, we found 87 974 nanoflares with an average formation rate of 6.0 × 10 ‒21 cm –2 s –1 . For solar latitudes from 0° up to 50° the nanoflare formation rate is approximately uniform with a standard deviation of about 25%. We have found an asymmetry in the rate of formation of nanoflares in the southern and northern hemispheres of the Sun: the rate of formation of nanoflares in the southern hemisphere was 34–42% higher than in the northern one. During this period, the same asymmetry was also observed for ordinary flares. We also found a weak dependence of the nanoflare formation rate on the solar cycle: the number of nanoflares increases with higher solar activity.

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 used data from the space telescope SOHO/EIT and the spectrometer VEIS on the Wind spacecraft to compare the solar wind (SW) speed near Earth's orbit with changes in the area of polar coronal holes (CHs) on the Sun during the 1996 solar activity minimum. We have found that in March 1996 the SW speed correlated with the southern CH area by a factor of 0.64. In September and October 1996, a correlation was revealed between the SW speed and the area of the northern CH (the coefficients are 0.64 and 0.85 respectively). We believe that this confirms the assumption that the solar wind from polar CHs can penetrate into the ecliptic plane at solar minimum. The SW speed was 460–500 km/s, which is lower than that from equatorial CHs (600–700 km/s).

The specific aspects of the design and operation of space instruments for recording radiation and images in extreme ultraviolet (EUV) range of the spectrum are considered. The main factors affecting the reliability of VUV instruments are contamination, the tempereture regime, pressure changes, acoustic and mechanical loads at the launch stage, weightlessness, and radiation-induced damage. These factors are often interconnected and operate in a complex manner. The analysis of the influence of these factors on the operating characteristics of the instruments and the possibilities of its reduction is carried out.

Pin-hole camera as a soft X-ray solar telescope is presented. The instrument is aimed on imaging of flares in the solar corona with angular resolution up to 40 angular seconds. It will allow to register temporal profiles of flares and determine spectra in soft X-ray. The telescope consist on pin hole 0.1 mm in diameter in tungsten disk, thin film Al/maylar filter and back illuminated CMOS based 2d detector. The telescope designed for 6U cubesat. The instrument itself has dimension of about 0.5U with extended tube from 20 to 50 cm length.

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.

Recent solar physics missions have shown the definite role of waves and magnetic fields deep in the inner corona, at the chromosphere-corona interface, where dramatic and physically dominant changes occur. HiRISE (High Resolution Imaging and Spectroscopy Explorer), the ambitious new generation ultra-high resolution, interferometric, and coronagraphic, solar physics mission, proposed in response to the ESA Voyage 2050 Call, would address these issues and provide the best-ever and most complete solar observatory, capable of ultra-high spatial, spectral, and temporal resolution observations of the solar atmosphere, from the photosphere to the corona, and of new insights of the solar interior from the core to the photosphere. HiRISE, at the L1 Lagrangian point, would provide meter class FUV imaging and spectro-imaging, EUV and XUV imaging and spectroscopy, magnetic fields measurements, and ambitious and comprehensive coronagraphy by a remote external occulter (two satellites formation flying 375 m apart, with a coronagraph on a chaser satellite). This major and state-of-the-art payload would allow us to characterize temperatures, densities, and velocities in the solar upper chromosphere, transition zone, and inner corona with, in particular, 2D very high resolution multi-spectral imaging-spectroscopy, and, direct coronal magnetic field measurement, thus providing a unique set of tools to understand the structure and onset of coronal heating. HiRISE’s objectives are natural complements to the Parker Solar Probe and Solar Orbiter-type missions. We present the science case for HiRISE which will address: i) the fine structure of the chromosphere-corona interface by 2D spectroscopy in FUV at very high resolution; ii) coronal heating roots in the inner corona by ambitious externally-occulted coronagraphy; iii) resolved and global helioseismology thanks to continuity and stability of observing at the L1 Lagrange point; and iv) solar variability and space climate with, in addition, a global comprehensive view of UV variability.

A small space telescope intended for imaging the Sun on board CubeSat small spacecraft has been developed and produced in the form of a ground-based model. A compact two-mirror optical system has been proposed for the telescope and tested. Using it, high-quality images of the Sun can be obtained with an angular resolution of better than 4″ in the EUV spectral region of the solar spectrum, where the main radiation of active solar processes lies. The expected quality was simulated for images that cannot be experimentally obtained and investigated under the ground conditions. The ability of the telescope to record all principal phenomena of the solar activity (flares and eruptive prominences) has been confirmed.

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.