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Based on observations conducted at the Ernest Gurtovenko Horizontal Solar Telescope, the height of the polar chromosphere of the Sun was determined for the period 2012–2023. The measurement was calculated as the difference between the positions of the maximum radial brightness gradients in the continuum and at the core of the Hα line. The results indicate that the height of the polar chromosphere is lower near the maximum of the solar cycle (approximately 4500 km, or 6.3″) and higher near the minimum of the cycle (approximately 5000 km, or 6.9″). The chromosphere's height at the southern pole in 2012–2013 and, particularly, 2016–2017 was higher than at the northern pole. This north–south asymmetry is likely related to differences in the dynamics and magnitude of the polar magnetic fields during Solar Cycle 24. The findings demonstrate that the time changes in the chromosphere's height closely correlate with sunspot numbers, the strength of the polar magnetic field, and chromospheric indices of solar activity. The correlation coefficient between the average annual height of the chromosphere and the smoothed relative sunspot number is –0.64 for the northern hemisphere and –0.75 for the southern hemisphere. The correlation coefficient between the average annual height of the chromosphere and the smoothed values of the polar magnetic field strength (based on data from the Wilcox Solar Observatory) is 0.86 for the northern hemisphere and 0.53 for the southern hemisphere (the latter value increases to 0.77). The correlation coefficient between the average annual height of the chromosphere and the chromospheric index IK2 reaches the highest values, 0.91, for the northern pole and 0.80 for the southern pole. [ABSTRACT FROM AUTHOR]

Ca ii K observations of the Sun have a great potential for probing the Sun's magnetism and activity, as well as for reconstructing solar irradiance. The Kodaikanal Solar Observatory (KoSO) in India, houses one of the most prominent Ca ii K archives, spanning from 1904 to 2007, obtained under the same experimental conditions over a century, a feat very few other sites have achieved. However, the KoSO Ca ii K archive suffers from several inconsistencies (e.g., missing/incorrect timestamps of observations and orientation of some images) which have limited the use of the archive. This study is a step towards bringing the KoSO archive to its full potential. We did this by developing an automatic method to orient the images more accurately than in previous studies. Furthermore, we included more data than in earlier studies (considering images that could not previously be analyzed by other techniques, as well as 2845 newly digitized images), while also accounting for mistakes in the observational date/time. These images were accurately processed to identify plage regions along with their locations, enabling us to construct the butterfly diagram of plage areas from the entire KoSO Ca ii K archive covering 1904 – 2007. Our butterfly diagram shows significantly fewer data gaps compared to earlier versions due to the larger set of data used in this study. Moreover, our butterfly diagram is consistent with Spörer's law for sunspots, validating our automatic image orientation method. Additionally, we found that the mean latitude of plage areas calculated over the entire period is 20.5 % ± 2.0 higher than that of sunspots, irrespective of the phase or the strength of the solar cycle. We also studied the north–south asymmetry showing that the northern hemisphere dominated plage areas during solar cycles 19 and 20, while the southern hemisphere dominated during Solar Cycles 21 – 23. [ABSTRACT FROM AUTHOR]

A distinct difference is seen between different parts of the solar chromosphere, including the bright points (BPs) at the boundary and within the network, and the darker regions formed the interior of the granules. In the photosphere, apart from sunspots and holes, there are direct and concentrated magnetic fields in the form of small flux tubes with fields of 1 to 2 kG. Chromospheric networks are highly dynamic regions with fine structures embedded in the magnetic flux. These structures are located inside the cell network and there are also many small and dark cells around it. BPs can be thought of as trails of spicules through which mass and energy from the lower layers of the Sun are transported to the corona as the solar wind. Although in the last two decades, despite high-resolution observations and theoretical advances, effective steps have been taken to determine dynamics of BPs and various thermodynamic parameters such as temperature and density, but the mechanisms responsible for their formation are still unknown. These ambiguities are due to the difference in their appearance as a result of observation at different spectral lines and wavelengths. In terms of their dimensions, lifetimes and physical conditions, there is indirect evidence that there is a relationship between the identity of spicules and BPs of the chromospheric network, which can be attributed to the position of the magnetic field around the points. BPs form smaller or larger groups that, due to their specific morphology, carry plasma flux tubes to the magnetic corona. The flow behavior along BPs and spikes is a debatable issue. However, observations of the contents of the sticks show periodic up and down movements. In this research, using images of the solar disc extracted by the IRIS space telescope, we investigate the periodic behavior of fast brightening on a small scale. By analyzing the movements of these points using the wavelet method, one can understand their periodicity. Examining these points in wavelengths of 1400, 1330, 2796 Å to the real identity and the effect of these points on the reactions obtained by the sun. The bright points of the grid are arranged for the light grains visible in the filters and in the roughly cellular designs that form the boundaries of the grid. According to the graphs of wavelet analysis, the three-minute peaks are quite clear. The 3-minute and 5-minute chromospheric peaks seen in these analyzes show that the considered bright points are P modes, which means compression mode. In the three-minute oscillations, the origin is chromospheric, and in the five-minute oscillations, the origin is photosphere. Origin of the p modes is (5-minute fluctuations) compressive forces. Spectral images were used to obtain Doppler velocities and these velocities were calculated as -20 to 30 km-1. The results of this study suggested that these bright points are short-lived sources of plasma that originate from the chromosphere or the lower atmosphere of the Sun. [ABSTRACT FROM AUTHOR]

Solar flares can be accompanied by high plasma velocities exceeding several hundred km/s. Detection and measurement of such velocities is limited by narrow-band and small wavelength range in most solar instruments. However, similar events with Doppler velocities exceeding two hundred km/s have been detected by the solar optical spectrographs at the Ondřejov Observatory. The results of the analysis of our multi-wavelength observation performed during the solar flare of June 7, 2011 and the calculation of several physical parameters of the coronal mass ejection fragments following the flare have been present. The calculation of the radiation of heated gas have been performed with allowance for self-absorption in the spectral lines of hydrogen and calcium. All the crucial processes of discrete level populating and depopulating have been taken into account in the balance equations. The theoretical radiation fluxes in the lines have coincided with those observed in the temperature range of 6300–10000 K at a gas concentration of ~ , a gas layer thickness of km, and a linear concentration of . [ABSTRACT FROM AUTHOR]

The results of the spectral observation analysis in the Hα line of a site of active region NOAA 11 024, which has been in the main phase of development given its sharply increased activity, are discussed. The studied site (its length is 10 Mm) has been located in the region of a new serpentine magnetic flow emergence. An arch filament system (AFS) has been formed on it, under which an Ellerman bomb (EB) emerged and developed, and a pore formed at a distance of approximately 7.2 Mm from the EB. The evolution of the AFS is studied, and the formation and development of all Hα-ejections that formed in its magnetic loops during the observations are investigated. Spectral data with high spatial (approximately 1″) and temporal (approximately 3 s) resolution were obtained with the THEMIS French–Italian solar telescope (Tenerife, Spain) on July 4, 2009. The observation time is 20 min (to 9:52–10:11 UT). We use the spectral region that contains the central part of the Hα chromospheric line. In all spectra, Hα ejections (surges) are visible both in the long wavelength and short wavelength wings of the absorption line. The changes in the Stokes I profiles shape are studied, which are very diverse and appreciably different from the profile for the undisturbed chromosphere. Depending on whether the ejection moved to the upward direction or to the downward one, the profile component corresponding to it is projected onto the blue or red line wing. Substantially broadened and dual lobed profiles appear close to the end of the observations, which indicates that both downward and upward plasma flows exist nearby. It is found that surges can be comprised of several jets that are formed during successive and periodic magnetic reconnections. Doppler shifts of the profile components are used to calculate the line-of-sight velocities (Vlos) of chromospheric matter in surges. The changes in the Vlos along the cross section of the surge jets at the place of their maximum intensity are analyzed. The Vlos of jets are different and probably depend on the magnetic field structure in the surge and the surrounding environment. The direction of jet movement is also different, since it depends on the phase of surge development. Most of the curves of Vlos changes consist of several segments. This indicates that the large jets are composed of several smaller jets, i.e., they had a fibrous structure. The flows of ascending and descending surges often occur simultaneously and coincide in time with the increase of the EB brightness. A vortex motion of the plasma is observed in one of the surges for approximately 3 min, as evidenced by the inclined dark streaks in the spectra. At the instant of the greatest brightness of the EB, there are seven surges in the studied site of AR, and the plasma moves downward with Vlos up to 77 km/s in three of them and upward with a much lower Vlos up to –35 km/s in five of them. During our observations, the maximum upward velocities of the chromospheric matter in the surges reach –110 km/s, and the downward velocities reach 90 km/s. In the upper part of the magnetic loops, the plasma velocities vary between –25 km/s and 22 km/s. The Vlos values in the site without active formations do not exceed ±2 km/s. It is also analyzed whether the processes of AFS evolution and EB development phases are related. The study is based on a detailed analysis of observational data obtained with high spatial and temporal resolutions, which allowed the authors to better understand the dynamics of the evolution of the arch filament system under which an Ellerman bomb has emerged and developed, and to reveal the features of the formation and development of surges that form in its magnetic loops. They are probably the result of successive and periodic magnetic reconnections, which are associated with the emergence of a new serpentine magnetic flux and occur when its loops interact with the preexisting magnetic field surrounding the active region or between the magnetic loops of the flux itself. [ABSTRACT FROM AUTHOR]

With the steady improvement of the observing capabilities and numerical simulations, an efficient data management of large data volumes has become mandatory. The Institute for Solar Physics (KIS) has developed the Science Data Centre (SDC), a data infrastructure to store, curate, and disseminate science-ready data from the German solar-observing facilities and other partner institutions. The SDC was also conceived to create and disseminate higher-level data products of added value like inversions from spectropolarimetric data. The SDC archive infrastructure consists of a back-end based on the Rucio science data-management and MongoDB systems and a front-end web interface that allows the user to search and discover data based on search parameters like instrument, date, wavelength range, and target. The SDC archive also provides data access via API and TAP services. The SDC currently offers access to 1299 science-ready datasets from the GRIS instrument at the GREGOR telescope (Tenerife) since 2014, a set of 610 spectra from the LARS at the Vacuum Solar Telescope (VTT, Tenerife) and 202 404 full-disc solar images from the Chromospheric Telescope (ChroTel). The SDC also offers to the community Milne–Eddington inversions of the GRIS spectropolarimetric archived data that can be downloaded as well as tools for data visualization and advanced analysis (e.g., GRISView tool). Many SDC activities have been carried out within the framework of large international data projects like the Horizon 2020 ASTERICS and ESCAPE EU-funded projects under the FAIR (Findable, Accessible, Interoperable, Reusable) principles. New and planned SDC activities include the ingestion of solar data from GREGOR context imaging instruments, flare observations from Ondřejov Observatory (Czech Republic), archiving and dissemination of in-house magnetohydrodynamic simulations, and creation of high-level data products using machine learning. The KIS Science Data Centre is a state-of-the-art data-management infrastructure that curates, archives, and provides access to ground-based science-ready spectropolarimetric and imaging solar data. SDC also provides advanced data visualization and analysis tools and invites data providers to publish their data to the solar and broader (astro)physics community via the SDC data archive. [ABSTRACT FROM AUTHOR]

This study investigates penumbrae and light bridges based on photospheric and chromospheric flow fields and photospheric magnetic fields in active region NOAA 13096. The improved High-resolution Fast Imager (HiFI+) and the GREGOR Infrared Spectrograph (GRIS) acquired high-resolution imaging and spectropolarimetric data at the 1.5-meter GREGOR solar telescope at the Observatorio del Teide, Izaña, Tenerife, Spain. Background-Subtracted Activity Maps (BaSAMs) have been used to locate areas of enhanced activity, Local Correlation Tracking (LCT) provides horizontal proper motions, and near-infrared full-Stokes polarimetry offers access to magnetic fields and line-of-sight velocities. The results show that the decaying active region is characterized by a triangular region between the three leading, positive-polarity sunspots with unfavorable conditions for penumbra formation. This region has a spongy appearance in narrow-band H α images, shows signs of enhanced activity on small spatial scales, is free of divergence centers and exploding granules, lacks well-ordered horizontal flows, has low flow speeds, and is dominated by horizontal magnetic fields. Umbral cores are inactive, but the interface between pores and penumbral filaments often shows enhanced activity. Moat flows and superpenumbrae are almost always observed, when penumbral filaments are present, even in very small penumbral sectors. However, evidence of the moat flow can also be seen around pores, surviving longer than the decaying penumbral filaments. Light bridges have mainly umbral temperatures, reaching quiet-Sun temperatures in some places, show strong intensity variations, and exhibit weak photospheric horizontal flows, while narrow-band H α flow maps show substantial inflows. [ABSTRACT FROM AUTHOR]

The torsional Alfvén wave is highly regarded as the carrier of the energy from the photosphere to the corona in the solar atmosphere. This paper presents a comprehensive linear analysis of the wave behavior and energy transfer within an open, twisted, divergent magnetic flux tube configuration, considering the impact of wave guide structure on the propagation of these waves using the magneto-hydrodynamic approach. The study shows that waves with frequencies between 0.001 Hz and 1 Hz can effectively penetrate the transition region, with the most efficient energy transfer occurring in the 0.1 Hz to 1 Hz frequency range. The research findings suggest that waves with certain intermediate frequencies are able to transmit energy to the coronal region of the Sun, contributing to its active heating. [ABSTRACT FROM AUTHOR]

The solar eclipse of 2017 August 21 was observed with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) on the Large Binocular Telescope (LBT), which is located at Mt. Graham International Observatory (MGIO), Arizona, USA. At this location, a partial eclipse was observed with maximum obscuration of about 61.6%. The 11-millimeter-aperture, binocular Solar Disk-Integrated (SDI) telescope, located on the kitchen balcony of the LBT building, feeds sunlight to PEPSI, which has recorded a total of 116 Sun-as-a-star spectra in the wavelength range of 5300 – 6300 Å, with a spectral resolution R ≈ 250 , 000 and signal-to-noise ratio of about 733:1. The temporal evolution of the Fraunhofer Na i D doublet at λ 5890/5896 Å is analyzed using contrast profiles that illustrate subtle changes in the spectral line, not obvious in the intensity profiles. Line bisectors are used to study the height-dependent signature of convective motions. Sun-as-a-star spectra illustrate the radial atmospheric stratification and are affected by limb darkening, solar differential rotation, convective motions, and magnetic activity. During a partial solar eclipse, the contribution of these features is modified by the passage of the Moon, resulting in a transit spectral signature. These observations are compared with synthetic Na i D spectra generated by the Spectropolarimetic NLTE Analytically Powered Inversion (SNAPI) code, based on state-of-the-art Bifrost atmospheric parameters, applied to a geometrically accurate model of the solar eclipse. The model is in qualitative agreement with the observations. However, the discrepancies indicate that models need to be improved, where high-resolution eclipse spectroscopy can serve as a benchmark. [ABSTRACT FROM AUTHOR]

This study explores nonlinear development of the magnetized Rayleigh–Taylor instability (RTI) in a prominence-corona transition region. Using a two-fluid model of a partially ionized plasma, we compare RTI simulations for several different magnetic field configurations. We follow prior descriptions of the numerical prominence model (Popescu Braileanu et al. 2021 Astron. Astrophys.646, A93 (doi:10.1051/0004-6361/202039053), Popescu Braileanu et al. 2021 Astron. Astrophys.650, A181 (doi:10.1051/0004-6361/202140425) and Popescu Braileanu et al. 2023 Astron. Astrophys.670, A31 (doi:10.1051/0004-6361/202142996)) and explore the charged-neutral fluid coupling and plasma heating in a two-dimensional mixing layer for different magnetic field configurations. We also investigate how the shear in magnetic field surrounding a prominence may impact the release of the gravitational potential energy of the prominence material. We show that the flow decoupling is strongest in the plane normal to the direction of the magnetic field, where neutral pressure gradients drive ion-neutral drifts and frictional heating is balanced by adiabatic cooling of the expanding prominence material. We also show that magnetic field within the mixing plane can lead to faster nonlinear release of the gravitational energy driving the RTI, while more efficiently heating the plasma via viscous dissipation of associated plasma flows. We relate the computational results to potential observables by highlighting how integrating over under-resolved two-fluid sub-structure may lead to misinterpretation of observational data. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'. [ABSTRACT FROM AUTHOR]

Solar partially ionized plasma is frequently modelled using single-fluid (1F) or two-fluid (2F) approaches. In the 1F case, charge-neutral interactions are often described through ambipolar diffusion, while the 2F model fully considers charge-neutral drifts. Here, we expand the definition of the ambipolar diffusion coefficient to include inelastic collisions (ion/rec) in two cases: a VAL3C one-dimensional model and a 2F simulation of the Rayleigh–Taylor instability (RTI) in a solar prominence thread based on [Lukin et al. 2024 Phil. Trans. R. Soc. A382, 20230417. (doi:10.1098/rsta.2023.0417)]. On one side, we evaluate the relative importance of the inelastic contribution, compared to elastic and charge-exchange collisions. On the other side, we compare the contributions of ion/rec, thermal pressure, viscosity and magnetic forces to the charge-neutral drift velocity of the turbulent flow of the RTI. Our analysis reveals that the contribution of inelastic collisions to the ambipolar diffusion coefficient is negligible across the chromosphere, allowing the classical definition of this coefficient to be safely used in 1F modelling. However, in the transition region, the contribution of inelastic collisions can become as significant as that of elastic collisions. Furthermore, we ascertain that the thermal pressure force predominantly influences the charge-neutral drifts in the RTI model, surpassing the impact of the magnetic force. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter Array (ALMA) allows for solar observations in the wavelength range of 0.3–10 mm, giving us a new view of the chromosphere. The measured brightness temperature at various frequencies can be fitted with theoretical models of density and temperature versus height. We use the available ALMA and Metsähovi measurements of selected solar structures (quiet sun (QS), active regions (AR) devoid of sunspots, and coronal holes (CH)). The measured QS brightness temperature in the ALMA wavelength range agrees well with the predictions of the semiempirical Avrett–Tian–Landi–Curdt–Wülser (ATLCW) model, better than previous models such as the Avrett–Loeser (AL) or Fontenla–Avrett–Loeser model (FAL). We scaled the ATLCW model in density and temperature to fit the observations of the other structures. For ARs, the fitted models require 9%–13% higher electron densities and 9%–10% higher electron temperatures, consistent with expectations. The CH fitted models require electron densities 2%–40% lower than the QS level, while the predicted electron temperatures, although somewhat lower, do not deviate significantly from the QS model. Despite the limitations of the one‐dimensional ATLCW model, we confirm that this model and its appropriate adaptations are sufficient for describing the basic physical properties of the solar structures. [ABSTRACT FROM AUTHOR]

Henri Deslandres initiated imaging spectroscopy of the solar atmosphere in 1892 at Paris Observatory. He developed two kinds of spectrographs: the 'spectrohéliographe des formes', i.e. the monochromatic instrument to reveal chromospheric structures (such as filaments, prominences, plages and active regions); and the 'spectrohéliographe des vitesses', i.e. the spectroheliograph to record line profiles of cross-sections of the Sun, in order to measure the Doppler shifts of dynamic features. Deslandres moved to Meudon and hired Lucien d'Azambuja in 1899; together they built the large quadruple spectroheliograph. CaII K systematic observations of the full Sun started in 1908 and were followed in 1909 by Hα with two dedicated 3-metre spectroheliographs. Daily observations were organized by d'Azambuja who also intensively used the large 7-metre spectroheliograph for his research and thesis (1930). This paper summarizes fifty years of investigations by Mr and Mrs d'Azambuja, who explored various photospheric and chromospheric lines, obtaining special spectroheliograms with the high dispersion 7-metre instrument. They also intensively observed filaments and prominences, reported results in an extensive memoir (1948) and recorded rare solar activity events with the two 3-metre spectroheliographs, until their retirement in 1959. [ABSTRACT FROM AUTHOR]

Oscillations are ubiquitous in sunspots and the associated higher atmospheres. However, it is still unclear whether these oscillations are driven by the external acoustic waves (p-modes) or generated by the internal magnetoconvection. To obtain clues about the driving source of umbral waves in sunspots, we analyzed the spiral wave patterns (SWPs) in two sunspots registered by IRIS MgII 2796 Å slit-jaw images. By tracking the motion of the SWPs, we found for the first time that two one-armed SWPs coexist in the umbra, and they can rotate either in the same or opposite directions. Furthermore, by analyzing the spatial distribution of the oscillation centers of the one-armed SWPs within the umbra (the oscillation center is defined as the location where the SWP first appears), we found that the chromospheric umbral waves repeatedly originate from the regions with high oscillation power and most of the umbral waves occur in the dark nuclei and strong magnetic field regions of the umbra. Our study results indicate that the chromospheric umbral waves are likely excited by the p-mode oscillations. [ABSTRACT FROM AUTHOR]

Observations have shown that the chromosphere networks are rich in features with density higher than the ambient atmosphere. To investigate the effect of horizontal density-inhomogeneity on spicules, here we carry out two-dimensional magnetohydrodynamic (MHD) simulations based on the shock scenario. In a gravitationally stratified solar atmosphere, we insert a vertical preexisting density structure (PeDS) that has higher density than the ambient regions, and then we drive a spicule by a velocity pulse at the bottom of the chromosphere. We find a horizontal flow of 2 km s−1 caused by a rarefaction wave that may have a certain material supplement effect for the spicules and a V-shaped shock front in the chromosphere. An interesting feature found in our experiment is that the existence of PeDS leads to the formation of multiple threads in spicules. Their formation results from the larger density, lower transition region, and higher speeds of magnetoacoustic waves in the PeDS than at its outer boundaries. Parameter studies show that multiple threads of a spicule can be more pronounced in cases with wider velocity pulses and a larger internal/external density ratio in the PeDS. Our study shows that the horizontal density-inhomogeneity in the solar atmosphere is an important factor that is responsible for the complexity of a spicule. [ABSTRACT FROM AUTHOR]

The mechanism of wave dissipation in different layers of the sun, as well as the solar granules inter-network bright points and internetwork bright points of chromosphoric solar granules, have been explored in this article. This article's data come from the Interface Region Imaging Spectrograph (IRIS). IRIS is a small exploration mission by NASA. That obtains spectra in nearultraviolet (NUV), far-ultraviolet 1 (FUV1), and far-ultraviolet 2 (FUV2), from 1332 to 2834 𝐴̇. Slit jaw images (SJIs) of IRIS, using various filters that can provide images centered on the Mg II h wing, Mg II k, Si IV 1403 𝐴̇, and C ii lines. To begin, bright points in the chromosphere are selected for this goal. These points were first chosen using IRIS Slit Jaw Images (SJI) Si IV 1403 𝐴̇. The time slices of the Doppler velocity of the Mg II k spectrum are then drawn at a specified velocity (+/- 20 km/s) and were fed into the wavelet transform function to perform the time-frequency analysis to obtain the oscillation periods of the Doppler velocity. The wavelet transform used for this purpose is Morlet 5 wavelet. The velocity oscillation period data are utilized to determine the bright points of the network and internetwork solar granules at the chromosphere. The Doppler shift diagram for the Si IV 1394 spectral line is then displayed in time. This graph is a Doppler shift various time graph with attenuation that is caused by wave loss in the solar layers. According to the obtained data, the network and internetwork bright points, the chromospheric solar granules have an attenuation time 3 to 5 min. The oscillations of the solar granules network bright points are damped more intensely than the solar granules inter-network bright points, and hence their damping lifetime is shorter. Si IV 1394 𝐴̇ Doppler velocity shift of bright points placed in the solar granules inter-network is dampened by a lower slope and so has a longer damping life time. Sadeghi and Tavabi researched about the kinetic energy above the bright points of the network and internetwork regions of the solar granules in a part of a paper titled "A new approach to kinetic energy flux at the different frequencies above the IRIS bright points" in 2022. They claim that a substantial percentage of the energy of the network bright points of solar granules is transferred to higher layers, namely the transition region corona, in the network bright points of solar granules. These findings are congruent with the findings of the current study. The oscillations in these locations are dampened for a brief period of time, and the energy is transmitted to higher layers with little loss. Furthermore, it is stated in the cited paper that the majority of the energy at the internetwork bright points of the solar granules does not transfer to higher layers of Sun. This suggests that the energy loss from bright points in the solar granules' internetwork region is rather high in the chromospheric layer. The time period of the wave damping is longer than the length of the damping time in the oscillations, according to the current study on the mechanism of wave dissipation in different layers of the sun and the network and internetwork bright points of the chromosphere. It has also been demonstrated that the time period of the wave damping is greater than the length of the damping time in the oscillations of the bright points of the granules' network region, which is the source of the most energy loss in this layer. [ABSTRACT FROM AUTHOR]

Chromosphere is the second layer of the Sun with high variability. The increase of the temperature and the decrease of the density are observed in this layer. This unusual behavior is one of the most important problems in the solar corona. Between the solar chromosphere and the corona, there is a thin transition zone in which the temperature rises very rapidly. Magnetohydrodynamic waves are thought to play an important role in this heating. The dissipation of Alfven waves has been investigated due to phase mixing in the presence of steady flow and sheared magnetic field in a solar stratified flux tube. The temperature variations with height (T0(z)) in the flux tube has been considered. The numerical calculations showed that the amplitude of the tube oscillations decreases with time. Hence, the wave damping takes place in the flux tubes. The temperature and the density variations enhances the wave damping rate compared to the case without temperature effect.

The similarities between Ca and x-ray jets suggest that their formation mechanism should be the same. Surges are almost cool plasma jets and are usually observed in Hα at ground-based observation, space observations also detect Surges. The structure and movement of the observed surges and jets necessitate a reconnection framework in which magnetic tension and the release of twisted energy contribute significantly. Convincing indications of reconnection are offered by whip-like movements, and the rotational behavior of the surges arises as an outcome of the relaxation of reestablished magnetic twist. The IRIS spectra spicules to determine one of the factors affecting the production and feeding of solar winds. Jets and spicules have been considered by examining several raster frames from the regions of the Sun. The characteristics of spicules, such as the rotational velocity of each spicule concerning the central axis of the spectrum , were computed using the output of the Doppler map. Then, using the Mg II spectrum simultaneously, Doppler maps for specific index samples were created at various speeds. Based on the results, a speed of 30 km/s was demonstrated.Our investigation into spicules unveiled a distinctive type that deviates from prior observations regarding speed, angle, and direction of movement. These spicules appear to result from vortical movements, which could also contribute to the propagation of Alfvén rotational waves and the transfer of energy to the Sun's upper atmosphere. In this study, it is essential to recognize that rotating spicules represent minute structures observable in exceedingly high-resolution imaging.

The Sun is a magnetically active star. It has a powerful magnetic field that shifts slightly from year to year until it reverses about every eleven years. The sun's magnetic field has many effects, the collection of which is called solar activity such as, sunspots, solar flares, and coronal mass ejections(CMEs). Currently, it is only possible to predict a magnetic storm 30 to 60 minutes before it occurs, which is a very short time. Solar flares and CMEs, massive eruptions of superheated plasma, are the two most energetic phenomenon which impulsively ejected and accelerated by releasing magnetic energy in the solar corona. Heliosphere, space weather, and the Earth are affected from transporting coronal plasma. Following the occurrence of these phenomenon in the Sun, the solar wind blows with greater speed and intensity and sends energetic charged particles into the space of the solar system. When these particles reach the Earth, their radiating electromagnetic waves interact with magnetic field of the Earth. Then various effects are observed, as shock waves and massive geomagnetic storms that disrupt satellites and power grids. Today, using the advancements of ground and space technology, the solar surface can be easily observed. Therefore, observation and prediction of solar storm will be possible more than in the past. In this paper, we review papers intended to collect comprehensive information about what has already been researched about fast solar jets and CMEs made by ground and space instruments over the last decades.

We analyzed the dynamics of the reconnection and energy release processes of the X2.2 and X9.3 flares that occurred on September 6, 2017 in the active region NOAA 12673. We used SDO/AIA 1600 Å images together with SDO/HMI magnetograms. As a proxy for the flare energy release rate, we used the KRIM and RSTN microwave time profiles, GOES soft X-rays and its time derivative. Assuming that the chromospheric flare ribbons are located at the footpoints of magnetic field lines reconnecting in the corona and that the magnetic flux is conserved from the photosphere to the corona, we obtained magnetic reconnection fluxes and rates of its change. The cumulative positive and negative magnetic fluxes involved in the reconnection process were balanced. Temporal correlations are found between the calculated reconnection rate and the observed microwave emissions from for both events. An analysis of the total cumulative magnetic flux and SXR fluence of events showed that the magnitude of the magnetic flux involved in the reconnection process was greater in more energetic events and less in weaker ones. [ABSTRACT FROM AUTHOR]

Solar eruptions, which generate various types of wave phenomena in the solar atmosphere, are sources of space weather perturbations. These waves not only transport a large amount of energy, but also illuminate the properties of the magnetic field and plasma on the path of propagation. Therefore, it is of great interest to investigate wave phenomena in the solar atmosphere, among which Moreton waves in the solar chromosphere and extreme ultraviolet (EUV) waves in the corona have attracted much attention in the past few decades. Moreton waves are characterized by bright fronts in the Hα line center and blue wing (or dark fronts in the Hα red wing), which propagate at speeds ranging from ~500 km/s to more than 2000 km/s. They were also observed in He I 10830 Å. EUV waves are characterized by bright fronts in the EUV images, which propagate at speeds ranging from ~10 km/s to more than 2000 km/s. Whereas the understanding of Moreton waves is rather mature, the nature of EUV waves and their relationship with Moreton waves are controversial and have been debated for more than two decades. Initially, it was proposed that EUV waves are the coronal counterparts of chromospheric Moreton waves; that is, they are fast-mode MHD waves. However, many EUV waves have been found to have speeds that are less than the sound speed, which means that some EUV waves cannot be accounted for by the fast-mode MHD wave model. Therefore, several alternate models have been proposed, such as slow-mode soliton waves, echoes of fast-mode waves, successive magnetic reconnection models, as well as hybrid models, which predict the existence of two components of EUV waves—a fast-mode and a slower pseudo-wave. The pseudo-wave is explained by the magnetic field line stretching model, which predicts that the fast-mode wave should be ~3 times faster than the pseudo-wave. With later high-cadence observations, EUV waves with two components have been revealed in many events, confirming the validity of the hybrid model. However, recent observations have revealed many new features that deserve further elaboration of the existing models, such as secondary EUV waves, small-scale EUV waves, patchy EUV waves, quasi-periodic EUV waves, multiple EUV waves, homologous EUV waves, and stationary EUV waves. In particular, the very recent observations of stationary EUV waves may indicate that the fast component of the EUV wave might experience a mode conversion from fast to slow mode when the wave crosses a region of weak magnetic field. In this article, we review the progress made in the research of Moreton and EUV waves and discuss in detail the discovery of the Moreton wave, the early classical model of Moreton waves (as well as recent modifications), the discovery of EUV waves, various observational features, and various models of EUV waves. Lastly, we offer our perspectives on the current research in Moreton and EUV waves and highlight the importance of this research.

Abstract The application of machine learning in solar physics has the potential to greatly enhance our understanding of the complex processes that take place in the atmosphere of the Sun. By using techniques such as deep learning, we are now in the position to analyze large amounts of data from solar observations and identify patterns and trends that may not have been apparent using traditional methods. This can help us improve our understanding of explosive events like solar flares, which can have a strong effect on the Earth environment. Predicting hazardous events on Earth becomes crucial for our technological society. Machine learning can also improve our understanding of the inner workings of the sun itself by allowing us to go deeper into the data and to propose more complex models to explain them. Additionally, the use of machine learning can help to automate the analysis of solar data, reducing the need for manual labor and increasing the efficiency of research in this field.

We have studied oscillation processes in the quiet Sun outside a coronal hole at different levels of the solar chromosphere. The study was based on spectroscopic observations of ionized calcium lines (K, H, and 849.8 nm) obtained by the Sayan Solar Observatory’s Automated Solar Telescope (AST). Spectral analysis of time series for some parameters of the lines has been carried out. We have compared the results of this work with the results of our study of oscillation processes in quiet regions located at the base of a coronal hole. The oscillation power was found to be higher in the region of the quiet Sun outside a coronal hole. At the same time, for the regions under study there is a common tendency for the oscillation power to decrease with height for all frequency ranges, except for the low-frequency one, in most chromospheric structures. In structures with a weak magnetic field, the power increases with height to the lower chromosphere and decreases somewhat to the upper chromosphere.

Solar radio observations provide a powerful diagnostic of the physical conditions of the solar atmosphere over a wide range of heights. In this paper, we report regular solar mapping made at the X-band (8.1 – 9.2 GHz) with the Arecibo 12-m radio telescope, covering a period between 13 December 2021 and 9 April 2023. This has demonstrated its potential for identifying active regions and tracking their brightness-temperature changes. The X-band results are discussed along with the near-simultaneous datasets available from space- and ground-based observations. A comparison of magnetic properties of active regions with their emission characteristics indicates that the X-band brightness temperature provides better information of the magnetic-field strength associated with the emission and a brightness temperature in excess of 13 000 K allows us to infer the possibility of intense flares (i.e., ≳ M1 class) and coronal mass ejections. The 'latitude–time' distribution of the brightness temperature reveals the three-dimensional evolution of quiet regions on the Sun, coronal holes, and eruptive sites, over many solar rotations in the ascending phase of the current Solar Cycle 25. [ABSTRACT FROM AUTHOR]

Our study focuses on analysing the coronal, transition and chromospheric activity of four rapidly rotating stars located within 50 pc in the solar neighborhood. We have used the multi-wavelength capabilities of AstroSat to investigate the outer atmospheres of AB Dor, BO Mic, DG CVn and GJ 3331. These stars, classified as M and K type active stars, are known for their short rotation periods, leading to increased surface magnetic activity. Our soft X-ray observations provide the coronal properties, such as emission measures, temperatures and elemental coronal abundances. We reported the detection of X-ray flares from AB Dor, BO Mic and DG CVn, while UV light curves reveal flares in both BO Mic and DG CVn. [ABSTRACT FROM AUTHOR]

In this article we describe the developement of an insulated box with a thermostat controller to protect the sensitive Baader Solar Spectrum H alpha filter in the low temperatures of winter. Our goal is to facilitate daily solar synoptic observations in all seasons. [ABSTRACT FROM AUTHOR]

Chromosphere is the second layer of the Sun with high variability. The increase of the temperature and the decrease of the density are observed in this layer. This unusual behavior is one of the most important problems in the solar corona. Between the solar chromosphere and the corona, there is a thin transition zone in which the temperature rises very rapidly. Magnetohydrodynamic waves are thought to play an important role in this heating. The dissipation of Alfven waves has been investigated due to phase mixing in the presence of steady flow and sheared magnetic field in a solar stratified flux tube. The temperature variations with height (T0(z)) in the flux tube has been considered. The numerical calculations showed that the amplitude of the tube oscillations decreases with time. Hence, the wave damping takes place in the flux tubes. The temperature and the density variations enhances the wave damping rate compared to the case without temperature effect. [ABSTRACT FROM AUTHOR]

Magnetohydrodynamic (MHD) waves play a crucial role in the plasma processes of stellar atmospheres and planetary magnetospheres. Wave phenomena in both media are known to have similarities and unique traits typical of each system. MHD waves and related phenomena in magnetospheric and solar physics are studied largely independently of each other, despite the similarity in properties of these media and the common physical foundations of wave generation and propagation. A unified approach to studying MHD waves in the Sun and Earth's magnetosphere opens up prospects for further progress in these two fields. The review examines the current state of research into MHD waves in the Sun’s atmosphere and Earth's magnetosphere. It outlines the main features of the wave propagation media: their structure, scales, and typical parameters. We describe the main theoretical models applied to wave behavior studies; discuss their advantages and limitations; compare characteristics of MHD waves in the Sun’s atmosphere and Earth’s magnetosphere; and review observation methods and tools to obtain information on waves in various media.

Broad-band observations of the solar photosphere began in Meudon in 1875 under the auspices of Jules Janssen. For his part, Henri Deslandres initiated imaging spectroscopy in 1892 at Paris observatory. He invented, concurrently with George Hale in Kenwood (USA), but quite independently, the spectroheliograph designed for monochromatic imagery of the solar atmosphere. Deslandres developed two kinds of spectrographs: the ' spectrohéliographe des formes ', that is, the narrow bandpass instrument to reveal chromospheric structures; and the ' spectrohéliographe des vitesses ', that is, the section spectroheliograph to record line profiles of cross sections of the Sun. This second apparatus was intended to measure the Dopplershifts of dynamic features. Deslandres moved to Meudon in 1898 to build the large quadruple spectroheliograph. The service of Hα and CaII K systematic observations was organized by Lucien d'Azambuja and continues today. The digital technology was introduced in 2002. The collection is one of the longest available: it contains sporadic images from 1893 to 1907 (during the development phase) and systematic observations along 10 solar cycles since 1908. This paper summarizes 130 years of observations, instrumental research and technical advances. [ABSTRACT FROM AUTHOR]

The application of machine learning in solar physics has the potential to greatly enhance our understanding of the complex processes that take place in the atmosphere of the Sun. By using techniques such as deep learning, we are now in the position to analyze large amounts of data from solar observations and identify patterns and trends that may not have been apparent using traditional methods. This can help us improve our understanding of explosive events like solar flares, which can have a strong effect on the Earth environment. Predicting hazardous events on Earth becomes crucial for our technological society. Machine learning can also improve our understanding of the inner workings of the sun itself by allowing us to go deeper into the data and to propose more complex models to explain them. Additionally, the use of machine learning can help to automate the analysis of solar data, reducing the need for manual labor and increasing the efficiency of research in this field. [ABSTRACT FROM AUTHOR]

The flare activity of the Sun has been studied for decades, using both space- and ground-based telescopes. The former have mainly focused on the corona, while the latter have mostly been used to investigate the conditions in the chromosphere and photosphere. The Interface Region Imaging Spectrograph (IRIS) instrument has served as a gateway between these two cases, given its capability to observe quasi-simultaneously the corona, the transition region, and the chromosphere using different spectral lines in the near- and far-ultraviolet ranges. IRIS thus provides unique diagnostics to investigate the thermodynamics of flares in the solar atmosphere. In particular, the Mg II h&k and the Mg II UV triplet lines provide key information about the thermodynamics of low to upper chromosphere, while the C II 1334 & 1335 Å lines cover the upper-chromosphere and low transition region. The Mg II h&k and the Mg II UV triplet lines show a peculiar, pointy shape before and during the flare activity. The physical interpretation, i.e., the physical conditions in the chromosphere, that can explain these profiles has remained elusive. In this paper, we show the results of a non-LTE inversion of such peculiar profiles. To better constrain the atmospheric conditions, the Mg II h&k and the Mg II UV triplet lines are simultaneously inverted with the C II 1334 & 1335 Å lines. This combined inversion leads to more accurate derived thermodynamic parameters, especially the temperature and the turbulent motions (micro-turbulence velocity). We use an iterative process that looks for the best fit between the observed profile and a synthetic profile obtained by considering non-local thermodynamic equilibrium and partial frequency redistribution of the radiation due to scattered photons. This method is computationally rather expensive (≈6 CPU-hour/profile). Therefore, we use the k-means clustering technique to identify representative profiles and associated representative model atmospheres. By inverting the representative profiles with the most advanced inversion code (STiC), in addition to recover the main physical parameters, we are able to conclude that these unique, pointy profiles are associated with a large gradient in the line-of-sight velocity along the optical depth in the high chromosphere.

Simultaneous observations of the Interface Region Imaging Spectrograph (IRIS) data, with a spatial resolution of less than one second consisting of ultraviolet (UV) spectra and images (SJI), make it possible to investigate solar chromosphere and transition region and provide valuable information about the dynamics of solar jets. IRIS combines numerical modeling, high resolution imaging, and UV spectroscopy. The interface region is the main place for the transfer of energy from the solar surface to the very hot corona. Of course, knowing the secret of energy transfer in the solar atmosphere is not the only goal of this mission, but also it examines the solar winds that is emitted from this area, which carry a rain of charged particles into space and also affect the Earth's climate. Information about the dynamic behavior of the physical phenomena of the solar atmosphere is obtained by studying the characteristics of spectral lines. For this purpose, it is necessary to obtain the information to identify and study spectral lines and how they are formed. The solar atmosphere is a plasma environment associated with a variety of transient events. Astrophysicists, especially in the field of solar dynamic physics, describe these events by magneto-hydrodynamics aspect. One of these phenomena is the bright spots of the solar atmosphere called jets. We identify and study the dynamics of a series of jets recorded on August 17, 2014, at Mg II k, C II and Si IV spectral lines corresponding to the 2796 A, 1336 A, and 1394 A wavelengths, respectively. Jets are small-scale dynamic events that can be detected by non-Gaussian profiles of lines in the solar chromosphere and transition region. The production mechanism of these plasma jets is still being investigated. We use the temporal evolution analysis method to track the path of these structures and determine their apparent velocity. To calculate the Doppler velocity we perform Gaussian fitting at the same time on the spectral intensity profiles. The apparent velocity results show that these jets have quasiperiodic motions with speeds of approximately 10 to 110 kms-1. Spectral investigation of these jets also shows the periodic behavior that is associated with increasing in blue and red wings at the three wavelengths as -65 to 40, 60 to 50, and 80 to 60 kms-1, respectively. Simultaneous enhancements in the blue and red wings of the spectrum can be caused by two-directional upward currents caused by magnetic reconnection and amplified by waves with p-modes (compression modes). According to these results, it is suggested that the fluctuations in these events with increasing on one side of the spectrum and both sides of the wing are signs of spiral and rotational motions, respectively. The results of this research show that by using the data of the IRIS Telescope, it is possible to identify and extract the physical components of jets at different wavelengths and identify their dynamic behavior. These specifications will help us better understand the stratification of the solar atmosphere and how heat and matter are transferred to the sun's surface and the effects of such transitions on the Earth's atmosphere. The application of this study will be the goal of space research and is very important in identifying space and Earth's climate. [ABSTRACT FROM AUTHOR]

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Spicules and jets occur in a typical emerging magnetical collimated flux tube, and we examine polar surges through limb observations in two wavelengths, revealing a more distinct view of the dynamic events high above the polar cap's chromosphere. Magnetic flux tube plays a key role and a wide range of jet-like activities in the solar atmosphere. The observational characteristics of these mass ejections are reviewed, and unified model based on magnetic reconnection or other alternative is presented to account for these different plasma ejections. Several excellent time sequences of images were taken with the Hinode Solar Optical Telescope (SOT) through the HCaII K and Ha line broadband filter and also X-Ray Telescope (XRT) with high spatial and temporal resolution was used. Image processing include the use of a madmax operator is used to reveal and improve the visibility of hair-like futures (especially for chromospheric small-scale anemone jets) and to a specific plasma motion in the vicinity of the X-null point. The intensive and rapid temporal observations imply a tight connection between Surges and similar jet-like phenomena, corroborating the theory that the acceleration and upward movement of these events are propelled by magnetic reconnection triggered by the emergence of flux. [ABSTRACT FROM AUTHOR]

The Sun is a magnetically active star. It has a powerful magnetic field that shifts slightly from year to year until it reverses about every eleven years. The sun's magnetic field has many effects, the collection of which is called solar activity such as, sunspots, solar ares, and coronal mass ejections (CMEs). Currently, it is only possible to predict a magnetic storm 30 to 60 minutes before it occurs, which is a very short time. Solar ares and CMEs, massive eruptions of superheated plasma, are the two most energetic phenomenon which impulsively ejected and accelerated by releasing magnetic energy in the solar corona. Heliosphere, space weather, and the Earth are affected from trans-porting coronal plasma. Following the occurrence of these phenomenon in the Sun, the solar wind blows with greater speed and intensity and sends energetic charged particles into the space of the solar system. When these particles reach the Earth, their radiating electromagnetic waves interact with magnetic field of the Earth. Then various effects are observed, as shock waves and massive geomagnetic storms that disrupt satellites and power grids. Today, using the advancements of ground and space technology, the solar surface can be easily observed. Therefore, observation and prediction of solar storm will be possible more than in the past. In this paper, we review papers intended to collect comprehensive information about what has already been researched about fast solar jets and CMEs made by ground and space instruments over the last decades. [ABSTRACT FROM AUTHOR]

In this study we review solar chromospheric jets and characteristics of them. The chromosphere and transition region are the interface between the solar photosphere and the corona, which plays a key role in the formation and acceleration of the solar wind. In recent years, scientists have made great efforts to understand the mechanism of energy transfer in the solar chromosphere and corona. The researchers suggest that the key to solving this problem may lie in understanding the nature of the small-scale transient events that are distributed across the surface of the Sun. Of these, solar spicules are the most prominent small-scale dynamical phenomena in the chromospheric regions that drive relatively cold material from the lower chromosphere to the corona. Spicules can heat the corona both by ejecting hot plasma and by energy transfer by magnetohydrodynamic (MHD) waves. These dynamical structures are formed when photospheric oscillations and convective flow along the magnetic field lines penetrate into the chromosphere.

We study point-like explosive events (EE), characterized by emission in the far wings of spectral lines, in a quiet region near the South Pole, using Interface Region Imaging Spectrograph (IRIS) spectra at two slit positions, slit-jaw (SJ) observations, and Atmospheric Imaging Assembly (AIA) images. The events were best visible in Si iv spectra; they were weak in SJs, occasionally visible in 1600 Å and 304 Å AIA images, and invisible in higher temperature AIA images. We identified EEs from position–time images in the far wings of the Si iv lines and measured their distance from the limb. A Gaussian model of the height distribution showed that EEs occur in a narrow (0.9′′) height range, centered at 3.2′′ above the continuum limb at 2832.0 Å. On the disk, we found that they occur in network boundaries. Further, we studied the line profiles of two bright EEs above the limb and one on the disk. We found that what appears as broad-band emission is actually a superposition of 2 – 3 narrow-band Gaussian components with well-separated line profiles, indicating that material is expelled towards and/or away from the observer in discrete episodes in time and in space. The expelled plasma accelerates quickly, reaching line-of-sight (LOS) velocities up to 90 km s−1. Overall, the motion was practically along the LOS, as the velocity on the plane of the sky was small. In some cases, tilted spectra were observed that could be interpreted in terms of rotating motions of up to 30 km s−1. We did not find any strong absorption features in the wing of the Si iv lines, although in one case, a very weak absorption feature was detected. No motions indicative of jets were detected in SJ or AIA images. Reconnection in an asymmetric magnetic-field geometry, in the middle or near the top of small loops, is a plausible explanation of their observational characteristics. [ABSTRACT FROM AUTHOR]

We present new results towards the explanation of the chromospheric-heating problem and the solar-wind origin, using a two-fluid model that takes into account the collisional interaction between ions (protons) and neutrals (hydrogen atoms). Our aim is to further reveal the mechanism behind chromospheric heating and plasma outflows. We simulate and analyse the propagation and evolution of Alfvén waves in the partially ionised solar chromosphere, consisting of ions + electrons and neutral fluids. The simplified model chromosphere is permeated by a vertical, uniform magnetic field. We perform numerical simulations in the framework of a quasi-1.5-dimensional (1.5D), two-fluid model in which Alfvén waves are excited by a harmonic driver in the transverse component of the ion and neutral velocities, operating in the chromosphere. In the case of a small-amplitude driver, Alfvén waves are weakly damped, and for the chosen wave periods of a few seconds, Alfvén waves manage to propagate through the chromosphere and enter the solar corona. Non-linear Alfvén waves excited by a large-amplitude driver cause significant chromospheric heating and plasma outflows. We thus conclude that two-fluid Alfvén waves with larger amplitudes can contribute to chromospheric heating and plasma outflows, which may result higher up in the solar-wind origin. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present intensity curves of solar flares obtained in the Hα hydrogen line and CaII H, CaIR 8542Å lines using multichannel spectrographs of Ondřejov Observatory (Czech Republic) for the period 2000–2012. The general behavior of observed intensity curves is practically the same for all flares and is consistent with temporal variations of X-ray emission. However, our results differ significantly from those obtained by other authors for selected flare stars, for example, AD Leo; EV Lac; YZ CMi. We tried to explain the difference in the behavior of Ca II and Hα radiation flux by appearance of a shock wave during a flare and slow heating of the plasma.