Physical conditions in the chromosphere of a two-strand solar flare with ejection

Chromosphere layers of solar flares were investigated according to the observed profiles of the H_α line. A two-strand flare was observed on September 4, 1990. Spectra were obtained with the ATsU-26 solar horizontal telescope at Terskol Peak Observatory (3100 m). Spectra photometry is performed for two bright nodes of one strand of the flare. Some profiles are superposed to the ejection. The observed profiles are characterized by high emissions in the wings of the H_α line (up to 10–12 Å) under relatively low intensity in the center of H_α (r = 0.35–0.6). To explain such profile behavior we calculated flare models with two or three components. Separate components of the model correspond to unresolved details in the flare area and therefore the averaged profile is calculated. Emission in the far wings is explained by model components with deep heating of chromosphere layers. These occupy 5–12% of the total area. Noticeable emission asymmetry is explained by ray velocities of up to 70 km/s and more. The models are determined by agreement of the observed and calculated profiles. We processed several photometric profiles for seven observations. The temperature in the models with deep heating in the lower cromosphere is increased by 1000–2500 K with respect to the model with an undisturbed chromosphere VAL-C. The second feature of the observed profiles is their high asymmetry and shift with respect to the undisturbed profiles. This can be explained by the opposite motion of the material. We revealed that for the most of the profiles the line-of-sight velocities were directed to the observer in the upper chromosphere (10–100 km/s) and from the observer in the lower chromosphere (5–20 km/s).     

Radial velocity field in the lower atmosphere of the solar active region during a flare with an ejection: The initial phase of the flare

Horizontal motion has been studied of the matter along the active region at different heights of the photosphere (115–580 km) in the initial phase of the two-ribbon solar flare on September 4, 1990, near the solar limb, accompanied by the ejection. Photospheric velocities varied in the range −3.5 ... 2.5 km/s. The direction of motion in the photosphere and the chromosphere was mainly toward the observer. Kinematic elements have been discovered in the structure of the horizontal velocity field. Their size reduced as they approached the maximum of the flare from 7–12 to 4–5 Mm, and the velocity amplitude decreased. Throughout the whole investigated active region, vortex motions were observed in the photosphere and chromosphere. Temporal changes in the horizontal velocity field in node areas and in their vicinity were oscillatory in nature and occurred almost simultaneously along the entire height of the photosphere.     

Radial velocities of the photospheric matter in a solar flare with matter ejection

We present results of a study of photospheric horizontal motions at the initial and main phases of the solar flare which happened on September 4, 1990, near the solar limb. The flare was accompanied by matter ejection. Spectra of the flare were obtained using the AZU-26 horizontal solar telescope at the MAO NAS (Terskol observatory). We found variations of the matter motion velocity’s value and direction at different stages of the photosphere during the flare development. The velocity changed in a range from −4 to 2 km/s. Comparisons of the obtained data with variations of the chromospheric radial velocities showed that the horizontal matter motions in the photosphere and chromosphere are mostly directed toward the observer but at particular time moments their direction changed. At two different knots, the time shift of the photospheric velocities is different. The highest velocities were observed at the main phase of the flare. At the initial phase of the flare, in the matter ejection region, we note a velocity increase compared with its preflare value and at the flare knots.     

Spectral study of a pair of Ellerman bombs

Results of the analysis of spectral observations of two Ellerman bombs in the Hα line are presented. These bombs (EB-1 and EB-2) appeared and evolved in the active region NOAA 11024 in the emerging magnetic flux area. The spectral data of a high temporal and spatial resolution were obtained with the French–Italian solar telescope THEMIS (Tenerife Island, Spain; THEMIS stands for Télescope Héliographique pour l’Etude du Magnétisme et des Instabilités Solaires) on July 4, 2009. The Hα-line profiles obtained for different periods of the Ellerman bombs (EBs) evolution were asymmetrical, demonstrating the emission excess in the long-wavelength wing. The intensity variations in the line wings indicate both the gradual and pulsed release of energy in the course of EBs. Temporal variations in the line-of-sight velocities V _los of the chromospheric material at a level of the Hα-core formation showed two periods in the velocity enhancement, containing several individual peaks. The maximum line-of-sight velocity of the material was–9 and 8 km/s toward and from the observer, respectively. Rapid upward and downward plasma streams (where V _los reaches–80 and 50 km/s, respectively) were sometimes observed. The Ellerman bombs were accompanied by small chromospheric ejections (surges) lasting for 0.5–1.5 min. A fine structure of EBs was found in the H_α-line spectra obtained during 4 min, when the intensity in the line wings sharply increased. The peculiarities of variations in the intensity of the H_α-line wings and the line-of-sight velocity of the chromospheric material suggest that two investigated EBs appeared and evolved as a physically connected pair. Our results support the model wherein the magnetic reconnection in the lower atmospheric layers is considered as a triggering mechanism for the EB formation.     

Preflare changes in the solar photosphere observed using the THEMIS telescope

The physical state of the photosphere 1 h 50 min before a C1 solar flare on May 24, 2012, was studied. The spectropolarimetric data from the French-Italian THEMIS telescope (Tenerife Island, Spain) were used. The modeling was carried out through the inversion method using SIR [B. Ruiz Cobo and J. C. del Toro Iniesta, Astrophys. J. 398, 375–385 (1992)] code. Height distributions of temperature, magnetic field strength, and line-of-sight velocity were obtained. Nine semiempirical models of the photosphere were constructed. Each model has a two-component (a magnetic field component and nonmagnetic surroundings) structure. According to the obtained models, the magnetic field parameters and thermodynamic parameters did change significantly in the course of observations that lasted for 8 min. The models contain layers with increased and decreased temperature values. The magnetic field strength in these models varied, on average, from 0.2 T (lower photospheric layers) to 0.13 T (upper layers). The line-of-sight velocities did not exceed 2 km/s in lower and middle photospheric layers and rose to 5–6 km/s in the upper layers. The differences in the physical state and its changes occurring at different sites within the active region prior to the flare were revealed.     

Atmosphere Dynamics of the Active Region NOAA 11024

We present results of the study of chromospheric and photospheric line-of-sight velocity fields in the young active region NOAA 11024. Multi-layer, multi-wavelength observational data were used for the analysis of the emerging flux in this active region. Spectropolarimetric observations were carried out with the telescope THEMIS on Tenerife (Canary Islands) on 4 July 2009. In addition, space-borne data from SOHO/MDI, STEREO and GOES were also considered. The combination of data from ground- and space-based telescopes allowed us to study the dynamics of the lower atmosphere of the active region with high spatial, spectral, and temporal resolutions. THEMIS spectra show strong temporal variations of the velocity in the chromosphere and photosphere for various activity features: two pores, active and quiet plage regions, and two surges. The range of variations of the chromospheric line-of-sight velocity at the heights of the formation of the Hα core was extremely large. Both upward and downward motions were observed in these layers. In particular, a surge with upward velocities up to ?73 km?s^?1 was detected. In the photosphere, predominantly upward motions were found, varying from ?3.1 km?s^?1 upflows to 1.4 km?s^?1 downflows in different structures. The velocity variations at different levels in the lower atmosphere are compatible with the emergence of magnetic flux.     

Dual chromospheric flows in the vicinity of a solar pore

Chromospheric line-of-sight velocities are investigated in a small pore and its vicinity on the part of the active region NOAA 11024 with a size of 5″. We used H_α spectra of the active region and undisturbed atmosphere obtained with the French–Italian solar telescope THEMIS (Tenerife, Spain). Significant line-of-sight velocity time variations are found. At the beginning of the observations, the investigated region consisted of two areas of oppositely directed flows. The first area had a bright point in the vicinity of the pore and the second area covered the pore. There were upflows in the former and downflows in the latter. Oppositely directed flows appeared in both areas 2.7 min after the start of observations. In the part of the active region with a length of 2Mm, two oppositely directed flows within the same resolution elements, the so-called dual flows, were observed. The size of the area occupied by the dual flows varied quickly. The area shifted toward the pore. The velocity of upflows and downflows reached 25 km/s. The downflows in the first area lasted only for approximately 1 min. Upflows in the second area gradually covered the pore and lasted for 2 min. The resulting velocity field distribution can be due to a new small-scale magnetic flux emergence.     

Plasma motions in the solar loop of emerging magnetic flux

The results of analyzing variations in the line-of-sight (LOS) velocities in the solar loop at photospheric and chromospheric levels in the region of emerging magnetic flux for the evolving active region NOAA 11024 are reported. The analysis combines the data of multiwave spectropolarimetric observations that were carried out on July 4, 2009, (Tenerife, Spain) using THEMIS solar telescope and the data obtained with GOES, SOHO, and STEREO cosmic satellites. A complex sequence of active events has been studied: formation of the Ellerman bomb, B1 X-ray microflare, and four chromospheric surges that were formed as a result of magnetic reconnection caused by new emerging magnetic flux. The Ellerman bomb was formed in the vicinity of a growing pore. Variations in the velocity V _LOS of the EB had an oscillation character for chromosphere and photosphere. Before the microflare, the average velocities of the upward and downward plasma fluxes in one leg of the magnetic loop were nearly the same—26 km/s. During the microflare, the velocity V _LOS of the ascending and descending flows increased up to ?33 and 50 km/s, respectively. Variations in line-of-sight velocity of a plasma in the second leg of the magnetic loop correlated well with variations of V _LOS in the region of microflare, but they occurred 1.5 minutes later. During the time of observations, four chromospheric ejections of matter were formed and three of them occurred in the region of Ellerman’s bomb formation. Sharp variations in the soft X-ray intensity occurred during these ejections. At photospheric level, variations in the line-of-sight velocity of plasma in the legs of the loop occurred in the opposite direction. In the region of the first leg, velocity V _LOS diminished from ?1.8 to ?0.4 km/s, while the velocity increased from ?0.6 to ?2.6 km/s in the region of the second leg.     

Influence of magnetic field on propagation of five-minute oscillations in the sun’s atmosphere: Phase shifts

From results of spectral (in Ba II λ 455.4-nm line) and spectropolarimetric (in Fe I λλ 1564.3–1565.8-nm lines) observations of the active region (an isolated faculae at the solar disk center) with the German vacuum tower telescope (VTT) at the Institute of Astrophysics on the Canary Islands, the peculiarities of propagation of five-minute oscillations from the photosphere base (h = 0 km) to the lower chromosphere (h = 650 km) were investigated. At the height of the continuum formation (h = 0 km), the nature of wave propagation in the active region does not differ much from that in the quiet region: 80–90% of the investigated areas are occupied by waves moving up and down. In the lower chromosphere (h = 650 km), differences in the behavior of the waves are fundamental. In a quiet area, the waves become standing for 90% of the cases. In contrast to this, in the presence of moderate and strong magnetic fields (B = 30–180 mT), in 47% of the cases, the waves are running upward, which gives the principal possibility to heat the active region. The investigations revealed the presence of the waves in the active region, for which the phase shift Φ _T,V of the temperature and velocity oscillations is between ?90° and 0°. These waves cannot propagate in a quiet atmosphere.     

A comparison between photospheric and chromospheric quiet-Sun magnetograms

Photospheric (Fei 5324.19 Å line) and chromospheric (H line) magnetic fields in quiet-Sun regions have been observed in the solar disk center by using the vector video magnetograph at Huairou Solar Observing Station of Beijing Astronomical Observatory. Observational results show that the quiet-Sun magnetic elements in the solar photosphere and chromosphere present similar magnetic structures. Photospheric and chromospheric magnetograms show corresponding time variations. This suggests that the magnetic fields in quiet-Sun regions present different 3-D magnetic configurations compared to those in solar active regions.     

Magnetic field effect on the fine structure of convective motions in the solar atmosphere

Statistical properties of solar granulation in an active region on the solar surface from the photosphere to the lower chromosphere are studied. We use the values of the velocity, intensity, and magnetic field that were obtained at different heights in the solar atmosphere according to the observation data on the VTT telescope at Observatorio del Teide, Tenerife. The changes in the line??s parameters (central depth of the line, halfwidth, equivalent width, and central depth shift) and convective velocity are presented as functions of the value of the magnetic field. We propose a 16-column model of solar granulation depending on the direction of motion of convective elements and on the sign of contrast at two heights??in the continuous spectrum and in the highest layer (h = 650 km). We found that the magnetic field impedes the change in the sign and motion direction of convective elements.     

PHYSICAL PARAMETERS OF A FLARE ON 22 DECEMBER 1999 DERIVED FROM THE Hα LINE WITH A MODIFIED CLOUD METHOD

The flare of 22 December 1999 was observed in the H line using the imaging spectrograph in the solar tower of Nanjing University. We present a new technique different from the classical cloud model to fit the H line profile which avoids using the background profile. We obtain the four parameters of the flare chromosphere: the source function, the optical thickness at line center, the line-of-sight velocity and the Doppler width. The observed asymmetry profiles have been reproduced well by the theoretical ones based on our model. A discussion is made about the reliability of the results we have obtained using the present method.     

Fine structure superimposed on millimeter-wavelength (23–18 GHz) spectra of solar active regions

A millimeter-wavelength (23–18 GHz) variable frequency radiometer with frequency resolution of 1 GHz and time resolution of 0.1–16 s has been developed in conjunction with a 13.7 m-diameter antenna. In this paper we describe briefly this new instrument, and its use to observe active regions.Spectra of four active regions show that: (i) spectra of the quiet-Sun region and those of the active region were of the same nature and the spectral index of both varied between –0.4 and +0.3; (ii) for two cases the spectra of the active region remained almost flat during the observing period of about one hour; (iii) spectra of the two other active regions exhibited frequency fine structures similar to the trough and crest type with a width of the order of 2 GHz in frequency and lasting about 40 to 90 min.The spectra of the quiet-Sun region and those of the active regions are attributed to bremsstrahlung emission. Fine structures in frequency type crest and trough are attributed to the radio signatures of the temperature plateau in the chromosphere.     

Flare model chromospheres and photospheres

Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of ～100 K over the temperature distribution in the pre-existing facula at a height of 300 km above τ₅₀₀₀ = 1. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We solve the transfer and statistical equilibrium equations for a three-level hydrogen atom and a five-level calcium atom in order to obtain the chromospheric flare models. The general properties of flares, including n _e, N ₂, linear thickness, and Lyman continuum intensity are approximately reproduced. We find that with increasing flare importance the height of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60–600 increase in pressure in these regions relative to the quiet Sun. The Ca II line profiles agree with observations only by assuming a macro-velocity distribution that increases with height. Also the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.     

Spatial structure and temporal development of a solar X-ray flare observed from Skylab on June 15, 1973

A solar flare on June 15, 1973 has been observed with high spatial and temporal resolution by the S-054 grazing-incidence X-ray telescope on Skylab. Both morphological and quantitative analyses are presented. Some of the main results are: (a) the overall configuration of the flare is that of a compact region with a characteristic size of the order of 30 at the intensity peak, (b) this region appears highly structured inside with complex systems of loops which change during the event, (c) a brightening over an extended portion of the active region precedes the flare onset, (d) the impulsive phase indicated by the non-thermal radio emission is a period during which a rapid brightening occurs in loop structures, (e) the X-ray emission is centered over the neutral line of longitudinal magnetic field, and the brightest structures at the flare onset bridge the neutral line, (f) loop systems at successively increasing heights form during the decay phase, finally leading to the large loops observed in the postflare phase, (g) different parts of the flare show distinctly different light curves, and the temporal development given by full disk detectors is the result of integrating the different intensity vs time profiles.The implications of these observations for mechanisms of solar flares are discussed. In particular, the flux profiles of different regions of the flare give strong evidence for continued heating during the decay phase, and a multiplicity of flare volumes appears to be present, in all cases consisting of loops of varying lengths.On leave from Arcetri Astrophysical Observatory, Florence, Italy.     

The Effect of Mass Motions Inside the Coronal Loops on Emission Line Profiles

The observed green coronal emission line profiles have been often found to have multi-components. Further examinations reveal that the occurrence of multi-components in line profiles is related to the solar cycle variations as well as the activity of the coronal region. The spatial correspondence between the intense loops in active regions and strong multi-components in line profiles suggests that the presence of loops affects the line shapes. The emission line profiles have been found to be fitted well with single or multi-Gaussians with line-of-sight velocities up to 70 km s–1. A simple radiative transfer model of coronal emission line profiles is developed which shows that coronal loops with mass motions inside may give rise to multi-components in line profiles. The effects of loop parameters such as electron density, flow velocity and kinetic temperature and the line-of-sight variations are studied. It is found that line profiles strongly reflect the physical conditions inside the loop.     

A qualitative interpretation of 7 August 1972 impulsive phase flare Hα line profiles

Tanaka's (1977) unique H profiles of the kernels of the 7 August 1972 flare were quantitatively interpreted by Brown et al. (1978; henceforth BCR) in terms of a thick target electron beam model. They found that this interpretation required beam inhomogeneity and/or partial precipation and large (60–100 km s^–1) macroturbulence. The latter requirement is somewhat suspect, since the only independent evidence also comes from efforts to understand the profiles of optically thick chromospheric lines. Relationships between model atmosphere parameters and line profile parameters calculated by Dinh (1980) show that these requirements could be considerably reduced, if not totally eliminated, if the actual chromospheric flare heating mechanism were simultaneously capable of pushing the flare transition region to greater column density and causing less heating of the residual chromosphere than the BCR models. This then implies that the chromosphere is heated primarily by a mechanism through which the heating effects do not penetrate as far below the flare transition region as is the case for a power-law spectrum of non-thermal electrons whose parameters are chosen appropriate to the nonthermal thick target interpretation of hard X-rays. Thermal conduction and optically thick radiation are examples of such a mechanism.     

Two-component photosphere models of a 2N/M2-class solar flare

Physical state of the photosphere during a 2N/M2 solar flare on July 18, 2000, was studied. We used Echelle Zeeman spectrograms obtained by V. G. Lozitsky in orthogonal circular polarizations with a solar spectrograph. Semiempirical photospheric models were constructed for three moments in time in the initial and main phases of the flare using the SIR code applied to Stokes I and V profiles of seven iron and chromium lines. The photospheric model of the flare contains two components: a magnetic-field component and nonmagnetic environment. The height distributions of the temperature, magnetic field, and line-of-sight velocity were derived. The temperature in the nonmagnetic component had a nonmonotonous run with height. The models include layers in the middle and upper photosphere in which temperature is enhanced relative to an unperturbed photosphere model. As the flare developed, the temperature in the lower layers was increasing by 500–800 K. The magnetic field increased by 0.05 T and 0.08–0.1 T in the lower and upper photosphere during the flare, respectively, with the vertical temperature gradient decreasing from 0.0012 to 0.0008 T/km. The model for the onset phase of the flare indicates that there were upflows and downflows of substance in the lower and upper photosphere, respectively. The flow velocities decreased appreciably in the main phase of the flare. The model parameters of the nonmagnetic environment were only slightly different from those of the unperturbed photosphere.     

Impulsive Phase He 10830 Spectra of a Large Solar Limb Flare of 16 August 1989*

We obtained simultaneously He i 10830 Å spectra, H filtergrams and microwave data of a large limb flare (2N/X20) in 1989. In this paper we characterize He i 10830 spectra in relation to the impulsive phase. All the He i 10830 spectra, except those of the surge, show blue shift or blue asymmetry. The velocities inferred from the spectra range from a few to 160 km s–1, implying that the horizontal motion is very likely present in the structure of this flare at different heights. The He i 10830 profiles of a flare are relatively broad and cannot be simulated by the Doppler broadening mechanism with a uniform flare model atmosphere. It is most likely that these characteristics are related to rapid and localized heating in the low and middle chromosphere. Comparing the SXR and microwave data with the optical data leads to the following scenario: the corona was already heated to some extent before the flare onset, and in the first 2 minutes of the impulsive phase, heat conduction was the main source or, at least, a competitive source, for chromospheric heating. However, the impulsive event, associated with the unusually broadened He i 10830 line (f>20 Å) and temporally correlated with a microwave burst, is probably caused by electron-beam heating.     

Observations and Modelling of the Pre-flare Period of the 29 March 2014 X1 Flare

On 29 March 2014, NOAA Active Region (AR) 12017 produced an X1 flare that was simultaneously observed by an unprecedented number of observatories. We have investigated the pre-flare period of this flare from 14:00 UT until 19:00 UT using joint observations made by the Interface Region Imaging Spectrometer (IRIS) and the Hinode Extreme Ultraviolet Imaging Spectrometer (EIS). Spectral lines providing coverage of the solar atmosphere from the chromosphere to the corona were analysed to investigate pre-flare activity within the AR. The results of the investigation have revealed evidence of strongly blue-shifted plasma flows, with velocities up to \(200~\mbox{km}\,\mbox{s}^{-1}\), being observed 40 minutes prior to flaring. These flows are located along the filament present in the active region and are both spatially discrete and transient. In order to constrain the possible explanations for this activity, we undertake non-potential magnetic field modelling of the active region. This modelling indicates the existence of a weakly twisted flux rope along the polarity inversion line in the region where a filament and the strong pre-flare flows are observed. We then discuss how these observations relate to the current models of flare triggering. We conclude that the most likely drivers of the observed activity are internal reconnection in the flux rope, early onset of the flare reconnection, or tether-cutting reconnection along the filament.