Spectropolarimetric investigation of an Ellerman bomb: 2. Photospheric models

Semiempirical models of the photosphere of an Ellerman bomb in the NOAA 11024 active region were obtained using profiles of Stokes parameters I, Q, U, and V of photospheric lines. Spectropolarimetric observations were conducted using the French–Italian THEMIS telescope (Tenerife, Spain). The SIR inversion code [28] was used in the modeling. The models have two components: a magnetic flux tube and nonmagnetic surroundings. The dependences of temperature, magnetic field strength, inclination of the magnetic field vector, and line-of-sight velocity in the tube on the optical depth were obtained. The models demonstrate that the thermodynamic parameters of the Ellerman bomb photosphere differ considerably from those of the quiet photosphere. The temperature in the tube model varied nonmonotonically with height and deviated by up to 700–900 K from its values for the quiet photosphere. Downflows were observed in the lower and the upper photospheric layers. The line-of-sight velocity in the upper layers of the photosphere was as high as 17 km/s. The magnetic field strength in the models varied from 0.1–0.13 T in the lower photospheric layers to 0.04–0.07 T in the upper ones. The physical state of the photosphere did change in the course of observations.     

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.     

Lower atmosphere in the main phase of a two-ribbon solar flare

This paper investigates the physical state of the photosphere in the main phase of the two-ribbon solar flare on June 3, 1979. The derived models show that the photosphere was in a disturbed state for a long time during the main phase of the flare. In the models, the temperature in the upper photospheric layers is higher and that in the lower layers is lower than in the quiet-sun model atmosphere. During the flare, the heating extends to the lower photospheric layers, and the upper layers cool down. A comparison of the obtained models to those for the two-ribbon solar flare on October 7, 1979, shows that the height distributions of the temperature in the main phase of the flares are strongly different.     

Physical State of the Photosphere at the Onset Phase of a Two-Ribbon Solar Flare

We study the physical state of the photosphere at about 30 minutes before and at the onset of a 2N/M2 two-ribbon solar flare. Semiempirical photospheric models are obtained for two Hα-kernels with the help of the SIR inversion code described by Ruiz Cobo and del Toro Iniesta (Astrophys. J. 398, 375, 1992). The models derived from the inversion reproduce spectral observations in seven Fraunhofer lines. The inferred models show variations in all photospheric parameters both before and at the onset of the flare relative to the quiet-Sun model. The temperature enhancement in the upper photospheric layers is found in the atmospheres in both kernels. The dynamical structure in the models reveals the variations at the onset of the flare relative to the preflaring ones. The inferred atmospheres show some difference in the thermodynamical parameters of two kernels.     

Photosphere model of 2N/2M solar flare: July 18, 2000

We investigate the photosphere parameters of a 2N/M2 solar flare that occurred in the NOAA 9077 active area on 18 July, 2000 before its maximum. We use Echelle Zeeman spectrograms obtained in orthogonal circular polarizations by means of a solar spectrograph of the astronomical observatory of Kiev National University, Ukraine (Kurochka, E.V., et. al, 1980). The photosphere is simulated by SIR software (Ruiz Cobo, B. and del Toro Inesta, J.C., 1992). The model of the flare??s photosphere is characterized by a two-component structure, including a magnetic flux tube and its nonmagnetic environment. For both components, we obtain the height distribution of the following parameters: temperature, magnetic field density and line-of-sight velocity. The temperature in the magnetic flux tube increases to approximately 5100 K in the upper photosphere layer of 250?C400 km. The magnetic field intensity decreases sharply from 2600 G (lower photosphere) to 100 G (middle photosphere) with a gradient of about 12 G/km. The model of the nonmagnetic environment differs slightly from the model of undisturbed photosphere.     

Temporal changes of physical conditions in the photospheric layers of a solar flare

An M4.1/1B solar flare on November 5, 2004, is investigated. The Stokes I ± V profiles of nine photospheric Fe I, Fe II, Sc II, and Cr II lines are studied for three instants of this flare (11 ^h 35 ^m , 11 ^h 39 ^m , and 11 ^h 45 ^m UT). The magnetic fields in the flare were measured in two ways: using the center-of-gravity method and by comparing the observed profiles with the theoretical ones computed with Baranovsky’s code. Analysis of the profiles reveals that the magnetic field strength peaked in the upper photosphere (logτ₅₀₀ = ?2.7) at the flare maximum (11 ^h 35 ^m ); this peak was smeared and shifted into the deeper photospheric layers as the flare evolved. The semiempirical model of the flare has two layers with an enhanced temperature: in the upper and middle photosphere. These layers also shifted deep into the photosphere as the flare evolved. The turbulent velocities at the distribution maximum increased by almost a factor of 5 compared to those in the undisturbed photosphere, while the plasma density both increased and decreased by a factor of 3–6.     

Changes in the photospheric magnetic field associated with solar flares

The flare-related, persistent and abrupt changes in the photospheric magnetic field have been reported by many authors during recent years. These bewildering observational results pose a challenge to the current flare theories in which the photospheric magnetic field usually remains unchanged in the eruption. In this paper, changes in the photosphere magnetic field during the solar eruption are investigated based on the catastrophe model. The results indicate that the projection effect is an important source that yields the change in the observed photospheric magnetic field in the line-of-sight. Furthermore one may observe the change in the normal component of magnetic field if the spectrum line used to measure the photospheric magnetic field does not exactly come from the photospheric surface. Our results also show that the significance of selecting the correct spectral lines to study the photospheric field becomes more apparent for the magnetic configurations with complex boundary condition (or background field).     

Ni I 6768 Å line profile variations during a solar flare and their effect on the SOHO/MDI magnetic field measurements

Observational data on the Ni I 6768 Å line profile variations during the impulsive and post-impulsive phases of the July 18, 2002 while light flare (WLF) in the kernel of WLF emission and in other flare kernels are presented. The line profiles at the sites of intense photospheric motions in active regions are also studied. The effect of the observed Ni I 6768 Å line profile variations on the SOHO/MDI magnetic field measurements is estimated. The following conclusions have been reached. (1) The thermodynamic structure of the photo-spheric layers changes significantly during the flare. As a result, the Ni I line profile changes, particularly at the site of WLF emission. At this time, the line depth decreases significantly, but the line does not show any emission reversal. Subsequently, a relatively slow return to the conditions of an undisturbed photosphere is observed. (2) The technique of SOHO/MDI magnetic field measurements is insensitive to such line variations. Therefore, the detected variations during the flare did not result in any noticeable errors in the MDI longitudinal magnetic field measurements. (3) The line profile is broadened, shifted as a whole, and asymmetric at the sites of active regions where intense photospheric motions appear. In the MDI measurements, such changes in the profile lead to an underestimation of the magnetic field by approximately 10% if the line-of-sight velocity of the photo-spheric ejection is about 1.6 km s^?1.     

Observed nonpotential magnetic fields and the inferred flow of electric currents at a location of repeated flaring

We have analyzed the vector magnetic field of an active region at a location of repeated flaring to determine the nature of the currents flowing in the areas where the flares initiated. The component of electric current density crossing the photosphere along the line-of-sight was derived from the observed transverse component of the magnetic field. The maximum concentrations of these currents occurred exactly at the sites of flare initiation and where the photospheric field was sheared the most. The calculated distribution of current density at the flare sites suggested that currents were flowing out of an area of positive magnetic polarity and across the magnetic inversion line into two areas of negative polarity. This interpretation was reinforced by a calculation of the source field, the magnetic field produced in the photosphere by the electric currents above the photosphere. In the vicinity of the flare sites, the calculated source field exhibited three particular characteristics: (1) maximum magnitudes at the sites of flare initiation, (2) a rotational direction where the vertical current density was concentrated, and (3) a fairly constant angular orientation with the magnetic inversion line. The source field was thus very similar to the field produced by two arcades of currents crossing the inversion line at the locations of greatest magnetic shear with orientations of about 60° to the inversion line. With this orientation, the inferred arcades would be aligned with the observed chromospheric fibrils seen in the H data so that the currents were field-aligned above the photosphere. The field thus exhibited a vertical gradient of magnetic shear with the shear decreasing upward from the photosphere. We estimated the currents in the two arcades by matching the source field derived from observations with that produced by a model of parallel loops of currents. We found that the loops of the model would each have a radius of 4500 km, a separation of 1830 km, and carry a current of 0.15 × 10¹² A. Values of vertical current densities and source fields appearing in the umbrae of the two large sunspots away from the flare sites were shown to lie at or below the level of uncertainty in the data. The main source of this uncertainty lay in the method by which the 180° ambiguity in the azimuth of the transverse field is resolved in umbral areas. We thus concluded that these quantities in large umbrae should be treated with a healthy skepticism. Finally, we found that the source field at the flare sites was produced almost entirely by the angular difference between the observed and potential field and not by the difference in field intensity.     

Magnetic reconnection,electric field,and particle acceleration in the July 14, 2000 solar flare

A topological model with magnetic reconnection at two separators in the corona is used to account for the recently discovered changes of the photospheric magnetic field in the active region NOAA 9077 during the July 14, 2000 flare. The model self-consistently explains the following observed effects: (1) the magnetic field strength decreases on the periphery of the active region but increases in its inner part near the neutral line of the photospheric magnetic field; (2) the center-of-mass positions of the fields of opposite (northern and southern) polarities converge; and (3) the magnetic flux of the active region decreases after the flare. The topological model gives not only a qualitative interpretation of the flare phenomena (the structure of the interacting magnetic fluxes in the corona, the location of the energy sources, the shape of the flare ribbons and kernels in the chromosphere and photosphere), but also correct quantitative estimates of the large-scale processes that form the basis for solar flares. The electric field emerging in the flare during large-scale reconnection is calculated. The electric field strength correlates with the observed intensity of the hard X-ray bremsstrahlung, suggesting an electron acceleration as a result of reconnection.     

Flare-associated magnetic changes in an active region

We report an instance of localized chromospheric polarity reversal in a rapidly-formed sunspot which appears to be part of new emerging flux. The chromospheric polarity reversal is preceded by extraordinarily fast growth of the transverse magnetic field and an increase in the line-of-sight magnetic flux of the newly formed sunspot in the photosphere. The strength of this reversal is more than 350 G at maximum, in contrast to approximately - 1300 G for the line-of-sight field and 400 G for the transverse field in the photosphere. Continued flare activity takes place around the site of the reversal with progressively increasing flare size and extent. It is suggested that a kinked or knotted flux loop, or a self-closed flux system developed above the fast-forming sunspot. So far, this phenomenon has been revealed in several active regions.     

A study of the magnetic and velocity fields in an active region

A time sequence of magnetograms and velocity-grams in the H and Fe i 6569 Å lines has been made at a rate of 12 h^–1 of McMath Region 10385 from 26 to 29 October, 1969. The 14 flares observed during this period have been studied in relation to the configuration and changes in the magnetic and velocity fields. There was little correlation between flare position and the evolutionary changes in the photospheric magnetic and velocity field, except at large central meridian distances where the velocity observations suggested shearing taking place at flare locations. At central meridian distances > 30° we found that flares are located in areas of low line-of-sight photospheric velocity surrounded by higher velocity hills. The one exception to this was the only flare which produced a surge. Blue-shifted velocity changes in the photosphere of 0.3 to 1 km s^–1 were observed in localized areas at the times of 8 of 14 flares studied.Visiting Astronomer, Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations

Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun’s atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun.     

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.     

The Hβ Chromospheric Magnetic Field in a Quiescent Filament

We observed the line-of-sight magnetic field in the chromosphereand photosphere of a large quiescent filament on the solar disk on September 6, 2001 using the Solar Magnetic Field Telescope in Huairou Solar Observing Station. The chromospheric and photospheric magnetograms together with Hβ filtergrams of the filament were examined. The filament was located on the neutral line of the large scale longitudinal magnetic field in the photosphere and the chromosphere. The lateral feet of the filament were found to be related to magnetic structures with opposite polarities. Two small lateral feet are linked to weak parasitic polarity. There is a negative magnetic structure in the photosphere under a break of the filament. At the location corresponding to the filament in the chromospheric magnetograms, the magnetic strength is found to be about 40-70 Gauss (measuring error about 39 Gauss). The magnetic signal indicates the amplitude and orientation of the internal magnetic field in the filament. We discuss several possible causes which may produce such a measured signal. A twisted magnetic configuration inside the filament is suggested .     

Measurement of the full Stokes vector of He I 10830 Å

First observations of the full Stokes vector in the upper chromosphere are presented. The He I 10830 Å line, which has been shown to give reliable measurements of the line-of-sight component of the magnetic field vector, has been used for this purpose. It is shown that the difference between the appearance of chromospheric and photospheric magnetic structures observed close to the solar limb is largely due to the difference in height to which they refer and projection effects. The observations do suggest, however, that the magnetic field above sunspot penumbrae is somewhat more vertical in the chromosphere than in the photosphere.The National Optical Astronomy Obervatories are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation     

Physical conditions in the chromosphere of a two-ribbon solar flare accompanied by a surge: 1

We studied changes in thermodynamic parameters of the chromosphere at the initial stage of the two-ribbon solar flare accompanied by a surge that occurred on September 4, 1990. The inhomogeneous semiempirical models of the flare chromosphere and surge are constructed for four observation moments. The spectra were obtained with the ATsU-26 horizontal solar telescope of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (Terskol Peak). Photometric transections of the spectra passed through two bright kernels of one of the flare ribbons and through the surge. The comparison of the observed profiles of the line H_α in the solar active and quiet-Sun regions reveals the substantial emission in the line wings (up to 1–1.2 nm) with a residual intensity of 0.6–0.77 at the center of the line profiles. Calculations within the two-component models of the chromosphere have shown that this may be the evidence of the existence of the details (unresolved by the telescope and occupying 5–12% of the total area) with a deep heating of the chromosphere layers. A strong asymmetry of the line profiles and the shift with respect to the line profile for the quiet-Sun region are explained by peculiarities of the line-of-sight velocity distribution over the height. It is found that the motion is directed to the observer in the upper chromosphere (10–30 km/s) and from the observer in the lower chromosphere (5–20 km/s) for the larger part of the active region under study. According to the models calculated for the surge, the line-of-sight velocities reach a value of 70 km/s.     

Non-Potentiality of the Magnetic Field Beneath the Eruptive Filament in the Bastille Event

Based on photospheric vector magnetograms obtained at Huairou Solar Observing Station (HSOS), non-potential characteristics of the magnetic field beneath the filament in active region NOAA 9077 are investigated. We focus on the structure and evolution of the magnetic field from 00:08 UT to 10:25 UT of 14 July before the Bastille event. Particular attention is paid to transverse field strength, shear degree and horizontal gradient of the line-of-sight magnetic field around the filament and filament channel. The following characteristics are found. (1) The magnetic non-potentiality has an obviously non-uniform distribution. The shear degree of the transverse field (Hagyard et al., 1984) is very large, up to 75° in some sites beneath the filament, such as the initial brightening site in TRACE 1600 Å images and the breaking site of the filament. The transverse field and the horizontal gradient of the line-of-sight field are very large in some parts corresponding to the hottest and continuously brightening portions. (2) The mean strength and mean shear angle of the transverse field and mean horizontal gradient of the line-of-sight field have obviously dropped in most parts beneath the filament for two hours before the filament eruption and onset of the X5.7/3B flare. After comparing simultaneous UV and EUV images, H filtergrams and Dopplergrams at solar atmosphere, we suggest that magnetic cancellation is likely to quickly transport the magnetic energy and complexity into the higher atmosphere in these two hours. This leads to magnetic instability in the filament and eventually causes the eruption of filament and onset of the flare.     

Irreversible rapid changes of magnetic field associated with the 2012 October 23 circular near-limb X1.8 Flare

It has been found that photospheric magnetic fields can change in accordance with restructuring of the three-dimensional magnetic field following solar eruptions.Previous studies mainly use vector magnetic field data taken for events near the disk center.In this paper,we analyze the magnetic field evolution associated with the 2012 October 23 X1.8 flare in NOAA AR 11598 that is close to the solar limb,using both the 45 s cadence line-of-sight and 12 min cadence vector magnetograms from the Helioseismic and Magnetic Imager on board Solar Dynamics Observatory.This flare is classified as a circular-ribbon flare with spine-fan type magnetic topology containing a null point.In the line-of-sight magnetograms,there are two apparent polarity inversion lines(PILs).The PIL closer to the limb is affected more by the projection effect.Between these two PILs there lie positive polarity magnetic fields,which are surrounded by negative polarity fields outside the PILs.We find that after the flare,both the apparent limb-ward and disk-ward negative fluxes decrease,while the positive flux in-between increases.We also find that the horizontal magnetic fields have a significant increase along the disk-ward PIL,but in the surrounding area,they decrease.Synthesizing the observed field changes,we conclude that the magnetic fields collapse toward the surface above the disk-ward PIL as depicted in the coronal implosion scenario,while the peripheral field turns to a more vertical configuration after the flare.We also suggest that this event is an asymmetric circular-ribbon flare:a flux rope is likely present above the disk-ward PIL.Its eruption causes instability of the entire fan-spine structure and the implosion near that PIL.     

A quantitative study relating observed shear in photospheric magnetic fields to repeated flaring

In this paper we present a quantitative evaluation of the shear in the magnetic field along the neutral line in an active region during an epoch of flare activity. We define shear as the angular difference in the photosphere between the potential magnetic field, which fits the boundary conditions imposed by the observed line-of-sight field, and the observed magnetic field. For the active region studied, this angular difference (shear) is non-uniform along the neutral line with maxima occurring at the locations of repeated flare onsets. We suggest that continued magnetic evolution causes the field's maximum shear to exceed a critical value of shear, resulting in a flare around the site of maximum shear. Evidently, the field at the site of the flare must relax to a state of shear somewhat below the critical value (but still far from potential), with subsequent evolution returning the field to the critical threshold. We draw this inference because several flares occurred at sites of maximum photospheric shear which were persistent in location.NOAA, Boulder, Colorado.