Ellerman Bombs Research Articles

Spectropolarimetric Observation of an Emerging Flux Region: Triggering Mechanisms of Ellerman Bombs

A high spatial resolution observation of an emerging flux region (EFR) was made using a vector magnetograph and a Hα Lyot filtergraph with the Domeless Solar Telescope at Hida Observatory on 2006 October 22. In Hα wing images, we could see many Ellerman bombs (EBs) in the EFR. Observations in two modes, slit scan and slit fixed, were performed with the vector magnetograph, along with the Hα filtergraph. Using the Hα wing images, we detected 12 EBs during the slit scan observation period and 9 EBs during the slit fixed observation period. With the slit scan observation, we found that all the EBs were distributed in the area where the spatial gradient of vertical field intensity was large, which indicates the possibility of rapid topological change in the magnetic field in the area of EBs. With the slit fixed observation, we found that EBs were distributed in the areas of undulatory magnetic fields, in both the vertical and horizontal components. This paper is the first to report the undulatory pattern in the horizontal components of the magnetic field, which is also evidence for emerging magnetic flux triggered by the Parker instability. These results allow us to confirm the association between EBs and emerging flux tubes. Three triggering mechanisms for EBs are discussed with respect to emerging flux tubes: 9 out of 21 EBs occurred at the footpoints of emerging flux tubes, 8 occurred at the top of emerging flux tubes, and 4 occurred in the unipolar region. Each case can be explained by magnetic reconnection in the low chromosphere.

Height Dependence of Gas Flows in an Ellerman Bomb

Abstract We performed spectroscopic observations of Ellerman bombs (EBs) in an active region of NOAA 10705 at Hida Observatory on 2004 November 24. The photospheric velocity fields of EBs have for the first time been investigated spectroscopically. From the Doppler shifts of a Ti II absorption line (6559.576 Å) and a broad H$\alpha$ emission line, we derived the photospheric velocity and the lower chromospheric velocity, respectively. The photospheric velocity of EBs was $\sim 0.2$kms$^{-1}$, indicating downward flow, on average. We found that the photospheric velocity variation of EBs has a good temporal correlation with the H$\alpha$ wing emission variation. On the other hand, the chromospheric velocity showed an upward flow of $\sim$1–3 kms$^{-1}$ on the average. From the characteristics of the flow field, we conclude that the observed EB occurred at the upper photospheric level. We suggest that it is important to know the motions of EBs in the photosphere because a plausible triggering mechanism of EBs is magnetic reconnection in the low-lying atmosphere.

Spectrophotometric analysis of Ellerman bombs in the Ca II, H$\mathsf{\alpha}$, and UV range

Context. Even if Ellerman bombs have been observed in the Hα line within emerging magnetic flux regions since the early 20th century, their origin and the mechanisms that lead to their formation have been strongly debated. Recently, new arguments in favor of chromospheric magnetic reconnection have been advanced. Ellerman bombs seem to be the signature of reconnections that take place during the emergence of the magnetic field.Aims. We have observed an active region presenting emergence of magnetic flux. We detected and studied Ellerman bombs in two chromospheric lines: Ca ii 8542 A and Hα . We investigated the link between Ellerman bombs and other structures and phenomena appearing in an emerging active region: UV bright points, arch filament systems, and magnetic topology.Methods. On August 3, 2004, we performed multi-wavelength observations of the active region NOAA 10655. This active region was the target of SoHO Joint Observation Program 157. Both SoHO/MDI and TRACE (195 A and 1600 A) were used. Simultaneously, we observed in the Ca ii and Na D1 lines with the spectro-imager MSDP mode of THEMIS. Alternately to the MSDP, we used the MTR spectropolarimeter on THEMIS to observe in Hα and in the Fe i doublet at 6302 A. We derived the magnetic field vectors around some Ellerman bombs.Results. We present the first images of EBs in the Ca ii line and confirm that Ellerman bombs can indeed be observed in the Ca ii line, presenting the same “moustache” geometry profiles as in the Hα line, but with a narrower central absorption in the Ca ii line, in which the peaks of emission are around ± 0.35 A. We noticed that the Ellerman bombs observed in the wings of Ca ii line have an elongated shape – the length about 50% greater than the width. We derived mean semi-axis lengths of 1.4'' 2.0'' . In the UV time profiles of the Ellerman bombs, we noticed successive enhanced emissions. The distribution of lifetimes of these individual impulses presents a strong mode around 210 s. Study of the magnetic topology shows that 9 out of the 13 EBs are located on the inversion line of the longitudinal field and that some typical examples might be associated with a bald patch topology.Conclusions. We provide new arguments in favor of the reconnection origin of Ellerman bombs. The different individual impulses observed in UV may be related to a bursty mode of reconnection. We also show that this Ca ii 8542 A chromospheric line is a good indicator of Ellerman bombs and can bring new information about these phenomena.

Detection of opposite polarities in a sunspot light bridge: evidence of low-altitude magnetic reconnection

A multiwavelength photometric analysis was performed in order to study the sub-structure of a sunspot light bridge in the photosphere and the chromosphere. Active region NOAA 8350 was observed on 1998 October 8. The data consist of a 100 min time series of 2D spectral scans of the lines Fe i 5576 A, Hα 6563 A, Fe i 6302.5 A, and continuum images at 5571 A. We recorded line-of-sight magnetograms in 6302.5 A. The observations were taken at the Dunn Solar Telescope at US National Solar Observatory, Sacramento Peak. We find evidence for plasma ejection from a light bridge followed by Ellerman bombs. Magnetograms of the same region reveal opposite polarity in light bridge with respect to the umbra. These facts support the notion that low-altitude magnetic reconnection can result in the magnetic cancellation as observed in the photosphere.

Ellerman Bombs and Jets Associated with Resistive Flux Emergence

Using two-dimensional (2D) magnetohydrodynamic simulations we study the effects of resistive processes in the dynamics of magnetic flux emergence and its relation to Ellerman bombs and other dynamic phenomena in the Sun. The widely accepted scenario of flux emergence is the formation and expansion of Ω-shaped loops due to the Parker instability. Since the Parker instability has the largest growth rate at finite wavelength λp ~ 10H-20H, where H is the scale height (≈200 km in the solar photosphere), a number of magnetic loops may rise from the initial flux sheet if it is sufficiently long. This process is shown in our numerical simulations. The multiple emerging loops expand in the atmosphere and interact with each other, leading to magnetic reconnection. At first reconnection occurs in the lower atmosphere, which allows the sinking part of the flux sheet to emerge above the photosphere. This reconnection also causes local heating that may account for Ellerman bombs. In the later stage, reconnection between the expanding loops occurs at higher levels of the atmosphere and creates high-temperature reconnection jets, and eventually a large (λp) coronal loop is formed. Cool and dense plasma structures, which are similar to Hα surges, are also formed. This is not because of magnetic reconnection but due to the compression of the plasma in between the expanding loops.

Spectral Analysis of Ellerman Bombs

By use of the high-resolution spectral data obtained with THEMIS on 2002 September 5, the characteristics of 14 well-observed Ellerman bombs (EBs) have been analyzed. Our results indicate that 9 of the 14 EBs are located near the longitudinal magneticpolarity inversion lines. Mostofthe EBsare accompaniedbymass motions. Themost obvious characteristic of the EB spectra is the two emission bumps at the two wings of both Hand Caii k8542. For the first time both thermal and nonthermal semiempirical atmospheric models for the conspicuous and faint EBs are computed. In computing the nonthermal models, we assume that the electron beam resulting from magnetic re- connection is produced in the lower chromosphere. The reasons are that it requires much lower energies for the injected particles and that it gives rise to a more profound absorption at the Hline center, in agreement with our observations. The common characteristic is the heating in the lower chromosphere and the upper photosphere. The temperatureenhancementisabout600-1300Kinthethermalmodels.Ifthenonthermaleffectsareincluded,thenthe required temperature increase can be reduced by 100-300 K. These imply that the EBs could probably be produced by the magnetic reconnection in the solar lower atmosphere. The radiative and kinetic energies of the EBs are estimated, and the total energy is found to be 10 26 to 5;10 27 ergs. According to the characteristics of EBs, we tentatively suggest that EBs could be called ''submicroflares.'' Subject headingg Sun: chromosphere

Investigation of pre-flare role of Ellerman bombs in solar active regions

Investigation of pre-flare role of Ellerman bombs in solar active regions

Emergence of undulatory magnetic flux tubes by small scale reconnections

With Flare Genesis Experiment (FGE), a balloon borne observatory launched in Antarctica on January 2000, series of high spatial resolution vector magnetograms, Dopplergrams, and Hα filtergrams have been obtained in an emerging active region (AR 8844). Previous analyses of this data revealed the occurence of many short-lived and small-scale H α brightenings called ‘Ellerman bombs’ (EBs) within the AR. We performed an extrapolation of the field above the photosphere using the linear force-free field approximation. The analysis of the magnetic topology reveals a close connexion between the loci of EBs and the existence of “Bald patches” (BP) regions (BPs are regions where the vector magnetic field is tangential to the photosphere). Some of these EBs/BPs are magnetically connected by low-lying field lines, presenting a serpentine shape. This results leads us to conjecture that arch filament systems and active regions coronal loops do not result from the smooth emergence of large scale Ω-loops, but rather from the rise of flat undulatory flux tubes which get released from their photospheric anchorage by reconnection at BPs, which observational signature is Ellerman bombs.

Spectro-Polarimetric Observations and Non-Lte Modeling of Ellerman Bombs

Ellerman bombs are bright emission features observed in the wings of Hα, usually in the vicinity of magnetic concentrations. Here we show that they can also be detected in the Ca II infrared triplet lines, which are easier to interpret and therefore allow for more detailed diagnostics. We present full Stokes observations of the 849.8 and 854.2 nm lines acquired with the new spectro-polarimeter SPINOR. The data show no significant linear polarization at the level of 3 × 10−4. The circular polarization profiles exhibit measureable signals with a very intricate pattern of peaks. A non-LTE analysis of the spectral profiles emerging from these features reveals the presence of strong downflows (∼10 {km s−1}) in a hot layer between the upper photosphere and the lower chromosphere.

The viewing angle effect on Hα-line impact polarisation in impulsive solar events

The effect of a viewing angle on the hydroden Hα-line impact polarisation is investigated in a plane vertical atmo- sphere arbitrary located on the solar disk. The impact polarisation is assumed to be caused by precipitating beam electrons with pitch-angular anisotropy steadily injected into the flaring atmosphere from its top. The polarisation is calculated for a 3 level plus continuum hydrogen atom affected by Zeeman splitting in a moderate magnetic field taking into account depolarising effects of diffusive radiation and collisions with thermal electrons. The Hα polarisation profiles are affected by electron beams only in the line cores whereas the wings are fully depolarized by the collisions with thermal electrons despite the extended wing emission, or moustaches, caused by beam electrons. The full (integrated in wavelength) Hα-line linear polarisation, caused by moderate electron beams, is shown to be 2−20% and either negative or positive depending on the position of a flaring loop on the solar disk and the direction of an emitted photon from the local magnetic field. The polarisation plane is projected onto a viewing angle ψ, being a superposition of the flare location on a solar disk and the magnetic field deviation from vertical on the solar surface. For viewing angles less then 50 ◦ the Hα-line impact polarisation is negative increasing up to −10% towards smaller angles, meaning that the polarisation is mostly perpendicular to the plane B × K where B is the magnetic field induction and K is the photon momentum vector. For viewing angles greater than 60 ◦ the measured impact polarisation becomes positive, sharply increasing up to 20% towards the limb. In the range of 50−60 ◦ the observed impact polarisation goes through a zero point despite the actual presence of beam electrons in the flaring atmosphere. The theoretical predictions of the dependence of polarisation degree on viewing angle fit remarkably well the observations of Hα-line linear polarisation in small-scale flaring events such as moustaches or Ellerman bombs, located in different positions on a solar disk.

Resistive Emergence of Undulatory Flux Tubes

During its 2000 January flight, the Flare Genesis Experiment observed the gradual emergence of a bipolar active region, by recording a series of high-resolution photospheric vector magnetograms and images in the blue wing of the Hα line. Previous analyses of these data revealed the occurrence of many small-scale, transient Hα brightenings identified as Ellerman bombs (EBs). They occur during the flux emergence, and many of them are located near moving magnetic dipoles in which the vector magnetic field is nearly tangential to the photosphere. A linear force-free field extrapolation of one of the magnetograms was performed to study the magnetic topology of small-scale EBs and their possible role in the flux emergence process. We found that 23 out of 47 EBs are cospatial with bald patches (BPs), while 15 are located at the footpoints of very flat separatrix field lines passing through distant BPs. We conclude that EBs can be due to magnetic reconnection, not only at BP locations, but also along their separatrices, occurring in the low chromosphere. The topological analysis reveals, for the first time, that many EBs and BPs are linked by a hierarchy of elongated flux tubes showing aperiodic spatial undulations, whose wavelengths are typically above the threshold of the Parker instability. These findings suggest that arch filament systems and coronal loops do not result from the smooth emergence of large-scale Ω-loops from below the photosphere, but rather from the rise of undulatory flux tubes whose upper parts emerge because of the Parker instability and whose dipped lower parts emerge because of magnetic reconnection. EBs are then the signature of this resistive emergence of undulatory flux tubes.

Polarization of Ellerman bombs and the arch structure of active region: some results of investigation

Polarization of Ellerman bombs and the arch structure of active region: some results of investigation

Emerging Flux and the Heating of Coronal Loops

We use data collected by a multiwavelength campaign of observations to describe how the fragmented, asymmetric emergence of magnetic flux in NOAA active region 8844 triggers the dynamics in the active-region atmosphere. Observations of various instruments on board Yohkoh, SOHO, and TRACE complement high-resolution observations of the balloon-borne Flare Genesis Experiment obtained on 2000 January 25. We find that coronal loops appeared and evolved rapidly ~6 ± 2 hr after the first detection of emerging magnetic flux. In the low chromosphere, flux emergence resulted in intense Ellerman bomb activity. Besides the chromosphere, we find that Ellerman bombs may also heat the transition region, which showed "moss" ~100% brighter in areas with Ellerman bombs as compared to areas without Ellerman bombs. In the corona, we find a spatiotemporal anticorrelation between the soft X-ray (SXT) and the extreme ultraviolet (TRACE) loops. First, SXT loops preceded the appearance of the TRACE loops by 30-40 minutes. Second, the TRACE and SXT loops had different shapes and different footpoints. Third, the SXT loops were longer and higher than the TRACE loops. We conclude that the TRACE and the SXT loops were formed independently. TRACE loops were mainly heated at their footpoints, while SXT loops brightened in response to coronal magnetic reconnection. In summary, we observed a variety of coupled activity, from the photosphere to the active-region corona. Links between different aspects of this activity lead to a unified picture of the evolution and the energy release in the active region.

Emerging Flux and the Heating of Coronal Loops

Observations of various instruments on board Yohkoh, SOHO, and TRACE complement high-resolution observations of the balloon-borne Flare Genesis Experiment, obtained on January 25, 2000. A subset of the TRACE loops are located in the vicinity of the SXT loops in the NOAA emerging active region 8844, but never coincide with them. We find that coronal loops appeared 6 ± 2 hr after the first detection of emerging magnetic flux. The loops evolved rapidly when the active region entered its impulsive flux emergence phase. In the low chromosphere, flux emergence was reflected in intense Ellerman bomb activity. Besides chromosphere, we find that Ellerman bombs may also heat the transition region, by contributing to the moss emission. Areas prolific in Ellerman bombs show moss ∼ 100% brighter than areas without Ellerman bombs. Only the strongest Ellerman bombs can heat their surroundings to coronal temperatures. In the corona, we find a spatio-temporal anti-correlation between the soft X-ray (SXT) and the extreme ultraviolet (TRACE) loops: First, the SXT loops preceded the appearance of the TRACE loops by 30 — 40 min. Second, the TRACE loops had different shapes and different footpoints compared to the SXT loops. The SXT and TRACE loops are probably formed independently.

High Spatial Resolution Observations of Pores and the Formation of a Rudimentary Penumbra

We present high spatial resolution observation of small-scale magnetic activity in solar active region NOAA 9539. The observations were obtained on 2001 July 15 using the 65 cm vacuum reflector and 25 cm refractor of the Big Bear Solar Observatory (BBSO). The data sets include time series of speckle reconstructed continuum images at 5200 A, Hα filtergrams (blue line wing, line center, and red line wing), and line-of-sight magnetograms. Two pores, separated by a light bridge, were located in the central part of NOAA 9539. The formation of penumbral filaments near the light bridge indicated a sudden change of the local magnetic field topology from almost vertical to strongly inclined magnetic fields, which allowed cool material previously suspended in a filament to stream downward. During the downward motion of the cool material, Hα Dopplergrams revealed twisted streamlines along the filament. Finally, there are several well-defined Hα brightenings, Ellerman bombs (EBs), occurred near the region where the downflow of materials fell in. The EBs reside near a magnetic inversion line and are stationary, as opposed to EBs associated with moving magnetic features. We also found that the horizontal flow field of the white-light images derived from local correlation tracking is different from the previous observations. The horizontal movements in the superpenumbrae of leading sunspot and the following sunspots are opposite.

What can we learn from lfff magnetic extrapolations

Observations of the Sun is done, up to now, in 2D and magnetic field is measured mainly in the photosphere. Magnetic extrapolation techniques allow us nevertheless to have a 3D view of the magnetic field. Different methods are available today. We present in this paper interesting and exploitable results obtained with linear force-free-field methods (lfff). The α parameter is assumed to be constant in the whole volume of computations. The results concern the global potential field in the corona (e.g. example of transequatorial loops), the magnetic topology of flaring active regions (e.g. flares and bald patch regions), twisted magnetic fields in filaments, and magnetic reconnection in emerging active region (e.g. Ellerman Bombs). We discuss on the limits of the validity of the methods.

A spectropolarimetric study of Ellerman bombs

We have studied four 10 to 40 minute sets of Hα spectropolarimetric observations of five Ellerman bombs (a total of 241 spectropolarimetric recordings) selected from data for more than 200 Ellerman bombs obtained with the Large Solar Vacuum Telescope at the Ba $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l}$$ kal Astrophysical Observatory. The degree of polarization P and azimuth of the plane of polarization were derived from quasi-simultaneous observations of the Stokes parameters Q/I and U/I. We interpret the observed linear polarization as impact polarization. A method for comparing the polarization parameters of Ellerman bombs observed at different times and distances from the center of the solar disk is suggested, which can be used to relate variations of the polarimetric parameters with the lifetimes of Ellerman bombs. A comparison between the observations and theoretical calculations indicates that the observed Ellerman bombs were due to electron beams with energies of several hundred keV.

Statistics, Morphology, and Energetics of Ellerman Bombs

We investigate the statistical properties of Ellerman bombs in the dynamic emerging flux region NOAA Active Region 8844, underneath an expanding arch filament system. High-resolution chromospheric Hα filtergrams (spatial resolution 08), as well as photospheric vector magnetograms (spatial resolution 05) and Dopplergrams, have been acquired by the balloon-borne Flare Genesis Experiment. Hα observations reveal the first seeing-free data set on Ellerman bombs and one of the largest samples of these events. We find that Ellerman bombs occur and recur in preferential locations in the low chromosphere, either above or in the absence of photospheric neutral magnetic lines. Ellerman bombs are associated with photospheric downflows, and their loci follow the transverse mass flows on the photosphere. They are small-scale events, with typical size 18 × 11 , but this size depends on the instrumental resolution. A large number of Ellerman bombs are probably undetected, owing to limited spatial resolution. Ellerman bombs occur in clusters that exhibit fractal properties. The fractal dimension, with an average value ~1.4, does not change significantly in the course of time. Typical parameters of Ellerman bombs are interrelated and obey power-law distribution functions, as in the case of flaring and subflaring activity. We find that Ellerman bombs may occur on separatrix, or quasi-separatrix, layers, in the low chromosphere. A plausible triggering mechanism of Ellerman bombs is stochastic magnetic reconnection caused by the turbulent evolution of the low-lying magnetic fields and the continuous reshaping of separatrix layers. The total energies of Ellerman bombs are estimated in the range (1027, 1028) ergs, the temperature enhancement in the radiating volume is ~2 × 103 K, and the timescale of radiative cooling is short, of the order of a few seconds. The distribution function of the energies of Ellerman bombs exhibits a power-law shape with an index ~-2.1. This suggests that Ellerman bombs may contribute significantly to the heating of the low chromosphere in emerging flux regions.

Three‐dimensional Solutions for Fan and Spine Magnetic Reconnection in Partially Ionized Plasmas

Under the approximation of incompressible flow, we solve both momentum equations (for neutrals and ions) and magnetic induction equation at three-dimensional level, obtaining solutions for steady spine and fan magnetic reconnection in partially ionized plasmas. In the case of weak coupling between the two species, sharp structures can form not only for the magnetic field but also for the ions' velocity, while for the neutrals the level of sharpness falls off by an order of ionization degree. In the strong coupling case when both species move together sharp structures appear only for magnetic field. The results could be used to understand the phenomena which are observed in the solar lower atmosphere, e.g., Ellerman bombs, type II white light flares, and photospheric magnetic flux cancellation which may be caused by magnetic reconnection. In an ideal approximation, the sharp structures have a power-law distribution: in the fan reconnection the field and both flows go up according to z(-\kappa\), while in the spine reconnection they obey the inverse square law r(-2). However, due to the existence of finite resistivity, the enhancements of currents, field, and flows should saturate at some level in the diffusion layer. When the ionization degree goes up (corresponds to epsilon --> 0 in this paper), our solutions recover previous formulation for wholly ionized case. Since the Ohmic resistivity is very small (similar to 10(-10)), the induced current becomes so large that, as we estimate, it will cause plasma micro-instability which could speed up the reconnection process significantly. So near the current sheet which is provided here usual MHD description must fail, we need another theory to join in, such as kinematic description, to account for violent phenomena (e.g., Ellerman bombs and type II white light flares) which occur in the Sun's lower atmosphere.

Ellerman Bombs, Type II White-light Flares and Magnetic Reconnection in the Solar Lower Atmosphere

Ellerman bombs and Type II white-light flares share many common features despite the large energy gap between them. Both are considered to result from local heating in the solar lower atmosphere. This paper presents numerical simulations of magnetic reconnection occurring in such a deep atmosphere, with the aim to account for the common features of the two phenomena. Our numerical results manifest the following two typical characteristics of the assumed reconnection process: (1) magnetic reconnection saturates in ~ 600--900 s, which is just the lifetime of the two phenomena; (2) ionization in the upper chromosphere consumes quite a large part of the energy released through reconnection, making the heating effect most significant in the lower chromosphere. The application of the reconnection model to the two phenomena is discussed in detail.