Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule

Context. Ellerman bombs (EBs) with significant Hα wing emissions and ultraviolet bursts (UV bursts) with strong Si IV emissions are two kinds of small transient brightening events that occur in the low solar atmosphere. The statistical observational results indicate that about 20% of the UV bursts connect with EBs. While some promising models exist for the formation mechanism of colder EBs in conjunction with UV bursts, the topic remains an area of ongoing research and investigation. Aim. We numerically investigated the magnetic reconnection process between the emerging arch magnetic field and the lower atmospheric background magnetic field. We aim to find out if the hot UV emissions and much colder Hα wing emissions can both appear in the same reconnection process and how they are located in the reconnection region. Methods. The open-source code NIRVANA was applied to perform the 2.5D magnetohydrodynamic (MHD) simulation. We developed the related sub-codes to include the more realistic radiative cooling process for the photosphere and chromosphere and the time-dependent ionization degree of hydrogen. The initial background magnetic field is 600 G, and the emerged magnetic field in the solar atmosphere is of the same magnitude, meaning that it results in a low- β magnetic reconnection environment. We also used the radiative transfer code RH1.5D to synthesize the Si IV and Hα spectral line profiles based on the MHD simulation results. Results. Magnetic reconnection between emerged and background magnetic fields creates a thin, curved current sheet, which then leads to the formation of plasmoid instability and the nonuniform density distributions. Initially, the temperature is below 8000 K. As the current sheet becomes more vertical, denser plasmas are drained by gravity, and hotter plasmas above 20 000 K appear in regions with lower plasma density. The mix of hot tenuous and much cooler dense plasmas in the turbulent reconnection region can appear at about the same height, or even in the same plasmoid. Through the reconnection region, the synthesized Si IV emission intensity can reach above 106 erg s−1 sr−1 cm−2 Å−1 and the spectral line profile can be wider than 100 km s−1, the synthesized Hα line profile also show the similar characteristics of a typical EB. The turbulent current sheet is always in a dense plasma environment with an optical depth larger than 6.5 × 10−5 due to the emerged magnetic field pushing high-density plasmas upward. Conclusions. Our simulation results indicate that the cold EB and hot UV burst can both appear in the same reconnection process in the low chromosphere, the EB can either appear several minutes earlier than the UV burst, or they can simultaneously appear at the similar altitude in a turbulent reconnection region below the middle chromosphere.

Ellerman bombs (EBs) are tiny brightenings often observed near sunspots. The most impressive characteristic of EB spectra is the two emission bumps in both wings of the Hα and Ca II 8542Å lines. High-resolution spectral data of three small EBs were obtained on 2013 June 6 with the largest solar telescope, the 1.6 m New Solar Telescope at the Big Bear Solar Observatory. The characteristics of these EBs are analyzed. The sizes of the EBs are in the range of 0.3′ – 0.8′ and their durations are only 3–5 min. Our semi-empirical atmospheric models indicate that the heating occurs around the temperature minimum region with a temperature increase of 2700–3000 K, which is surprisingly higher than previously thought. The radiative and kinetic energies are estimated to be as high as 5 × 1025 − 3.0 × 1026 erg despite the small size of these EBs. Observations of the magnetic field show that the EBs just appeared in a parasitic region with mixed polarities and were accompanied by mass motions. Nonlinear force-free field extrapolation reveals that the three EBs are connected with a series of magnetic field lines associated with bald patches, which strongly implies that these EBs should be produced by magnetic reconnection in the solar lower atmosphere. According to the lightcurves and the estimated magnetic reconnection rate, we propose that there is a three phase process in EBs: pre-heating, flaring and cooling phases.

Magnetic reconnection in the solar lower atmosphere

Context.Ultraviolet (UV) bursts and Ellerman bombs (EBs) are small-scale magnetic reconnection events taking place in the highly stratified, low solar atmosphere. The plasma density, reconnection mechanisms, radiative cooling and transfer processes clearly differ from one layer of the atmosphere to the next. In particular, EBs are believed to form in the upper photosphere or the low chromosphere. It is still not clear whether UV bursts have to be generated at a higher atmospheric layer than the EBs or whether both UV bursts and EBs can occur in the low chromosphere.Aims.We numerically studied the lowβmagnetic reconnection process around the solar temperature minimum region (TMR) by including more realistic physical diffusions and radiative cooling models. We aim to find out whether UV bursts may occur in the low chromosphere and to investigate the dominant mechanism that accounts for heating in the UV burst in the chromosphere.Methods.We used the single-fluid magnetohydrodynamic (MHD) code NIRVANA to perform the simulations. The time-dependent ionization degrees of hydrogen and helium are included in the code, which lead to a more realistic magnetic diffusion caused by electron-neutral collision and ambipolar diffusion. A more realistic radiative cooling model is also included in the simulations. The initial mass density and temperature are 1.66057 × 10−6kg m−3and 4400 K, respectively, values that are typical for the plasma environment around TMR.Results.Our results in high resolution indicate that the plasmas in the reconnection region are heated up to more than 20 000 K if the reconnecting magnetic field is as strong as 500 G, which suggests that UV bursts can be generated in the dense low chromosphere. The dominant mechanism for producing the UV burst in the low chromosphere is heating, as a result of the local compression in the reconnection process. The thermal energy occurring in the reconnection region rapidly increases after the turbulent reconnection mediated by plasmoids is invoked. The average power density of the generated thermal energy in the reconnection region can reach over 1000 erg cm−3s−1, which is comparable to the average power density accounting for a UV burst. With the strength of the reconnecting magnetic field exceeding 900 G, the width of the synthesized Si IV 1394 Å line profile with multiple peaks can reach up to 100 km s−1, which is consistent with observations.

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.

Ellerman bombs (EBs) and ultraviolet (UV) bursts are two of the smallest observed solar activities triggered by magnetic reconnection in the lower solar atmosphere, typically associated with flux emergence regions. Joint observations from the Interface Region Imaging Spectrograph (IRIS) satellite and ground-based solar telescopes reveal that approximately 20% of hot UV bursts are temporally and spatially connected with the cooler EBs. Using 3D radiation magnetohydrodynamic (RMHD) simulations with the MURaM code, we investigated the spontaneous emergence of a magnetic flux sheet, leading to complex magnetic field structures and diverse high-temperature activities due to magnetic reconnection. The simulations show that opposite-polarity magnetic fields converge in the lower solar atmosphere, forming thin current sheets and triggering plasmoid instability, which results in small twisted magnetic flux ropes and highly nonuniform plasma density and temperature. Hot plasmas (>20,000 K) emitting strong UV radiation coexist with cooler plasmas (

Context. Ellerman bombs (EBs), observed in the photospheric wings of the Hα line, and UV bursts, observed in the transition region Si IV line, are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere. Aims. We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5 Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship. Methods. The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the 1.5D RH code and the fully 3D MULTI3D code. Results. The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic spectra in the Hα and Si IV 139.376 nm lines both show characteristics that are typical of the observations. These synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.

Ellerman bombs are transient brightenings in the wings of Hα 6563 Å that pinpoint photospheric sites of magnetic reconnection in solar active regions. Their partial visibility in the 1600 Å and 1700 Å continua registered routinely by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) offers a unique opportunity to inventory such magnetic-field disruptions throughout the AIA database if a reliable recipe for their detection can be formulated. This is done here. We have improved and applied an Hα Ellerman bomb detection code to ten data sets spanning viewing angles from solar disc centre to the limb. They combine high-quality Hα imaging spectroscopy from the Swedish 1 m Solar Telescope with simultaneous AIA imaging around 1600 Å and 1700 Å. A trial grid of brightness, lifetime and area constraints is imposed on the AIA images to define optimal recovery of the 1735 Ellerman bombs detected in Hα. The best results when optimising simultaneously for recovery fraction and reliability are obtained from 1700 Å images by requiring 5σ brightening above the average 1700 Å nearby quiet-Sun intensity, lifetime above one minute, area of 1–18 AIA pixels. With this recipe 27% of the AIA detections are Hα-detected Ellerman bombs while it recovers 19% of these (of which many are smaller than the AIA resolution). Better yet, among the top 10% AIA 1700 Å detections selected with combined brightness, lifetime and area thresholds as many as 80% are Hα Ellerman bombs. Automated selection of the best 1700 Å candidates therefore opens the entire AIA database for detecting most of the more significant photospheric reconnection events. This proxy is applicable as a flux-dynamics tell-tale in studying any Earth-side solar active region since early 2010 up to the present.

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.

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 Telescope Heliographique pour l’Etude du Magnetisme et des Instabilites 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 Vlos 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 Vlos 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 Hα-line spectra obtained during 4 min, when the intensity in the line wings sharply increased. The peculiarities of variations in the intensity of the Hα-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.

Ellerman bombs are short-lived brightenings of the outer wings of Hα that occur in active regions with much flux emergence. We point out fads and fallacies in the extensive Ellerman bomb literature, discuss their appearance in various spectral diagnostics, and advocate their use as indicators of field reconfiguration in active-region topography using AIA 1700 Å images.

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.

Ellerman bombs (EBs) are transient brightenings of the extended wings of the solar Balmer lines in emerging active regions. We describe their properties in the ultraviolet lines sampled by the Interface Region Imaging Spectrograph (IRIS), using simultaneous imaging spectroscopy in H with the Swedish 1-m Solar Telescope (SST) and ultraviolet images from the Solar Dynamics Observatory for Ellerman bomb detection and identification. We select multiple co-observed EBs for detailed analysis. The IRIS spectra strengthen the view that EBs mark reconnection between bipolar kilogauss fluxtubes with the reconnection and the resulting bi-directional jet located within the solar photosphere and shielded by overlying chromospheric fibrils in the cores of strong lines. The spectra suggest that the reconnecting photospheric gas underneath is heated sufficiently to momentarily reach stages of ionization normally assigned to the transition region and the corona. We also analyze similar outburst phenomena that we classify as small flaring arch filaments and ascribe toreconnection at a higher location. They have different morphologies and produce hot arches in million-Kelvin diagnostics.

We conducted coordinated observations of Ellerman bombs (EBs) between Hinode Satellite and Hida Observatory (HOP12). Ca II H broad-band filter images of NOAA 10966 on 2007 August 9 and 10 were obtained with the Solar Optical Telescope (SOT) aboard the Hinode Satellite, and many bright points were observed. We identified a total of 4 bright points as EBs, and studied the temporal variation of their morphological fine structures and spectroscopic characteristics. With high-resolution Ca II H images of SOT, we found that the EBs, thus far thought of as single bright features, are composed of a few of fine subcomponents. Also, by using Stokes $I$/$V$ filtergrams with Hinode/SOT, and Ca II H spectroheliograms with Hida/Domeless Solar Telescope (DST), our observation showed: (1) The mean duration, the mean width, the mean length, and the mean aspect ratio of the subcomponents were 390 s, 170 km, 450 km, and 2.7, respectively. (2) Subcomponents started to appear on the magnetic neutral lines, and extended their lengths from the original locations. (3) When the Ca II H line of EBs showed the characteristic blue asymmetry, they are associated with the appearance or re-brightening of subcomponents. Summarizing our results, we obtained an observational view that elementary magnetic reconnections take place one by one successively and intermittently in EBs, and that their manifestation is the fine subcomponents of the EB phenomena.

The paper examines the prospects of using laser plasmas for studying novel regimes of the magnetic field line reconnection and charged particle acceleration. Basic features of plasma dynamics in the three-dimensional configurations relevant to the formation of current sheets in a plasma are addressed by analyzing exact self-similar solutions of the magneto-hydrodynamics and electron magneto-hydrodynamics equations. Then the magnetic field annihilation in the ultrarelativistic limit is considered, when the opposite polarity magnetic field is generated in collisionless plasma by multiple laser pulses, in the regime with a dominant contribution of the displacement current exciting a strong large-scale electric field. This field leads to the conversion of the magnetic energy into the kinetic energy of accelerated particles inside a thin current sheet. Charged particle acceleration during magnetic field reconnection is discussed when radiation friction and quantum electrodynamics effects become dominant.