On the structure and dynamics of Ellerman bombs

Abstract

Aims. We study the structure and dynamics of three Ellerman bombs (EBs) observed in an evolving active region. Methods. The active region NOAA 11271 was observed with the Vacuum Tower Telescope at Observatorio del Teide/Tenerife on August 18, 2011. We used the two-dimensional Triple Etalon SOlar Spectrometer (TESOS) to obtain time sequences of the active region and of EBs in Hα at a cadence of 15 s. Simultaneously, we obtained full Stokes profiles with the Tenerife Infrared Polarimeter (TIP II) in the two magnetically sensitive Fe i infrared lines (IR) at 1.56 μ, scanning spatial sections of the area with cadences of 28−46 s. The Hα data were reconstructed with speckle methods to study the evolution of the atmospheric stratification. Two methods were used to extract magnetic field information from the IR Stokes profiles: 1) fitting of the (Q, U, V) profiles by Gaussians; and 2) applying the Milne-Eddington approximation, assuming two separate magnetic structures in the resolution element and fitting by trial and error some profiles from the EB areas. Data from SDO-HMI and -AIA were also used. We performed two-dimensional (2D) non-LTE radiative transfer calculations of Hα in parameterised models of EBs. Results. The three EBs studied in detail occurred in a complex active region near sunspots. They were very bright with a factor of 1.5-2.8 brighter than the nearby area. They lived for 1/2 h and longer. They were related to broadband faculae, but the latter were not the brightest features in the field of view. The EBs occurred in magnetic field configurations with opposite polarity close together. One EB was located at the outskirts of a penumbra of a complex sunspot and showed repeated "flaring" in SDO-AIA data. Another was close to a strong field patch and moved into this during the end of its lifetime. The third EB showed clear changes of field structure during the time it was observed. We obtained from the 2D modelling that heating and increase in Hα opacity are likely to occur at heights of 300-800 km. Line shifts and asymmetries can well be reproduced by velocities at these heights and also at much larger heights. Conclusions. The three EBs occurred at sites with magnetic fields of opposite polarity, which were likely the cause of the Hα brightening upon reconnection.

Hαfeatures with hot onsets

“…However, it is essential that linear Stark broadening, a specific agent for Balmer lines, is included in the line formation computation since without it the Hα wing has less extinction than the continuum. This is demonstrated by the lowest solid curve, which results from only accounting for radiation damping and Van der Waals broadening as done by Bello González et al (2013).…”

“…Kitai (1983), Berlicki et al (2010), Bello González et al (2013) and Berlicki & Heinzel (2014) performed Hα synthesis from best-fit ad-hoc perturbations of a static 1D standard model of the solar atmosphere to reproduce observed Hα wing brightenings. These modeling efforts were summarized in Paper IV together with their failures: nonreproduction of the observed photospheric anchoring of EBs in internetwork lanes (Paper I), non-reproduction of EB brightening in IRIS Si iv and C ii lines (Paper III), and non-reproduction of EB non-visibility in the Na i D and Mg i b lines (Paper IV).…”

Multiwavelength spectropolarimetric observations of an Ellerman bomb

“…Payne (1993), Berlicki et al (2010), and Bello González et al (2013) found them to lie around the temperature minimum and above it. Some authors suggested that EBs form in the low chromosphere (Matsumoto et al 2008), while others found that they are solely a photospheric phenomena (Watanabe et al 2011;Vissers et al 2013).…”

“…Some studies made a link between EBs and undulating field topologies in emerging flux regions (Pariat et al 2004;Watanabe et al 2008). Bello González et al (2013) found evidence for the existence of opposite magnetic polarities at the sites of three EBs.…”

“…Ellerman bombs have a typical lifetime of a few minutes, but can live up to an hour (Kurokawa et al 1982;Georgoulis et al 2002;Pariat et al 2007;Bello González et al 2013). The brightening peaks at about Hα ± 1.2 Å and fades away toward ±5 Å (Berlicki et al 2010;Kneer 2010).…”