On the structure and dynamics of Ellerman bombs

Detailed study of three events and modelling of Hα

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
e-mail: nbello@kis.uni-freiburg.de
² Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
e-mail: danilovic@mps.mpg.de
³ Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: kneer@astro.physik.uni-goettingen.de

Received: 3 April 2013
Accepted: 6 July 2013

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.