Statistics, Morphology, and Energetics of Ellerman Bombs

Abstract

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 H filtergrams (spatial resolution 0>8), as well as photospheric vector magnetograms (spatial resolution 0>5) and Dopplergrams, have been acquired by the balloon-borne Flare Genesis Experiment. H 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 1>8 Â 1>1 , 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 (10 27 , 10 28 ) ergs, the temperature enhancement in the radiating volume is $2 Â 10 3 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 $À2.1. This suggests that Ellerman bombs may contribute significantly to the heating of the low chromosphere in emerging flux regions.

Formation of Ellerman bombs due to 3D flux emergence

“…Therefore, it is expected that QSLs may occur in the low atmosphere and EBs are triggered somewhere along QSLs. EB triggering by magnetic reconnection on QSLs was also favoured by the observational data obtained by Georgoulis et al (2002).…”

“…Kitai (1983) found that a density enhancement (by a factor of 5) and a hot temperature gradient (≈1500 K) may account for the line broadening in EBs. Georgoulis et al (2002) and Pariat et al (2004) also found that the temperature increases by ≈2000 K at the sites of EBs. In our simulations, the density enhancement is close to the value reported by Kitai (1983).…”

“…The observations by Pariat et al (2004) and Georgoulis et al (2002) correspond to the central region of an emerging active region. Based on these observations Pariat et al (2004) proposed a resistive flux emergence model where EBs are connected with Article published by EDP Sciences low-lying fieldlines with a serpentine shape.…”

“…Dara et al (1997) also reported strong upflows in the chromosphere. Georgoulis et al (2002) investigated the statistical properties of EBs in an emerging flux region and found that EBs were correlated with downflows (≈0.5 km s −1 ) at the photosphere. Also, non-LTE analysis of spectral profiles of EBs showed strong downflows (≈10 km s −1 ) between the upper photosphere and the low chromosphere (Socas-Navarro et al 2006).…”

“…These loops get rid of the over-dense photospheric plasma by reconnection at BPs. The brightenings at the regions where reconnection occurs may account for the observational appearance of EBs (Ding et al 1998;Georgoulis et al 2002;Pariat et al 2004;Fang et al 2006). The undulatory shape of the rising magnetic field and the reconnection at the low atmosphere may also be used for the explanation of moving magnetic features and the EBs observed surrounding sunspots.…”