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

Abstract

A high spatial resolution observation of an emerging flux region ( EFR) was made using a vector magnetograph and a H Lyot filtergraph with the Domeless Solar Telescope at Hida Observatory on 2006 October 22. In H 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 H filtergraph. Using the H 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.

Ellerman Bombs at High Resolution. I. Morphological Evidence for Photospheric Reconnection

“…Selected recent Ellerman bomb studies of particular relevance here, in addition to the comprehensive study of Georgoulis et al (2002), are those by Pariat et al (2004Pariat et al ( , 2007, Isobe et al (2007), Watanabe et al (2008), Matsumoto et al (2008), Hashimoto et al (2010), Morita et al (2010), and Guglielmino et al (2010). These studies provide evidence that the Ellerman bomb phenomenon occurs at sites of and is due to magnetic reconnection, and characteristically has elongated shapes reminiscent of the "chromospheric anemone jets" of Shibata et al (2007).…”

“…In particular, Pariat et al (2004Pariat et al ( , 2007 and Watanabe et al (2008) found evidence for undulating "serpentine" field topologies with Ellerman bombs formed at the dip of field lines (called "bald patches" in the French literature), where the fields are nearly antiparallel. The fact that Ellerman bombs are reported to occur only in emerging flux regions speaks for an antiparallel reconnection scenario.…”

Evolution of Active Regions

“…At the beginning of flux emergence (first day) there is no single magnetic inversion line, but small (≈ 2 Mm) individual bipoles appear all over the AR in between the growing magnetic concentrations at the edges of the emerging flux Bernasconi et al, 2002). The small-scale pattern is suggestive of undulatory (serpentine) flux formed of an alternating series of small Ω and U-loops breaking through the photosphere (Strous et al, 1996;Pariat et al, 2004;Watanabe et al, 2008). These small-scale flux emergence sites may be recurrent and they may appear with a wavelike regularity (with a = 8 Mm) in between the two main opposite polarity concentrations (Strous and Zwaan, 1999).…”

Formation of Ellerman bombs due to 3D flux emergence

“…From a set of identified serpentine lines in their model, Pariat et al (2004) found that the distance between successive EBs along the undulating flux tubes has a peak around 3 Mm and a cutoff for values smaller than 2 Mm. Watanabe et al (2008) reported that magnetic fields around EBs are undulatory with a characteristic wavelength of ≈4 Mm. These sizes are larger than the convective scales for granulation (≈1 Mm) or less than supergranulation (≈10 Mm) and are indicative of the presence of some instability.…”