Spectrophotometric analysis of Ellerman bombs in the Ca II, H$\mathsf{\alpha}$, and UV range

Abstract

Context. Even if Ellerman bombs have been observed in the Hα line within emerging magnetic flux regions since the early 20th century, their origin and the mechanisms that lead to their formation have been strongly debated. Recently, new arguments in favor of chromospheric magnetic reconnection have been advanced. Ellerman bombs seem to be the signature of reconnections that take place during the emergence of the magnetic field. Aims. We have observed an active region presenting emergence of magnetic flux. We detected and studied Ellerman bombs in two chromospheric lines: Ca ii 8542 Å and Hα. We investigated the link between Ellerman bombs and other structures and phenomena appearing in an emerging active region: UV bright points, arch filament systems, and magnetic topology. Methods. On August 3, 2004, we performed multi-wavelength observations of the active region NOAA 10655. This active region was the target of SoHO Joint Observation Program 157. Both SoHO/MDI and TRACE (195 Å and 1600 Å) were used. Simultaneously, we observed in the Ca ii and Na D1 lines with the spectro-imager MSDP mode of THEMIS. Alternately to the MSDP, we used the MTR spectropolarimeter on THEMIS to observe in Hα and in the Fe i doublet at 6302 Å. We derived the magnetic field vectors around some Ellerman bombs.Results. We present the first images of EBs in the Ca ii line and confirm that Ellerman bombs can indeed be observed in the Ca ii line, presenting the same "moustache" geometry profiles as in the Hα line, but with a narrower central absorption in the Ca ii line, in which the peaks of emission are around ±0.35 Å. We noticed that the Ellerman bombs observed in the wings of Ca ii line have an elongated shape -the length about 50% greater than the width. We derived mean semi-axis lengths of 1.4 × 2.0 . In the UV time profiles of the Ellerman bombs, we noticed successive enhanced emissions. The distribution of lifetimes of these individual impulses presents a strong mode around 210 s. Study of the magnetic topology shows that 9 out of the 13 EBs are located on the inversion line of the longitudinal field and that some typical examples might be associated with a bald patch topology. Conclusions. We provide new arguments in favor of the reconnection origin of Ellerman bombs. The different individual impulses observed in UV may be related to a bursty mode of reconnection. We also show that this Ca ii 8542 Å chromospheric line is a good indicator of Ellerman bombs and can bring new information about these phenomena.

Formation of Ellerman bombs due to 3D flux emergence

“…An interesting result of the work by Pariat et al (2007) was that the undulatory magnetic flux system seemed to be connected to the photosphere by bald patches (BPs). BPs are associated with fieldlines having dips tangent to the photosphere and, thus, it is likely to be related with the U segments of the undulating field.…”

Multiwavelength spectropolarimetric observations of an Ellerman bomb

“…While some authors found them to be co-spatial with facular features and network bright points (Bruzek 1972;Kitai & Muller 1984;Stellmacher & Wiehr 1991), others argued that not every brightening in the wing of Hα line is a signature of EBs (Watanabe et al 2011;Rutten et al 2013). For instance, the EBs shown by Roy & Leparskas (1973) match fairly well with the ones shown by Pariat et al (2007). However, it is not clear if these objects are indeed the same features reported by Ellerman (1917) or Socas-Navarro et al (2006).…”

“…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).…”

“…There are indications that some EBs are essentially moving magnetic features (Harvey & Harvey 1973) in the moat of sunspots (e.g., Nindos & Zirin 1998). Fang et al (2006) and Pariat et al (2007) found signatures of EBs in the Ca ii IR lines and in Hα. These studies showed that EBs exhibit enhanced brightenings not only in the wing of the Hα line, but also in the wings of the Ca ii IR line.…”

Fast imaging spectroscopy with MSDP spectrometers. Vector magnetic maps with THEMIS/MSDP

“…But velocity fields and magnetic fields observed at high levels strongly depend on photospheric values, so spectropolarimetry of different wavelength ranges are often required. Time scales smaller than one minute characterize local events such as the triggering of waves across sunspots (Tziotziou et al 2007) or emerging fluxes and Ellerman bombs in active regions (Pariat et al 2007). But short time scales not only concern small-size events.…”