Spectrophotometric analysis of Ellerman bombs in the Ca II, H, and UV range

¹ LESIA, Observatoire de Paris - Meudon, 92195 Meudon Cedex, France e-mail: etienne.pariat@obspm.fr
² Université Paris 7 - Denis Diderot, 75251 Paris Cedex 05, France
³ Naval Research Laboratory, Space Science Division, 20375 Washington, DC, USA
⁴ Astronomical Institute, Wrocław University, 51-622 Wrocław, Poland
⁵ National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
⁶ THEMIS - CNRS UPS 833, C/ vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁷ Department of Astronomy, Beijing Normal University, Beijing 100875, PR China

Received: 22 December 2006
Accepted: 25 June 2007

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.