Ellerman Bomb on Light Bridge Seen in Brackett

Alan Clark and Marcel Bergman (University of Calgary), and Doug Rabin and Claude Plymate (NSO) used the NIM infrared camera on the main spectrograph of the McMath-Pierce Telescope to image an active region at HI Brackett wavelengths for the first time. The active region NOAA 8350 was observed during a period of excellent seeing on 8 October 1998, at a time when a bright Ellerman bomb appeared on a light bridge crossing the main sunspot.

Spectroheliograms were assembled from a series of 200 spectral-spatial frames in which the 256 × 256 array imaged about 50" of the solar surface in the spatial direction and about 3 cm^-1 around the Br position at 2467.7 cm^-1, or 4.05 m, in the spectral direction. The pixel spacing on each image was 0.2" and 0.012 cm^-1 respectively. The telescope was translated by 0.8" in a direction perpendicular to the slit between each of the 200 frames of a spatial swath.

Although light clouds interrupted parts of the observing, the seeing was excellent during the imaging of the central sunspot. Images of small, bright Ellerman bombs on the fringe of the penumbra are about 1 arcsec in diameter, almost matching the limiting resolution of the telescope at these IR wavelengths. These features show the characteristic "moustache" spectral signature for spectral lines. However, the most interesting feature, a bright Ellerman bomb, can be seen on the light bridge which crosses the sunspot. The spectral frame centered on this feature, which is crossed by several N₂O absorption lines from the Earth's atmosphere, clearly shows the moustache emission extending over most of the 3 cm^-1 spectral range. A spectroheliogram at a wavelength near the peak of the emission clearly shows the intense brightness as well as the structure of the transient event, while a spectroheliogram centered on Br shows no brightening, as expected. The matching CaII K image also shows this bright feature.

Caption: Spectral Frame centered on the Ellerman bomb. Caption: Image at "moustache" peak intensity.

The appearance of the sunspot in Br is interesting in its own right. The dark umbra is surrounded by a bright penumbra, which itself is surrounded by plage regions which appear dark against the quiet photosphere. This behavior has been seen in subsequent observations of other sunspots.

Caption: Image at Br Line Center. Caption: Ca II K-line image.

Two other lines in this spectral region with potential as diagnostic tools for the study of sunspot penumbral regions are atomic SiI lines. The Zeeman splitting of one of these lines can be clearly seen on the left-hand side of the spectral frame. The lines have modest g-values (1-2) and appear to follow the behavior of the 12 m MgI lines by disappearing at high magnetic field strength. These characteristics may eventually limit their usefulness, but consistent field strength values of about 800 Gauss have been measured with both lines on test spectra over the penumbral region.

These results are part of a broader infrared spectroscopic imaging program to explore the distribution across active regions and the limb of many metal and molecular lines in order to develop new diagnostic tests for atmospheric models of both quiet and active regions of the Sun.