Small, but perfectly formed


Fig. 1: Image in the H-alpha wing showing the two ribbons of the flare.

Our recent service mode observational run at the Dunn Solar Telescope was successful in catching a very tiny flare on 3rd May! The event was a B9 event, with a two-ribbon morphology as viewed with the IBIS instrument (Fig 1, left), and a detectable signature in RHESSI. As can be seen, the ribbons are very narrow, particularly where they enter the sunspot umbra.

This IBIS image in the wing of H-alpha shows a mixture of chromospheric  (ribbons, fibrils) and photospheric features (spot). The observation run included broad-band spectra from the Horizontal Spectrograph, but we have yet to establish if the slit crossed the flare kernels, so we don’t yet know whether there is ‘true’ continuum emission present, but that is one of the things that we will be looking for carefully.

A preliminary look at other data shows a similar morphology. The 304 A channel of the AIA instrument on SDO has two ribbons (note, the IBIS image, which is aligned with the parallactic angle, needs to be rotated clockwise by about 45 degrees to match up), and RHESSI images indicate X-rays up to 25keV from the north/west ribbon with perhaps a part of a loop extending towards the ribbon in the spot. RHESSI spectroscopy also indicates a thermal and a non-thermal component.


Fig 2: SDO/AIA 304 A image in the background, with RHESSI contours superposed showing indications of thermal and non-thermal emission at the `fat’ ribbon, but no observable hard X-rays from the ribbon in the spot.

Well, it’s not as spectacular as the best-observed solar flare captured at the Dunn Solar Telescope on 29th April, but at least it has the virtue of being small and simple and perhaps easy to model!


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