A microflare in the corona

Science nugget: Jan 1, 1999

This nugget, the year's first, is only happening because there was a nice observation of an over-the-limb microflare on Dec. 21 that might as well be pointed out. This week (1998 Dec. 30 to 1999 Jan. 4), Yohkoh is in its normal extended holiday period over New Year's, with minimal observation, no operations staff, and little data analysis. Check out next week's for really new material.

Here's what caught my eye, as shown in the image sequence for one filter (SXT was operating four filters in its quiet mode at this time):


The eye can easily spot the bright blob appearing as time evolves (one frame every two "digital minutes", 128-sec cadence). Is this

  • (a) a Feldman blob (Acton, Feldman et al., PASJ 44, L71, 1992),
  • (b) a Masuda source (Masuda et al., Nature 371, 495, 1994),
  • (c) an example of coronal loop-loop interaction (e.g. Smartt et al., Solar Physics 148, 139-151, 1993) or
  • (d) an artifact of 3D geometry?
  • I think it's the latter. The flaring loop was not visible before the event, but probably existed in close to its observed location; the orientation of the loop was roughly east-west, and line-of-sight integration explains the rest. To confirm the EW orientation we looked at a KPNO magnetogram from a week earlier, when the active region was near disk center:


    Sure enough, at the latitude of the event (about -33), the polarity separation is roughly EW. Thus the tangent to the loop top would have been approximately in the line of sight on Dec. 21, at the limb, and the apparent loop-top brightening would be easily explainable by mere geometry. So, the answer to the quiz is (d), unfortunately the least interesting from a physical point of view!

    Incidentally, the KPNO archive image has an artifact - very unusual in this well-maintained database, for which we're extremely grateful - that makes the active region look like a two-ribbon unipolar magnetic flare! Apparently the strips composing the image were slightly misaligned, resulting in two copies of the active region plus a bite taken out of the W limb. We only show this to point out that two-ribbon unipolar magnetic flares are extremely unlikely ever to occur.

    Comments to: hudson@isass0.solar.isas.ac.jp