A fifth kind of dimming? Or a second?

Science Nugget: December 10, 1999


The sudden dimming of parts of the corona, as observed by Yohkoh's SXT, give us a clue as to the origin of mass ejected into the outer corona and solar wind. A "coronal mass ejection" or CME may spectacularly result. In the ancient history of this subject, as far back as 1997, we recognized four types of dimming:

  • Transient coronal holes (discovered with Skylab observations)
  • "Above-the-arcade" dimming
  • "Enveloping" dimming
  • Moving cloud

    These are just morphological classifications, not physical, and it might be that all refer to the same basic process. The figures below show the prototype events:

    The full explanation of these is in a paper (.ps file) published in the Chapman Conference on Coronal Mass Ejections.

    A new variety of dimming

    Recently J.I. Khan has found three examples of dimming signature with strikingly different characteristics: the dimming occurs in trans-equatorial loops, the subject of frequent science nuggets of this type. Briefly, he found three examples of the sequence flare => global wave => loop disruption => prompt loop reformation. Here's an example:

    The three panels show, left to right, "before", "after", and the difference image, comparing two SXT images from May 6, 1998 (04:50 and 11:21 UT). The actual disappearance of the structure was much faster, but this image pair gives a clean view - the big flare in the southern active region produced a great amount of scattered light.

    What does it mean?

    Although the details are not given here, this large-scale loop rises into the corona and beyond, forming the front of a beautiful CME. Unlike many CMEs, but in agreement with the early work of Richard Harrison, the CME occurs non-concentrically with respect to its flare. It should be noted that in these cases there is strong evidence that the flare caused the CME - timing, morphology, and a plausible mechanism all support this view.

    These events also do not show much evidence of helical structure, which we know to play a role in many eruptive flares and CMEs. So if you have read this far, you understand the title of this nugget - All of the earlier four classes of X-ray dimming events might be different manifestations of flux-rope eruption, whereas these interconnecting loops might be quite different.
    December 10, 1999 Hugh Hudson (hudson@isass1.solar.isas.ac.jp)