Three X-ray global waves

Science Nugget: September 7, 2001


By "global waves" we mean large-scale coronal waves of the sort detected as Type II radio bursts, H-alpha Moreton waves, or EIT waves. Arguably, sometimes, these are all manifestations of the same thing (see below): the presence of an weak fast-mode magnetohydrodynamic (MHD) shock wave launched by yet-mysterious processes in the heart of a solar flare. There are also Type II signatures from interplanetary shocks, but we now believe that these are probably akin to bow waves driven ahead of the CMEs rather than free-running "blast waves" of the sort seen in the corona. The CME ("coronal mass ejection") is a whole separate topic currently of great research interest.

There have now been at least two interesting new developments in this area:

  • Yohkoh was launched, and its soft X-ray telescope SXT nicely imaged the corona but did not immediately see these global waves, which should have been there; and
  • SOHO was launched, and its extreme ultraviolet imager EIT did see global waves. But, they were different, in that they did not move as fast as the Moreton waves or their Type II radio-burst counterparts did.

    Part of this problem has now been solved. It turns out that most EIT waves are in fact not just Moreton waves, but more usually coronal restructurings. In fact, EIT does not observe frequently enough to catch the fast waves as a rule. The restructuring waves happen more slowly and are closely related to the initial motions of CMEs. Interestingly, the insight that something other than MHD shocks could be involved was itself deceptive; all three of the insight events also had MHD shock signatures as well as CME restructurings. It has been hard for EIT to catch the true Moreton wave phenomenon, just as it has been for SXT, but we show one of the several examples of good agreement that now exist (see here for the original clear evidence that some EIT wave observations were indeed Moreton waves). In the meanwhile, fans of these science nuggets will remember our enthusiastic report of the first good soft X-ray observation.

    In this nugget we report a couple of additional X-ray detections, both from flares in the same active region. In at least one of them we find an close timing match between H-alpha, EUV, and soft X-ray wave fronts, conclusively identifying the existence of shock-wave signatures in the coronal images.

    Three waves from one region, then a zinger

    One particular active region produced several extremely impulsive flares in November, 1997, and culminated with a major X-class event on November 6. The plot below shows the time history of soft X-ray flux, with arrows pointing to the wave events and to the zinger (an X9 flare) on November 6. A previous science nugget on this active region, NOAA number 8100, contained a spectacular movie of the build-up to this major flare.

    The X-class flare itself, somehow omitted from past nuggets, is well worth discussion itself because of spectacular hard X-ray observations. As one can see from the flare parameters or the plot above, the wave-associated flares started small. It would strain credulity to imagine that there were no wave from the X9.4 event, but there you have it - the NOAA listing missed the Potsdam report of its Type II burst.

    Below we show one miscellaneous image from each of the three wave events:

    On the left, a flare-mode image (10 arc min field of view), heavily saturated but showing the ghostly wave front in the upper left. The movie allows the eye to follow the wave slightly better. In the middle - an SXT full-frame image. Yohkoh did not enter flare mode, so catching this wave was kind of accidental. On the right, a sketch of the Moreton wave on November 4, taken from very nice Japanese-language Web pages prepared by graduate student S. Eto of Kyoto University's Kwasan Observatory. From these pages I have also imported an H-alpha movie showing the event on the right; beware, though, because even using .mpg the movie file still is large.


    These data convincingly show that impulsive solar flares can produce global waves, probably by Uchida's fast-mode MHD shock mechanism, that are properly detectable in soft X-rays. The event on the left above illustrates one of SXT's major problems in this regard, namely that the scattered light from the flare core masks the wave at its brightest; the one in the middle illustrates another problem, namely that sampling outside flare mode is sketch; the GOES plot for the Eto event illustrates a third problem: nice flares often happen in orbit night. However in spite of these and other technical problems regarding global waves and soft X-rays, we have some results and are feverishly writing them up. The X-ray data are particularly important because of their relatively honest reflection of the physical parameters in the magnetic solar corona where the waves propagate, so we are hoping to learn a great deal.

    September 9, 2001
    H. Hudson <> J. Khan <>