Coronal oscillations and type II bursts
The solar corona, we believe, is composed mainly of magnetism - at least, this is the key structural element. It is interesting that something so ethereal can be mechanically important, and it may be that this leads to one's intuition sometimes going awry. In any case one talks about "magnetic pressure" (isotropic) and "magnetic tension" (along "field lines") as though these field lines were real, rather than just traces of a vector field pervading the medium. The wonderful discovery of a class of oscillating coronal loops via TRACE observations makes this all seem reasonable, though. This nugget explains these observations briefly and points to a possible explanation of the oscillations. As with last week's nugget, this takes us a bit further afield from Yohkoh, but not totally....
As one can see from any arbitrary TRACE image, such as the one below, the corona is threaded by many fine features. They are bright because of hot gas trapped in the magnetic flux tubes, ie bundles of magnetic field lines.
|Example of a TRACE flare observation, in fact the first one for which the oscillations were detected. The field of view is a few arc minutes, and the shape of the bright loops gives a good idea of the three-dimensionality of the structures. In this case the structure is predominantly bipolar, a bundle of flux tubes coming up out of the photosphere on the left and then going back in on the right. The diagonal streaks are caused by diffraction resulting from the very bright flare emission.|
This image, and many more things, are available on the TRACE POD ("picture of the day") pages for the first of the scientific papers in a set now numbering many - perhaps as many as ten - as people follow this important business up.
|A dissection of one of the oscillating loops in this event; the panels at the top show the selected loop (left) and its motions (right), from which one can understand (since the amplitude is only a few pixels at most) why TRACE resolution was instrumental in this discovery. Click to make visible.|
After this first event caused its ripples in the community, many other examples appeared -- a list of seventeen! Please check this link, because it contains the new result of this nugget: the association of coronal oscillations with type II solar radio bursts.
|Example of solar radio bursts, in which the emission frequency is plotted against time. The almost vertical traces are type III bursts, identified with streams of electrons; the diagonal ones are type II bursts, identified with "blast waves" (MHD shock waves) launched by solar flares. The time axis is given in minutes.|
In the list of seventeen events we have tacked on a II, denoting the occurrence of a type II burst, and its time. A total of 6 out of 17 events had such associations, a very strong result given the rarity of type II bursts and the incompleteness of our records of them.
It appears that the coronal loop oscillations result from the jiggling of the corona induced by the passage of the blast wave from the flare, at least circumstantially. This is an exciting addition to an already exciting subject: via "coronal loop seismology", the measurement of oscillation periods, one can learn quite precise things about the mysterious coronal magnetic fields. The blast waves are also quite mysterious, but now (it appears) we have a new means to recognize them and measure their effects - the speculation would be that the other 11 of the events also resulted from MHD disturbances originating in nearby flares, but that the conditions for type II radio emission (quite complex) were not met properly in these cases.
August 23, 2002
Hugh Hudson (email@example.com)