Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 19, 2002)

Science Nugget: May 10, 2002

Complicated radio waves from an X-class flare


The remarkable flare that we introduced in the April 19 science nugget will continue to amaze and astound us, partly because the "Max Millennium" organizational machinery worked splendidly, so that many of the world's observatories were looking at the right place at the right time. In this nugget we present complicated data from ground-based observatories in China and Japan. The purpose here is mainly to give a feeling for the range of data people need to consider while they work out explanations for these things. The intention is to return to this flare again with future nuggets, and no doubt this well-observed phenomenon will be the subject of an actual workshop in which people meet physically, rather than through the ether as we are doing now.

Radio time profiles

The plot below shows a sample of three time profiles at radio frequencies of 1, 17, and 34 GHz, from the Nobeyama solar radio observatory.

Microwave time profiles

Many complicated things can be seen in this plot:

  • At 1 GHz (red) a huge flux, due (we think) to a plasma emission mechanism capable of making the apparent temperature (hence radio flux) far greater than its natural thermal temperature;
  • At 17 GHz (purple) the gyrosynchrotron spectrum, also inherently non-thermal, produced by fast electrons gyrating in coronal magnetic fields; and
  • At 34 GHz (yellow), barely detected because of drifting background levels, the thermal signal - bremsstrahlung or free-free radiation are other terms for this - that is the low-frequency tail of what we see in X-rays, actually.

    Radio spectra

    Even more complicated, almost to the point of inexplicability, one has various kinds of radio spectra, such as the following from Huairou:

    This represents a narrow range (1.2 GHz) of frequencies not far from the red line in the plot above), over a short time interval (about 3 minutes). It contains a myriad of drifting structures. Radio events with drifting fine structure occur, and in principle their occurrences tells us many things. Because of the frequencies we know that the sources of these emissions do not lie far up in the corona, but rather in the dense core of the flaring active region. So probably we would learn about flare dynamics directly if only we could understand them. But, this will require a new instrument: a Frequency Agile Solar Radiotelescope capable of true imaging spectroscopy, with high resolution in all parameters.


    No conclusions, really, except to remind data analyzers of the great complexity of the radio spectrum of a solar flare. This complexity is unfortunate, but of course it points to complex knowledge that will be gained once the phenomena are understood.

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    May 10, 2002

    Hugh Hudson and David McKenzie(hudson@isass1.solar.isas.ac.jp)