The Physical Properties of a Flaring Loop

We have used high-resolution radio observations to study the physical parameters of a flaring loop. The loop is visible at radio wavelengths due to gyrosynchrotron emission by nonthermal electrons (energies typically above 500 keV) accelerated by the flare. We are able to measure the loop thickness and length with a precision better than 1 00. We find that the loop length increases from about 60 00 initially to about 80 00 in the decay phase of the event. The loop (averaged along its length) is just 3 00 wide. The soft and hard X-ray data obtained with the SXT and HXT telescopes on the Yohkoh satellite are consistent with the same loop as observed at radio wavelengths (although the soft X-ray morphology has some small differences early in the event). This event was accompanied by a coronal mass ejection and a coronal depletion visible in SOHO/EIT images, so it involved a very large volume of the corona, yet the radio observations clearly indicate that much of the energy release in the low corona was restricted to a region apparently no more than 2000 km across. As the event proceeds the loop develops a bright feature at the looptop in both the radio and soft X-ray images that cannot be reproduced in gyrosynchrotron loop models in which the electron distribution has relaxed by pitch angle scattering to fill the loop. This prevents us from using the flare properties to measure the magnetic field strength and variation along the loop. The bright looptop source may require that trapping of electrons take place at the looptop late in the event.