X-ray and radio observations of the activation stages of an X-class solar flare

We report interesting developments prior to the impulsive phase of an X-class solar flare that occurred on September 24, 2001. Our multiwavelength study makes use of X-ray data from the Yohkoh satellite, the Ondrejov radio spectral observations in the decimetric band, and the new Hard X-Ray Spectrometer instrument (HXRS) on board the MTI satellite. The GOES time history of this event showed a "precursor" phase starting as early as two hours prior to the impulsive phase, and we have used various data sets to identify what parts of this development could be associated with the flare itself. The most interesting time interval was identified roughly one hour before the main peak when an unusual drifting radio continuum was observed together with two radio sources (at 327 and 164 MHz) in positions corresponding to expanding loops seen in Yohkoh/SXT and SOHO/EIT images, accompanied by a filament disappearence during the same period. Hard X-ray observations revealed a soft spectrum that we interpret as non-thermal, located within loop structures observed in soft X-rays along the magnetic neutral line. The hard X-ray emission continued for more than one hour, as observed in turn by the two spacecraft. In the initial phase of the flare itself, the hard X-ray emission arose in structures closely identifiable with the early soft X-ray loops, which appeared to evolve smoothly into the post-flare loop system of the flare maximum. The decimeter spectra showed loosely-correlated spiky emission at frequencies consistent with the densities inferred from soft X-rays, but with rapid drifts implying motions along field lines. From all these data we infer that the initiation of the flare involved non-thermal processes extending along the neutral line in the photosphere, systematically including open magnetic field lines as shown by the occurrence of interplanetary Type III bursts observed by the WAVES spectrometer on board the WIND spacecraft.