DECIMETRIC SPIKES-TO-MICROWAVE CONTINUUM CORRELATION IN SOLAR FLARES

G.D.Fleishman

D.E.Gary, G.Nita

We report the results of comparative study of decimetric spike bursts occured simultaneously to microwave gyrosynchrotron bursts. Since the properties of gyrosynchrotron emission are well-known [1,2,3], we use the microwave emission as a \"reference frame\" to specify the respective properties of the decimetric spike clusters. We analyzed the entire list of events recorded by Owens Valley Solar Array (OVSA) during 2001 and found that all spike events display the following properties:
  1. No event has steep low-frequency microwave spectrum, thus, no indication of the Razin suppression is found, and, respectively, the plasma frequency to gyrofrequency ratio is systematically lower for spike-producing events than for other events
  2. sense of polarization of the decimetric spikes is extraordinary wave-mode, the degree of polarization varies in broad limits
  3. The degree of polarization of the optically thin microwave emission is systematically stronger than for other gyrosynchrotron bursts, indicating both magnetic asymmetry of the source and pitch-angle anisotropy of the fast electrons of the loss-cone type [8]
  4. The flux-to-flux correlation is observed sometimes, but not for all events
  5. The decimetric to microwave flux ratio is strongly correlated for all events with high-frequency spectral index of the microwave emission, while the sign of the correlation is different for quasi-transverse and quasi-parallel to the magnetic field sources of gyrosynchrotron radiation: in the quasi-parallel case the soft microwave spectrum is more favorabale for the decimetric spike production, while in the quasi-transverse case, the hard spectrum is more favorabale for the decimetric spike production. Since the gyrosynchrotron spectral index is related to the electron energy spectrum in the quasi-transverse case, while to pitch-angle anisotropy in the quasi-parallel case [3], we conclude the solar millisecond spikes to originate in relatively tenuous plasma, when the fast electrons have the hardest energy and the most anisotropic pitch-angle distributions. In essense, these are the properties of electron cyclotron maser emission generated by trapped fast electrons with power-law distribution over momentum [4].
Thus, the reported spike properties constitute the most direct evidence obtained so far for millisecond radio spikes to be generated by electron cyclotron maser emission mechanism. The results obtained are compared with those found earlier in spike-to-hard X-ray correlation study [5] and discussed together with the complete list of spike properties [4]. The work was supported by the Russian Foundation for Basic Research, grant No. 02-02-39005, and NSF grant AST-9987366 to New Jersey Institute of Technology.

References

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