Radio Spectral Evolution of an X-Ray-poor Impulsive Solar Flare: Implications for Plasma Heating and Electron Acceleration

We present radio and X-ray observations of an impulsive solar flare that was moderately intense in microwaves, yet showed very meager EUV and X-ray emission. The flare occurred on 2001 October 24 and was well observed at radio wavelengths by the Nobeyama Radioheliograph (NoRH), the Nobeyama Radio Polarimeters (NoRP), and the Owens Valley Solar Array (OVSA). It was also observed in EUV and X-ray wavelength bands by the TRACE, GOES, and Yohkoh satellites. We find that the impulsive onset of the radio emission is progressively delayed with increasing frequency relative to the onset of hard X-ray emission. In contrast, the time of flux density maximum is progressively delayed with decreasing frequency. The decay phase is independent of radio frequency. The simple source morphology and the excellent spectral coverage at radio wavelengths allowed us to employ a nonlinear χ2-minimization scheme to fit the time series of radio spectra to a source model that accounts for the observed radio emission in terms of gyrosynchrotron radiation from MeV-energy electrons in a relatively dense thermal plasma. We discuss plasma heating and electron acceleration in view of the parametric trends implied by the model fitting. We suggest that stochastic acceleration likely plays a role in accelerating the radio-emitting electrons.

[1]  S. White,et al.  Updated Expressions for Determining Temperatures and Emission Measures from Goes Soft X-Ray Measurements , 2005 .

[2]  A. Veronig,et al.  A Coronal Thick-Target Interpretation of Two Hard X-Ray Loop Events , 2004 .

[3]  T. Lüthi,et al.  First observation of a solar X-class flare in the submillimeter range with KOSMA , 2004 .

[4]  V. Petrosian,et al.  Stochastic Acceleration of Electrons and Protons. I. Acceleration by Parallel-Propagating Waves , 2004, astro-ph/0401585.

[5]  G. Fleishman,et al.  Gyrosynchrotron Emission from Anisotropic Electron Distributions , 2003 .

[6]  J. Raulin,et al.  First detection of the impulsive and extended phases of a solar radio burst above 200 GHz , 2002 .

[7]  K. Shibasaki,et al.  Spatial Structure of Simple Spiky Bursts at Microwave/Millimeter Wavelengths , 2001 .

[8]  Haimin Wang,et al.  Correlation of Microwave and Hard X-Ray Spectral Parameters , 2000 .

[9]  D. Gary,et al.  Solar Microwave Bursts and Injection Pitch-Angle Distribution of Flare Electrons , 2000 .

[10]  M. Kundu,et al.  Observations and Models of a Flaring Loop , 2000 .

[11]  K. Shibasaki,et al.  Magnetic Trapping and Electron Injection in Two Contrasting Solar Microwave Bursts , 2000 .

[12]  Jay A. Bookbinder,et al.  The transition region and coronal explorer , 1998 .

[13]  D. Gary,et al.  RADIO EMISSION FROM SOLAR FLARES , 1998 .

[14]  L. Belkora.  Time Evolution of Solar Microwave Bursts , 1997 .

[15]  Stephen G. Benka,et al.  Critical issues for understanding particle acceleration in impulsive solar flares , 1997 .

[16]  V. Petrosian,et al.  Stochastic Acceleration of Low-Energy Electrons in Cold Plasmas , 1996, astro-ph/9610148.

[17]  James A. Miller,et al.  Stochastic Electron Acceleration by Cascading Fast Mode Waves in Impulsive Solar Flares , 1996 .

[18]  H. Nakajima,et al.  Gamma-ray and millimeter-wave emissions from the 1991 June X-class solar flares , 1994 .

[19]  K. Shibasaki,et al.  The Nobeyama radioheliograph , 1994, Proc. IEEE.

[20]  Gordon J. Hurford,et al.  Coronal temperature, density, and magnetic field maps of a solar acitve region using the Owens Valley Solar Array , 1994 .

[21]  N. Vilmer,et al.  Electron trapping in evolving coronal structures during a large gradual hard X-ray/radio burst , 1994 .

[22]  R. Hamilton,et al.  Stochastic acceleration of electrons. I. Effects of collisions in solar flares , 1992 .

[23]  G. Holman,et al.  A Thermal/Nonthermal Model for Solar Microwave Bursts , 1992 .

[24]  Gordon J. Hurford,et al.  Multifrequency Observations of a Remarkable Solar Radio Burst , 1991 .

[25]  P. Sturrock,et al.  Classification of Solar Flares , 1989 .

[26]  K. Kai,et al.  Energetic electrons in impulsive and extended solar flares as deduced from flux correlations between hard X-rays and microwaves , 1988 .

[27]  E. Cliver,et al.  Solar gradual hard x-ray bursts and associated phenomena , 1986 .

[28]  K. Klein,et al.  Microwave diagnostics of energetic electrons in flares , 1986 .

[29]  S. Wu,et al.  Inhibition of conductive heat flow by magnetic constriction in the corona and transition region: Dependence on the shape of the constriction , 1985 .

[30]  B. Dennis,et al.  Characteristics of gamma-ray line flares , 1985 .

[31]  I. N. Toptygin.  Cosmic rays in interplanetary magnetic fields , 1985 .

[32]  G. Dulk,et al.  Radio Emission from the Sun and Stars , 1985 .

[33]  T. Kosugi,et al.  Long time delay between the peaks of intense solar hard X-ray and microwave bursts , 1983 .

[34]  D. Melrose.  Plasma astrophysics: Nonthermal processes in diffuse magnetized plasmas. Volume 1 - The emission, absorption and transfer of waves in plasmas , 1980 .

[35]  G. S. Vaiana,et al.  A survey of soft X-ray limb flare images: the relation between their structure in the corona and other physical parameters. , 1977 .

[36]  J. Brown,et al.  Precipitation in Trap Models for Solar Hard X-ray Bursts , 1976 .

[37]  K. R. Lang,et al.  Astrophysical Formulae: A Compendium for the Physicist and Astrophysicist , 1974 .

[38]  R. Ramaty,et al.  Free-free absorption of gyrosynchrotron radiation in solar microwave bursts. , 1972 .

[39]  R. Ramaty.  Gyrosynchrotron emission and absorption in a magnetoactive plasma , 1969 .

[40]  V. Ginzburg,et al.  Developments in the Theory of Synchrotron Radiation and its Reabsorption , 1969 .