NARROWBAND GYROSYNCHROTRON BURSTS: PROBING ELECTRON ACCELERATION IN SOLAR FLARES

Recently, in a few case studies we demonstrated that gyrosynchrotron microwave emission can be detected directly from the acceleration region when the trapped electron component is insignificant. For the statistical study reported here, we have identified events with steep (narrowband) microwave spectra that do not show a significant trapped component and at the same time show evidence of source uniformity, which simplifies the data analysis greatly. Initially, we identified a subset of more than 20 radio bursts with such narrow spectra, having low- and high-frequency spectral indices larger than 3 in absolute value. A steep low-frequency spectrum implies that the emission is nonthermal (for optically-thick thermal emission, the spectral index cannot be steeper than 2), and the source is reasonably dense and uniform. A steep high-frequency spectrum implies that no significant electron trapping occurs; otherwise a progressive spectral flattening would be observed. Roughly half of these radio bursts have RHESSI data, which allows for detailed, joint diagnostics of the source parameters and evolution. Based on an analysis of radio-to-X-ray spatial relationships, timing, and spectral fits, we conclude that the microwave emission in these narrowband bursts originates directly from the acceleration regions, which have relatively strong magnetic field, high density, and low temperature. In contrast, the thermal X-ray emission comes from a distinct loop with smaller magnetic field, lower density, but higher temperature. Therefore, these flares occurred likely due to interaction between two (or more) magnetic loops.

[1]  E. Kontar,et al.  ACCELERATION, MAGNETIC FLUCTUATIONS, AND CROSS-FIELD TRANSPORT OF ENERGETIC ELECTRONS IN A SOLAR FLARE LOOP , 2011, 1102.3664.

[2]  V. Zaitsev,et al.  On the Origin of Pulsations of Sub-THz Emission from Solar Flares , 2014 .

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

[4]  D. Gary,et al.  The secondary spectral component of solar microwave bursts , 1990 .

[5]  D. A. Guidice,et al.  Spectral distributions of microwave bursts , 1975 .

[6]  K. Shibasaki,et al.  Loop-Top Nonthermal Microwave Source in Extended Solar Flaring Loops , 2002 .

[7]  C. G. Giménez de Castro,et al.  A Burst with Double Radio Spectrum Observed up to 212 GHz , 2012, 1208.5904.

[8]  G. J. Hurford,et al.  Deducing Electron Properties from Hard X-ray Observations , 2011, 1110.1755.

[9]  Gordon J. Hurford,et al.  Multifrequency Observations of a Solar Microwave Burst with Two-dimensional Spatial Resolution , 1990 .

[10]  J. Brown,et al.  Nonsolar astronomy with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) , 2003, SPIE Astronomical Telescopes + Instrumentation.

[11]  H. Zirin,et al.  Flat microwave spectra seen at X-class flares , 1994 .

[12]  D. Gary,et al.  A COLD, TENUOUS SOLAR FLARE: ACCELERATION WITHOUT HEATING , 2011, 1103.2705.

[13]  Gordon J. Hurford,et al.  A frequency-agile interferometer for solar microwave spectroscopy , 1984 .

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

[15]  D. Gary,et al.  Peak Frequency Dynamics in Solar Microwave Bursts , 2008 .

[16]  D. Gary,et al.  A simple solar microwave burst observed with high spectral resolution , 1988 .

[17]  A. Magun,et al.  Spectral Flattening During Solar Radio Bursts At Cm–mm Wavelengths and the Dynamics of Energetic Electrons in a Flare Loop , 1998 .

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

[19]  C. G. Giménez de Castro,et al.  Solar flares at submillimeter wavelengths , 2013, The Astronomy and Astrophysics Review.

[20]  J. Michael Klopf,et al.  THE CONTRIBUTION OF MICROBUNCHING INSTABILITY TO SOLAR FLARE EMISSION IN THE GHz TO THz RANGE OF FREQUENCIES , 2014, 1406.5499.

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

[22]  Lindsay Glesener,et al.  MEASUREMENTS OF THE CORONAL ACCELERATION REGION OF A SOLAR FLARE , 2010 .

[23]  D. Gary,et al.  ENERGY PARTITIONS AND EVOLUTION IN A PURELY THERMAL SOLAR FLARE , 2015, 1502.00315.

[24]  F. Benvenuto,et al.  Determination of the Acceleration Region Size in a Loop-structured Solar Flare , 2012, 1206.0477.

[25]  P. Kaufmann.  Observations of Solar Flares from GHz to THz Frequencies , 2012 .

[26]  Joaquim E. R. Costa,et al.  A very narrow gyrosynchrotron spectrum during a solar flare , 2006 .

[27]  G. J. Hurford,et al.  High-resolution microwave spectra of solar bursts , 1989 .

[28]  D. Gary,et al.  PROBING DYNAMICS OF ELECTRON ACCELERATION WITH RADIO AND X-RAY SPECTROSCOPY, IMAGING, AND TIMING IN THE 2002 APRIL 11 SOLAR FLARE , 2013, 1303.4098.

[29]  K. A. Marsh,et al.  Simplified expressions for the gyrosynchrotron radiation from mildly relativistic, nonthermal and thermal electrons , 1982 .

[30]  E. Kontar,et al.  SUB-THz RADIATION MECHANISMS IN SOLAR FLARES , 2009, 0911.5335.

[31]  D. Gary,et al.  Radio Spectral Evolution of an X-Ray-poor Impulsive Solar Flare: Implications for Plasma Heating and Electron Acceleration , 2007, 0704.2413.

[32]  D. Gary,et al.  Statistical Study of Two Years of Solar Flare Radio Spectra Obtained with the Owens Valley Solar Array , 2004 .

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

[34]  E. Kontar,et al.  ON THE VARIATION OF SOLAR FLARE CORONAL X-RAY SOURCE SIZES WITH ENERGY , 2014, 1404.1962.

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

[36]  Richard A. Schwartz,et al.  Compton backscattered and primary X-rays from solar flares: angle dependent Green's function correction for photospheric albedo , 2005 .

[37]  M. Aschwanden,et al.  Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares , 2011, 1109.6496.

[38]  Yan Xu,et al.  RHESSI Hard X-Ray Imaging Spectroscopy of Extended Sources and the Physical Properties of Electron Acceleration Regions in Solar Flares , 2008 .

[39]  V. Mel’nikov.  Electron acceleration and capture in impulsive and gradual bursts: Results of analysis of microwave and hard x-ray emissions , 1994 .