9o Encontro Brasileiro de Fısica dos Plasmas / ID: 113-1. 1. Broadband coherent synchrotron radiation contribution to solar microwave bursts produced by.
9o Encontro Brasileiro de F´ısica dos Plasmas / ID: 113-1
1
Broadband coherent synchrotron radiation contribution to solar microwave bursts produced by bunched high energy electron beams Pierre Kaufmann(1) CRAAM/EE, Universidade Presbiteriana Mackenzie, S˜ ao Paulo, SP, Brazil
J. Michael Klopf Jefferson Lab, Free Electron Laser Facility, Newport News, VA, 23606 USA
Jean-Pierre Raulin, Marta M. Cassiano, C. Guillermo Gim´enez de Castro, Arline M. Melo(1) CRAAM/EE, Universidade Presbiteriana Mackenzie, S˜ ao Paulo, SP, Brazil
The recent discovery of sub-THz peaks in the emission from solar flares challenges interpretations using existing models. Observation of these spectral signatures may bring new insights to the understanding of flare emissions. Although there are controversies about the nature of solar flare accelerators, they are known to produce relativistic electrons beams (E > 10 MeV) injected into helix-like magnetic fields. This has striking similarities to the coherent radiation produced by relativistic electron bunches in laboratory accelerators. In accelerators and solar flares, broadband incoherent synchrotron radiation (ISR) is emitted from the accelerated electrons. If the beam traverses a quasi-periodic magnetic structure, two processes can result in coherent emission. Firstly, coherent narrowband emission resonant with the magnetic structure can be produced. Solar magnetic structures may be comparable to undulators used in laboratory accelerators, but much less uniform, possibly broadening the narrowband emission. In such structures, density perturbations can group the electrons into very short bunches (microbunches). Radiation at wavelengths equal to or longer than the microbunch produces broadband coherent synchrotron radiation (CSR). The peak frequency of the CSR thus depends on the microbunch size, which was characterized in laboratories only in recent years. The CSR scales as N 2 (N = number of electrons), so microbunching of even a fraction of the particles could produce substantial CSR in the cm-mm wavelength range. This interpretation is consistent with other flare descriptions and models, and presents one plausible explanation for the “solar flare electron number paradox” (i.e. the discrepancy in electron number computed from microwave and hard X-ray observations). Further, both microwave solar bursts and emission from laboratory helical undulators exhibit circular polarization. To improve the solar flare accelerator model, a number of theoretical and observational developments are needed. Tests for coherence and the presence of rapid pulsations in solar bursts could indicate electron beam microbunching. Similarly, microwave, THz, and infrared observations of brightness, temperature, flux, and polarization with improved spectral and spatial resolution; complemented by flare description in the visible, UV, X- and gamma-rays will enable testing of this proposed mechanism. (1) also at CCS, Universidade Estadual de Campinas, Campinas, SP, Brazil
