Spatial Evidence for Transition Radiation in a Solar Radio Burst

Abstract

Microturbulence, i.e. enhanced fluctuations of plasma density, electric and magnetic fields, is of great interest in astrophysical plasmas, but occurs on spatial scales far too small to resolve by remote sensing, e.g., at ~ 1-100 cm in the solar corona. This paper reports spatially resolved observations that offer strong support for the presence in solar flares of a suspected radio emission mechanism, resonant transition radiation, which is tightly coupled to the level of microturbulence and provides direct diagnostics of the existence and level of fluctuations on decimeter spatial scales. Although the level of the microturbulence derived from the radio data is not particularly high, <\Delta n^2 >/n^2 ~ 10^{-5}$, it is large enough to affect the charged particle diffusion and give rise to effective stochastic acceleration. This finding has exceptionally broad astrophysical implications since modern sophisticated numerical models predict generation of much stronger turbulence in relativistic objects, e.g., in gamma-ray burst sources.Comment: 13 pages, 4 figures, ApJL accepte

Remote Sensing of Gamma‐Ray Emission from Solar Energetic Proton Interactions with the Solar Wind

“…Nonthermal electrons with energies of tens of kilovolts and higher are detected in the microwave range by their gyrosynchrotron, plasma, and transition radiation as they interact with coronal magnetic fields, plasmas, and turbulence, respectively (Bastian et al 1998;Nita et al 2005). Flare hard (E k 20 keV ) X-rays from electron bremsstrahlung have been observed by instruments on a number of spacecraft ( Hudson & Ryan 1995).…”

Cosmic Electrodynamics

“…The EM map reveals the existence of a magnetic loop or arcade of loops filled with hot and dense plasma, which is consistent with the magnetic and radio topology presented in the left bottom panel. The 2 GHz RCP radio source and the 7.4 GHz LCP kernel are well aligned with the most dense section of the loop (Nita et al 2005). Reproduced by permission of the AAS.…”

“…Here we give only one example of RTR produced in a continuum burst of decimeter radio emission from a solar flare (Nita et al 2005). Figure 9.18, upper panel, presents dynamic spectra of the 2001 April 06 solar radio burst in intensity (top) and circular polarization (bottom), observed with OVSA.…”

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Generation of Emissions by Fast Particles in Stochastic Media

“…This emission process, giving rise to enhanced low-frequency (at lower frequencies than accompanying synchrotron emission) continuum radio emission, has recently been reliably confirmed in a subclass of two-component solar radio bursts [21,22]. This finding is of particular importance for diagnostics of the number density, the level of small-scale turbulence, and the dynamics of low-energy fast particles in solar flares.…”