Spatial Evidence for Transition Radiation in a Solar Radio Burst

Microturbulence, i.e. enhanced fluctuations of plasma density, electric andmagnetic fields, is of great interest in astrophysical plasmas, but occurs onspatial scales far too small to resolve by remote sensing, e.g., at ~ 1-100 cmin the solar corona. This paper reports spatially resolved observations thatoffer strong support for the presence in solar flares of a suspected radioemission mechanism, resonant transition radiation, which is tightly coupled tothe level of microturbulence and provides direct diagnostics of the existenceand level of fluctuations on decimeter spatial scales. Although the level ofthe 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 particlediffusion and give rise to effective stochastic acceleration. This finding hasexceptionally broad astrophysical implications since modern sophisticatednumerical models predict generation of much stronger turbulence in relativisticobjects, e.g., in gamma-ray burst sources.Comment: 13 pages, 4 figures, ApJL accepte