Non‐Gaussian Radio‐Wave Scattering in the Interstellar Medium

It was recently suggested by Boldyrev & Gwinn that the characteristics ofradio scintillations from distant pulsars are best understood if theinterstellar electron-density fluctuations that cause the time broadening ofthe radio pulses obey non-Gaussian statistics. In this picture the densityfluctuations are inferred to be strong on very small scales ($\sim 10^8-10^{10}{cm}$). We argue that such density structures could correspond to the ionizedboundaries of molecular regions (clouds) and demonstrate that the power-lawdistribution of scattering angles that is required to match the observationsarises naturally from the expected intersections of our line of sight withrandomly distributed, thin, approximately spherical ionized shells of thistype. We show that the observed change in the time-broadening behavior forpulsar dispersion measures $\lesssim 30 {\rm pc} {\rm cm}^{-3}$ is consistentwith the expected effect of the general ISM turbulence, which should dominatethe scattering for nearby pulsars. We also point out that if the clouds areionized by nearby stars, then their boundaries may become turbulent on accountof an ionization front instability. This turbulence could be an alternativecause of the inferred density structures. An additional effect that mightcontribute to the strength of the small-scale fluctuations in this case is theexpected flattening of the turbulent density spectrum when the eddy sizesapproach the proton gyroscale.Comment: 15 pages, 3 figures, accepted to Ap