Scintillation-induced Intermittency in Seti

We use scattering theory, simulations, and empirical constraints on interstellar scintillations to discuss the intermittency of radio signals from extraterrestrial intelligence (ETI). The number of ETI sources in the Galaxy has a direct influence on the expected dynamic range of fluxes in a survey, through inverse square-law effects and, equally importantly, by the number of independent statistical trials made on modulations caused by interstellar scintillations. We demonstrate that scintillations are very likely to allow initial detections of narrowband signals, while making redetections extremely improbable, a result that follows from the skewed, exponential distribution of the modulation. This conclusion holds for relatively distant sources but does not apply to radio SETI toward nearby stars (∼< 100 pc). Recent SETI has found non-repeating, narrowband events that are largely unexplained. We consider three models in order to assess these events and to analyze large surveys in general: (I) Radiometer noise fluctuations; (II) A population of constant Galactic sources which undergo deep fading and amplification due to interstellar scintillation, consistent with ETI transmissions; and (III) Real, transient signals (or hardware errors) of either terrestrial or extraterrestrial origin. We derive likelihood and Bayesian tests of the models for individual events and globally on entire surveys. Applying them to The Planetary Society/Harvard META data, we find that Models II and III are both highly preferred to Model I, but that Models II and III are about equally likely. In the context of Model II, the likelihood analysis indicates that candidate events above threshold (∼ 32σ) are combinations of large amplitude noise fluctuations Department of Astronomy, Cornell University National Astronomy & Ionosphere Center, Cornell University Center for Radiophysics and Space Research, Cornell University The Planetary Society, Pasadena, CA 91106 Deceased