SCINTILLATION-INDUCED INTERMITTENCY IN SETI

We use scattering theory, simulations, and empirical constraints on in terstellar scintillations to discuss the intermittency of radio signal s 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 im portantly, by the number of independent statistical trials made on mod ulations caused by interstellar scintillations. We demonstrate that sc intillations are very likely to allow initial detections of narrowband signals, while making redetections extremely improbable, a result tha t follows from the skewed, exponential distribution of the modulation. This conclusion holds for relatively distant sources but does not app ly to radio SETI toward nearby stars (less than or similar to 100 pc). Recent SETI has found nonrepeating, narrowband events that are largel y unexplained. We consider three models in order to assess these event s and to analyze large surveys in general: (model I) radiometer noise fluctuations; (model II) a population of constant Galactic sources tha t undergo deep fading and amplification due to interstellar scintillat ion, consistent with ETI transmissions; and (model III) real, transien t signals (or hardware errors) of either terrestrial or extraterrestri al origin. We derive likelihood and Bayesian tests of the models for i ndividual events and globally on entire surveys. Applying them to The Planetary Society/Harvard META data, we find that models II and III ar e both highly preferred to model I, but that models II and III are abo ut equally likely. In the context of model II, the likelihood analysis indicates that candidate events above threshold (similar to 32 sigma) are combinations of large amplitude noise fluctuations and scintillat ion gains, making it highly probable that events seen once will only v ery rarely be seen again. Ruling out model II in favor of model III is difficult-to do so, many more reobservations (e.g., thousands) are ne eded than were conducted in META (hundreds) or the reobservation thres hold must be much lower than was used in META. We cannot, therefore, r ule out the possibility that META events are real, intrinsically stead y ETI signals. Our formalism can be used to analyze any SETI program. We estimate the number of reobservations required to rule out model II in favor of model III, taking into account that reobservations made p romptly sample the same scintillation gain as in the original detectio n, while delayed reobservations sample a decorrelated scintillation mo dulation. The required number is a strong function of the thresholds u sed in the original survey and in reobservations. We assess optimal me thods for applying statistical tests in future SETI programs that use multiple site and multiple beam observations as well as single site ob servations. We recommend that results be recorded on many more events than have been made to date. In particular, we suggest that surveys us e thresholds that are far below the false-alarm threshold that is usua lly set to yield a small number of noise-induced ''detections'' in a m assive survey. Instead, large numbers of events should be recorded in order to (1) demonstrate that background noise conforms to the distrib ution expected for it; and (2) investigate departures from the expecte d noise distribution as due to interference or to celestial signals. I n this way, celestial signals can be investigated at levels much small er than the false-alarm threshold. The threshold level for archiving c andidate intensities and their corresponding sky positions is best def ined in terms of the recording and computational technology that is av ailable at a cost commensurate with other survey costs.