Scintillation-induced Intermittency in SETI
Scintillation-induced Intermittency in SETI
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DOI：
10.1086/304620
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发表时间：
1997-07
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影响因子：
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通讯作者：
J. Cordes;T. Lazio;C. Sagan
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文献类型：
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作者：
J. Cordes;T. Lazio;C. Sagan
我们使用散射理论，模拟和经验约束星际湍流讨论来自地外文明（ETI）的无线电信号的不稳定性。银河系中ETI源的数量直接影响着调查中通量的预期动态范围，通过逆平方律效应，同样重要的是，通过对星际闪烁引起的调制进行的独立统计试验的数量。我们证明，reflectilations是非常有可能允许初始检测的窄带信号，而重新检测极不可能，结果如下从偏斜，指数分布的调制。这一结论适用于相对遥远的源，但不适用于无线电SETI对附近的恒星（100 pc）。最近的SETI发现了非重复的窄带事件，这些事件在很大程度上无法解释。我们考虑三个模型，以评估这些事件，并分析大型调查一般：（模型I）辐射计噪声波动;（模型II）人口不断的银河系源，经历深衰落和放大，由于星际闪烁，符合ETI传输;和（模型III）真实的，无论是地球或外星起源的瞬态信号（或硬件错误）。我们得出的可能性和贝叶斯检验的模型，为个别事件和全球的整个调查。将它们应用于行星协会/哈佛Meta数据，我们发现模型II和III都比模型I更受欢迎，但模型II和III的可能性大致相同。在模型II的上下文中，似然分析表明，阈值（~32 σ）以上的候选事件是大幅度噪声波动和闪烁增益的组合，使得很有可能一次看到的事件将非常罕见地再次看到。排除模型II而支持模型III是困难的--要做到这一点，需要更多的重新观察（例如，在META中进行的重新观测（数百）必须比在META中进行的重新观测（数千）更少，或者重新观测阈值必须比在META中使用的低得多。因此，我们不能排除Meta事件是真实的、本质上稳定的ETI信号的可能性。我们的形式主义可以用来分析任何SETI计划。考虑到重新观测及时采样与原始检测相同的闪烁增益，而延迟的重新观测采样去相关的闪烁调制，我们估计排除模型II有利于模型III所需的重新观测的数量。所需数量是原始调查和重新观察中使用的阈值的重要函数。我们评估的最佳方法，在未来的SETI计划，使用多个站点和多波束观测以及单站点观测应用统计检验。我们建议记录比迄今为止更多事件的结果。特别是，我们建议，调查使用的阈值是远远低于假警报阈值，通常被设置为产生少量的噪声引起的“检测”在一个大规模的调查。相反，应该记录大量的事件，以便（1）证明背景噪声符合预期的分布;（2）调查由于干扰或天体信号而偏离预期噪声分布的情况。通过这种方式，可以在比假警报阈值小得多的水平上研究天体信号。存档候选强度及其相应天空位置的阈值水平最好根据记录和计算技术来定义，该技术的成本与其他调查成本相称。
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 nonrepeating, narrowband events that are largely unexplained. We consider three models in order to assess these events and to analyze large surveys in general: (model I) radiometer noise fluctuations; (model II) a population of constant Galactic sources that undergo deep fading and amplification due to interstellar scintillation, consistent with ETI transmissions; and (model 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 (~32 σ) are combinations of large amplitude noise fluctuations and scintillation gains, making it highly probable that events seen once will only very 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 needed than were conducted in META (hundreds) or the reobservation threshold must be much lower than was used in META. We cannot, therefore, rule out the possibility that META events are real, intrinsically steady 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 promptly sample the same scintillation gain as in the original detection, while delayed reobservations sample a decorrelated scintillation modulation. The required number is a strong function of the thresholds used in the original survey and in reobservations. We assess optimal methods for applying statistical tests in future SETI programs that use multiple site and multiple beam observations as well as single site observations. We recommend that results be recorded on many more events than have been made to date. In particular, we suggest that surveys use thresholds that are far below the false-alarm threshold that is usually set to yield a small number of noise-induced “detections” in a massive survey. Instead, large numbers of events should be recorded in order to (1) demonstrate that background noise conforms to the distribution expected for it; and (2) investigate departures from the expected noise distribution as due to interference or to celestial signals. In this way, celestial signals can be investigated at levels much smaller than the false-alarm threshold. The threshold level for archiving candidate intensities and their corresponding sky positions is best defined in terms of the recording and computational technology that is available at a cost commensurate with other survey costs.