RUI: Using the Earth as a Gravitational Wave Detector

物理学家和天文学家在未来十年面临的重要挑战之一是直接探测引力波(GW)。在过去的二十年里，大多数的实验努力都是为了开发探测器，这种探测器对预计会伴随着恒星物体引力崩塌而产生的毫秒级辐射爆发很敏感。用来比较随机背景的极限的一个有用的基准是宇宙的封闭密度。能量密度远大于此的背景的存在将与当前对减速参数的限制相冲突。脉冲星计时和微波背景各向同性所暗示的极限都比每倍频程的闭合密度低几个数量级。地面地震观测对毫赫兹随机背景的限制要高得多，每倍频程的闭合密度是几倍；然而，使用地球作为GW探测器的优点是提供对探测器内部波动的区分。对于上述两种方法来说，这几乎是不可能的。在美国国家科学基金会之前的一次拨款中，鲍恩教授分析了IDA项目全球地震仪网络收集的100多个站年的数据，以寻找引力辐射的随机背景对地球低频四极模的异常激发。由于没有OS2和4S2模式的平均激发，他将引力波背景的上限设定在0.31 MHz和1.72 MHz，这是每倍频程宇宙临界密度的几倍，比以前的上限低了几个数量级。根据这一奖项，他将通过分析最安静的IDA台站在低地震背景噪声期间的数据来寻找OS2模式的脉冲激励。背景地震噪声施加的灵敏度极限对应于h 10-13的无量纲应变幅度。为了评估地球作为引力波探测器的最终用途，他将计算地球来源对OS2模式的预期激发。

One of the important challenges facing physicists and astronomers in the next decade is the direct detection of gravitation waves (GW's). Most of the experimental effort in the last twenty years has been directed at developing detectors which are sensitive to the millisecond bursts of radiation expected to accompany the gravitational collapse of stellar objects. A useful benchmark with which to compare limits on a stochastic background is the closure density of the universe. The existence of a background with an energy density much larger than this would conflict with current limits on the deceleration parameter. The limits implied by pulsar timing and microwave background isotropy are both several orders of magnitude below closure density per octave. Limits on a millihertz stochastic background derived from terrestrial seismic observations are much higher, several times closure density per octave; however, using the earth as a GW detector has the advantage of providing discrimination against intrinsic fluctuations in the detector. This is virtually impossible for the above two methods. Under a previous NSF grant Prof. Boughn analyzed more than 100 station-years of data collected by the Project IDA global network of seismometers in order to look for anomalous excitation of the low frequency quadrupole modes of the earth by a stochastic background of gravitational radiation. The absence of an average excitation of the oS2 and 4S2 modes allowed him to set upper limits on a background of gravitational waves at 0.31 mHz and 1.72 mHz of a few times the critical density of the universe per octave which is several orders of magnitude lower than previous limits. Under this award, he would search for pulse excitation of the oS2 modes by analyzing the data of the quietest IDA stations during periods of low seismic background noise. The sensitivity limit which is imposed by background seismic noise corresponds to a dimensionless strain amplitude of h 10-13. In order to evaluate the ultimate usefulness of the earth as a gravitational wave detector he will compute the expected excitation of the oS2 mode by terrestrial sources.