[1] The first measurements from a new globally distributed extremely low frequency magnetic field sensor network are presented. The recorded data demonstrate that the system observed lightning with continuing currents on a global scale with a timing accuracy of ∼30 ms. The network consists of four state of the art instruments at sites in Scotland, the United States, South Africa and Australia. Each instrument records the two horizontal magnetic field components (Bx and By) with a sampling frequency of 4 kHz. The first results show the typical electromagnetic signature of a transient airglow increase (sprite) above a thunderstorm in southern Europe which is simultaneously imaged with a video camera. A similar electromagnetic signature is recorded from a lightning discharge in central Africa, and it is also attributed to a sprite occurrence. Studies using this global network should advance lightning and sprite research considerably.Citation: Whitley, T., et al. (2011), Worldwide extremely low frequency magnetic field sensor network for sprite studies,
Optical fibers, stretched between anchors attached to the Earth and buried in boreholes and shallow trenches, offer an inexpensive and robust means to observe Earth strain. We have deployed several optical fiber strainmeters (OFSs) in boreholes, where thermal stability helps mitigate the noise from temperature changes. We have also constructed horizontal OFSs in 1‐m‐deep trenches of lengths 221 and 174 m. In these, a second optical fiber having a contrasting temperature coefficient provides a means to separate true strain from thermal noise. Signals in the seismic band from the horizontal strainmeters agree well with records from collocated seismometers, and the predominant ambient noise recorded by both techniques are consistent. Solid Earth strain tides with amplitude of 30 nε (nanostrain) are observed in these horizontal OFSs that agree well with those observed by collocated vacuum laser strainmeters. Noise at periods of a few days amounts to 50 nε. The two horizontal strainmeters show drift rates of up to 1 µε per month, and occasional false signals (evident from comparisons with the vacuum laser strainmeters) can accumulate occasionally to the order of 50 nε over times from hours to weeks. Results to date indicate that this technology on land could detect slow‐slip events of amplitude 100 nε and higher.
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