LIGO Document T1800506-v2
- Part of PEM vetting of GW150914: a lightning strike with a ~1/10^4 probability of coincidence with the candidate event was causing concerns that it might have produced the candidate.
1) The magnetic fields from the 504 kA Burkina Faso strike, and the 59 other strikes in the second containing GW150914, were at least 3 orders of magnitude too small to produce an event with the amplitude of GW150914. This is demonstrated with injected magnetic chirps.
2) The fields from lightning do not evolve in time and frequency like GW150914. The magnetic fields are broadband, shorter and do not evolve in frequency. Interactions with the magnetosphere may produce electromagnetic signals (tweaks and whistlers) that evolve in frequency, but in the opposite direction of our chirp.
3) There were 11 much closer lightning strikes in Dec. 2015 with larger peak currents that occurred when at least one detector was observing (41 to 1500 km vs. about 10,000 km; 508 kA - 734 kA vs. 504 kA). No events were detected on the gravitational wave channels during these 11 larger and closer strikes.
4) Studies of lightning coupling over the last 15 years have indicated that lightning could produce a DARM event only if the current is extremely high and the strike is within kilometers of a site.
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