When we reported another Russian fireball on Saturday, we weren't sure if it was real or a hoax. Now, thanks to geophysical stations in the region, we can confirm that the event coincides with a sound-blast, and even pinpoint a probable location on the Kola Peninsula.
Fireball observed in Murmansk, Russia on April 19th, 2014. Clip extracted from WorldNews247.
You've seen earthquake seismographs before, every time a major city gets hit by a quake. Seismographs don't just record the amplitude of an earthquake or other ground-rattling event: they also record the arrival times of various types of seismic waves. The time-difference between the arrival of speedy P-waves and slower S-waves allows seismologists to calculate the distance from the seismograph and the source event. Microbarographs are like seismographs for the atmosphere. The geophysical instrument measures tiny changes in atmospheric pressure (instead of ground pressure), and distances can be calculated from the arrival-times.
NORSAR is a Norwegian research foundation specializing in geophysics and tasked with monitoring any violations of the Comprehensive Nuclear-Test-Ban Treaty. They've analyzed microbarograph data from Norway, Sweden and Finland from the time of the alleged fireball. The sensors used to detect nuclear blasted detected an airblast — possibly produced by the fireball exploding in the atmosphere — at the same time as the event recorded in the video footage.
A single seismogram provides you only with a distance, where the source event could be anywhere on a circle with that radius out from the seismograph. But through the geometry of triangulation, if you have at least three seismic stations all recording the same event, you can use the overlapping circles to pinpoint a surface location. (For an earthquake, you need a fourth seismograph to also determine the depth).
Microbarographs aren't seismographs — they measure atmospheric sound, not vibrations in the ground. However, the principles of measuring distance from travel-time of a compressional wave travelling through a known material (air or earth) are the same. NORSAR's Steven Gibbons provided me with the following translation of their recent work:
Low frequency sound waves (infrasound) were observed at several array stations in the region, at distances of up to 1000 km. The figures below show the signals on NORSAR's infrasound array in Bardufoss, together with a f-k scan from which you can estimate the directions from which the soundwaves originate. By performing triangulation of the observations from several stations in the region, the source (a presumed meteor explosion) can be located in the western part of the Kola Peninsula, just to the east of the Finnish-Russian border.
Data from KIR (Kiruna), JAM (Jämtön), and SDK (Sodankylä) stations are courtesy of the Swedish Institute for Space Physics.
Taking a Fourier transform over time and space of the microbarographs (an f-k transform) tidies the data up, pinpointing the signal source as the Kola Peninsula in northwestern Russia.
Using signals from a network built to detect illegal nuclear testing, NORSAR has found supporting evidence for the recent Russian fireball, and located it in the far-northwestern tip of Russia. This agrees with the various videos of the fireball from dashboard and security cameras, even though few people were awake to report additional observations. At what point do we get to stop calling this "alleged," and just call it a fireball?
Images credit: NORSAR. Special thanks to Steven Gibbons for his assistance! Read more about the Infrasound Monitoring Network. Have you noticed how often we get smacked by rocks? Might be time to think about funding the B621 Foundation to protect Earth from impact events.