Sounds from the Arizona Fireball
June 29, 2009 2 Comments
Last Tuesday’s fireball over southern Arizona was accompanied by many reports of sounds. These sounds ranged from bangs and rumbles to a crackling sound. The map below is an updated version of one posted last week. Though the path of the fireball has not changed and is still considered uncertain, many sounds reports have been added.
The majority of sound reports are of the distant thunder, low rumble variety. These sounds were heard 1-3 minutes after the fireball was seen and were strong enough to rattle houses in some cases. Some observers likened it to a sonic boom. In fact, that is exactly what they heard, a series of sonic booms. As the fireball descended through Earth’s atmosphere, its broke into numerous pieces. As these pieces decelerated from supersonic to subsonic speeds, they created a sonic boom. There could easily have been dozens to hundreds of small sonic booms though they probably all merged into one.
It is interesting that the sonic boom reports (circles with a heartbeat symbol on the map below) are mainly located to the north and west of the fireball’s path. Except for reports from the Sierra Vista area immediately to the east of the path, there are no other reports from the East (Benson, Bisbee, Tombstone, Douglas, Willcox, etc.). It is unknown if this is due to something intrinsic to the fireball or just a population density bias (fewer people living out there, hence fewer eyewitnesses reports).
The purple triangles denote a different kind of sound phenomenon. These are sounds heard by the observer while the fireball was visible. Most sounds are created at the fireball and then take time to travel to the observer since the speed of sound is many times slower than the speed of light. Hearing sounds at the same time as seeing the fireball suggests that the sound traveled as fast as light. Well, not quite. The shock wave produced by a fireball is so hot that it radiates across most of the EM spectrum. Sometimes structures near an observer (houses, trees, etc) act as antenna and pick up these waves and vibrate. As a result, a buzzing or crackling sound can be heard at the same time as the fireball is seen.