A large‐amplitude traveling ionospheric disturbance produced by the May 18, 1980, explosion of Mount St. Helens

Abstract

A remarkable long-lived, large-scale traveling ionospheric disturbance (TID), excited by the May 18, 1980, explosion of Mount St. Helens, has been detected in total electron content monitor data. Oscillatory perturbations in the electron column density of the ionosphere with amplitudes about 10% of the nominal daytime content were detected at three stations whose ionospheric penetration points lie between 1610 and 1890 km from Mount St. Helens. Smaller perturbations were detected at five of six additional stations between 3760 and 4950 km away. The period of the TID increased linearly with great-circle distance from Mount St. Helens, ranging from •37 min at the nearest station to • 116 min at the most distant one. The TID persisted for at least four cycles at the three close stations and three cycles at the more distant stations and was qualitatively similar to TIDs produced by the low-altitude thermonuclear detonations of the 1960's. The disturbance front of this TID accelerated from an average velocity of •350 m/s between Mt. St. Helens and the close stations to an average velocity of •550 m/s to the more distant ones. In analogy with the bomb-excited disturbances, this TID is interpreted as the ionospheric fluctuations induced by a gravity wave, excited by the explosion of Mount St. Helens, propagating in the neutral atmosphere. A model based on the free wave response of an isothermal atmosphere to a point disturbance provides a good fit to the data at the three closest stations, but no such model can account for all of the data. Modeling of the long-distance behavior of the Mount St. Helens TID in terms of upper-atmosphere guided gravity waves is complicated by the requirement of exciting them by a ground-level explosion. A comprehensive explanation would probably include the propagation of energy to ionospheric heights in free modes and the excitation thereby of guided modes. There was no evidence for a strong supersonic shock wave in the ionosphere. As a result, the Mount St. Helens disturbance may prove to be a cleaner test of detailed theories of the point excitation and propagation of gravity waves in a realistic atmosphere than were TIDs excited by thermonuclear weapons.

Model of traveling ionospheric disturbances

“…The data thus obtained are given in Table 1. The space-time constancy of V i values was experimentally confirmed by Breitling & Kupferman (1967), Row (1967), Kanellakos (1967), Albee & Kanellakos (1968), Lomax & Nielson (1968), Roberts et al (1982aRoberts et al ( , 1982b, Igarashi et al (1994) for distances from the disturbance source up tõ 14 000 km. A i values are taken from Bristow et al (1994), calculated for a delay relative to the onset of AGW excitation equal to 3.333 h. This delay is close to the average value used in our experimental data.…”

“…The invariability of the propagation velocity of the wave train extrema was deduced from theoretical treatments and was confirmed experimentally 45 years ago for TIDs generated by nuclear tests and high-power volcanic explosions (Row 1967;Breitling & Kupferman 1967;Kanellakos 1967;Albee & Kanellakos 1968;Lomax & Nielson 1968;Roberts et al 1982aRoberts et al , 1982bIgarashi et al 1994). However, some scientists can hardly accept the fact that this velocity is practically constant up to 14 000 km.…”

“…The aforementioned model, equivalent isothermal atmosphere adapted, as it is named by Roberts et al (1982a) was successfully tested by Roberts et al (1982b), using experimental data obtained after the explosion of the volcano St. Helens. For the diagnosis of disturbances the Faraday effect of pulsar PSR 0329 +54 signals, recorded by a highly sensitive radio interferometer, was used.…”

“…Such approach was implemented by Roberts et al (1982aRoberts et al ( , 1982b. In the first article the total electron content was monitored by measuring the Faraday rotation angle of linearly polarized VHF signals radiated by geostationary satellites.…”

“…Following Albee & Kanellakos (1968), we have used the formula s 0 = 2pC/(c g) (Row 1967). We substitute the parameters from an equivalent isothermal atmosphere adopted model (Roberts et al 1982a) and obtained s 0 = 3.95 min. The slope of the AGW-TID front in the vertical plane for the MS TID (5) and LS TID (6) was obtained from Fedorenko et al (2010b).…”