Open AccessInvestigating seismoionospheric effects on a long subionospheric path
Author(s) -
Clilverd Mark A.,
Rodger Craig J.,
Thomson Neil R.
Publication year - 1999
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/1999ja900285
Subject(s) - sunrise , amplitude , geology , sunset , maxima and minima , seismology , terminator (solar) , ionosphere , very low frequency , geodesy , physics , geophysics , atmospheric sciences , astronomy , optics , mathematics , mathematical analysis
We examine the possibility of earthquake precursors influencing the subionospheric propagation of VLF transmissions. We consider the long (12 Mm) path from northeastern United States to Faraday, Antarctica (65°S, 64°W), during 1990–1995 and investigate the subionospheric amplitude variation of signals from the NAA communication transmitter (24.0 kHz, 1 MW) in Cutler, Maine, with particular emphasis on possible changes induced by seismic events occurring in South America. We have analyzed the changes in timing of modal minima generated by the passage of the sunrise terminator over the Andes, i.e., the “VLF terminator time” (TT) method. The anomalous variations in timing throughout the year are of a size and occurrence frequency similar to those previously reported, i.e., ±0.5–1 hour and 1–2 per month. However, we find that in these anomalous cases, the time of the sunrise modal minimum does not change significantly, but rather, the minimum becomes insufficiently deep to be detected, and the time of the next nearest minimum is logged. Our analysis indicates that the occurrence rate of successful earthquake predictions using the TT method cannot be distinguished from that of chance. Additionally, the level of false earthquake prediction using the TT method is high.