1988/05/01 by V Courtillot, Jean‐Louis Le Mouël · 2 citations
Biochemistry, Genetics and Molecular Biology · Earth and Planetary Sciences · Mathematics · #Geomagnetism and Paleomagnetism Studies #Earthquake Detection and Analysis #Geophysical and Geoelectrical Methods #Secular variation #Earth (classical element) #Earth's magnetic field #Geology #Geophysics #Field (mathematics) #Astrobiology #Geodesy #Magnetic field #Physics #Astronomy #Mathematics
paper · doi:10.1146/annurev.ea.16.050188.002133
openalex publication_date 1988/05/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/06/11
Variations in time of the Earth's natural magnetic field have a frightfully wide spectrum, ranging over more than 20 orders of magnitude: They extend from well over 103 Hz, where they merge into the radioelectric noise spectrum, to probably more than 100 m.y. In rough summary, the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere, whereas the longer periods arise from internal sources, primarily fluid motions in the core at depths in excess of 2900 km. Methods for separating external and internal signals have been available since the time of Gauss ( 1 838), but the accuracy, the space distribution, and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations. Although periods shorter than 1 yr should clearly be assigned to external sources, the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress. Currie ( 1966, 1 968) proposed that there was little overlap and a cutoff of ' 3.7 yr. A long debated, l l -yr contribution due to the solar cycle was clearly demonstrated by Currie ( 1973a), Courtillot & Le MoueI ( 1976a,b) and Alldredge ( 1976), and the cutoff period was thus rejected to at least ,20 yr. The situation seemed to be confused again when Courtillot et al ( 1978a) provided evidence for sharp impulses of 1 yr duration i n the internal, so-called secular, variation. The debate that followed over the reality of brief internal events is one of the topics addressed in this review. We first provide a short historical account of successive discoveries