An analysis of the spectra of geomagnetic variations having periods from 5 min to 4 hours

Abstract

The geomagnetic field variation spectra for the period range of 5 min to 4 hours were analyzed at 64 world observatories to discover systematic behavior in the spectral composition that could be associated with the time of day, season, solar cycle, activity level, component direction, and geographical location of the station. There was found no consistent frequency location for peaks in the spectral composition. Rather, the displays of spectral amplitudes A were mostly ‘linear’ in form, often obeying A ∼ Tm, where T is the period. The spectral slope m was usually close to 1.0 but varied at times from 0.5 to 2.0. The amplitudes always showed a maximum at the auroral zone latitudes, a minimum near 20°−40°, and a minor maximum near the equator. The positions shifted equatorward with increasing activity. The relative growth in amplitude with rising activity varied with latitude. Seasonal peaks in activity were found at equinoctial months. A summertime minimum occurred at auroral zone stations; elsewhere a summertime enhancement occurred. General findings emphasizing the North American hemisphere and the year 1965 are presented in a number of tables and graphs.

Spatial distribution of the geomagnetic spectral composition for disturbed days.

“…This study complements an earlier publication (CAMPBELL, 1976) which focused upon the monthly average spectral characteristics.…”

A statistical analysis of ionospheric velocity and magnetic field power spectra at the time of pulsed ionospheric flows

“…[46] Our suggested magnetic field power law exponent of 1.8 is within the range of 1.0-4.0 found by Campbell [1976] though somewhat smaller than the other studies listed in Table 1. With the exception of the Buchert et al [1999] study, all the listed studies use reasonable amounts of data and there are no particular aspects of their methodology which would give us reason to expect different exponents.…”

“…The results of these studies are summarized in Table 1. In the largest such study, using 123 data years from 64 geomagnetic observatories over a wide range of latitudes and longitudes, Campbell [1976] concluded that the power spectra from individual observatories in the period range from 5 min to 4 hours (0.07 -3.3 mHz) were mostly power law in form with an exponent range 1 a 4.0, but usually a % 2.0, and that no consistent frequency location for spectral peaks was observed.…”

Annual and semiannual variations of the lunar semidiurnal geomagnetic field components at North American locations.

“…One purpose of the investigation is to determine the predictable characteristics of field levels on undisturbed days in order to evaluate the limitations of accuracy obtainable in magnetic mapping, to improve the selection of favorable times for survey observations, to adjust various field measurements to more reproducible baselines, and to find the modifications of the measurements by the induced fields. The studies started at the short period end of the spectra at 0.3 sec (WERTZ and CAMPBELL, 1976) and have already extended to field variations with periods of 4 hr (CAMPBELL, 1976(CAMPBELL, a, b, 1977. Before investigating the solar-quiet (Sq) diurnal and semidiurnal variations (24-and 12-hr periods) it was necessary to extract the lunar contributions to the semidiurnal field changes.…”