Spectral characteristics of field variations during geomagnetically quiet conditions.

Abstract

Using data from 15 observatories at a wide distribution of latitude locations in North and South America the spectra of geomagnetic field variations having periods from 5min to 4hr were examined for six days of 1965 when the Ap index values were 3 ol less. At high latitudes, where the maximum amplitudes occurred more poleward on low Ap days than on high Ap days, the ratio of the short to long period amplitudes was higher than at other latitudes. For all spectral samples of the data, a minimum in activity was found near 30° geomagnetic latitude and a dayside enhancement was found at the equator. At all latitudes most amplitudes were larger during the local daytime hours and smallest at the predawn hours. East-west component amplitudes were about the same size as the north-south component amplitudes averaged over the day yet regular diurnal variations were found in the ratio of these two components. On the average there was a 10 to 20 percent reduction of the east-west field component in the region of the ionospheric equatorial and auroral electrojet currents. The vertical component of field was about 40 percent less than the horizontal field. This reduction seemed to be accounted for by an effective crustal conductivity of about 2×10-3mhos/meter extending to an equivalent depth of about 300km. The general characteristics of the quiet-time spectra seemed to match the projection of the changes in active-day spectra to low Ap values. Empirical relationships between the spectral features and the Ap index are provided to allow some prediction of geomagnetic amplitude levels as a function of period, time, and location on magnetically quiet days.