Signals and noise in magnetic observatory annual means: Mantle conductivity and jerks

Abstract

Geomagnetic temporal variations can yield valuable information on the electrical conductivity of Earth's mantle and on motion of core fluid. The external‐source signal in first differences of magnetic observatory annual means is primarily due to a degree‐one spherical harmonic closely aligned with Earth's magnetic dipole axis. The transfer function between the electromagnetically induced degree‐one internal Gauss coefficient (Schmidt seminormalized) and the inducing degree‐one external Gauss coefficient is 0.089 ± 0.020 with a phase shift of 0° ± 45° for a 2‐year period. This transfer function is consistent with a nearly insulating mantle and a highly conducting core for which the theoretical transfer function is 0.082 with no phase shift. The temporal power spectrum for noise in first differences of magnetic observatory annual means is approximately white. Third differences of annual means are primarily noise and degree‐one external‐source signal; thus, when the degree‐one external‐source signal is removed from annual means third differences, the root‐mean‐square residuals for a given field component and time interval at a given observatory are a good indicator of noise for the relevant component, observatory, and time interval. These rms residuals were used as weights for construction of spherical harmonic models of geomagnetic secular variation. European secular variation graphs for the 1962–1983 time interval exhibit prominent changes of slope (geomagnetic jerks) in the geomagnetic east component at approximately 1970 and 1978. The jerk of 1970 (but not 1978) is evident on the geomagnetic north and vertical components. The vertical component exhibits additional slope changes at approximately 1966 and 1975.