Sq effect on the electromagnetic response functions in the period range between 104 and 105 s

Abstract

SUMMARY Geomagnetic induction responses such as geomagnetic depth sounding (GDS), magnetotelluric (MT), and horizontal geomagnetic transfer function (HTF) at long periods are used to estimate the electrical conductivity in the deep mantle. The responses in the period range that are shorter than 10 5 s (about 1 day) are in many cases considered to be local or regional induction problems in which the source field is approximated by plane waves and therefore the sphericity of Earth is not taken into account. In the period range between 10 4 and 10 5 s, the most dominant signature of the magnetic field variation is the solar quiet daily (Sq) variation and its higher harmonics. Therefore, when we obtain the responses due to the quasi-white background spectrum composed of plane waves, we regard the Sq field variations as noises in estimating the responses, and line spectra of the variations are removed from the observed time-series before the responses are calculated. However, with this approach, the calculated responses tend to possess a discontinuity at a period of about 10 4 s, and the response functions show common features at longer periods irrespective of the location of the observation site. It is particularly well known that the imaginary part of the induction vector tends to have a significant westward component for periods ranging between 10 4 and 10 5 s. Such features cannot be easily explained by the effect of the electrical conductivity structure alone. Examination of the phase of the HTF implies that the responses in the same period range are affected by the signature of sources of finite wavelength moving westward. Thus, it is suggested that the response functions in this period range were under the effect of the Sq field variations, even though the line spectra of them were removed and the responses were estimated at periods separate from the harmonic periods of Sq field variation. We examined how the influences of the Sq field appear on the responses numerically by a forward modelling. Results show most of the characteristic features in observed response functions can be ascribed to Sq source effects.