Comments on use of anomalous stations for IMS

Abstract

One of us has recently reviewed the general problem of the effects of induction in the earth upon the measurement of external time‐varying magnetic fields [Gough, 1974]. In this context we are concerned about the inclusion of Rapid City, South Dakota, and Alert and Mould Bay, N.W.T., Canada, in magnetometer networks for the International Magnetospheric Study, as described by Lanzerotti et al [1976]. Inhomogen eities in the electrical conductivity of the earth's crust cause drastic distortions in the variation fields recorded at these stations.Rapid City is on top of an anomaly which more than doubles the D component variations, at periods of tens of minutes; Zis also anomalous [Alabi et al., 1975]. At Alert the horizontal variation vector is strongly confined in the northwest‐southeast direction [Niblett et al., 1974]. Observations at Rapid City and Alert will certainly be dominated by the very large anomalies caused by induced currents channeled in the underlying crust. At Mould Bay the vertical variations are unusually attenuated by crustal layers of anomalously high conductivity [DeLaurier et al., 1974].