| Popis: |
The effect of a coastline on geomagnetic variations associated with the daytime equatorial electrojet for Sq (24 h) and substorm (20 and 60 min) periods has been investigated with the aid of a thin sheet algorithm developed at the University of Victoria. The numerical model comprises a thin sheet at the surface of and electrically connected to a two-layer structure representing a 30 km thick crust of conductivity 0.005 S m −1 and an underlying mantle of conductivity 0.1 S m −1 . The sheet includes lateral variations of conductance corresponding to a land-sea interface: (1) parallel to, (2) perpendicular to and (3) inclined at an angle to the dip-equator. The electrojet is represented by a gaussian distribution of ionospheric currents flowing eastwards, of half-width (standard deviation) 150 km, both with and without the inclusion of northern and southern return current flows. It is found that even at Sq periods, anomalies in the northward horizontal and vertical components of the magnetic field, H and Z respectively, are apparent for the electrojet source without return currents. The effects are less when return currents are included in the external source field. Anomalies in the eastward horizontal component D for both types of sources are very small except near the periphery of the jet, where they become significant ( > 2 nT) when the coastline is perpendicular to the dip-equator. The computed value of the ratio of internal to external parts of H for the electrojet source without return currents is 0.24 near the centre of the electrojet, regardless of the direction of the coastline, which is in close agreement with the value 0.28 obtained from rocket and ground data. Changing, the half-width of the electrojet from 50 to 300 km caused the ratio to vary between 0.14 and 0.34. The induced effects are found to be more prominent at substorm periods than for the longer-period Sq variation and they are highly dependent on the strike of conductivity contrast. |
