ALBERT

Description: The electromagnetic response of the Assistance Bay region (N.W.T., Canada) is studied using laboratory analogue model and field station measurements. Magnetic field components for the frequency range 0.1-15 Hz, which includes the well known lightning induced Schumann Resonances, were measured at 8 sites along a 6 km line through the central region of Assistance Bay and roughly perpendicular to the Cornwallis Island coastline. Two of the stations were on land and six were off-shore on the ice. Although the field data were examined in the three bandwidths 0.1-6 Hz, 6-10 Hz, and 10-15 Hz, the present work deals only with the 8 Hz Schumann Resonance component within the 6-10 Hz range. Calculated average amplitudes and phases of the three magnetic field components show the relative response of the region at stations along the 6 km line. The response is discussed in terms of the effect of the on shore topography, the coastline, and the bathymetry. A laboratory analogue model of the Assistance Bay region was constructed and model measurements carried out for a simulated frequency of 8 Hz for E-Polarization (electric field in the east-west direction) and H-Polarization (electric field in the north-south direction) of an overhead uniform horizontal inducing field. The magnetic field amplitudes Hx, Hy, Hz for both polarizations for 10 traverses perpendicular to the coastline are presented and the anomalous fields attributed to the response of the coastline bay, capes, bathymetry and source field polarization. The analogue model and the field station coast effects (vertical to horizontal field ratio) are compared for the model traverse coinciding with the 6 km line of field stations. The analogue model results show a maximum enhancement of approximately 1.3 directly over the coastline with rapidly decreasing values seaward followed by a second enhancement over the sharp ocean depth gradient beyond the 55 m depth contour. The field station results do not show maximum at the coastline, but rather an increasing value seaward, reaching a value of 1.7 at the station over the sharp ocean depth gradient near the 55 m depth contour, indicating the effective coastline to be at the deep water edge. This difference may in part be attributed to the shallow water conductivity being smaller than simulated in the laboratory model. The effect of the 2m layer of ice, as well as the accumulation of fresh water from streams in the Assistance Bay region, may lead to a significantly lower effective conductivity than assumed for the analogue model. An effective decrease in the shallow water conductivity would lead to decreased enhancement at the coastline and relative increased enhancement off-shore at the deep water edge. Other factors possibly contributing to the seemingly anomalous seaward coast effect could be subsurface geologic and permafrost structure, and the nature of the source field. The laboratory overhead inducing source is adequate for simulating ionospheric currents of large extent but may not be suitable for the Schumann source field associated with distant lightning strokes due to lack of a significant vertical electric field component. This source field effect requires further study. © 1983, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.