Abstract

The paper by Thiel and Chant reports the well-known sunrise and sunset effect which in micropulsation studies is referred to as the dawn and dusk “chorus.” However, we disagree with the interpretation of these authors. Their choice of wavetilt as the diagnostic function is slightly misleading since they are essentially measuring at the surface of the earth despite the unfortunate location of their magnetic sensor at a height of 4 m and presumably in a building. In this case the wavetilt  
We=Eh120πHh
according to their equation (1) is equivalent to  
We=Z1Z0,
where Z1 is the impedance of the earth and Z0=µ0/ε0 is the impedance of free space. This result is independent of the mode of propagation but certainly at their distance of 4000 km is predominantly the sky wave except during the dawn and dusk periods. The turbulence due to the formation or breakup of the D-layer in the ionosphere virtually destroys the ionospheric reflected component which is the dominant contributor to the field incident on the earth. We suggest that during most of the day the authors are measuring the impedance of the earth scaled by a numerical constant.
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