The Ionospheric Absorption of the VLF Emissions at the Auroral Zone

抄録

Ionospheric absorptions of VLF emissions at the auroral zone have been calculated along a geomagnetic line of force connecting with the earth's surface at geomagnetic latitude of 64° by using the longitudinal approximation of the Appleton-Hartree formula and data of rocket sounding at Ft. Churchill and of auroral luminosity profile at College, Alaska.
Ionospheric absorptions at 4kc/s between 50km and 170km height are of 33.1dB, 15.4dB, 16.9dB, and 2.2dB for a daytime ionosphere during total polar black-out, a quiet ionosphere near noon, a midnight ionosphere during diffuse aurora, and a quiet ionosphere at winter midnight respectively. Therefore the lack of VLF emissions during total polar black-out at high latitudes may be due to the marked increase of the absorption in the ionosphere, especially in D layer.
On the other hand, it seems likely that particles producing auroras and geomagnetic disturbances around E layer do not cause considerable absorption of the VLF emissions, and that the power of VLF emissions is originally weak in the exosphere on night side at the auroral zone. The absorption of 15.4dB in the quiet ionosphere near noon suggests that the original power of VLF emissions in the exosphere is of the order of 3.5×10^-13-3.5×10^-15W.m.^-2(c/s)^-1 in the daytime. The diurnal variation in maximum chorus frequency at high latitudes may be explained by relative increase of ionospheric absorption during the daytime for frequencies above a few kilo-cycles.