ELF (500–1000 cps) Emissions at High Latitude

Abstract

Naturally, occurring electromagnetic radiation in the audio- and subaudio-frequency range have been studied in recent years at Kiruna Geophysical Observatory (geomagnetic latitude 65.3°N). Many emission bands, which are quite distinct from the normal background signals and are obtained as strong enhancements in portions of the spectrum, have been recorded. The main results observed for the emissions between 500 and 1000 cps are:

• 1) The frequency of the maximum signal strength of the emission band is almost constant in time; e.g. at 650 cps ± 150 cps.

• 2) The amplitude of the emission at 650 cps is often ten times higher than the normal background noise level at this frequency range.

• 3) The average half-intensity bandwidths of these 650 cps emissions are ± 200 cps.

• 4) A pronounced morning maximum (between 04 and 10 MET) is found for the different seasons.

• 5) The diurnal variation, as well as the correlation with micropulsations, indicate that the behavior of the 650 cps enhancements are different from all other audio-frequencies investigated at the Kiruna Observatory.

• 6) It seems likely that the 650 cps radiation is an auroral zone phenomenon.

• 7) A marked Doppler broadening band shift is found for many of the emission events.

It is considered probable that these emissions consist of electromagnetic radiation generated outside the E-layer by arrival of protons, which is propagated in the extraordinary mode along the lines of force of the earth’s magnetic field.