Features of power line harmonic radiation (PLHR) related narrow band ELF-emissions at very high latitudes

J. Manninen1), A. Oikarinen2), and T. Turunen1)
1) Sodankylš Geophysical Observatory, Sodankylš, Finland
2) University of Oulu, Dept. Physical Sciences, Oulu, Finland

Power line harmonic radiation (PLHR) is always detectable in high sensitivity ELF-VLF receiving systems even when measuring at location tens of kilometers away from the nearest power lines. The observed PLHR is either from direct radiation from the power lines or it is from the magnetosphere, where it has experienced amplification in wave-particle interactions. We have studied properties of PLHR measured at extremely low local noise surrounding at L-value of about 6. The nearest power line systems are about 40 km away and distance to those carrying strong currents is more than one hundred kilometers. Suddenly looking PLHR appears, as it was direct wave from power lines. Detailed study shows, however, that at least in some limited frequency bands the line intensities follow geophysical activity. Disappearance of strong PLHR lines after substorm onset and related auroral hiss event strongly supports the interpretation that the observed lines are of magnetospheric origin. Detailed studies show that very narrow band emissions, new lines, appear at low intensity level. The original PLHR lines, the “mother lines”, become weaker or temporarily disappear below the detection level in this process and they get wider bandwidth in the process. The new “daughter lines” are, on contrary, extremely narrow band emissions and seem to appear at “subharmonics” of the mains frequency. More detailed study shows that the widening of the original “mother lines” is result of the appearance of another type of “daughter lines”, which are only fractions of Hz away from the “mother line” frequency. The number of these daughter lines is of the order of ten or less, they seem to appear on one side of the mother line, and apparently the mother line is also no more at the original frequency but shifted by a fraction of Hz towards the daughter line group. The exact frequency where the mains harmonic should exist, may be totally empty and a question arises if the observed “mother line” is the original mother line at all.

Sometimes weak hiss band is evident in normal VLF-analysis and it is limited to frequencies, where the new line generation processes take place. Some PLHR lines often seem to control the upper and lower boarders of the frequencies, where the “hiss” is seen. Questions, however, arise about the nature of this hiss when looking it at subhertz spectral resolutions. Many of the mentioned phenomena have been reported already earlier. In this study we have made attempt to go further by using very high spectral resolution like 0.05 Hz and by searching the relevant time scales of the intensity variation of the phenomena. We have also made some estimates about the arrival directions, estimates on the polarization and relations to other geophysical phenomena.

The PLHR coming from the magnetospheric source is perhaps all the time present and in quiet locations dominates over the direct radiation. PLHR may turn out to be more important in the magnetospheric processes than sometimes assumed.