ULF tree potentials and geomagnetic pulsations

Abstract

HIGH-SENSITIVITY measurements of ultra-low-frequency (ULF; frequencies less than 5 Hz) geomagnetic pulsations¹ usually require elaborate receiving antennas ranging from large air-cored coils² through multi-turn steel, or mumetal-cored solenoids^3–5 to small superconducting loops immersed in liquid helium⁶. Pairs of electrodes inserted in the ground have also been used as antennas^7,8. The need for a large spacing between the electrodes (varying from hundreds to thousands of metres) and the difficulty of calibrating the measurements absolutely have resulted in the almost universal use of the more compact and easily calibrated coil-type antennas in recent years. I describe here a new method for measuring ULF geomagnetic pulsations, which requires a minimum of elaborate equipment. The method is based on the use of trees, or, more specifically, on the use of pairs of electrodes inserted into trees, as ULF receiving antennas.