Natural electromagnetic field fluctuations in the 3.0- to 0.02-cps range

Using large loop antennas the ultra-low-frequency research group at the National Bureau of Standards has studied the upper atmosphere phenomena of geomagnetic micropulsations. Data taken at a number of world stations on both direct reading chart and magnetic tape indicate a division into three contributing phenomena for the frequency range of 3.0 to 0.02 cps. Very regular oscillations of 2.0 to 0.2 cps are a strange pulsation phenomenon most likely of outer atmospheric origin but apparently unrelated to solar-terrestrial disturbances. Sudden bursts of large amplitude field fluctuations spread throughout the frequency range are closely related to high latitude particle precipitation, enhanced ionospheric absorption, and auroral luminosity; these fluctuations seem to be of ionospheric origin. Regular oscillations between 0.2 and 0.03 cps appearing over broad sections of the earth with related phase on days of high solar-terrestrial activity are presently the best candidate for magneto-hydrodynamic interpretation. During the International Quiet Sun Year (IQSY) a configuration of world stations will be operated along a latitudinal line covering about 180° at three sites in the boreal auroral zone, along a longitudinal line near 75° to 80°W longitude with conjugate stations corresponding to L shell values of about 6.5 and 4, and at an equatorial site.