ALBERT

Notes: Abstract World maps of the occurrence of VLF emissions obtained by the satellites Ariel 3 and 4 reveal maxima above industrial regions of high power consumption in North America and Euro-Asia. A study of the generation and radiation of power line harmonics indicates that these may be a major source of the observed signals. The latter propagate in the whistler mode into the geomagnetically conjugate regions in the southern hemisphere. A particularly prominent zone of emission is obtained at VLF (3.2 kHz) over North America where frequent magnetospheric wave amplification/stimulated emission, up to 50 dB and typically 10 to 20 dB above a baseline level that we ascribe to power harmonic radiation (PLHR), is obtained at invariant latitudes 45 to 55° (2 〈 L 〈 3) centred on the electron slot. It appears that PLHR may be responsible for pitch angle diffusion of energetic electrons (E ≥ 100 keV) at large pitch angles by first-order resonance and thereby contribute to the formation of the electron slot. There is a strong seasonal variation in wave-amplification/stimulated emission which we suggest may be due to a variation in the ability of the waves to become entrapped in ducts where wave-amplification occurs through a phase-bunching process. There is a strong correlation between D ST and signal intensity, the latter lagging by ∼ 1–5 hr in the morning and ∼ 10 hr in the evening; here again wave-amplification appears to depend on duct formation and wave trapping therein. One or two (or multi) hop emissions occur with about equal probability at 3.2 kHz; at 9.6 kHz one hop are predominant.

Notes: Abstract In-situ spectral observations of power-line harmonic radiation (PLHR) are still quite rare and almost all the detailed characteristics have been derived from studies at Antarctic stations such as Siple and Halley, and their conjugates in North America. Because of the lack of more direct satellite evidence of PLHR and the difficulties in interpretation of morphological studies, such as those of Ariel 3 and 4, there is considerable controversy concerning the relative importance of PLHR and its contribution to wave-particle interactions (WPI) in the magnetosphere. The early Ariel 3 and 4 global surveys indicated that, in terms of true mean wave energy, there is no longitudinal localisation, the contribution of world-wide intense VLF emissions, associated with magnetic storms, being dominant. Also, the most intense wave emission, that of plasmaspheric hiss at ELF (〈 1 kHz) exhibits little evidence of localisation. The PLHR phenomenon is most conspicuous by its persistence in quiet times (Kp ≤ 2+) at 45° 〈 Λ 〈 55° over North America and its conjugate region, even though the less frequent strongest emissions, to which it gives rise in the summer, are located polewards at 3 〈 L 〈 5. In the northern winter, when spheric activity over both North America and its conjugate are low, there is a high occurrence of strong discrete emissions, which are more sharply localised than in the summer, on the NE industrial U.S.A. field line. The most recent Ariel 4 studies, particularly on the spheric wavefield over North America (using data from the Appleton Laboratory impulse counters) and on the character of the wavefield over the mainland and over the Atlantic immediately to the east (where the spheric contribution is similar) throw new light on the problem. It appears that the principal role of the PLHR may be to sustain duct structure and multihop propagation which is relatively much rarer over the Atlantic. Typical industrial PLHR consists of a series of narrow pulses at twice the mains frequency. It is suggested that these ‘artificial spherics’ may help to sustain the WPI and multihop duct structure. At L = 4, Yoshida et al. (1980) have shown that there is a strong, sharp maximum for WPIs originating in spherics, at f ≅ 3 kHz, in good agreement with Siple observations.