A STUDY OF QUASI-PERIODIC ELF-VLF EMISSIONS AT 3 ANTARCTIC STATIONS -EVIDENCE FOR OFF-EQUATORIAL GENERATION

The spatial extent and temporal behaviour of quasi-periodic (QP) inten sity modulations of 0.5-2 kHz ELF-VLF signals were investigated in a c omparative study of data collected at the Antarctic stations of South Pole (L = 14), Halley (L = 4), and Siple (L = 4). Frequently, the wave forms of ELF-VLF signals simultaneously received at each site were ide ntical. Although of similar frequency structure, the waveforms of the accompanying Pc3 magnetic pulsations did not show a one-to-one associa tion. Whereas both are dayside phenomena, QP emissions occur over a sm aller range of local times, and have a maximum of occurrence later in the day closer to local noon. QP emissions are identified with the per iodic modulation of the electron pitch-angle distribution by the propa gation of ULF compressional fast-mode waves through a region. However, contrary to previous ideas, rising-tone emissions do not represent th e frequency-time signatures of such waves. In additions to generation close to the equatorial plane, we propose an additional high-latitude source of QP emissions. These emissions are associated with regions of minimum B produced by the dayside compression of the magnetosphere cl ose to the magnetopause. Model magnetic field calculations of these mi nimum-B regions as a function of magnetic local time and invariant lat itude are presented.