PERIODIC AND QUASI-PERIODIC ELF VLF EMISSIONS OBSERVED BY AN ARRAY OFANTARCTIC STATIONS/

This paper describes amplitude modulations in the frequency range 0-50 0 mHz of ELF/VLF (0.5-4.0 kHz) radio wave power recorded throughout 19 93 and 1995 at Halley and South Pole stations, Antarctica, which lie i n approximately the same magnetic meridian and at geomagnetic latitude s (Lambda) of 61 degrees and 74 degrees, respectively. Data from the i ntermediate automatic geophysical observatories P2 and P3 (Lambda = 70 degrees and 72 degrees, respectively) were also analyzed where availa ble. In agreement with earlier work, spectrograms have revealed the fr equent day-time (typically 0700-1700 MLT) occurrence of modulations ly ing almost entirely within the two period ranges: 10-60 s and 4-6 s. T he first range corresponds to quasiperiodic (QP) emissions, while the latter is typical of the two-hop whistler mode echo period in the plas matrough, and the events are termed periodic emissions (PEs). QP occur rence rates higher than some earlier studies (335 station-days out of 667 examined) may be attributable to the sensitive spectral analysis t echnique. The type I QPs (i.e., those correlated with geomagnetic puls ations observed at South Pole and/or P2/P3) were consistent with an up stream wave driver, controlled by the IMF cone angle. Type II QPs (unc orrelated with magnetic pulsations) were always accompanied by PEs, su ggesting a link between the two, reinforced by a frequently observed s teady increase in period in both phenomena, especially during the morn ing, possibly associated with increasing densities due to upward flow of photoionized plasma from the ionosphere after dawn. Here we propose that type II QPs are driven by field line resonant ULF waves which in turn are generated by field-aligned currents arising from PE induced electron precipitation.