Periodic and quasiperiodic ELF/VLF emissions observed by an array of Antarctic stations

This paper describes amplitude modulations in the frequency range 0–500 mHz of ELF/VLF (0.5–4.0 kHz) radio wave power recorded throughout 1993 and 1995 at Halley and South Pole stations, Antarctica, which lie in approximately the same magnetic meridian and at geomagnetic latitudes (Λ) of 61° and 74°, respectively. Data from the intermediate automatic geophysical observatories P2 and P3 (Λ = 70° and 72°, respectively) were also analyzed where available. In agreement with earlier work, spectrograms have revealed the frequent day-time (typically 0700-1700 MLT) occurrence of modulations lying almost entirely within the two period ranges: 10–60 s and 4–6 s. The first range corresponds to quasiperiodic (QP) emissions, while the latter is typical of the two-hop whistler mode echo period in the plasmatrough, and the events are termed periodic emissions (PEs). QP occurrence rates higher than some earlier studies (335 station-days out of 667 examined) may be attributable to the sensitive spectral analysis technique. The type I QPs (i.e., those correlated with geomagnetic pulsations observed at South Pole and/or P2/P3) were consistent with an upstream wave driver, controlled by the IMF cone angle. Type II QPs (uncorrelated with magnetic pulsations) were always accompanied by PEs, suggesting a link between the two, reinforced by a frequently observed steady increase in period in both phenomena, especially during the morning, 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.

Details