抄録
Cosmic radio noise data, acquired with a fast-response 30MHZ riometer at Siple Station, Antarctica (L≈4), show evidence of weak impulsive quasiperiodic variations in ionospheric absorption. Significant amplitude variations approaching 0.02dB, approximately the statistical noise limit of the measurement, have been measured. An event typically will consist of 3 to 5 cycles of oscillation (-60 to 90sec period) and can occur during locally disturbed or quiet geomagnetic conditions. Usually the oscillations are not related closely to magnetic field fluctuations as measured on the ground at Siple or in the conjugate area. Conjugate bremsstrahlung X-ray data, available for one event, suggest that these absorption variations are caused by the impact on the ionosphere of a modulated flux of energetic electrons from the magnetosphere. Limited statistics on the location of the plasmapause suggest that these events occur close to, but on either side of, the boundary of the plasmasphere. These absorption oscillations may be another manifestation of the process that produces short-term intensity and phase variations in fixedfrequency sub-ionospheric LF and VLF wave propagation. Further studies of this unique modulated precipitation/absorption phenomenon and its relationship to hydromagnetic wave disturbances in the magnetosphere will be conducted with a set of riometer and magnetometer stations appropriately sited in a latitude array.
