Large simultaneous disturbances (LSDs) in the Antarctic ionosphere

A high frequency radio Doppler experiment was deployed in the Antarctic Peninsula region, centred on Argentine Islands (65°15′S, 64°16′W; L = 2.3), to investigate the morphology and sources of ionospheric disturbances. The experiment consisted of a three-transmitter dual frequency network which permits horizontal and vertical propagation velocities to be estimated over a north-south baseline of 200 km and an east-west baseline of 100 km. A new class of ionospheric disturbance has been observed, in the period range 10 min−1 h. These disturbances are characterised by unusually good correlation between perturbations on all available Doppler signals, but are apparently non—propagating and occur simultaneously at each reflection point. Several of these events display large (2 Hz at about 5 MHz transmitted frequency) Doppler shifts, thus we have labelled them Large Simultaneous Disturbances (LSDs). Criteria for identification of LSDs are established and the analysis of one event is described in detail. The occurrence statistics of the LSDs are presented, including their seasonal and diurnal distributions. There is no clear general relationship between LSDs and local geomagnetic field perturbations. However, examination of the magnetic indices AE and IRC indicates that there is a loose association between the occurrence and amplitude of LSDs and magnetic activity. Several possible mechanisms for the generation of LSDs at middle latitudes are reviewed. The most likely explanation is that high latitude electric fields penetrate to magnetic middle latitudes and drive the ionospheric plasma via the E × B drift.

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