Plasmaspheric whistler duct positions

Abstract

More than 3000 whistlers recorded at four New Zealand stations between invariant latitudes 47° and 80° have been analysed by whistler extrapolation to determine the equatorial L-value of ducts ( L-duct). A detailed analysis is given of the accuracy of the results. Most of the whistlers are observed on ducts in the range L = 2.6 to 3.6 ( Λ = 55 ± 3°), independently of the station positions. There is a small seasonal variation in the position of the whistler ducts, with maximum L-duct in winter and minima near the equinoxes; any summer pattern is not obvious. There is little diurnal variation; in particular there is no evidence for an evening bulge. There is a dependence on magnetic activity with ducts occurring at lower latitudes as activity increases. At Scott Base whistlers are observed from two distinct latitude ranges. The whistlers near L = 3 are also observed at the other stations, suggesting they emerge from ducts near the New Zealand meridian. Whistlers near L = 3.8, however, seem to emanate from ducts away from New Zealand possibly near 225°E geographic. The evening bulge has been clearly observed from stations near 270°E, where knee whistlers occur often, but not from near 180°E. The reason could be that the 270°E stations are near geomagnetic meridians that pass through the South Atlantic geomagnetic anomaly, and particle effects associated with the anomaly influence duct and whistler propagation conditions. The whistler duct properties observed near 180°E seem more closely related to the plasmapause at ISIS altitudes (~3000 km) rather than near the Equator.