Pc 3‐4 geomagnetic pulsations observed at very low latitude conjugate stations

Abstract

In order to investigate the properties of Pc 3‐4 geomagnetic pulsations at very low latitudes, a conjugate study at 23° geomagnetic latitude was conducted over one month in 1988. The results show that highly polarized Pc 3‐4 pulsations occur mainly in the morning from 0400 to 1000 LT. This occurrence distribution appears to be a consequence of hydromagnetic wave source and/or propagation characteristics rather than ionospheric effects. Most of the observed signals have small ellipticities, and the east‐west component is enhanced near local sunrise. The polarization major axes show mirror symmetry in opposite hemispheres. Calculations of the toroidal mode eigenfrequency using actual ionospheric data to fit plasma density models show that the observed signal frequencies at 23° geomagnetic latitude are similar to the eigenfrequencies at 40°–50° latitude. Events observed simultaneously between 23° and 45° latitude suggest that Pc 3‐4 at the very low latitude conjugate stations may be associated with field line resonances near 45° latitude. Possible source mechanisms of these pulsations are global mode resonances or trapped fast mode waves in the plasmasphere driving forced field line oscillations at very low latitudes.