Properties and sources of low and very low latitude Pi2 pulsations

Abstract

Data from a low‐latitude ground array of 10 fluxgate magnetometer stations spanning a geomagnetic latitude range from 38°N through the equator to 47°S, with a geomagnetic longitudinal extent from 185°E to 227°E covering ∼3 hours in magnetic local time, and an additional high‐latitude station (65°N) are used to study the spatial and conjugate properties of Pi2 geomagnetic pulsation wave frequency, amplitude, phase, and polarization. Using fast fourier transform and pure state vector analysis techniques, multistation observations show that individual frequency elements of the Pi2 waves are constant with latitude and longitude, but the dominant frequency may vary. In fact, the higher frequencies often dominate at low latitudes, and the lower frequencies often dominate at higher latitudes. The D component amplitude shows a clearly decreasing trend to a near‐zero minimum at the equator, while the H component amplitude does not show a simple behavior. The H component phase is nearly constant at low latitudes, while the high‐latitude station may be out of phase or in phase with the low‐latitude stations for different events. The D component phase is constant in each hemisphere with a 180° phase difference between hemispheres. Azimuthal wave numbers m obtained from interstation phase differences are small, typically |m| < 3, and show a dominance of westward propagation at most longitudes. It is concluded that the main features of low‐latitude Pi2 properties are most easily explained by magnetospheric cavity or wave guide mode resonances, and field line resonances are most likely responsible for the high‐latitude Pi2s. This supports the idea that midlatitude and low‐latitude Pi2s are two‐source phenomena.