Are low-latitude Pi2 pulsations cavity/waveguide modes?

The plasmasphere seems the most likely magnetospheric region in which compressional hydromagnetic waves may be trapped to form cavity modes. It has been suggested that substorm-associated Pi 2 magnetic pulsations have a cavity mode character at low latitudes. Recent detailed observations of night-side Pi 2 events at low latitudes have also suggested a cavity or waveguide character. We apply a well-tried numerical hydromagnetic wave coupling model at low latitudes, and compare the model output with these observations. We find that the qualitative amplitude and phase characteristics of the model magnetic fields in latitude and longitude fit well with the observations, provided the ionospheric boundary condition at low latitudes gives negligible rotation of magnetic field components between ionosphere and ground. The comparison supports the idea that low-latitude Pi 2 pulsations are cavity/waveguide modes. It also suggests that the magnetic H component observed at the ground near the equator may be a combination of the radial and compressional components above the ionosphere.