Polarization characteristics of hydromagnetic waves at low geomagnetic latitudes

A field campaign, using three magnetometer stations spaced in latitude around the equatorial magnetic field distance L approximately 1.9, was conducted in early 1979 to investigate the polarization characteristics of hydromagnetic waves at low geomagnetic latitudes. The magnetic pulsations are observed to have periods in the range approximately 20-25 s and to occur primarily in the local morning hours. Statistically, the polarizations at all three stations were predominantly left handed in the local morning hours and right handed in the local afternoon. At the highest-latitude station (L approximately 2) the orientation of the major axis of the polarization ellipse changed from a predominantly NW-SE direction in the local morning to a mixed NW-SE/NE-SW direction in the afternoon. These two statistical results are consistent with the excitation of the waves by the Kelvin-Helmholtz instability at the magnetopause. However, frequent changes in phase are often observed in the magnetic variations, which result in polarization variations on the time scale of minutes, a situation not readily reconcilable with the Kelvin-Helmholtz instability. The existence of the waves at very low latitudes, furthermore, places constraints on the damping rate of externally excited surface waves inside the magnetosphere. We conclude that present theories for hydromagnetic waves in the geomagnetosphere cannot readily incorporate all of these low-latitude results.