Wave characteristics of Ps 6 magnetic variations and their implications for convective flow in the magnetotail

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Ps 6 magnetic variations are fluctuations with periods of 10–30 min and amplitudes of tens to hundreds of nT observed in the dawn sector auroral oval during substorm activity. The causative current system appears to be associated with auroral omega bands which follow the substorm breakup, and drift eastward from the midnight sector with typical velocities of 0.5–2.0 km s −1 . Quadrature techniques are applied to Ps 6 waveforms to separate their group and phase velocities, and to obtain their wave characteristics. The Ps 6 phenomenon seems to be confined to the 03:00–09:00 L.T. sector, and over the range of longitudes involved, its wave properties are relatively constant in the ionosphere. The group velocity has an average value of 1.0 km s −1 and, on the average, marginally exceeds the phase velocity by about 15% ; both these velocities are directed eastward. When the Ps 6 wave parameters observed in the ionosphere are mapped to the equatorial magnetospheric plane, the wavelength of the disturbance shows, on an average, values ranging from ∼47 Re for the 03:00–04:00 L.T. sector to ∼12 Re for the 08:00–09:00 L.T. sector; the phase velocity changes from ∼ 300 km s −1 in the deep magnetotail to ∼ 20 km s −1 near the dawn meridian. Assuming the disturbance to originate through a Kelvin-Helmholtz instability, at the interface between the low-latitude boundary layer (LLBL) and the central plasma sheet (cps), the width of the momentum shear zone narrows from ∼ 4.5 R E at 03:00–04:00 L.T. to ∼ 1.2 R E at 08:00–09:00 L.T. The inferred large sunward convection velocity of ∼ 300 km s −1 for plasma in the 03:00 L.T. region of the LLBL/cps interface is interpreted as confirmation of magnetic field line reconnection at a neutral line antisunward of the region of formation of Ps 6 in the magnetotail.