SPRG Seminars
November 16, 2010:
"Cluster observation of shear-mode surface waves in the outflows from magnetotail reconnection"
Dr. Lei Dai,
University of Minnesota
This talk will present a Cluster spacecraft study of the plasma-sheet- confined surface waves diverging from reconnection and in strong earthward and tailward reconnection outflows in the geomagnetic tail at ~17 Re. Using phase lag analysis with multi-spacecraft measurements, we quantitatively determine the wavelength and phase velocity of the waves with spacecraft frame frequencies from 0.03 Hz to 1 Hz and wavelengths from much larger (x40) than to comparable to the H^+ gyroradius scale (~300 km). The wave phase velocities, nearly perpendicular to the local magnetic field, track the perpendicular component of outflow velocity projected in the plane of the plasma sheet. The propagation directions and wavelength of the observed surface waves resemble those of flapping waves of magnetotail current sheet, the propagation speed of observed waves is almost one order magnitude larger than the typical speed of flapping waves.
Analysis of electric and magnetic field data shows that observed waves are associated with strong field-aligned Alfvenic Poynting flux directed away from magnetotail reconnection toward Earth. Evidence of velocity shear is found in the perpendicular component of the reconnection outflows. These observations present a scenario in which the observed surface waves/structures are driven and convected through velocity-shear type instability by reconnection outflows tending to slow down due to energy loss through Poynting flux. The mapped Poynting flux (100 ergs/cm2 s) and longitudinal scales (10-100 km) to ionosphere suggest that the observed waves and their motions are an important boundary condition in determining both the energetics of the aurora and their complex motions in the night sky.