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Alfvénic Turbulence as a Driver of Mass Outflow into the Magnetosphere C. C. Chaston 1, V. Genot 2, J. W. Bonnell 1, C. W. Carlson 1, R. E. Ergun 3, J. P. Mc. Fadden 1 and R. J. Strangeway 4 1. SSL, University of California, Berkeley. 2. CESR, Toulouse 3. LASP, University of Colorado, Boulder 4. IGPP, University of California, Los Angeles 1. Identification of Turbulence 2. Ion Acceleration 3. Wave Focusing and Cavity Properties 4. Is this important?
FAST Auroral Oval Crossing Containing Electromagnetic Turbulence
Interferometry shows that wave power has P~k-1. 7 dependency Electron Acceleration Ion Acceleration Dispersion is similar to a dispersive (inertial) shear Alfvén wave.
Results from 2 -fluid MHD simulations above auroral oval -assumption of periodicity transverse to Bo in E and B and uniformity in density Current contours transverse to Bo at FAST altitude (E 2~k-1. 7) Test O+ ion energy
2 -fluid test-particle result shows that ions may be extracted from the ionosphere on time scales of a few minutes
So the observed electron and ion acceleration is dependent on the transport of energy from MHD scales to scale similar to le and ri but how does this occur-? Inhomogeneity in the transverse Alfven speed profile may be important Case Study Cavity
Plasma properties inside a single density cavity Complete evacuation of cold ionospheric plasma Large amplitude parallel electric field inside cavity Intense field-aligned currents Ion acceleration and outflow from cavity (mass loss from the ionosphere) Field-aligned electron acceleration
Wave Properties Inside the Cavity Wave Energy Flux/Group speed is directed downwards towards the ionosphere Transverse Wave Group speed is directed inwards towards the center of the cavity Transverse Wave Phase speed is directed outwards from the center of the cavity
Ionospheric Density Cavity Schematic
Statistics of plasma properties within Alfvenic cavities Density depletion typically 3050% but statistically significant number have >90% Te~Ti with the cavities Drift speed of electrons in cavities sufficient to excite ion waves For 2% of events electron drift in the cavity may be similar to or larger than the electron thermal speed which may lead to the formation of double layers
Cavity formation occurs on electron inertial scales and wave energy density increases with cavity depth Wave Current increases with cavity depth Transverse scale of the current ~2 ple
So is this important? 12 18 6 0 Total Alfvénic Ion Outflow Flux Total Alfvén Wave Poynting Flux Survey results based on 4000 FAST traversals of the auroral oval. Statistics indicate that ~30% of ion outflow from the auroral oval occurs in correlation with Alfvén waves
Conclusion: Ionospheric density irregularities are unstable to incoming Alfvén wave Poynting flux. This leads to ionospheric erosion and the population of the magnetosphere with ionospheric ions. For more see: Chaston et al. , Ionospheric Erosion by Alfvén Waves, J. Geophys. Res. , 111, A 03206, doi: 10. 1029/2005 JA 011367, 2006.