University of Maryland
Atlantic Building, Room 2400 4:30 PM Monday, April 27, 2009
Coffee, Tea & Snacks 4:15-4:30 PM

Fouad Sahraoui
NASA/GSFC, LPP/CNRS-Ecole Polytechnique

Dispersive cascade and dissipation of solar wind turbulence at electron scales: recent observations and theoretical modeling

Magnetic turbulence is known to play a key role in the microphysical processes (e.g., energy dissipation, particle acceleration and magnetic reconnection) that occur in space and astrophysical plasmas. Consequently, identifying the properties of turbulence has been one of a major goal of previous space missions. In the solar wind, large-scales turbulence, which can be described in the magnetohydrodynamic (MHD) limit, has been studied extensively. Its power spectra are known to obey the Kolmogorov scaling, k-5/3, down the local proton gyrofrequency (~0.1Hz). However, turbulence at frequencies above fci has not been investigated thoroughly and remains far less well understood. Above fci the spectrum generally steepens to ~f-2.5 and a debate exists as to how the turbulence is dissipated. One possibility is that the turbulence becomes dispersive following either kinetic Alfvén mode or whistler mode before it is dissipated at smaller scales. Another possibility is that turbulence is merely dissipated at ion scale by ion Landau and/or cyclotron dampins. Here, I will show recent results obtained from Cluster data that illustrates the nature of this small-scale turbulence. This analysis has been made possible by combining the high resolution magnetic and electric field data from Cluster. I will show spectra of turbulence that extend over more than five decades, ranging from 10-3 Hz to 102 Hz (in the spacecraft reference frame). Below fci, the results confirm the Kolmogorov scaling k-5/3. Above fci new dispersive inertial range is evidenced and characterizes the energy cascade down to the Doppler shifted electron gyroscale fre, where dissipation is shown to occur. Using the (gyro)kinetic Vlasov theory, we show that these observations are remarkably consistent with theoretical predictions of a quasitwo-dimensional cascade into kinetic Alfvén waves, and subsequent dissipation via electron Landau damping (Sahraoui et al., Physical Review Letters, revised). I will discuss the implications of these results for the heating problem of the solar wind and solar corona.