New insight into short wavelength solar wind fluctuations from Vlasov theory
Publication date: 13 March 2012
Authors: Sahraoui, F., et al.
Journal: The Astrophysical Journal
Volume: 748
Issue: 2
Page: 100 (11pp)
Year: 2012
Copyright: American Astronomical Society
The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with w < w_ci, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., w > w_ci) is more relevant. Here we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically b_i > b_e ~1 and for high oblique angles of propagation 80° d theta_kB d 90° as observed from Cluster spacecraft data. The linear properties of plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (theta_kB < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales k-rho_i e1 to frequencies either larger or smaller than w_ci, depending on the anisotropy k_para/k_perp. This extension into small scales is more readily called whistler (w > w_ci) or KAW (w < w_ci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.
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