Alfvénic turbulence associated with density and temperature filaments

Abstract

Abstract.A systematic laboratory study of controlled density and temperature filaments having transverse scale length comparable to the electron skin-depth has been performed in the large plasma device (LAPD) at UCLA. It is found that large amplitude shear Alfvén waves develop spontaneously and are localized within the filaments. As the plasma conditions change (e.g., lowering the plasma beta parameter or increasing the heating power) the highly coherent eigenmodes develop into broad band Alfvénic turbulence. A kinetic description that includes the effect of coulomb collisions has been developed to understand the linear properties of the modes. Excellent agreement with the measured eigenfunctions is found for the density filaments in the higher beta regime in which the modes remain strongly coherent. The similarity between the broad band fluctuation spectra generated in a variety of plasma configurations suggest the possibility of a universal process involving filamentary structures and spontaneously generated Alfvénic turbulence.

Alfvén wave filamentation and particle acceleration in solar wind and magnetosphere

“…In the solar wind region and over a wide range of heliospheric distances, the quantity ∣ δB y 0 ∣/ B 0 tends to be small (≈5%) [ Matthaeus and Goldstein , 1982; Roberts et al , 1987]. Morales et al [1999] studied density and temperature filaments in laboratory at fluctuating levels of ∣ δB ⊥ ∣/ B 0 ≈ 10 −3 , under conditions of relevance to the auroral ionosphere. The characteristic scales of the filament size for all the three regions are of the order of 2 ρ s (at e −1 of the intensity peak) and the separation of the filaments is of the order of 10 3 ρ s for lower values of g .…”

“… Chmyrev et al [1988] found the perpendicular scale size of the filaments of Alfvén vortex tubes propagating transverse to the background magnetic field of the order of ρ s when β > m e / m i and λ e when β < m e / m i . Morales et al [1999] analyzed filamentary structures in laboratory nonthermal plasmas with transverse scale size of the order of λ e , spontaneously generating Alfvénic turbulence spatially localized to the filament region. In case of polar cusp, the scale size of the filament is 2 ρ s (= 20 km) in our result.…”

Transient Evolution of Nonlinear Localized Coherent Structures of Kinetic Alfvén Waves

“…For these typical parameters at ω/ω ci = 0.02 and k x ρ s = 0.001, the magnitudes of the fluctuations are |δB y0 |/B 0 = 0.003 and = 0.02. Morales et al (1999) studied density and temperature filaments in the laboratory at fluctuation levels of |δB ⊥ |/B 0 ≈ 10 −3 , under conditions of relevance to the auroral ionosphere. The observed filamentary structures of Figure 5 have the characteristic transverse scale size of the order of λ e (at e −1 of the intensity peak).…”

“…The observed filamentary structures of Figure 5 have the characteristic transverse scale size of the order of λ e (at e −1 of the intensity peak). Morales et al (1999) analyzed filamentary structures in laboratory nonthermal plasmas with transverse scale size of the order of λ e , spontaneously generating Alfvénic turbulence spatially localized in the filament region.…”

“…It has long been known that Alfvén waves are commonly observed in space plasmas, especially in the solar wind (Coleman, 1966;Unti and Neugebauer, 1968;Belcher and Davis, 1971), the magnetosheath (Sahraoui et al, 2003), and the auroral regions (Norqvist et al, 1996;Stasiewicz et al, 2000). At the equatorial magnetosphere, where the magnetospheric plasma is hot and the electron thermal speed exceeds the Alfvén speed, the kinetic Alfvén wave (KAW) is the appropriate limit.…”