Local instabilities of Alfvén waves in high speed streams |
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| Authors: | B Bavassano M Dobrowolny G Moreno |
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| Institution: | (1) Laboratorio per il Plasma nello Spazio, CNR, 00044 Frascati, C.P. 27 Via G. Galilei, Italy |
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| Abstract: | A two fluid stability analysis of an inhomogeneous solar wind plasma leads to prediction of possible instabilities of both Alfvénic and magnetoacoustic waves driven by local velocity gradients. The waves predicted to be possibly unstable have short wavelengths in comparison with the length scale of the gradients and, with different thresholds for the value of velocity shear, may have different directions of propagation with respect to the background magnetic field.We have performed a detailed study, based on Pioneer 6 magnetic and plasma data relative to several high speed streams in the solar wind, on the direction of propagation of the transverse waves which are found within the streams and on their association with velocity gradients within the stream structure. The analysis leads to the conclusion that the observed Alfvén waves may be consistent with the hypothesis of local generation through one of the above mentioned instabilities where velocity shear leads in fact to excitation of incompressible waves in directions almost parallel to the magnetic field. |
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