科研之友
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摘要
We investigate the conditions under which parallel-propagating Alfven/ion-cyclotron waves are driven unstable by an isotropic (T-perpendicular to alpha = T-parallel to alpha) population of alpha particles drifting parallel to the magnetic field at an average speed U-alpha with respect to the protons. We derive an approximate analytic condition for the minimum value of U-alpha needed to excite this instability and refine this result using numerical solutions to the hot-plasma dispersion relation. When the alpha-particle number density is similar or equal to 5% of the proton number density and the two species have similar thermal speeds, the instability requires that beta p greater than or similar to 1, where beta(p) is the ratio of the proton pressure to the magnetic pressure. For 1 less than or similar to beta p less than or similar to 12, the minimum U-alpha needed to excite this instability ranges from 0.7 nu(A) to 0.9 nu(A), where nu(A) is the Alfven speed. This threshold is smaller than the threshold of similar or equal to 1.2 nu A for the parallel magnetosonic instability, which was previously thought to have the lowest threshold of the alpha-particle beam instabilities at beta(p) greater than or similar to 0.5. We discuss the role of the parallel Alfvenic drift instability for the evolution of the alpha-particle drift speed in the solar wind. We also analyze measurements from the Wind spacecraft%26apos;s Faraday cups and show that the U-alpha values measured in solar-wind streams with T-perpendicular to alpha approximate to T-parallel to alpha are approximately bounded from above by the threshold of the parallel Alfvenic instability.
- 出版日期2013-8-10
