科研之友
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摘要
Turbulence in the solar wind is believed to generate an energy cascade that is supported primarily by Alfven waves or Alfvenic fluctuations at MHD scales and by kinetic Alfven waves (KAWs) at kinetic scales k(vertical bar)rho(i) greater than or similar to 1. Linear Landau damping of KAWs increases with increasing wavenumber and at some point the damping becomes so strong that the energy cascade is completely dissipated. A model of the energy cascade process that includes the effects of linear collisionless damping of KAWs and the associated compounding of this damping throughout the cascade process is used to determine the wavenumber where the energy cascade terminates. It is found that this wavenumber occurs approximately when |gamma/omega| similar or equal to 0.25, where omega(k) and gamma(k) are, respectively, the real frequency and damping rate of KAWs and the ratio gamma/omega is evaluated in the limit as k(perpendicular to) >> k(parallel to). For plasma parameters typical of high-speed solar wind streams at 1 AU, the model suggests that the KAW cascade in the solar wind is almost completely dissipated before reaching the wavenumber k(perpendicular to)rho(i) similar or equal to 25. Consequently, an energy cascade consisting solely of KAWs cannot reach scales on the order of the electron gyro-radius, k(perpendicular to)rho(e) similar to 1. This conclusion has important ramifications for the interpretation of solar wind magnetic field measurements. It implies that power-law spectra in the regime of electron scales must be supported by wave modes other than the KAW.
- 出版日期2010-3-20
