科研之友
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摘要
On 3 August 2004, the enhancement of Ultra-Low-Frequency (ULF) waves with frequency less than the ion cyclotron frequency and the high-speed flows are observed by TC-1 at X-GSM similar to -12R(E) in the plasma sheet of the magnetotail. During the high-speed flows, the amplitude of the perpendicular component is roughly unchanged. For the parallel component, the amplitude accompanied with high peak flow is larger than that accompanied with low peak flow. After numerically analyzing the perturbed two-fluid model for drift-driven electromagnetic instability by the fully magnetized ions, we found, (1) For perpendicular propagation, the ratio of the cross-field drift to the Alfven speed plays a role in the growth rate of the instability and the excited wave frequency. With the increase of the ratio, the growth rate increases and the excited wave frequency turns to positive from negative. (2) For parallel propagation, the ratio of the ion thermal velocity to the Alfven speed plays a role in the growth rate of the instability and the excited wave frequency, but the instability mode keeps unchanged. However, when the temperature is isotropic, the ratio of the cross-field ion drift to the Alfven speed affects the growth rate and the excited wave frequency as well as the instability mode and the corresponding branches. When the plasma density, temperature, bulk velocity and magnetic field observed by TC-1 are input into the model equations, the numerical solutions show that the enhancements of the power spectrum density (PSD) and the excited wave frequency are well consistent with what the theoretical model predicts for perpendicular propagation; while the enhancements of the power spectrum are consistent with what the theoretical model predicts with the former frequency less than the latter for parallel propagation. In addition to considering whether other factors such as the ion flow parallel to the magnetic field will affect the instability and the excited wave frequency, it is necessary to statistically further verify the correlation of ULF waves with the instabilities accompanied with high-speed flows.
