A Study on Sunward Propagating Alfvénic Fluctuations With a Power Law Spectrum Observed by theWind Spacecraft

摘要

Sunward propagating Alfvénic fluctuations with a power law spectrum (SAFP) have been recently observed in the upstream region of the Earth’s bow shock. However, some physical properties of these fluctuations such as anisotropy remain unclear. Here we develop a new method for identifying SAFPs, and present for the first time the anisotropy of SAFPs power and spectral index. In this method, the propagation direction determination of SAFPs does not rely on a radial magnetic geometry but the pitch angle distribution of strahl electron outflow. Therefore, the SAFPs with any value of θ_(RB) (angle between the global mean magnetic field and the Sun-to-Earth radial direction) can be identified, so that enables the study of the spectral anisotropy. We find 508 SAFPs using the Wind spacecraft measurements from 1995 to 2014. We show that the SAFP has an averaged spectral index of ?1.77 ± 0.28 and the index changes continuously from ?2.18 ± 0.21 when θ_(RB) = 0°–10° to ?1.71 ± 0.03 when θ_(RB) = 80°–90°. These SAFPs are observed more frequently in the slow solar wind especially at solar minimum. We also select antisunward propagating Alfvénic fluctuations with a power law spectrum using the same method for comparison. The results indicate that the power spectrum of SAFP is steeper, and the spectral intensity as well as the power anisotropy of SAFP is weaker. These new findings may provide information on the generation of turbulence in the upstream region.