The upstream-propagating Alfvenic fluctuations with power-law spectra in the upstream region of the Earth’s bow shock
Xin Wang1, Chuanyi Tu1, Linghua Wang1, Jiansen He1, and Eckart Marsch2
1 School of Earth and Space Sciences, Peking University, 100871 Beijing, China; wanglhwang@gmail.com
2 Institute for Experimental and Applied Physics, Christian Albrechts University at Kiel, D-24118 Kiel, Germany
Abstract
Based on theories, the beam instability induced by shock-accelerated ions can generate upstream-propagating Alfven waves (UPAWs) with a power spectral bump near 0.03 Hz, while the nonlinear wave-wave interaction favors an inverse cascade to create a power-law spectrum. Here we present the first observational evidence for the upstream-propagating Alfvenic fluctuations (UPAFs) with power-law spectra. We utilize a new criterion to identify the upstream-propagating Alfvenic intervals: the propagation direction is opposite to that of solar wind strahl electron outflow. Besides 35 UPAWs, we find 47 UPAFs with power-law spectra, and ~47% of these UPAFs are associated with energetic ion events (>30 keV). These UPAWs and UPAFs are mostly observed in the slow solar wind. However, their occurrence rate and power behave differently in dependence on the radial distance from the Earth. These results provide new clues on understanding the dynamic equilibrium between the non-linear inverse cascade and the linear ion-beam instability.
Acknowledgments This work at Peking University is supported by NSFC under contracts 41421003, 41474147, 40931055, 41231069, 41274172, 41474148, and 41222032. Wind data were obtained from SPDF (http://spdf.gsfc.nasa.gov).
Citation: Wang, X., C. Tu, L. Wang, J. He, and E. Marsch (2015), Geophys. Res. Lett., 42, doi:10.1002/2015GL063893.