SH33A-2449
Alfvénic fluctuations with power-law spectra propagating sunward within the region from L1 to the Earth's bow shock
Wednesday, 16 December 2015
Poster Hall (Moscone South)
Honghong Wu1, Xin Wang1, Linghua Wang1,2, Chuanyi Tu1,2, Jiansen He1,2 and Eckart Marsch3,4, (1)Peking University, Beijing, China, (2)Peking University, Institute of Space Physics and Applied Technology, Beijing, China, (3)University of Kiel, Kiel, Germany, (4)Institute for Experimental and Applied Physics, Kiel, Germany
Abstract:
According to several theories, the beam instability induced by shock-accelerated ions can generate upstream-propagating Alfvén waves (UPAWs) with a bump near 0.03 Hz in the power spectrum, 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 Alfvénic fluctuations (UPAFs) with power-law spectra by using measurements from the WIND spacecraft in year 1995. We utilize a new criterion to identify the upstream-propagating Alfvénic 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. The spectral indices of UPAFs are between -3 and -2. These power-law spectra cannot be explained by the linear ion-beam instability. The results provide new clues on understanding the dynamic equilibrium between the non-linear inverse cascade and the linear ion-beam instability.
