Sunward propagating Alfvén waves in association with sunward drifting proton beams in the solar wind

Sunward Propagating Alfvén Waves in Association with Sunward Drifting Proton Beams in the Solar Wind

Jiansen He1 , Zhongtian Pei1, Linghua Wang1 , Chuanyi Tu1, Eckart Marsch2, Lei Zhang1, and Chadi Salem3

1 School of Earth and Space Sciences, Peking University, Beijing, 100871, China; jshept@gmail.com2 Institute for Experimental and Applied Physics, Christian-Albrechts-Universität zu Kiel, D-24118 Kiel, Germany 3 Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA

ABSTRACT

Using measurements from the WIND spacecraft, here we report the observation of sunward propagating Alfvén waves (AWs) in solar wind that is magnetically disconnected from the Earthʼs bow shock. In the sunward magnetic field sector, we find a period lasting for more than three days in which there existed (during most time intervals) a negative correlation between the flow velocity and magnetic field fluctuations, thus indicating that the related AWs are mainly propagating sunward. Simultaneous observations of counter-streaming suprathermal electrons suggest that these sunward AWs may not simply be due to the deflection of an open magnetic field line. Moreover, no interplanetary coronal mass ejection appears to be associated with the counter-streaming suprathermal electrons. As the scale goes from the magnetohydrodynamic down to the ion kinetic regime, the wave vector of magnetic fluctuations usually becomes more orthogonal to the mean magnetic field direction, and the fluctuations become increasingly compressible, which are both features consistent with quasi-perpendicular kinetic AWs. However, in the case studied here, we find clear signatures of quasi-parallel sunward propagating ion- cyclotron waves. Concurrently, the solar wind proton velocity distribution reveals a sunward field-aligned beam that drifts at about the local Alfvén speed. This beam is found to run in the opposite direction of the normally observed (anti-sunward) proton beam, and is apparently associated with sunward propagating Alfvén/ion- cyclotron waves. The results and conclusions of this study enrich our knowledge of solar wind turbulence and foster our understanding of proton heating and acceleration within a complex magnetic field geometry.

Figure-1. Diagnosed patches of sunward and anti-sunward Alfvén waves and slow waves in the time and period diagram. (click to zoom in)

Figure-2. Associated sunward drifting proton beams. (click to zoom in)

Acknowledgement: This work at Peking University is supported in part by NSFC under contracts 41222032, 41174148, 41231069, 41274172, 41474148, and 41421003. We thank the 3DP team and MFI team on WIND for sharing data with us. We thank Lynn Wilson III for helpful discussions on the WIND/3DP data analysis.

Citation: He, J.-S., Pei, Z.-T., Wang, L.-H., Tu, C.-Y., Marsch, E., Zhang, L., and Salem, C.: Sunward Propagating Alfvén Waves in Association with Sunward Drifting Proton Beams in the Solar Wind, Astrophys. J., 806, 176, doi:10.1088/0004-637X/805/2/176, 2015.