973 program [2012CB825602]
; NNSFC [41574169, 41231067]
; Youth Innovation Promotion Association of the Chinese Academy of Sciences
; Specialized Research Fund for State Key Laboratories of China
英文摘要:
The mixture/interaction of anti-sunward-propagating Alfvenic fluctuations (AFs) and sunward-propagating Alfvenic fluctuations (SAFs) is believed to result in the decrease of the Alfvenicity of solar wind fluctuations with increasing heliocentric distance. However, SAFs are rarely observed at 1 au and solar wind AFs are found to be generally outward. Using the measurements from Voyager 2 and Wind, we perform a statistical survey of SAFs in the heliosphere inside 6 au. We first report two SAF events observed by Voyager 2. One is in the anti-sunward magnetic sector with a strong positive correlation between the fluctuations of magnetic field and solar wind velocity. The other one is in the sunward magnetic sector with a strong negative magnetic field-velocity correlation. Statistically, the percentage of SAFs increases gradually with heliocentric distance, from about 2.7% at 1.0 au to about 8.7% at 5.5 au. These results provide new clues for understanding the generation mechanism of SAFs.
