Journal
ASTROPHYSICAL JOURNAL
Volume 778, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/778/2/177
Keywords
magnetohydrodynamics (MHD); plasmas; solar wind; turbulence
Categories
Funding
- NASA Postdoctoral Program at the Goddard Space Flight Center
- STFC
- NASA [NNN06AA01C, NAS5-02099]
- Leverhulme Trust Network for Magnetized Plasma Turbulence
- STFC [ST/F002505/2, ST/I002138/1, ST/H002383/1] Funding Source: UKRI
- Science and Technology Facilities Council [ST/H002383/1] Funding Source: researchfish
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We show that the scaling of structure functions of magnetic and velocity fields in a mostly highly Alfvenic fast solar wind stream depends strongly on the joint distribution of the dimensionless measures of cross helicity and residual energy. Already at very low frequencies, fluctuations that are both more balanced (cross helicity similar to 0) and equipartitioned (residual energy similar to 0) have steep structure functions reminiscent of turbulent scalings usually associated with the inertial range. Fluctuations that are magnetically dominated (residual energy similar to -1), and so have closely anti-aligned Elsasser-field vectors, or are imbalanced (cross helicity similar to 1), and so have closely aligned magnetic and velocity vectors, have wide 1/f ranges typical of fast solar wind. We conclude that the strength of nonlinear interactions of individual fluctuations within a stream, diagnosed by the degree of correlation in direction and magnitude of magnetic and velocity fluctuations, determines the extent of the 1/f region observed, and thus the onset scale for the turbulent cascade.
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