Alignment and Scaling of Large-Scale Fluctuations in the Solar Wind

R. T. Wicks, A. Mallet, T. S. Horbury, C. H. K. Chen, A. A. Schekochihin, and J. J. Mitchell

Phys. Rev. Lett. 110, 025003 – Published 11 January 2013

Abstract

We investigate the dependence of solar wind fluctuations measured by the Wind spacecraft on scale and on the degree of alignment between oppositely directed Elsasser fields. This alignment controls the strength of the nonlinear interactions and, therefore, the turbulence. We find that at scales larger than the outer scale of the turbulence the Elsasser fluctuations become on average more antialigned as the outer scale is approached from above. Conditioning structure functions using the alignment angle reveals turbulent scaling of unaligned fluctuations at scales previously believed to lie outside the turbulent cascade in the “ $1 / f$ range.” We argue that the $1 / f$ range contains a mixture of a noninteracting antialigned population of Alfvén waves and magnetic force-free structures plus a subdominant population of unaligned cascading turbulent fluctuations.

Received 24 July 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.025003

Authors & Affiliations

R. T. Wicks^1,*, A. Mallet², T. S. Horbury³, C. H. K. Chen⁴, A. A. Schekochihin², and J. J. Mitchell³

¹NASA Postdoctoral Program Fellow, Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
²Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, United Kingdom
³Space and Atmospheric Physics Group, Imperial College London, London SW7 2AZ, United Kingdom
⁴Space Sciences Laboratory, University of California, Berkeley, California 94720, USA