Alignment and scaling of large-scale fluctuations in the solar wind

TL;DR

This study examines how the alignment between Elsasser fields influences turbulence in the solar wind, revealing that the 1/f range includes both non-interacting structures and cascading turbulent fluctuations.

Contribution

It demonstrates the scale-dependent behavior of Elsasser field alignment and its impact on turbulence scaling in the solar wind, especially within the 1/f range.

Findings

Elsasser fluctuations become more anti-aligned near the outer scale.

The 1/f range contains both non-interacting structures and turbulent fluctuations.

Alignment conditioning reveals turbulent scaling in the 1/f range.

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 non-linear 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 anti-aligned 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 non-interacting anti-aligned population of Alfv\'{e}n waves and magnetic force-free structures plus a subdominant population of unaligned cascading turbulent fluctuations.