A new scenario with two subranges in the inertial regime of solar wind turbulence

¹ School of Earth and Space Science and Technology, Wuhan University, Wuhan, PR China
² School of Earth and Space Sciences, Peking University, Beijing, PR China
³ SIGMA Weather Group, State Key Laboratory for Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, PR China
⁴ Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden
⁵ Institute for the Science and Technology of Plasmas, National Research Council, Bari, Italy
⁶ School of Space and Earth Sciences, Beihang University, Beijing, PR China

^⋆ Corresponding author: honghongwu@whu.edu.cn

Received: 22 January 2025
Accepted: 26 March 2025

Abstract

Context. The solar wind provides a natural laboratory for plasma turbulence. The core problem is the energy cascade process in the inertial range, which has been a fundamental long-standing question. Much effort has been put into theoretical models to explain the observational features in the solar wind. However, there are still many questions that remain unanswered.

Aims. Here, we report the observational evidence for the existence of two subranges in the inertial regime of the solar wind turbulence and show the scaling features for each subranges.

Methods. We performed multi-order structure function analyses for one high-latitude fast solar wind interval at 1.48 au measured by Ulysses and one slow but Alfvénic solar wind at 0.17 au measured by the Parker Solar Probe (PSP). We also conducted statistical analyses on 103 fast solar wind intervals observed by Wind.

Results. We identify the existence of two subranges in the inertial range according to the distinct scaling features of the magnetic field. The multi-order scaling indices versus the order for the two subranges demonstrates a clear disparity, with the second-order scaling index being 1/2 in the larger-scale subrange 1 and 2/3 in the smaller-scale subrange 2. Both subranges display apparent but different anisotropies. The velocity exhibits similar features as the magnetic field. The PSP interval shows that subrange 1 follows Yaglom scaling law, while subrange 2 does not. The Ulysses interval shows that the intermittency abruptly grows to a maximum 5% of the interval from subrange 1 to subrange 2.

Conclusions. Based on the observational features, we propose a new scenario that the inertial regime of the solar wind turbulence consists of two subranges. The observational evolution of the scaling as the solar wind expands may be a consequence of observing different subranges at different radial distances.