The Ion Transition Range of Solar Wind Turbulence in the Inner Heliosphere: Parker Solar Probe Observations

TL;DR

This study uses Parker Solar Probe data to analyze the ion transition range in solar wind turbulence, revealing variable spectral indices and correlations with turbulence properties, advancing understanding of turbulence scaling in the inner heliosphere.

Contribution

First detailed analysis of the ion transition range in solar wind turbulence using high-resolution PSP data, highlighting spectral variability and turbulence property correlations.

Findings

Spectral index varies between -3 and -5.7 at sub-ion scales.

Higher power amplitudes correlate with steeper spectra.

Steep spectra correlate with increased normalized cross helicity.

Abstract

The scaling of the turbulent spectra provides a key measurement that allows to discriminate between different theoretical predictions of turbulence. In the solar wind, this has driven a large number of studies dedicated to this issue using in-situ data from various orbiting spacecraft. While a semblance of consensus exists regarding the scaling in the MHD and dispersive ranges, the precise scaling in the transition range and the actual physical mechanisms that control it remain open questions. Using the high-resolution data in the inner heliosphere from Parker Solar Probe (PSP) mission, we find that the sub-ion scales (i.e., at the frequency f ~ [2, 9] Hz) follow a power-law spectrum f^a with a spectral index a varying between -3 and -5.7. Our results also show that there is a trend toward and anti-correlation between the spectral slopes and the power amplitudes at the MHD scales, in agreement with previous studies: the higher the power amplitude the steeper the spectrum at sub-ion scales. A similar trend toward an anti-correlation between steep spectra and increasing normalized cross helicity is found, in agreement with previous theoretical predictions about the imbalanced solar wind. We discuss the ubiquitous nature of the ion transition range in solar wind turbulence in the inner heliosphere.