The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere (1912.02348v1)

Abstract: The first two orbits of the Parker Solar Probe (PSP) spacecraft have enabled the first in situ measurements of the solar wind down to a heliocentric distance of 0.17 au (or 36 Rs). Here, we present an analysis of this data to study solar wind turbulence at 0.17 au and its evolution out to 1 au. While many features remain similar, key differences at 0.17 au include: increased turbulence energy levels by more than an order of magnitude, a magnetic field spectral index of -3/2 matching that of the velocity and both Elsasser fields, a lower magnetic compressibility consistent with a smaller slow-mode kinetic energy fraction, and a much smaller outer scale that has had time for substantial nonlinear processing. There is also an overall increase in the dominance of outward-propagating Alfv\'enic fluctuations compared to inward-propagating ones, and the radial variation of the inward component is consistent with its generation by reflection from the large-scale gradient in Alfv\'en speed. The energy flux in this turbulence at 0.17 au was found to be ~10% of that in the bulk solar wind kinetic energy, becoming ~40% when extrapolated to the Alfv\'en point, and both the fraction and rate of increase of this flux towards the Sun is consistent with turbulence-driven models in which the solar wind is powered by this flux.

The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere

The paper conducted by Chen et al. presents a comprehensive investigation into the properties, evolution, and implications of solar wind turbulence within the inner heliosphere, specifically leveraging data from the Parker Solar Probe (PSP). This paper marks a significant advancement in our understanding of solar wind dynamics by extending observations closer to the Sun than previously possible, reaching down to 0.17 au.

The PSP data reveal substantial increases in turbulence energy levels as proximity to the Sun decreases, suggesting the existence of intense nonlinear interactions. Particularly notable is the spectral index of -3/2 observed in magnetic, velocity, and Elsasser fields at 0.17 au, deviating from the -5/3 index typically seen at greater heliocentric distances. This suggests a potentially different cascade process near the sun which is not fully captured by existing models. Additionally, a noticeable reduction in magnetic compressibility and the slow-mode fraction is observed, indicating weaker compressive fluctuations near the Sun and potentially a different balance of modes compared to farther solar wind locales, possibly implicating wave-driven models.

A striking aspect of the research is the observed dominance of outward-propagating Alfvénic fluctuations—estimated to constitute approximately 10% of the turbulence energy flux relative to the bulk solar wind kinetic energy at 0.17 au, rising to 40% if extrapolated to the Alfvén point. This notable increase supports theoretical models in which solar wind acceleration is powered by turbulent magnetic energy flux.

The radial evolution of inward-propagating fluctuations also supports these reflections as they are quantitatively consistent with wave reflection models driven by large-scale magnetic field gradients, postulated to be a key mechanism at smaller heliocentric distances.

Moreover, the paper underscores the importance of understanding changes in the turbulence's outer scale and its relation to solar wind speed and travel time. The break in turbulent scaling appears consistent with nonlinear interactions, challenging simple interpretations of an undisturbed wave spectrum in regions past the break.

This paper provides new perspectives into the coupling of solar wind streams, the dynamics of Alfvénic turbulence, and the critical interactions driving solar wind acceleration. Future PSP orbits closer to the Sun promise further insights into the underlying mechanisms of solar wind turbulence, contributing to our theoretical understanding of plasma turbulence and solar atmosphere dynamics.

In summary, the research by Chen et al. provides a significant step in characterizing solar wind turbulence near the Sun, showing that its evolution and role are consistent with turbulence-driven solar wind models, and invites further paper to refine the understanding of turbulence dynamics in this region of the heliosphere.