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

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 R ⊙ ). 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 Alfvénic 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 Alfvén 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 Alfvén 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.

On Alfvénic Slow Wind: A Journey From the Earth Back to the Sun

“…On the other hand, at intermediate scales (say the inertial range), the radial evolution suggests a transition from a Kraichnan‐like scaling of the power density toward a Kolmogorov‐like scaling when the heliocentric distance increases. This feature is in agreement with recent studies of the radial evolution of solar wind turbulence limited to PSP data (Chen et al., 2020; Duan et al., 2020). The same transition is recovered in the power spectrum of velocity fluctuations (not shown) but spread over a larger range of distances.…”

“…Chen et al. (2020) observed an increased turbulence energy level by more than an order of magnitude and a lower magnetic compressibility compared to the plasma measured at 1 AU. Moreover, in the inertial range of the turbulent cascade, they found for the Alfvénic slow stream a typical spectral index of about −3/2 for both magnetic and velocity fields and a dominance of outward‐propagating Alfvénic fluctuations with respect to the inward‐propagating ones.…”

DC and Low‐Frequency Electric Field Measurements on the Parker Solar Probe

“…Note that the much lower power at 50 solar radii is evidence that the electric field noise floor is a few orders of magnitude below the data at perihelion. A study of the radial dependence of Alfvenic turbulence in magnetic field data below 1 Hz has also been made (Chen et al, 2020), but this study did not discuss the >1‐Hz frequencies found in this work.…”

Power Spectral Density Background Estimate and Signal Detection via the Multitaper Method

“…In general, even when the physical process at work in the creation of the PSD is well known, any individual instances may not produce a fully developed PSD of that type, so the flexibility provided by our algorithm may still be needed. For example, the PSD spectral slope of the solar wind parameters in the inertial range evolves with increasing distance from the Sun, steepening from −3/2 to −5/3 for the velocity (Roberts, 2010) and magnetic field (Chen et al., 2020), or may tend toward −2 in the presence of discontinuities (Roberts, 2010) or anisotropies (Horbury et al., 2012). We use the statistical properties of the adaptive MTM to develop a maximum likelihood determination of the PSD background as in Vaughan (2010).…”