Abstract
Abstract The first two orbits of the Parker Solar Probe spacecraft have enabled the first in situ measurements of the solar wind down to a heliocentric distance of 0.17 au (or 36 ). 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 toward the Sun are consistent with turbulence-driven models in which the solar wind is powered by this flux.
Highlights
The solar wind is observed to contain a turbulent cascade at distances from the closest previous in situ measurements to the Sun at 0.29 au (Tu & Marsch 1995) out to the edge of the heliosphere and beyond (Fraternale et al 2019)
The properties of solar wind turbulence were measured in situ down to a heliocentric distance of 0.17 au for the first time
While many of the measured properties are shared with measurements nearer 1 au, significant differences include increased power levels, a −3/2 spectral index in all fields, a significantly smaller compressive component of the turbulence, a much smaller outer scale at which the nonlinear time is less than the travel time from the Sun, and an increase in the turbulence imbalance that is consistent with the generation of the inward-propagating component by reflection
Summary
The solar wind is observed to contain a turbulent cascade at distances from the closest previous in situ measurements to the Sun at 0.29 au (Tu & Marsch 1995) out to the edge of the heliosphere and beyond (Fraternale et al 2019). The evolution of all of these features is consistent with an active cascade occurring throughout the solar wind, which is consistent with the observed nonadiabatic temperature profile suggesting continual heating of the plasma (Mihalov & Wolfe 1978; Gazis & Lazarus 1982; Marsch et al 1982; Freeman 1988; Richardson et al 1995; Matthaeus et al 1999b; Cranmer et al 2009; Hellinger et al 2011) In addition to this heating far from the Sun, turbulence is proposed to play a key role in the heating of the solar corona and acceleration of the solar wind itself.
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