Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations

Abstract

Plasma velocity and magnetic field measurements from the Voyager 2 mission are used to study solar wind turbulence in the slow solar wind at two different heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to provide information on the radial evolution of this turbulence. The magnetic helicity and the cross-helicity, which express the correlation between the plasma velocity and the magnetic field, are used to characterize the turbulence. Wave number spectra are computed by means of the Taylor hypothesis applied to time resolved single point Voyager 2 measurements. The overall picture we get is complex and difficult to interpret. A substantial decrease of the cross-helicity at smaller scales (over 1-3 hours of observation) with increasing heliocentric distance is observed. At 5 AU the only peak in the probability density of the normalized residual energy is negative, near -0.5. At 29 AU the probability density becomes doubly peaked, with a negative peak at -0.5 and a smaller peak at a positive values of about 0.7. A decrease of the cross-helicity for increasing heliocentric distance is observed, together with a reduction of the unbalance toward the magnetic energy of the energy of the fluctuations. For the smaller scales, we found that at 29 AU the normalized polarization is small and positive on average (about 0.1), it is instead zero at 5 AU. For the larger scales, the polarization is low and positive at 5 AU (average around 0.1) while it is negative (around - 0.15) at 29 AU.

14 pages 5 figures. Accepted for publication on European Journal of Mechanics B/Fluids (5/8/2015)

References in corpus (6)

• Dynamic Alignment in Driven Magnetohydrodynamic Turbulence
• Validity of the Taylor Hypothesis for Linear Kinetic Waves in the Weakly Collisional Solar Wind
• Dynamic Alignment and Exact Scaling Laws in MHD Turbulence
• MHD Turbulence, Turbulent Dynamo and Applications
• Role of Cross Helicity in Cascade Processes of MHD turbulence
• Turbulence in the solar wind: spectra from Voyager 2 data at 5 AU

Cited by in corpus (6)

• Diagnosing solar wind origins using in-situ measurements in the inner heliosphere
• A generalized two-component model of solar wind turbulence and ab initio diffusion mean free paths and drift lengthscales of cosmic rays
• Magnetic turbulence spectra and intermittency in the heliosheath and in the local interstellar medium
• Turbulence in the outer heliosphere
• Voyager 2 solar plasma and magnetic field spectral analysis for intermediate data sparsity
• The structure of magnetic turbulence in the heliosheath region observed by Voyager 2 at 106 AU