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

TL;DR

This study analyzes Voyager 2 data to examine how solar wind turbulence, characterized by magnetic and cross-helicity, evolves between 5 and 29 astronomical units, revealing complex changes in turbulence properties with distance.

Contribution

It provides new insights into the radial evolution of solar wind turbulence by analyzing magnetic and cross-helicity at two different heliocentric distances using Voyager 2 data.

Findings

Cross-helicity decreases with increasing heliocentric distance.

Normalized residual energy shows a shift from single to double peaks.

Polarization varies with scale and distance, indicating turbulence complexity.

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.