Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations

TL;DR

This study analyzes high-speed solar wind streams from the HELIOS satellites between 0.3 and 1 AU, revealing detailed radial evolution patterns of plasma density, magnetic field, and temperature, with implications for solar wind heating.

Contribution

It provides revised observations of pure high-speed solar wind streams, challenging previous assumptions about plasma density decrease and magnetic field behavior, and offers new insights into solar wind heating mechanisms.

Findings

Proton density decreases as expected for radial expansion.

Magnetic field components deviate from Parker predictions.

Double-adiabatic invariants are violated, indicating entropy increase.

Abstract

Spacecraft observations have shown that the proton temperature in the solar wind falls off with radial distance more slowly than expected for an adiabatic prediction. Usually, previous studies have been focused on the evolution of the solar-wind plasma by using the bulk speed as an order parameter to discriminate different regimes. In contrast, here, we study the radial evolution of pure and homogeneous fast streams (i.e. well-defined streams of coronal-hole plasma that maintain their identity during several solar rotations) by means of re-processed particle data, from the HELIOS satellites between 0.3 and 1 AU. We have identified 16 intervals of unperturbed high-speed coronal hole plasma, from three different sources and measured at different radial distances. The observations show that, for all three streams, (i) the proton density decreases as expected for a radially expanding plasma, unlike previous analysis that found a slower decrease; (ii) the magnetic field deviates from the Parker prediction, with the radial and tangential components decreasing more slowly and quickly than expected, respectively; (iii) the double-adiabatic invariants are violated and an increase of entropy is observed; (iv) the proton-core temperature anisotropy is constrained by mirror mode instability; (v) the collisional frequency is not constant, but decreases as the plasma travels away from the Sun. The present work provides an insight into the heating problem in pure fast solar wind, fitting in the context of the next solar missions, and, especially for Parker Solar Probe, it enables us to predict the high-speed solar-wind environment much closer to the Sun.