Alpha particle thermodynamics in the inner heliosphere fast solar wind

Abstract

Context. Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of different ion species. However, most in-situ observations of positive ions in the solar wind are taken at 1 AU, where information on their solar source properties may have been irreversibly erased. Aims. In this study we aimed to use the properties of alpha particles at heliocentric distances between 0.3 and 1 AU to study plasma processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous remote sensing observations of the plasma closer to the Sun. Methods. We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial variation of alpha particle temperatures in fast solar wind observed between 0.3 and 1 AU. Results. Between 0.3 and 1 AU alpha particles are heated in the magnetic field perpendicular direction, and cooled in the magnetic field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.

The origin of slow Alfvénic solar wind at solar minimum

“…The sense of temperature anisotropy is the same in the fast and slow Alfvénic winds, with T ⊥ > T for protons but T ⊥ < T for alpha particles, however the proton temperature anisotropy is weaker in the slow Alfvénic wind compared to the fast wind (see also Stansby et al 2019b). Note that at least in the fast wind, these features in alpha particle temperatures are not observable at 1 AU, due to the effect of plasma microinstabilities on the evolution of alpha particle temperatures (Stansby et al 2019c). Figure 4 shows the joint distributions of proton and al- pha particle temperatures in each of the three types of wind, with the same colour coding as figure 3.…”

“…Alpha particle parameters were calculated using the bi-Maxwellian fitting routine described in Stansby et al (2019c), with minor modifications to adapt the fitting to work well in both slow and fast solar winds 1 . Proton temperatures and velocities are bi-Maxwellian fits to the proton core from the dataset of Stansby et al (2018) 2 .…”

Enhanced Energy Transfer Rate in Solar Wind Turbulence Observed near the Sun from Parker Solar Probe

“…Another subtle complexity arises due to the presence of alpha particles in the ion population. While solar wind alpha particles are a minority population (e.g., Kasper et al 2007) in terms of their number densities, they are important in terms of their mass contribution (e.g., Robbins et al 1970;Kasper et al 2007;Stansby et al 2019). So the alpha particles can have a large effect on the estimation of the denominator in equation 1.…”

Turbulence versus Fire-hose Instabilities: 3D Hybrid Expanding Box Simulations

“…Observed radial profiles of proton temperatures indicate an important heating which is often comparable to the estimated turbulent energy cascade rate (MacBride et al 2008;Cranmer et al 2009;Hellinger et al 2013). Alpha particles also need to be heated (Stansby et al 2019) whereas for electrons it is much less clear because of their strong heat flux (Štverák et al 2015).…”