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Description of the Objective

Ultraviolet observations of the solar corona reveal that most ions reach their maximum temperature within about five solar radii above the photosphere (Cranmer et al., 1997; Kohl et al., 2006; Telloni et al., 2007). On the other hand, the observed solar-wind temperature and anisotropy profiles beyond 0.3 au suggest that ion heating continues throughout the heliosphere (Cranmer et al., 2009; Matteini et al., 2012; Hellinger et al., 2013), while the electron temperature profiles are consistent with a predominantly non-local heat-flux deposition (Stverak et al., 2015). In addition to the heat-flux deposition, non-thermal features in the electron distribution function can contribute to the acceleration of the solar wind through ambipolar diffusion, the exospheric effect, and velocity filtration (Lemaire & Scherer, 1971; Scudder, 1994; Maksimovic et al., 2001; Maksimovic et al., 2005; Zouganelis et al., 2004, 2005). It is unclear, however, as to how and to which degree these different processes contribute to the acceleration and heating of the solar wind. A major step forward in our understanding of the solar wind would be the quantitative comparison between exospheric effects and wave/turbulence processes for their role in the acceleration of the plasma.


Needed Observations

Detailed studies of the radial evolution of the ion and electron distribution functions are the cornerstone for determining the spatial dependence of the energy deposition in the solar wind. In a first step, these measurements will allow us to perform an energy budget analysis like the one presented by Cranmer et al. (2009) and Stverak et al. (2015). 

Beyond these budget analyses, which are based on moment calculations, we will measure the radial evolution of the shape of the observed ion distributions with high resolution in velocity space through case studies to learn about heating mechanisms at different distances. For example, the predominant occurrence of quasi-linear plateaus would indicate cyclotron-resonant dissipation (Marsch & Tu, 2001; Marsch & Bourouaine, 2011), while a Moyal distribution would indicate energy deposition through stochastic heating (Klein & Chandran, 2016). These high-resolution ion case studies are an important addition to the moment analyses since they will help us to understand the nature of the heating processes rather than just the presence of a heating process.

High-resolution measurements of the electron distributions and the determination of their third-order velocity moments will allow us to investigate the radial profiles of the electron heat flux as well as its evolution through free-streaming, collisions, and kinetic instabilities (see Gary et al., 1999; Bale et al., 2013; Landi et al. 2014). Furthermore, the observation of truncated electron distributions can lead to a quantification of exospheric effects and their contribution to the solar-wind acceleration by comparison with exospheric-model predictions (Pierrard et al., 1999; Lie-Svendsen & Leer, 2000).