1997), the kappa value, and thus the strength of the suprathermal population, is not constant but varies in time with solar wind conditions.
Throughout the heliosphere, the kappa of the halo population generally decreases with increasing solar distance. Values of κ = 7 are frequently found closer than 1 AU, but rarely beyond 1 AU. A similar trend is found for kappa values around 5, and κ = 3 is hardly found below 1.5 AU, and more frequently beyond 1.5 AU. The strahl population can follow a different trend.
This reduction of kappa corresponds to an increase in kinetic temperature. Therefore, some mechanism for energizing these electrons has to be present in the solar wind.
The presence of non‐thermal populations in many space plasmas suggests a universal mechanism for their creation. This concerns mainly electrons, but also the ions in the solar wind. Observations of electron suprathermal tails in the solar wind suggest their existence in the solar corona, because the electron mean free path in the solar wind is around 1 AU. The ion‐charge measurements also confirm that suprathermal electrons can already be present in the corona. In any case, measurements made by Helios a radial distances as low as 0.3 AU confirm the presence of suprathermal electrons, even if they are characterized by a larger kappa index for the halo population at low radial distances (Pierrard et al., 2016). Solar Orbiter will have the advantage of measuring the distributions at low distances inside and outside the ecliptic plane. Up to now, the distributions at low distances have come from Helios, and those at large distances from Ulysses. Even if useful to study the average evolution with the radial distances, the observations were made at very different time periods (1970s for Helios, 1990s for Ulysses) and by different instruments measuring the distributions on different energy ranges.
Different scenarios have been suggested to generate the suprathermal particles (see Pierrard & Lazar, 2010, for a review). Turbulence associated with the long range of the Coulomb interactions has been proposed. Models including the generation of suprathermal electrons by resonant interaction with whistler waves in the solar corona and wind have been developed (Pierrard et al., 2011; Vocks & Mann, 2003). Recent scenarios use the kinetic theory to show that Kappa‐like distributions could correspond to a particular thermodynamic equilibrium state. Kappa‐like distributions can result as a consequence of the entropy generalization in nonextensive statistics (Leubner & Vörös, 2005), physically related to the long‐range nature of the Coulomb potential, turbulence, and intermittency.
Suprathermal particles have important consequences for plasma fluctuations, resonant and nonresonant wave–particle acceleration, the heat flux, and plasma heating. These effects are well described by the kinetic approaches, where no closure requires the distributions to be nearly Maxwellian. The heating of the corona can be naturally explained by the velocity filtration, without depositing wave or magnetic field energy. Indeed, if suprathermal particles are present, their ratio over thermal ones increases as a function of altitude in an attraction field and automatically lead to an increase in temperature. This process works for electrons, but also protons and other ions if they are characterized by an enhanced population of suprathermal particles already in the low corona. The mass‐to‐charge ratio then determines the temperature of each ion species in good agreement with observations (Pierrard & Lamy, 2003). At larger radial distances (typically above 1.5 solar radii), the wind starts to blow, and the temperature reaches a maximum before starting to decrease. The presence of suprathermal electrons also increases the solar wind velocity as shown by Pierrard and Pieters (2014), in comparison to what is obtained with a simple thermal population characterized by a Maxwellian distribution.
1.6. THE EVOLUTION OF THE SOLAR WIND FROM THE INNER TO OUTER HELIOSPHERE
There are far fewer measurements of the solar wind in the outer heliosphere than in the inner heliosphere; however, measurements of the solar wind taken by the Pioneer 10/11 and Voyager 1/2 spacecraft showed that the heliosphere changes in two important ways. First, beyond 1 AU, solar wind interaction regions begin to dominate the heliosphere. Around 2–3 AU (Gosling et al., 1976; Hundhausen & Gosling, 1976; E. J. Smith & Wolfe, 1976), the dynamic compression of the slow wind by the fast wind creates a high‐pressure region that expands in the reference frame of the solar wind, producing a forward fast shock and a reverse shock. The expansion of the compression region into the fast stream continually removes plasma from the high‐speed stream to create larger, simpler structures. As the wind continues outward, the slower streams become more prominent, and the interaction regions broaden ahead of the fast wind.
At approximately 7 to 10 AU, by which time the fast and slow wind streams have given way to large interaction regions (Burlaga et al., 1995), successive interaction regions begin to interact themselves, forming “merged interaction regions.” On average, there exists a single merged interaction region per solar rotation near 10 AU (Hundhausen & Gosling, 1976). Around 20 AU, these merged interaction regions and their associated shocks are replaced by corotating pressure enhancements that slowly form large pressure rings concentric to the Sun (Whang & Burlaga, 1990). These have been called “global merged interaction regions” and, together with merged interaction regions, play a significant role in the modulation of galactic cosmic rays (Burlaga et al., 1993; Wang et al., 2006).
The second important difference in the solar wind of the outer heliosphere is the existence of a new population of ions called “pick‐up ions.” Beyond several tens of AUs, these pick‐up ions actually modify the properties of the solar wind and must be accounted for. A pick‐up ion is created when a neutral atom, streaming into the heliosphere from the local interstellar medium, becomes ionized (Vasyliunas & Siscoe, 1976; Isenberg, 1987). The ionization processes include charge exchange with solar wind ions, photoionization from solar photons, and impact ionization from solar wind electrons. Out of these, charge exchange is the dominant interaction process, whereby a singly charged ion passes close to the neutral atom and captures one of its electrons.
Once ionized, the new pick‐up ion starts to gyrate around the magnetic field. The ion starts with a very small velocity as compared with the solar wind flow (about 23 km/s). Therefore, the ion experiences a motional electric field caused by the moving solar wind plasma. The pick‐up ion gains energy from this electric field until its guiding center reaches a velocity that is equivalent to the solar wind flow velocity, at which point it becomes part of the bulk flow and is convected outward with the expanding solar wind. After the pick‐up, the ions experience gyrotropization and isotropization by either ambient or self‐generated, low‐frequency electromagnetic fluctuations in the solar wind plasma. In the spacecraft frame, this process appears as if those new ions are “picked up” by the solar wind flow.
We have not thoroughly grasped the transport of pick‐up ions in the solar wind due to the lack of measurements of keV‐ions in the outer heliosphere. The production of the suprathermal tails on the velocity distribution functions, in particular, remains a puzzle (Chalov et al., 2003; Fahr & Fichtner, 2011). It is believed that during their propagation through the outer heliosphere, pick‐up ions undergo pitch‐angle scattering and stochastic acceleration by interactions with different kinds of solar wind turbulences (Chalov et al., 1997). Pick‐up ions may also play a significant role in mediating the solar wind interaction with the interstellar medium, possibly changing the structure of the heliosheath through charge exchange (Zank, 1999), but more measurements are needed.
1.7. OUTSTANDING QUESTIONS AND FUTURE PROSPECTS
We have decades of in situ measurements of the solar wind plasma, composition, and magnetic field within 1 AU, and we have decades of remote imaging and spectroscopy of the solar corona and young solar wind. NASA’s Parker Solar Probe and ESA’s Solar Orbiter missions provide an amazing opportunity to finally unite these disparate data sets and make a direct connection between the remotely observed corona and the “ground truth” in situ solar wind observations.
These missions will enable
