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 highly detailed studies of the mysterious solar wind heating. Not only will these provide new constraints on kinetic models of solar wind ions and elections and their interaction with plasma waves, but also direction comparison of the data with kinetic model results will be possible. Additionally, because together Solar Orbiter and the Parker Solar Probe will cover such a wide range of solar distances, it will be possible to study (a) the radial evolution of plasma turbulence in the expanding solar wind, (b) temperature anisotropy evolution for different particle species, and (c) to distinguish whether power‐law suprathermal tails are formed due to local turbulent processes in the solar wind or have their origin in the solar corona. Similarly, we will be able to investigate the nature of the formation of in situ mesoscale structures, weighing the importance of turbulent processing or magnetic reconnection that occurs during transit against time‐dynamic magnetic reconnection in the corona or stationary coronal streamer substructure.
Despite the many unanswered questions surrounding the formation and propagation of the solar wind, it is obvious that heliophysics is poised on the threshold of an exciting era of discovery.
ACKNOWLEDGMENTS
OA is supported by the French Centre National d’Etude Spatiales (CNES). VP and CV thank ISSI for the project on kappa distributions. APR, YU, BL, RP, and ML acknowledge support from the French space agency (Centre National des Etudes Spatiales; CNES; https://cnes.fr/fr) that funds activity in plasma physics data center (Centre de Données de la Physique des Plasmas; CDPP; http://cdpp.eu/) and the Multi Experiment Data & Operation Center (MEDOC; https://idoc.ias.u‐psud.fr/MEDOC), and the space weather team in Toulouse (Solar‐Terrestrial Observations and Modelling Service; STORMS; http://storms‐service.irap.omp.eu/). This includes funding for the data mining tools AMDA (http://amda.cdpp.eu/), CLWEB (clweb.cesr.fr/), and the propagation tool (http://propagationtool.cdpp.eu). APR, YW, BL, RP, and ML also acknowledge financial support from ERC for the project SLOW_SOURCE ‐ DLV‐819189 as well as ANR project COROSHOCK No ANR − 18 − ERC1 − 0006 − 01.
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