In situ measurements of the solar wind have been available for almost 60
years, and in that time plasma-physics simulation capabilities have
commenced, and ground-based solar observations have expanded into
space-based solar observations. These observations and simulations have
yielded an increasingly improved knowledge of fundamental physics and
have delivered a remarkable understanding of the solar wind and its
complexity. Yet there are longstanding major unsolved questions.
Synthesizing inputs from the solar wind research community, nine
outstanding questions of solar-wind physics are developed and discussed
in this commentary. These involve questions about the formation of the
solar wind, about the inherent properties of the solar wind (and what
the properties say about its formation), and about the evolution of the
solar wind. The questions focus on (1) origin locations on the Sun, (2)
plasma release, (3) acceleration, (4) heavy-ion abundances and charge
states, (5) magnetic structure, (6) Alfven waves, (7) turbulence, (8)
distribution-function evolution, and (9) energetic-particle transport.
On these nine questions we offer suggestions for future progress,
forward looking on what is likely to be accomplished in near future with
data from Parker Solar Probe, from Solar Orbiter, from the Daniel K.
Inouye Solar Telescope (DKIST), and from Polarimeter to Unify the Corona
and Heliosphere (PUNCH). Calls are made for improved measurements, for
higher-resolution simulations, and for advances in plasma-physics