Nine Outstanding Questions of Solar Wind Physics

Abstract

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 theory.Plain Language Summary The Sun's atmosphere, called the solar corona, is a very hot plasma (ions and electrons) that reaches temperatures of 1000000 K or more. The coronal plasma continually expands away from the Sun, carrying solar magnetic field with it. This is the solar wind. It reaches speeds of hundreds of kilometers per second and fills the solar system. The space carved out by the solar wind flow defines the heliosphere. The formation of the solar wind and its evolution as it flows away from the Sun is fundamental to how the Sun and stars get rid of stressed magnetic fields and involves physical processes that operate throughout the universe. Additionally, the solar wind constantly bombards Earth's magnetic field and plasma environment, driving dynamics called space weather. The solar wind is the medium through which larger space weather events from solar storms propagate. Understanding the solar wind is therefore key for understanding the space environment around Earth. In this paper, we synthesize input from the heliophysics community on the outstanding questions of solar wind physics. We describe the current state of research, an updated framework for understanding solar wind formation, and future needs and opportunities for progress, including 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). Introduction: What Have Been the...

The Magnetic Structure of the Solar Wind: Ionic Composition and the Electron Strahl

“…(See also Li, , and Li et al, , for support of this picture.) The origin of this magnetic structure and its current sheets (directional discontinuities) is an outstanding issue of solar wind physics (e.g., Neugebauer & Giacalone, , ; Owens et al, ; Tu et al, ; Viall & Borovsky, ). Two candidate ideas about the origin are (1) the fossil magnetic structure from the corona (Borovsky, , ) and (2) the intermittent structure created in situ by MHD turbulence (Greco et al, ; Vasquez et al, ; Zhdankin et al, ).…”

Measurement of Magnetic Field Fluctuations in the Parker Solar Probe and Solar Orbiter Missions

“…Recently, two major space missions have been launched with the aim of better understanding the highly dynamic inner heliosphere and the influence on it of the Sun. This largely unexplored region of our solar system continuously affects our near‐Earth environment and also challenges us with some of the major unsolved questions in contemporary physics (Viall & Borovsky, 2020).…”

On the Causes of the Slow Solar Wind: 1. The Solar Unipolar Induction Currents

“…There have already very extensive and recent reviews on the progress on the cause of the solar wind, including those by McComas et al. (1998, 2013); Miralles and Sanchez Almeida (2011); Cranmer (2009) and Viall and Borovsky (2020).…”

“…Cranmer (2009) and Viall and Borovsky (2020) summarized the present understanding and discussions by stating that there are two groups in terms of energization and release mechanisms. Such a distinction was theoretically discussed earlier by Pneman (1972) and observationally by Bame et al.…”

“…The second group considers magnetic reconnection/loop opening for hotter, denser wind from low latitudes (Sushino et al., 2008; Wang et al., 2000). Studies of the second group was detailed by Viall and Borovsky (2020).…”