Hot plasma in the Sun’s atmosphere flows radially outward into interplanetary space to form the solar wind, filling the solar system and blowing a cavity in the interstellar medium known as the heliosphere. During solar minimum, large-scale regions of a single magnetic polarity in the Sun’s atmosphere – polar coronal holes – open into space and are the source of high speed (~700 km/s), rather steady solar wind flows. There is also a slow wind (300-500 km/s) that emanates from magnetically complex regions at low latitudes and the periphery of coronal holes. It is highly variable in speed, composition, and charge state. The origin of the slow wind is not known. At solar maximum, this stable bimodal configuration gives way to a more complex mixture of slow and fast streams emitted at all latitudes, depending on the distribution of open and closed magnetic regions and the highly tilted magnetic polarity inversion line.
The fast wind from the polar coronal holes carries magnetic fields of opposite polarity into the heliosphere, which are then separated by the heliospheric current sheet (HCS) embedded in the slow wind. Measurements over a range of latitudes far from the Sun show that this boundary is not symmetric around the Sun’s equator, but is on average displaced southward. This offset must reflect an asymmetry on the Sun; but since there cannot be a mismatch between the inward and outward magnetic flux on the Sun, its origin is unclear. In situ, the HCS is warped and deformed by the combined effects of solar rotation and inclination of the Sun’s magnetic axis, effects that are even more prominent at solar maximum.
The energy that heats the corona and drives the wind comes from the mechanical energy of convective photospheric motions, which is converted into magnetic and/or wave energy. In particular, both turbulence and magnetic reconnection are implicated theoretically and observationally in coronal heating and acceleration. However, existing observations cannot adequately constrain these theories, and the identity of the mechanisms that heat the corona and accelerate the solar wind remains one of the unsolved mysteries of solar and heliospheric physics. How the coronal plasma is generated, energized, and the way in which it breaks loose from the confining coronal magnetic field are fundamental physical questions with crucial implications for predicting our own space environment, as well as for the understanding of the natural plasma physics of other astrophysical objects, from other stars, to accretion disks and their coronae, to energetic phenomena such as jets, X- and gamma-ray bursts, and cosmic-ray acceleration.
The solar wind contains waves and turbulence on scales from millions of kilometres to below the electron gyroradius. The turbulence scatters energetic particles, affecting the flux of particles that arrives at the Earth; local kinetic processes dissipate the turbulent uctuations and heat the plasma. Properties of the turbulence vary with solar wind stream structure, reflecting its origins near the Sun, but the turbulence also evolves as it is carried into space with the solar wind, blurring the imprint of coronal conditions and making it difficult to determine its physical origin. The inner heliosphere, where Solar Orbiter will conduct its combination of remote-sensing and in-situ observations, provides the ideal laboratory for understanding the magnetohydrodynamic turbulence of natural plasmas expected to be ubiquitous in astrophysical environments.
In the following sections we discuss in more detail three interrelated questions which flow down from this top-level question: What are the source regions of the solar wind and the heliospheric magnetic field? What mechanisms heat and accelerate the solar wind? What are the sources of turbulence in the solar wind and how does it evolve?