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?
- 1.1 What are the source regions of the solar wind and heliospheric magnetic field?
- 1.1.1 Source regions of the fast solar wind
- 1.1.2 Source regions of the slow solar wind
- 1.1.2.1 Does slow wind originate from the over-expanded edges of coronal holes?
- 1.1.2.2 Does slow and intermediate solar wind originate from coronal loops outside of coronal holes?
- 1.1.2.3 Abundance of minor ions as a function of height in the corona as indicator of slow or fast wind
- 1.1.2.4 Study of density fluctuations in the extended corona as a function of the outflow velocity of the solar wind while evolving in the heliosphere
- 1.1.2.5 Structure and evolution of streamers
- 1.1.2.6 Disentangle the spatial and temporal variability of the slow wind
- 1.1.2.7 Trace streamer blobs and other structures through the outer corona and the heliosphere.
- 1.1.2.8 Determine the velocity, acceleration profile and the mass of the transient slow wind flows
- 1.1.3 Source regions of the heliospheric magnetic field
- 1.1.4 Transverse themes
- 1.1.4.1 Reconnection
- 1.1.4.1.1 Interchange reconnection between open and closed field lines and its role in slow wind generation
- 1.1.4.1.2 Identify coronal reconnection sites by measuring impulsive event material
- 1.1.4.1.3 Identify reconnection exhausts in the solar wind
- 1.1.4.1.4 Current sheets inferred by determining the magnetic field geometries at local chromospheric heating sites
- 1.1.4.1.5 Identify and characterise the solar wind reconnection physics in current sheets with thickness down to the ion scales and smaller
- 1.1.4.1.6 Photospheric reconnection
- 1.1.4.1.7 Electron acceleration in coronal reconnection regions
- 1.1.4.1.8 Formation of flux ropes/CMEs via magnetic reconnection in the corona
- 1.1.4.1 Reconnection
- 1.2 What mechanisms heat the corona and heat and accelerate the solar wind?
- 1.2.1 What mechanisms heat the corona?
- 1.2.1.1 Energy flux in the lower atmosphere
- 1.2.1.2 Energy and mass flux in the corona.
- 1.2.1.3 Contribution of flare-like events on all scales
- 1.2.1.4 Observe and explore flare-like ‘heating events’ from the quiet corona
- 1.2.1.5 Determine whether coronal heating is spatially localized or uniform, and time steady or transient or impulsive for a wide range of magnetic loops with different spatial scales.
- 1.2.1.6 Resolve the geometry of fine elemental loop strands
- 1.2.1.7 Detect and characterise waves in closed and open structures
- 1.2.1.8 Investigate the role of small scale magnetic flux emergence in energizing the above laying layers
- 1.2.1.9 Multi-temperature diagnostics of flaring coronal loops
- 1.2.1.10 Heating in flaring loops vs heating in active regions
- 1.2.2 What mechanisms heat and accelerate the solar wind?
- 1.2.2.1 Determine where energy is deposited in the solar wind
- 1.2.2.2 What drives the evolution of the solar wind distribution functions in situ?
- 1.2.2.3 What is the nature and origin of waves, turbulence and small-scale structures?
- 1.2.2.4 Solar wind reconnection physics
- 1.2.2.5 Magnetic reconnection in the chromosphere, the transition region and the corona
- 1.2.2.6 Study fast plasma flows from the edges of solar active regions discovered with Hinode/EIS
- 1.2.2.7 Study the correlation degree between velocity and magnetic field fluctuations in the interplanetary space
- 1.2.2.8 What determines the azimuthal flow of the near-Sun solar wind?
- 1.2.1 What mechanisms heat the corona?
- 1.3 What are the sources of solar wind turbulence and how does it evolve?