Overview

Target Description

The supersonic, super-Alfvenic solar wind arises from the million-Kelvin solar corona, where the heating processes generating these temperatures and the role of small-scale waves, turbulence and field dynamics are far from being understood. In-situ solar wind turbulence observations show a dissipation range, which is direct evidence of ongoing turbulent heating believed to operate throughout the heliosphere, from the low corona out to the heliosheath. Subsurface solar convection powers all its mass loss, generates magnetic fields, excites solar flares through magnetic reconnection, and drives coronal mass ejections, Alfvénic waves, ion–cyclotron waves, and the various turbulent processes that evolve throughout the heliosphere. Understanding the origin, acceleration and evolution of the solar wind is critical for predicting virtually all forms of space weather. This Focused Science Topic (FST) directly relates to LWS Strategic Science Areas (SSA)-0, which focuses on physics-based understanding of the variability of solar magnetic fields and particles.

This FST covers the array of physical processes involved in the solar wind’s origin and evolution: the sources of different solar wind types and their connection to different coronal structures; the micro-physics of particle velocity distribution functions, their anisotropies and nonthermal characteristics; the role of turbulence and wave-particle interactions in heating and acceleration; and the energization driven by structures, such as shocks, current sheets and/or magnetic reconnection.

This FST addresses a range of science questions, including: What specific observables can be derived from and used to test solar wind models? What existing observations can be used to validate solar wind models, ranging from the kinetic to the AU scales? Furthermore, in preparation for the next decade of exploration of the inner heliosphere and corona with Parker Solar Probe and Solar Orbiter, how can the anticipated observations drive theoretical developments?

Goals and Measures of Success

The primary goal of this FST is to advance our understanding of the origin, acceleration and evolution of the solar wind for future predictive models.

A key component of this FST will be the inter-comparison and testing of competing solar wind models, better constraining them using an array of solar wind in-situ and remote sensing observations, and the development of observational metrics to evaluate their strengths and limitations. The outcome will improve solar wind modeling capabilities. Direct observations across a range of temporal and/or spatial scales may be used to determine how large-scale features evolve in the origins of solar wind. Measures of success include, but are not limited to the:

• Determination of how magnetic scales couple to enable the release of material that form the wind
• Clarification of how plasma turbulence evolves and dissipates to heat and accelerate solar wind plasma
• Evaluation of how energy propagates across different regions of the corona and through the transition region
• Determination of the relationship between charge-states and elemental abundances and how they are set
• Understanding of nano-flares and magnetic reconnection and how stored electromagnetic energy is transferred to particles
• Evaluation of processes that heat and accelerate the solar wind plasma in the low corona

It is anticipated that future observations may transform our understanding of the origins and acceleration of the solar wind. In preparation, models need to be defined and tested and to establish specific metrics that can be used for validation. This will allow future predictive models to be developed and tested efficiently as new observations of the solar wind emerge.

Types of Investigations

The nature of this research effort requires the interdisciplinary combination of observational, theoretical, and numerical studies, including the following subtopics:

• waves, turbulence, and/or structures and their role in the heating of the solar wind plasma
• reconnection as an energy source that drives and/or heats the solar wind
• electron transport and heat conduction
• minor ions and their role in the origin and the evolution of the solar wind
• non-Maxwellian velocity distribution functions and their role in non-equilibrium solar wind thermodynamics
• small-scale energy release processes (nano-flares, etc.) and their role in the origin of the solar wind
• solar wind source models based on charge state and elemental composition
• mass flux, solar wind power, and their relationship to the large-scale magnetic field and small-scale dynamics
• differential studies of the spectrum of solar wind types that arise from different global-scale magnetic topologies
• evolution of solar wind properties through the solar cycle

Studies within this program will combine theoretical, numerical, and observational methods. The successful outcome of each research effort will rely on high-quality data analyses from past and present missions — such as Helios 1 and 2, Wind, ACE, Ulysses, STEREO, SOHO, SDO, IRIS, DSCOVR, etc. — to facilitate the robust comparison and constrain models with measurements. The effort could also rely on high-performance computing to facilitate multi-scale modeling activities.