The Wind spacecraft is a critical element in NASA’s Heliophysics System Observatory (HSO) – a fleet of spacecraft created to understand the dynamics of the sun-Earth system – owing to the combination of its longevity (>25 years in service), its diverse compliment of instrumentation, and high resolution and accurate measurements. Wind has over 55 selectable public data products with over ~1100 total data variables (including OMNI data products) on SPDF/CDAWeb alone. These data have led to paradigm shifting results in studies of statistical solar wind trends, magnetic reconnection, large-scale solar wind structures, kinetic physics, electromagnetic turbulence, the Van Allen radiation belts, coronal mass ejection topology, interplanetary and interstellar dust, the lunar wake, solar radio bursts, solar energetic particles, and extreme astrophysical phenomena such as gamma-ray bursts. This review introduces the mission and instrument suites then discusses examples of the contributions by Wind to these scientific topics that emphasize its importance to both the fields of heliophysics and astrophysics.

Solar wind electrons play an important role in the energy balance of the solar wind acceleration by carrying energy into interplanetary space in the form of electron heat flux. The heat flux is stored in the complex electron velocity distribution functions (VDFs) shaped by expansion, Coulomb collisions, and field-particle interactions. We investigate how the suprathermal electron deficit in the anti-strahl direction, which was recently discovered in the near-Sun solar wind, drives a kinetic instability and creates whistler waves with wave vectors that are quasi-parallel to the direction of the background magnetic field. We combined high-cadence measurements of electron pitch-angle distribution functions and electromagnetic waves provided by Solar Orbiter during its first orbit. Our case study is based on a burst-mode data interval from the Electrostatic Analyser System (SWA-EAS) at a distance of 112 RS (0.52 au) from the Sun, during which several whistler wave packets were detected by Solar Orbiter’s Radio and Plasma Waves (RPW) instrument. The sunward deficit creates kinetic conditions under which the quasi-parallel whistler wave becomes unstable. We directly test our predictions for the existence of these waves through solar wind observations. We find whistler waves that are quasi-parallel and almost circularly polarised, propagating away from the Sun, coinciding with a pronounced sunward deficit in the electron VDF. The cyclotron-resonance condition is fulfilled for electrons moving in the direction opposite to the direction of wave propagation, with energies corresponding to those associated with the sunward deficit. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.

The solar wind is a continuous outflow of plasma from the Sun, which expands into the space between the planets in our solar system and forms the heliosphere. The solar wind is inherently turbulent and characterised by kinetic micro-instabilities on a range of scales. The Sun also intermittently ejects mass (coronal mass ejections; CMEs) or solar energetic particles (SEPs) which change their trajectory or energy due to their interactions with the solar wind. When these violent particle events hit the Earth’s magnetosphere, they can cause “space weather” with both long- and short-term impacts on our natural and technological environment. This presentation investigates the behaviour and the effects of kinetic instabilities in a turbulent plasma with particular emphasis on energy transfer processes. We utilise the unprecedented observations from ESA’s Solar Orbiter spacecraft, launched in February 2020, to advance our understanding of the Sun, the solar wind, and space weather. Large-scale compressions (ubiquitous in solar-wind turbulence) create conditions for proton, alpha-particle and electron microinstabilities, which then transfer energy to small-scale fluctuations. These instability-driven small-scale fluctuations, including those driven by turbulence and other sources of free energy (e.g. particle beams, differential flows, heat fluxes, temperature anisotropies), make a significant contribution to the fluctuation spectrum at kinetic scales, where energy dissipation occurs. We consider instabilities driven by turbulence in the plasma by using statistical methods to analyse the Solar Orbiter data and characterise the turbulence at the relevant scales and amplitude. Specifically, we evaluate the processes by which turbulence combined with temperature anisotropy causes instability, comparing theoretical calculations with the high resolution data available from the Solar Orbiter MAG and SWA instruments.