The solar wind is a supersonic, magnetized plasma of mainly electrons, protons, and alpha particles streaming outward from the Sun, carrying with it the interplanetary magnetic field and inflating the Heliosphere that envelopes the Solar System. Near Earth (1 AU), typical proton densities are of order 1–10 cm⁻³ and bulk speeds commonly range from about 300–500 km/s for slow wind to 500–800 km/s for fast wind, values documented by in situ spacecraft and summarized in authoritative references. According to Encyclopaedia Britannica, representative quiet-time conditions at 1 AU include densities in that range and speeds of roughly 350–700 km/s, while NOAA’s Space Weather Prediction Center highlights fast streams of 500–800 km/s linked to coronal holes. These outflows embed and stretch the Sun’s field into the Parker spiral pattern that threads the solar system. Britannica; NOAA SWPC; NASA GSFC Cosmicopia. (britannica.com)

Origins and theory

The modern theory of a thermally driven, continuous coronal outflow was established by Eugene N. Parker, who showed that a hot corona must expand supersonically and that the Sun’s rotation would wind the interplanetary magnetic field into an Archimedean spiral. Parker’s seminal paper formalized the transonic “solar wind” solution and predicted the spiral IMF later confirmed by spacecraft observations. [Parker’s 1958 paper](journal://The Astrophysical Journal|Dynamics of the Interplanetary Gas and Magnetic Fields|1958); ANGEO review; NASA NTRS (Pioneer-era IMF evidence). (angeo.copernicus.org)

Structure in the heliosphere

As the wind streams outward, it drags the interplanetary magnetic field (IMF) into the Parker spiral, producing sector structure and a wavy heliospheric current sheet—the largest coherent structure in the Solar System. Typical mean IMF magnitudes near 1 AU are about 3–4 nT, though transients may produce larger excursions. ANGEO review; GSFC education page; NOAA SWPC. (angeo.copernicus.org)

Sources and types of solar wind

Two canonical regimes are identified: a fast, compositionally uniform wind linked to open magnetic-field regions called coronal holes, and a slower, more variable wind associated with streamers and active-region boundaries. NASA summarizes these associations and their typical speeds; the slow/fast dichotomy is also distinguished by plasma composition and charge-state signatures in reviews. NASA Science: What Is the Solar Wind?; NASA SVS explainer; Living Reviews in Solar Physics. (science.nasa.gov)

Composition and plasma properties

The solar wind is a weakly collisional plasma primarily composed of electrons and protons, with helium (alpha particles) at a few percent by number and trace heavy ions whose abundances and charge states encode coronal source temperatures and processes. In the fast wind, alpha particles are typically hotter than protons and comprise about 5% by number, corresponding to a substantial mass fraction. A&A research article; NASA NTRS (abundances, FIP effect); Living Reviews in Solar Physics. (aanda.org)

Close‑Sun discoveries

NASA’s Parker Solar Probe has revealed pervasive “switchbacks,” brief reversals of the radial magnetic field accompanied by jet-like velocity spikes, offering new clues about coronal heating and the energization of the wind. Initial results released December 4, 2019, and subsequent analyses link switchbacks to magnetic reconnection and Alfvénic structures. NASA summaries and research highlight these findings and their potential origins in small-scale jets. NASA feature; NASA news release; NASA switchbacks explainer. (nasa.gov)

Interaction with planets and space weather

At Earth, the solar wind is largely deflected by the Magnetosphere, but southward IMF and dynamic pressure enhancements enable magnetic reconnection and energy entry that drive geomagnetic activity and Aurora. NASA explains how gusts of solar wind compress the magnetosphere and how reconnection and particle precipitation produce auroral emissions; NOAA emphasizes that fast streams and stream interaction regions can intensify geomagnetic disturbances. NASA Science: What Is the Solar Wind?; NASA aurora page; NOAA SWPC. (science.nasa.gov)

Transients, comets, and the outer boundary

Beyond the continuous wind, transient Coronal Mass Ejection ejecta carry plasma and magnetic fields that can merge with the ambient flow and perturb the heliosphere; individual CMEs may expel ~10¹³ kg of plasma and can produce severe geomagnetic storms. Comet ion tails align with the IMF embedded in the wind, revealing its structure. At the periphery, the wind slows at the termination shock before encountering the interstellar medium at the heliopause; Voyager 1 and 2 crossed the termination shock at ~94 AU (December 2004) and ~84 AU (August 2007), respectively. NASA Basics of Space Flight; NASA on Comet Encke and solar wind; NASA Interstellar Mission page. (science.nasa.gov)

Monitoring and forecasting

Near‑real‑time monitoring of upstream conditions at the Sun–Earth L1 point underpins space‑weather alerts. NASA’s Wind and ACE missions and NOAA/NASA’s DSCOVR measure solar‑wind plasma and IMF, typically providing 15–60 minutes of lead time; NOAA’s SWFO‑L1 (launched September 24, 2025) will expand operational capabilities. NASA Wind mission; NASA ACE mission; NOAA on DSCOVR real‑time solar wind; NOAA SWFO‑L1 update. (wind.nasa.gov)

Key parameters and variability

– Typical IMF magnitude at 1 AU is a few nT (often ~3–4 nT) with sector structure set by the Parker spiral; dynamic pressure, composition, and temperature vary with solar cycle and source region. – Speeds generally cluster into slow (~300–500 km/s) and fast (~500–800 km/s) regimes, but composition and charge states provide a more robust classification than speed alone. – Heavy‑ion and helium fractions, temperature anisotropies, and Alfvénic turbulence reflect kinetic processes central to heating and transport. ANGEO review; NASA SVS fast/slow explainer; Living Reviews in Solar Physics; A&A alpha‑particle study. (angeo.copernicus.org)