Solar Wind

A continuous stream of charged particles released from the Sun's outermost atmospheric layer — the corona. Consists primarily of electrons, protons, and alpha particles with trace heavy ions and nuclei. The solar wind inflates the heliosphere, sculpts interplanetary space, shapes comet tails, drives space-weather, and erodes planetary atmospheres lacking magnetic fields.

Discovery

The existence of particles flowing from the Sun was first suspected by Richard Carrington after the 1859 solar storm. The concept was formalized by Eugene Parker in 1958 — initially rejected by two peer reviewers ("utter nonsense") but published by editor Subrahmanyan Chandrasekhar, who found no errors. First direct detection: Luna 1 (1959), confirmed by Luna 2 and Venera 1; two-speed structure confirmed by Mariner 2.

Two States

Slow solar wind (~400 km/s at 1 AU)

• ~3× denser than fast wind; more variable
• Originates from the equatorial "streamer belt" where coronal streamers produce magnetic flux draped over closed loops
• Temperature ~10⁵ K; composition matches the corona

Fast solar wind (~750 km/s at 1 AU)

• Originates from coronal holes — funnel-shaped regions of open magnetic field lines, most prevalent at solar poles
• Temperature ~10⁶ K; composition matches the photosphere

At Earth's orbit (1 AU): 250–750 km/s; 3–10 particles/cm³; 10⁴–10⁶ K. Supersonic from a few solar radii outward. Becomes subsonic at the termination shock (~75–94 AU).

Acceleration Mechanism

Thermal energy alone cannot explain solar wind speeds — the corona at >1 MK gives particles enough energy to escape solar gravity, but an additional unknown acceleration mechanism (likely magnetic) is needed. Parker's 1958 hydrodynamic model showed that a hot extended corona cannot remain static: pressure forces drive radial expansion that transitions from subsonic to supersonic — analogous to a de Laval nozzle. Recent evidence (2023) points to nanoflare jetlets and small-scale magnetic reconnection events at the coronal base as contributors.

The Parker Spiral

Because the Sun rotates (~25 day period), the outward-flowing solar wind carries the interplanetary magnetic field into a spiral pattern in the ecliptic — the Parker spiral. Voyager data confirmed the spiral extends far into the outer heliosphere.

Effects

Comets — Solar wind pressure and radiation pressure push cometary material away from the Sun, always pointing the comet's plasma tail away from the Sun regardless of direction of travel.

Planetary atmospheres — Planets without magnetic fields have their atmospheres gradually stripped. NASA's MAVEN mission measured Mars losing ~100 g/s of atmosphere to solar wind stripping. The Moon (no field, no atmosphere) has its surface directly implanted with solar wind particles — Apollo missions confirmed lunar regolith enrichment with solar wind atomic nuclei.

CMEs — Large fast-moving plasma bursts that interrupt both slow and fast wind streams, causing geomagnetic storms. CMEs cause shock waves in heliospheric plasma, accelerating particles and preceding the main ejecta.

Co-rotating Interaction Regions (CIRs) — Where fast wind overtakes slower wind originating from the Sun's rotation, turbulent compressed regions form, creating wave motions and accelerated particles that affect Earth's magnetosphere more gently than CMEs.

Mass Loss

The Sun loses ~1.3–1.9 million tonnes per second to the solar wind — equivalent to one Earth mass every 150 million years. Since formation, the Sun has lost only ~0.01% of its initial mass this way. Other stars have far stronger stellar winds with much higher mass-loss rates.