Solar Wind

Full definition

The Sun is not a static fireball radiating only light: it constantly 'blows' plasma at all latitudes. That flow is the solar wind. It's composed of charged particles — protons (≈ 95 %), electrons, helium nuclei (≈ 4 % α), plus traces of heavier elements (carbon, oxygen, iron). At 1 AU (Earth's orbit), these particles pass by at typical speeds of 400-600 km/s, with densities of 3-10 particles per cm³ and temperatures around 100,000 K.

The physical origin is the solar corona, that extremely hot envelope (1-3 million kelvin) surrounding the Sun above the photosphere. Famous paradox: the corona is 200 times hotter than the visible surface (5,770 K), and we still don't fully understand why (magnetohydrodynamic waves? nanoflare reconnections?). This enormous heat makes the corona partly evaporative: the fastest particles reach solar escape velocity (618 km/s at the surface) and leave. That's the solar wind as Parker modelled it in 1958.

The solar wind sculpts the entire Solar System's architecture. It carves a protective bubble around the Sun — the heliosphere — pushing back the interstellar medium to the heliopause, at ~120 AU. It forms comet ion tails (always pointing away from the Sun, whatever the comet's direction). It sweeps dust from protoplanetary discs. And crucially, it interacts with every planet's magnetosphere, producing polar auroras, radiation belts, geomagnetic storms.

On Earth, the magnetic field protects us: solar wind particles are deflected around the magnetosphere and only a few penetrate via the poles, creating the boreal and austral auroras (at 100-300 km altitude). But when the Sun erupts violently (CME, coronal mass ejection), billions of tons of plasma can strike Earth and disrupt satellites, the power grid, and radio communications.

Values, formulas, orders of magnitude

Typical solar wind parameters at 1 AU (Earth's orbit):

• Speed: 300-900 km/s (average ≈ 400 km/s) • Particle density: n ≈ 3-10 cm⁻³ • Proton temperature: T_p ≈ 10⁵ K • Electron temperature: T_e ≈ 1-2 × 10⁵ K • Interplanetary magnetic field (IMF): B ≈ 5-10 nT at 1 AU • Total mass loss rate from the Sun: ≈ 1.3-1.9 × 10⁹ kg/s (≈ 10⁻¹⁴ M☉/year) • Sun-to-Earth travel time: 2 to 6 days depending on speed

Kinetic energy of a particle:

E_k = (1/2) m v²

For a 500 km/s proton: E_k ≈ 1.3 keV. For a 2,000 km/s CME proton: E_k ≈ 21 keV.

Total thermal/kinetic power: ≈ 2-3 × 10²⁰ W, about 10⁻⁶ of the Sun's total luminosity (3.828 × 10²⁶ W).

Total mass lost over 4.6 Gyr: ≈ 2 × 10²⁶ kg ≈ 10⁻⁴ M☉ — negligible on solar-mass scales (2 × 10³⁰ kg).

For comparison: massive O or B stars lose up to 10⁻⁶-10⁻⁵ M☉/year via stellar winds, 10⁷-10⁸ times more than the Sun. Wolf-Rayet stars can shed half their mass over their lifetime.

The different regimes

The solar wind is not uniform: several regimes coexist.

Slow solar wind. 300-500 km/s, denser (5-10 cm⁻³), warmer in ionic composition (traces of heavy elements), emerging from equatorial regions and the coronal 'streamer belt'. Irregular, turbulent, rich in structure. About 50 % of total volume flux.

Fast solar wind. 600-900 km/s, less dense (~3 cm⁻³), cooler ionic composition. Emerges from 'coronal holes' (regions with magnetic field lines open to space). During solar minimum, polar coronal holes dominate and fast wind fills high latitudes; at maximum, the geometry grows complicated.

Coronal mass ejection (CME). Abrupt eruption of coronal plasma, typically linked to a solar flare and magnetic reconnection. A CME can launch 10¹²-10¹³ kg of plasma at 500-3,000 km/s. About 1 to 5 CMEs per day on average (varying with the 11-year solar cycle). When a CME hits Earth, it triggers a geomagnetic storm — sometimes major (e.g. Carrington event of September 1859, or Halloween 2003).

Solar Energetic Particles (SEP). Protons and ions accelerated by flare- and CME-associated shocks, reaching energies of 10 MeV to 10 GeV. Dangerous for astronauts outside the magnetosphere (Apollo, future lunar or martian missions) and for satellite electronics.

Solar cycle. Solar wind activity varies on an 11-year cycle (Schwabe cycle). The current cycle 25 began in December 2019 and its maximum was expected in 2024-2026 — we're in it right now. A full magnetic polarity reversal occurs every 22 years (Hale cycle).

How do we observe/measure it?

The solar wind is probed by a fleet of dedicated missions.

In-situ measurements (since 1959). Luna 1 (USSR, January 1959) detected the first flows; Mariner 2 (NASA, August 1962, Venus flyby) confirmed Parker's theory over 4 months of measurements. Since then, dozens of probes measure density, speed, temperature and magnetic field directly: Ulysses (ESA/NASA, 1990-2009, polar passes), ACE (NASA, 1997-, L1 point), Wind (NASA, 1994-), Cluster (ESA, 2000-, magnetospheric study), STEREO A and B (NASA, 2006-).

Parker Solar Probe (NASA, launched August 2018). Dives into the solar corona, passing within 9.86 R☉ (≈ 6.9 million km) of the Sun since December 2024. First spacecraft to 'touch' the corona. Already revealed magnetic 'switchbacks' and pinned down the origin of the slow wind.

Solar Orbiter (ESA/NASA, launched February 2020). Complementary mission, orbit at 0.28 AU, observes the Sun in UV/X-ray imaging and samples plasma in-situ. First instrument to film the solar poles in 2025-2026.

SOHO (ESA/NASA, 1995-). At L1, continuously watches the Sun. More than 5,000 comets serendipitously discovered via its LASCO camera (2024), mostly Kreutz sungrazers.

Helioseismology and imaging. Ground-based GONG and space-based SDO/HMI map the photospheric magnetic field to predict active regions and coronal holes.

Operational prediction. NOAA SWPC (US) and the ESA Space Weather Service Network issue alerts when a CME threatens Earth (warning time: 15-60 min via L1 → Earth). Our space weather tool displays the Kp index and real-time solar wind flux with auroral alerts.

What about amateurs? You don't 'see' the solar wind directly, but its manifestations: polar auroras (visible from Kp 5 at ≈ 55° N), HF radio disturbances, comet haloes. A Lapland or Iceland aurora hunt during the cycle 25 peak is an excellent alibi.

Not to be confused with

Several solar phenomena coexist.

Solar wind vs solar radiation. Radiation is electromagnetic (light, UV, X), travels at c and reaches us in 8 min 20 s. Solar wind is plasma (massive particles), travels at 400-900 km/s and reaches us in 2-6 days. Very different effects: radiation warms; wind erodes and electrically charges.

Solar wind vs solar flare. A flare is a brief, violent electromagnetic event (UV/X/radio burst) caused by coronal magnetic reconnection. It may or may not be accompanied by a CME. The solar wind is a continuous flow — flares are gusts.

Solar wind vs coronal mass ejection (CME). A CME is a gust of solar wind, typically 10-1,000 times denser, 2-5 times faster. CMEs drive severe geomagnetic storms (Carrington 1859, Halloween 2003, Gannon event of May 2024). The 'ordinary' solar wind only causes low-intensity storms.

Solar wind vs cosmic rays. Cosmic rays are ultra-high-energy particles (GeV to ZeV) coming from across the Universe (supernovae, active galactic nuclei, etc.), not specifically the Sun. The solar wind, at just 1-10 keV, modulates them but does not resemble them.

Solar wind vs Parker radiation. The 'Parker spiral' is the shape taken by the interplanetary magnetic field (IMF) due to solar rotation — not a form of radiation. Field lines dragged by the solar wind form a 'lawn-sprinkler' spiral.