Solar Wind

Solar wind is the continuous, transonically accelerated outflow of ionized plasma and embedded magnetic flux expanding radially from the corona of the Sun throughout the Solar System. Composed of protons (~95% by number), alpha particles (He²⁺, typically 3% to 5%), free electrons, and trace heavy ions in high charge states (such as O⁶⁺, O⁷⁺, C⁵⁺, and Fe⁹⁺ to Fe¹⁴⁺), the solar wind carries away ~1.3 × 10⁹ to 2.0 × 10⁹ kg of solar mass per second (~2 × 10⁻¹⁴ M_☉ yr⁻¹). Although this mass loss represents less than 0.01% of the Sun's mass over its main-sequence lifetime, the magnetized outflow exerts an electromagnetic torque that has spun down solar rotation since the Sun's youth and inflates the 120-AU heliosphere within the local interstellar medium.

In 1951, German astronomer Ludwig Biermann deduced from the antisolar orientation of comet ion tails that the Sun emits a continuous corpuscular flow far stronger than radiation pressure. In 1958, American astrophysicist Eugene N. Parker formulated the hydrodynamic theory of this outflow, coining the term solar wind and proving that a million-kelvin corona cannot remain in static equilibrium. Parker's prediction was confirmed in situ by the Soviet Luna 1 probe in January 1959 and measured continuously by Marcia Neugebauer and Conway Snyder aboard NASA's Mariner 2 during its 1962 cruise to Venus.

Hydrostatic Breakdown and the Sonic Transition Point

In a spherically symmetric solar corona at T ≈ 1.5 × 10⁶ K, high electron thermal conductivity (κ ∝ T^5/2) maintains an extended temperature profile (T(r) ∝ r^−2/7). Integrating the hydrostatic momentum equation shows that the asymptotic coronal gas pressure at infinity (r → ∞) remains finite (~10⁻⁴ Pa)—roughly eight orders of magnitude larger than the confining pressure of the local interstellar medium (~10⁻¹³ Pa). Consequently, the corona cannot be confined statically and must expand outward.

Combining steady-state radial mass conservation (4πr²ρv = constant) and momentum balance for an isothermal plasma of sound speed c_s = (2k_BT / m_p)^1/2 yields Parker's momentum equation:

(v² − c_s²) (1 / v) (dv / dr) = (2c_s² / r) − (GM_☉ / r²)

This equation possesses a transonic singularity at the sonic radius (r_c), where the outflow speed equals the local sound speed (v = c_s ≈ 150–180 km s⁻¹) and the right-hand side vanishes:

r_c = GM_☉ / (2c_s²)

For coronal temperatures of 1.0 × 10⁶ to 2.0 × 10⁶ K, r_c lies at 3.5 to 6.0 R_☉. Only the transonic Parker wind solution—accelerating from subsonic speeds (v < c_s) in the low corona through r_c (dv/dr > 0) to supersonic speeds (v > c_s) beyond r_c while pressure vanishes at infinity—satisfies both solar and interstellar boundary conditions. Additional momentum and heating from magnetohydrodynamic Alfvén waves, turbulent dissipation, and ambipolar electric fields accelerate the fast wind up to 750–800 km s⁻¹.

Fast versus Slow Solar Wind and Corotating Interaction Regions

Out-of-ecliptic polar passes by the ESA/NASA Ulysses spacecraft (1990–2009) demonstrated that the ambient solar wind is organized into two bimodal states:

• Fast Solar Wind (~650–800 km s⁻¹): Originates inside dark, unipolar coronal holes along open magnetic field lines. Fast wind is steady, tenuous (n_p ≈ 2.5–3 cm⁻³ at 1 AU), exhibits low heavy-ion freeze-in temperatures (O⁷⁺/O⁶⁺ < 0.05), and has near-photospheric elemental composition.
• Slow Solar Wind (~300–450 km s⁻¹): Emerges near the helmet streamer belt, pseudostreamers, and active-region boundaries. Slow wind is denser, enriched by a factor of 2 to 4 in elements with a low first ionization potential (FIP < 10 eV, such as Fe, Mg, and Si), and exhibits higher ionization states indicative of interchange reconnection between closed coronal loops and open flux.

When a fast solar wind stream from a low-latitude coronal hole overtakes slower upstream wind along the same radial trajectory as the Sun rotates, the collisionless plasmas cannot interpenetrate across their frozen-in magnetic fields, forming a compressed corotating interaction region (CIR) that recurs every 27-day Carrington rotation.

Frozen-In Flux and the Archimedean Parker Spiral

Because high electrical conductivity yields a magnetic Reynolds number >10¹², magnetic flux is frozen into the expanding solar wind per Alfvén's theorem. While a plasma parcel travels radially outward at speed v_sw, its magnetic footpoint rotates with the solar corona at angular velocity Ω_☉ ≈ 2.7 × 10⁻⁶ rad s⁻¹, winding the interplanetary magnetic field into an Archimedean Parker spiral:

r − r₀ = −(v_sw / Ω_☉) (φ − φ₀)

By magnetic flux conservation (∇ · B = 0), the radial component decays as B_r(r) ∝ r⁻², whereas the azimuthal component decays as B_φ(r) = −B_r(r) (Ω_☉ r sin θ / v_sw) ∝ r⁻¹. At 1 astronomical unit (AU), the spiral "garden-hose" angle tan ψ = |B_φ / B_r| is ~45° (B ≈ 5 nT), whereas at Jupiter (5.2 AU) and Saturn (9.5 AU) the field wraps nearly azimuthally (ψ > 80°).

Parker Solar Probe, the Alfvén Surface, and Switchbacks

The boundary where the radial solar wind speed first exceeds the local Alfvén wave speed (v_A = B / [μ₀ρ]^1/2, where Alfvén Mach number M_A = v_sw / v_A = 1) is the Alfvén surface, dividing the magnetically dominated solar corona from the interplanetary wind. On April 28, 2021, NASA's Parker Solar Probe crossed below the Alfvén surface at 18.8 R_☉ (M_A ≈ 0.7–0.85, B ≈ 100 nT). Its inner passes down to 9.86 R_☉ revealed ubiquitous S-shaped radial magnetic field reversals called switchbacks—Alfvénic velocity and magnetic kinks organized on ~30,000-km supergranular scales by interchange reconnection.

Magnetospheric Deflection versus Atmospheric Sputtering

Planetary interactions with the supersonic solar wind fall into three regimes:

1. Intrinsic Magnetospheres (Mercury, Earth, Jupiter, Saturn): Planetary dipole fields stand off the solar wind behind a collisionless bow shock and magnetopause, coupling energy into the magnetotail via dayside magnetic reconnection during southward interplanetary fields and coronal mass ejections or solar flares.
2. Unmagnetized Atmospheres (Venus and Mars): Solar wind fields induce currents in the dayside ionosphere, forming an induced magnetosphere while pickup ion scavenging and sputtering strip upper-atmospheric volatiles over geologic time.
3. Airless Bodies (the Moon and Asteroids): Solar wind protons strike bare silicate regolith directly, reducing Fe²⁺ into nanophase metallic iron (npFe⁰) during space weathering.

How fast does the solar wind travel from the Sun to Earth?

Across 1 AU (149.6 million km), a slow solar wind stream moving at 400 km s⁻¹ takes ~4.3 days (104 hours) to reach Earth, a fast solar wind stream at 750 km s⁻¹ arrives in ~2.3 days (55 hours), and fast interplanetary coronal mass ejections (>2,000 km s⁻¹) can transit 1 AU in 15 to 20 hours.

Why doesn't the solar wind strip away Earth's atmosphere like it did on Mars?

Earth's liquid outer-core geodynamo sustains a global magnetosphere extending ~10 Earth radii (~64,000 km) sunward, deflecting solar wind protons around the upper atmosphere. Mars lost its core dynamo ~4.0–4.1 Ga ago, exposing its exosphere directly to solar wind pickup ion stripping.

What are the magnetic "switchbacks" discovered by Parker Solar Probe?

Switchbacks are abrupt S-shaped folds in the near-Sun magnetic field where the radial field vector flips by 90° to nearly 180° for seconds to tens of minutes alongside a spike in outward plasma velocity, generated where open and closed magnetic loops reconnect above photospheric supergranules.

1. Parker, E. N. (1958). "Dynamics of the interplanetary gas and magnetic fields." The Astrophysical Journal, 128, 664–676. https://doi.org/10.1086/146579
2. Neugebauer, M., & Snyder, C. W. (1966). "Mariner 2 observations of the solar wind: 1. Average properties." Journal of Geophysical Research, 71(19), 4469–4484. https://doi.org/10.1029/JZ071i019p04469
3. McComas, D. J., Elliott, H. A., Schwadron, N. A., et al. (2003). "The three-dimensional solar wind around solar maximum." Geophysical Research Letters, 30(10), 1517. https://doi.org/10.1029/2003GL017136
4. Bale, S. D., Badman, S. T., Bonnell, J. W., et al. (2019). "Highly structured slow solar wind emerging from an equatorial coronal hole." Nature, 576(7786), 237–242. https://doi.org/10.1038/s41586-019-1818-7
5. Kasper, J. C., Klein, K. G., Lichko, E., et al. (2021). "Parker Solar Probe enters the magnetically dominated solar corona." Physical Review Letters, 127(25), 255101. https://doi.org/10.1103/PhysRevLett.127.255101