Solar System
The Alfvén Critical Surface: Where the Sun Loses Grip on Its Wind
On 28 April 2021, at a distance of about 19.7 solar radii — roughly 13 million kilometres, or 0.09 AU — NASA's Parker Solar Probe flew below an invisible boundary no spacecraft had ever crossed and briefly entered the Sun's magnetically dominated atmosphere. That boundary is the Alfvén critical surface: the wrinkled, ragged shell around the Sun where the outflowing solar wind, still accelerating, finally matches the speed of the Alfvén waves that carry information along the magnetic field.
Inside it, the magnetic field is strong enough to dictate the plasma's motion, forcing the corona to co-rotate with the Sun and letting disturbances travel back down to the surface. Outside it, the wind wins: the flow becomes super-Alfvénic, the field is dragged outward into the Parker spiral, and the Sun permanently loses that parcel of plasma — and a share of its angular momentum — forever.
- RegimeBoundary between sub- and super-Alfvénic solar wind
- Key criterionAlfvén Mach number M_A = v_wind / v_A = 1
- Driven byWind acceleration outpacing the falling Alfvén speed
- Typical distance~10–20 R☉ (0.05–0.1 AU); PSP crossed at ~19.7 R☉
- First describedWeber & Davis, 1967 (angular-momentum loss)
- First crossedParker Solar Probe, 28 April 2021
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What the Alfvén critical surface is — and why it matters
The Sun does not have a sharp edge. Its million-kelvin corona streams outward as the solar wind, a supersonic plasma of protons and electrons flowing at 400–800 km s⁻¹. Threading that outflow is the Sun's magnetic field. Close in, the field is strong and the wind slow, so the field controls the plasma. Far out, the wind is fast and the field weak, so the plasma controls the field. The Alfvén critical surface is the boundary where these two influences balance exactly.
It matters because it is the true outer boundary of the solar atmosphere in a physical sense — the last place where the corona and Sun remain magnetically connected. Below it, plasma and field co-rotate and the corona behaves as an extension of the star; above it, each parcel of wind is severed from the Sun and cannot send information back. Crossing it, a spacecraft literally passes from the Sun's atmosphere into interplanetary space.
The physics, step by step: when the wind outruns its own waves
The controlling quantity is the Alfvén speed, v_A = B / √(μ₀ ρ), the speed at which magnetic tension propagates transverse (Alfvén) waves through a plasma of mass density ρ and field strength B. Near the Sun, B is large and the wind speed v is small, so v ≪ v_A: the flow is sub-Alfvénic. Any kink or wave in the field can travel back down toward the Sun, keeping the plasma magnetically tethered and co-rotating.
As the wind accelerates outward, v rises. Meanwhile ρ falls roughly as r⁻² and B falls even faster (as r⁻² for the radial field, faster still overall), so v_A drops. At some radius the two curves cross: v = v_A, the Alfvén Mach number M_A = v/v_A = 1. This is the Alfvén point. Beyond it the flow is super-Alfvénic: it moves faster than any wave can travel back, so the field can no longer hold the plasma or force co-rotation. The wind has escaped the Sun's magnetic grip.
Characteristic numbers, scales, and the governing criterion
The defining criterion is simply M_A = v / v_A = 1, where v_A = B/√(μ₀ρ). In practice the Alfvén surface sits at roughly 10–20 solar radii, or about 0.05–0.1 AU. It is not a smooth sphere: modeling and Parker Solar Probe data show a corrugated, ragged shell whose radius varies with solar longitude and the underlying magnetic structure, dipping inward under quiet coronal-hole flows and bulging outward under dense streamers.
Representative in-situ values near the crossing are B ≈ 100–400 nT (about 10⁻³ Gauss), proton densities of tens per cm³, wind speeds of a few hundred km s⁻¹, and plasma β (gas-to-magnetic pressure ratio) dropping below one. A related benchmark is the Alfvén radius r_A in angular-momentum theory: because plasma co-rotates out to roughly r_A, the Sun loses angular momentum with a large lever arm, which explains why the Sun has spun down over ~4.6 Gyr far more than simple mass loss alone would predict.
How it is detected: in-situ crossings and remote imaging
The Alfvén surface can be probed two ways. The direct method is in-situ measurement: NASA's Parker Solar Probe (launched 2018) carries the FIELDS magnetometer and the SWEAP particle instruments, which measure B, density and velocity simultaneously — everything needed to compute v_A and compare it to v. On 28 April 2021 PSP recorded sustained intervals with M_A < 1 at about 19.7 R☉, the first confirmed sub-Alfvénic encounters (Kasper et al., Physical Review Letters, 2021). Later perihelia found more crossings, some ragged and multiple within a single pass.
The indirect method is remote imaging plus modeling. White-light coronagraphs — SOHO/LASCO, STEREO, and the WISPR imager on PSP — track density features carried out by the wind; where those features stop showing inward-propagating disturbances marks the surface. Combined with MHD models (constrained by photospheric magnetograms from SDO/HMI), these techniques map a statistically averaged Alfvén surface across the solar cycle.
Where it operates, and how it differs from related boundaries
An Alfvén critical surface exists around any magnetized, wind-driving body, not just the Sun. Magnetized stars, T Tauri stars with accretion-powered winds, pulsars, and even accretion-disk winds all have one, and it governs their magnetic braking and spin-down. In stellar-rotation studies the Sun's example calibrates how field-anchored winds carry angular momentum away, shaping gyrochronology — dating stars by their spin.
It should not be confused with several nearby ideas. The sonic critical point (from Parker's 1958 wind solution) is where the wind first goes supersonic; it lies much closer in, around a few solar radii, and involves the sound speed, not the Alfvén speed. The heliopause is the far outer boundary (~120 AU) where the solar wind meets the interstellar medium. The Alfvén wing is a related but distinct structure formed when a body like a moon moves through a sub-Alfvénic flow. The Alfvén surface is uniquely defined by M_A = 1.
Open questions and broader significance
Even after PSP's crossings, the surface's true geometry is debated. Is it a single ragged shell, or does the corona possess multiple nested and disconnected sub-Alfvénic pockets? PSP found that a single orbit can dip in and out several times, hinting the boundary is fractal and dynamic, reshaped by streamers, coronal-hole boundaries and transient events. How coronal mass ejections and switchbacks distort it locally remains open.
The surface is also central to the coronal heating and wind-acceleration problem: most of the wind's acceleration and much of the corona's heating happen below it, so it brackets the region where turbulence, Alfvén-wave dissipation and magnetic reconnection deposit their energy. Pinning down where and how the wind crosses M_A = 1 constrains which heating mechanism dominates. And because the crossing sets the Sun's angular-momentum-loss lever arm, understanding it refines models of stellar spin-down across the main sequence, feeding directly into how we age Sun-like stars.
| Property | Below the surface (sub-Alfvénic) | Above the surface (super-Alfvénic) |
|---|---|---|
| Alfvén Mach number M_A | < 1 (flow slower than Alfvén speed) | > 1 (flow faster than Alfvén speed) |
| Who controls the motion | Magnetic field dominates the plasma | Plasma inertia drags the field |
| Rotation | Near co-rotation with the Sun | Angular momentum decoupled and lost |
| Information flow | Alfvén waves can travel back sunward | Waves swept outward; no sunward signal |
| Magnetic field geometry | Largely radial, magnetically 'stiff' | Wound into the Parker spiral |
| Distance from Sun | ≲ 10–20 R☉ | Beyond ~20 R☉ out to the heliopause |
Frequently asked questions
What exactly defines the Alfvén critical surface?
It is the surface around the Sun where the radial speed of the outflowing solar wind equals the local Alfvén speed, v_A = B/√(μ₀ρ). Equivalently, it is where the Alfvén Mach number M_A = v/v_A equals 1. Inside it the flow is sub-Alfvénic and magnetically controlled; outside it the flow is super-Alfvénic and inertia-dominated.
When and where did Parker Solar Probe first cross it?
On 28 April 2021, during its eighth solar encounter, at roughly 19.7 solar radii (about 0.09 AU, or 13 million km). Its FIELDS and SWEAP instruments recorded sustained intervals with M_A below 1 — the first time a spacecraft was confirmed to be inside the Sun's magnetically dominated corona. The result was published by Kasper and colleagues in Physical Review Letters in 2021.
Why does the Alfvén surface control the Sun's rotation and angular-momentum loss?
Below the surface the magnetic field is stiff enough to force the plasma to co-rotate with the Sun, so escaping wind is flung outward with the Sun's angular velocity out to a large lever arm (the Alfvén radius). This carries away far more angular momentum per unit mass than the plasma itself would, which is why the Sun and Sun-like stars spin down substantially over billions of years — the basis of magnetic braking and gyrochronology.
How is the Alfvén surface different from the sonic critical point?
They are two distinct critical points of the solar wind. The sonic (Parker) critical point is where the wind first exceeds the sound speed, and it sits close in, at a few solar radii. The Alfvén surface is farther out (10–20 R☉) and is defined by the wind exceeding the Alfvén speed. The wind is already supersonic long before it becomes super-Alfvénic.
Is the Alfvén surface a smooth sphere?
No. Both MHD models and PSP measurements show it is a wrinkled, corrugated shell whose distance from the Sun varies with longitude and with the underlying coronal magnetic structure — closer under fast coronal-hole wind, farther out under dense streamers. A single spacecraft pass can cross it multiple times, suggesting a ragged, possibly fractal boundary rather than a clean surface.
Do other stars and objects have an Alfvén surface?
Yes. Any magnetized body that drives a wind — magnetized main-sequence stars, T Tauri stars, pulsars, and even magnetized accretion-disk winds — has an Alfvén critical surface that sets how efficiently it sheds angular momentum. The Sun's well-measured case serves as the calibration point for modeling magnetic braking and rotational evolution across the stellar population.