Solar Physics
The Evershed Effect: The Radial Outflow in Sunspot Penumbrae
On January 5, 1909, at the Kodaikanal Observatory in southern India, John Evershed noticed that spectral lines from the striped outer skirt of a sunspot were tilted — Doppler-shifted by a couple of kilometres per second, gas visibly streaming outward across the solar surface. It was the first time anyone had measured a velocity field in a magnetized astrophysical plasma, and the flow now bears his name.
The Evershed effect is a nearly horizontal, radial outflow of gas through the penumbra — the filamentary grey collar surrounding a sunspot's dark umbra. Reaching 1–2 km/s in the photosphere and up to 6–8 km/s in the deepest, most horizontal channels, it is a signature of magnetoconvection under a strongly inclined magnetic field, and after more than a century it remains a benchmark test for models of how heat and mass move through concentrated solar magnetic fields.
- TypeRadial plasma outflow in sunspot penumbra
- DiscoveredJanuary 1909, John Evershed (Kodaikanal)
- Photospheric speed~1 km/s (inner) to ~2 km/s (mid-penumbra)
- Deep-channel speed6–8 km/s (supersonic, shock-forming)
- DriverOverturning magnetoconvection in inclined field
- Observed inPhotospheric penumbrae; inverted in chromosphere
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What the Evershed Effect Is
A sunspot has two zones: the dark central umbra, where a vertical magnetic field of 2,500–3,500 gauss chokes off convection and cools the gas to roughly 4,000 K, and the surrounding penumbra, a bright-and-dark filamentary ring where the field fans out to become nearly horizontal (inclination 70–90° from vertical) with strengths of 700–1,500 gauss.
The Evershed effect is the observation that gas flows outward along these penumbral filaments, roughly parallel to the tilted field. It reveals itself as a Doppler shift: spectral lines from the limb-side penumbra are redshifted (gas receding) and lines from the disk-centre side are blueshifted (gas approaching), the classic signature of a horizontal outflow seen in projection.
- The flow is almost purely radial and horizontal, confined to the penumbra.
- Speed rises from ~1 km/s at the umbra–penumbra border to a mid-penumbral peak, then decays to near zero at the outer edge.
- It is a steady, large-scale flow — not turbulence — carrying mass through a strong-field region.
The Mechanism: Magnetoconvection in an Inclined Field
Two families of models have competed to explain the flow. The older siphon-flow / flux-tube picture (Meyer & Schmidt 1968; Montesinos & Thomas 1997) treats the Evershed flow as gas driven along an arched magnetic flux tube by a gas-pressure difference between its two footpoints. Because magnetic pressure B²/8π and gas pressure must balance the surroundings, the footpoint with the stronger field has the lower gas pressure, and gas siphons toward it: roughly, ΔP_gas ≈ (B_outer² − B_inner²)/8π sets the driving.
The modern consensus, from high-resolution spectropolarimetry and radiative-MHD simulations (Scharmer & Spruit 2006; Rempel 2011), is that the Evershed flow is instead the horizontal, field-aligned component of overturning magnetoconvection. Hot plasma rises in the bright filament cores, is deflected outward by the near-horizontal field, cools as it travels, and plunges back down at the filament ends. The outward branch is the Evershed flow; the same convection carries the penumbra's heat and builds its filaments.
Key Quantities and a Worked Estimate
Characteristic penumbral numbers: filament width ~150–300 km, length up to ~5,000 km; field inclination ~70–90°; flow temperature slightly above the surrounding penumbra. Speeds span 1–2 km/s for the bulk photospheric flow and 6–8 km/s in the deepest, most horizontal channels — fast enough to exceed the local sound speed and form shocks where the gas dives back down.
Worked example — is the flow supersonic? The sound speed is c_s = √(γ k_B T / μ m_H). Take the penumbral photosphere at T ≈ 5,500 K, γ = 5/3, mean molecular weight μ ≈ 1.3 for partially ionized gas:
- c_s ≈ √(1.67 × 1.38e-23 × 5500 / (1.3 × 1.67e-27)) ≈ 7 km/s.
So the bulk 1–2 km/s flow is comfortably subsonic, but the 6–8 km/s deep channels reach or exceed c_s — consistent with the observed downflow shocks at filament ends and the redshifted transition-region emission (~5 km/s) seen above them.
How It Is Observed
The Evershed effect is fundamentally a spectroscopic measurement. Evershed used a spectrograph to detect the line shift; today's instruments add full Stokes polarimetry to recover velocity and magnetic vector together.
- Center-to-limb test: the outflow only produces a Doppler signal when the spot is away from disk centre; the limb-side/disk-centre red/blue asymmetry confirms the flow is horizontal, not vertical.
- Line asymmetries: the flow is strongest in the deep, dark filaments, so photospheric lines show characteristic asymmetric, sometimes multi-lobed profiles.
- Instruments: the Swedish 1-m Solar Telescope, Hinode's spectropolarimeter, GREGOR, the Dunn Solar Telescope, DKIST, and space UV spectrographs (IRIS) have mapped it from photosphere to transition region at sub-0.2-arcsecond (~150 km) resolution.
These data reveal the uncombed penumbra: nearly horizontal, flow-carrying filaments interleaved with a more vertical background field — the fine structure the flow both traces and helps create.
Related Flows and Regimes
Several sunspot flows are easily confused with the Evershed effect:
- Inverse Evershed flow: in the overlying chromosphere the direction reverses — gas flows inward along superpenumbral fibrils at 3.8–6 km/s, tilting ~20° downward as it approaches the spot and shocking as it lands. It is driven by pressure differences along field lines arching ~3 Mm high over ~13 Mm.
- Moat flow: a slower (~0.5–1 km/s) outflow outside the spot, across the annular moat. Simulations (Rempel 2015) suggest it has a largely independent origin — a mass-flux imbalance from suppressed downflows — rather than being the Evershed flow continued.
- Counter-Evershed flow: rare channels where gas runs inward within the penumbra itself, reproduced in MHD models as a transient convective mode.
Distinguishing these requires tracking both height (photosphere vs chromosphere) and location (inside vs outside the penumbral boundary).
Significance and Open Questions
The Evershed effect matters far beyond bookkeeping. It was the first velocity field ever measured in a cosmic magnetic structure, and it remains the sharpest observational probe of magnetoconvection — the same physics that governs starspots, accretion-disk coronae, and any place where strong fields throttle convection.
Yet a century on, real puzzles persist:
- How is the outflow's mass budget closed? The abrupt downflows at filament ends, some locally supersonic, are hard to fully reconcile with observations.
- Do siphon and convective drivers coexist, and in what proportion at different depths?
- Why does the flow reverse to the inverse Evershed pattern in the chromosphere, and what sets that transition height?
India honoured the discovery with a commemorative stamp in 2008. Modern facilities — DKIST's 4-metre aperture and MURaM-class simulations — are now resolving individual filament roots and downflow shocks, and are steadily closing the gap between the observed flow and a first-principles model of a sunspot penumbra.
| Flow | Layer / location | Direction | Typical speed |
|---|---|---|---|
| Evershed flow | Photospheric penumbra | Radially outward | 1–2 km/s (up to 6–8 in deep channels) |
| Inverse Evershed flow | Chromosphere / superpenumbra | Radially inward, tilted ~20° down | 3.8–6 km/s |
| Moat flow | Photosphere outside spot | Outward across moat cell | 0.5–1 km/s |
| Umbral/photospheric downflow | Outer penumbral filament ends | Downward into surface | few km/s (locally supersonic) |
| Counter-Evershed flow | Rare penumbral channels | Radially inward | ~1–2 km/s |
Frequently asked questions
Who discovered the Evershed effect and when?
British astronomer John Evershed discovered it in January 1909 at the Kodaikanal Observatory in India. He noticed that spectral lines from a sunspot's penumbra were Doppler-shifted, indicating gas flowing radially outward across the solar surface — the first velocity field ever measured in an astrophysical magnetic structure.
Which way does the Evershed flow go?
In the photosphere the flow is outward, running from the inner umbra–penumbra boundary toward the penumbra's outer edge, roughly along the near-horizontal magnetic filaments. In the overlying chromosphere the direction reverses into the 'inverse Evershed flow,' with gas streaming inward toward the sunspot.
How fast is the Evershed flow?
The bulk photospheric flow runs about 1 km/s at the inner border, peaks near 2 km/s in the mid-penumbra, and fades to nearly zero at the outer edge. In the deepest, most horizontal channels speeds reach 6–8 km/s, which is transonic or supersonic and produces shock fronts where the gas dives back down.
What causes the Evershed effect?
The leading explanation is overturning magnetoconvection in a strongly inclined magnetic field: hot gas rises in bright filaments, is bent outward by the near-horizontal field, cools, and sinks at the filament ends. An older siphon-flow model attributes it to gas-pressure differences between the two footpoints of an arched magnetic flux tube; both mechanisms may contribute.
What is the inverse Evershed flow?
It is the chromospheric counterpart: instead of flowing outward, gas flows inward along dark superpenumbral fibrils toward the sunspot at roughly 3.8–6 km/s, tilting about 20 degrees below horizontal as it nears the spot and shocking as it lands. It occurs higher in the atmosphere than the classic photospheric Evershed flow.
How is the Evershed flow different from the moat flow?
The Evershed flow is a magnetized outflow confined inside the penumbra at a few km/s. The moat flow is a slower (~0.5–1 km/s) outflow in the annular 'moat' just outside the sunspot. Simulations indicate the moat flow arises largely independently, from a mass-flux imbalance, rather than being the Evershed flow simply continuing beyond the spot.