Solar Atmosphere
Chromosphere
The Sun's red middle atmosphere — 2000 km thick, 4000 K at its base, 25000 K at its top
The chromosphere is the Sun's middle atmospheric layer, sandwiched between the 5778 K photosphere below and the million-K corona above. It rises from 4000 K to 25000 K across about 2000–10000 km, glows red in Hα, and appears as a crimson rim during eclipse totality.
- Height range2000–10000 km above photosphere
- Temperature4000 K (base) → 25000 K (top)
- Density10⁻⁴ to 10⁻⁸ kg/m³ (drops 10000× with height)
- Color (Hα)Crimson red — 6562.8 Å hydrogen line
- Visible duringTotal eclipses (flash spectrum); via Hα filter anytime
- Pierced by~10⁶ spicules at any moment, each ~5 min lifetime
Interactive visualization
A cross-section of the solar limb, showing the photosphere, chromosphere, and corona in their characteristic colors.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Where the chromosphere sits
The Sun's atmosphere is layered. From bottom to top:
- Photosphere — ~300 km thick, 5778 K. The visible surface, dominated by granulation cells and the occasional sunspot. Optical depth at 5000 Å is about 1 here.
- Chromosphere — ~2000–10000 km thick, 4000–25000 K. Dominated by neutral hydrogen and Ca II emission, pierced everywhere by spicules.
- Transition region — ~100 km thick, 25000–10⁶ K. A very thin layer where the plasma becomes fully ionized and emission lines of C, N, O, Si dominate.
- Corona — extends millions of km, 1–3 × 10⁶ K. Highly ionized iron and other heavy elements emit in EUV and X-ray.
The chromosphere's name comes from the Greek chroma = "color" — coined by Norman Lockyer in the 1860s. From a ground-based eclipse view, the chromosphere flashes pink-red at second contact (just before totality) and again at third contact (just after) as the lunar limb sweeps past the deep chromospheric layers, briefly revealing them above the now-blocked photosphere. This is the famous "flash spectrum," first systematically observed by Charles Young in 1870.
Temperature structure — the inversion begins
The semi-empirical VAL-C model (Vernazza, Avrett & Loeser 1981) and its successor FAL-C provide the canonical 1D average temperature profile through the quiet-Sun chromosphere:
| Height above photosphere (km) | Temperature (K) | Electron density (m⁻³) | Dominant emission |
|---|---|---|---|
| 0 (photospheric τ = 1) | 6420 | 1.5 × 10²⁰ | Continuum |
| ~500 (T minimum) | 4400 | 2 × 10¹⁸ | CO infrared bands |
| ~1000 (low chromosphere) | 6000 | 2 × 10¹⁷ | Ca II H/K wings |
| ~1800 (mid chromosphere) | 7000 | 4 × 10¹⁶ | Hα, Ca II K core |
| ~2200 (upper chromosphere) | 9000 | 2 × 10¹⁶ | Hα core, Lyα wings |
| ~2500 (chromospheric plateau) | ~25000 | 3 × 10¹⁵ | Lyα core, He II 304 Å |
| ~2600 (transition region base) | ~10⁵ | 10¹⁴ | C IV, Si IV, O VI |
The "plateau" at the top of the chromosphere — the few hundred km where temperature stays roughly at 20000–25000 K before exploding into the transition region — is a key diagnostic. It is the height range where Lyα (1216 Å) is optically thick and where the cooling rate of hydrogen by Lyα emission balances the mechanical heating rate. This plateau is universal: it appears in every star with a chromosphere, including K and M dwarfs.
Why specific emission lines
The chromosphere's spectrum is dominated by a few strong lines because of the layer's temperature and density. Three lines deserve attention:
- Hα at 6562.8 Å. The Balmer-α line — the n=3 to n=2 transition of neutral hydrogen. Strong in the upper chromosphere where T ≈ 8000–10000 K populates the n=3 level. Optically thick across the chromosphere; the line core forms at 1500–2000 km above the photosphere, the wings at the temperature minimum.
- Ca II H and K at 3933.7 and 3968.5 Å. Resonance lines of singly ionized calcium. Strong in the mid-chromosphere; their cores form at ~1000–1500 km. The line widths and asymmetries are sensitive to chromospheric oscillations (especially the 3-minute oscillations) and to magnetic field strength via the Zeeman effect.
- Lyman-α at 1216 Å. The n=2 to n=1 transition of neutral hydrogen. The brightest line in the solar spectrum (UV); forms at the top of the chromosphere where T ≈ 20000 K. The Lyα plateau anchors the chromosphere-to-corona transition. Important for Earth's upper atmosphere because Lyα ionizes the D-region of the ionosphere.
Dynamics — a non-uniform, jet-pierced layer
The 1D VAL-C model is a useful reference but the real chromosphere is highly inhomogeneous and very dynamic. Three populations of features dominate:
- Spicules — about 10⁶ at any moment, each ~500 km wide and 6000–10000 km tall, launched at 20–50 km/s with lifetimes of ~5 minutes. They cluster along the chromospheric network at the edges of supergranulation cells (Beckers 1968). Visible at the limb as a "burning prairie" of hair-like jets.
- Type-II spicules — De Pontieu et al. (2007) on Hinode/SOT. Faster (50–150 km/s), shorter-lived (less than 100 s), and contain hotter plasma (some reach 10⁵ K) than type-I spicules. Proposed as a coronal heating channel.
- Fibrils — Hα-dark structures along magnetic field lines in active regions. They trace the chromospheric magnetic geometry. In sunspot canopies they form the "superpenumbra."
- Plages — bright, magnetic chromospheric regions in active regions. Higher density and emission than quiet-Sun chromosphere; visible in Hα and Ca II K. Photospheric magnetic field strength of 100–1500 Gauss.
Worked example: chromospheric scale height
How thick should the chromosphere be from simple hydrostatic balance? In an isothermal atmosphere with mean molecular weight μ, the pressure scale height is:
H = k_B · T / (μ · m_H · g_⊙)
k_B = 1.381 × 10⁻²³ J/K (Boltzmann)
m_H = 1.673 × 10⁻²⁷ kg (hydrogen mass)
g_⊙ = 274 m/s² (solar surface gravity)
μ ≈ 1.3 (mean weight, neutral H + He)For the photospheric base at T = 6000 K:
H = (1.381×10⁻²³ × 6000) / (1.3 × 1.673×10⁻²⁷ × 274)
= 8.29×10⁻²⁰ / 5.96×10⁻²⁵
≈ 1.39 × 10⁵ m
≈ 140 kmFor the upper chromosphere at T = 20000 K:
H = (1.381×10⁻²³ × 20000) / (1.3 × 1.673×10⁻²⁷ × 274)
≈ 460 kmSo you'd expect about 5 hydrostatic scale heights — roughly 2000 km — between τ = 1 in the continuum and the top of the chromosphere. This matches the observed thickness in 1D models. The fact that real spicules reach 6000–10000 km — well above the hydrostatic scale heights — shows that mechanical forces (waves and magnetic acceleration) dominate over gravity in the upper chromosphere. The hydrostatic estimate sets the "quiescent" thickness; spicules and plages add much more.
Three solar atmospheric layers at a glance
| Property | Photosphere | Chromosphere | Corona |
|---|---|---|---|
| Thickness | ~300 km | ~2000–10000 km | Millions of km |
| Temperature | 5778 K (uniform) | 4000 → 25000 K (rising) | 1–3 × 10⁶ K |
| Density | ~10⁻⁴ kg/m³ | 10⁻⁴ to 10⁻⁸ kg/m³ | 10⁻¹⁵ kg/m³ |
| Dominant ionization | Neutral atoms | Partially ionized hydrogen | Fully ionized; iron stripped of 13+ electrons |
| Visible color | Yellow-white continuum | Crimson red (Hα) | Pearly white (Thomson scattering) |
| Best ground observation | White-light continuum | Hα filter, Ca II K filter | Coronagraph or eclipse |
| Best space mission | SDO/HMI | IRIS | SDO/AIA, Hinode/EIS |
| Why the temperature is what it is | Radiative equilibrium with interior | Acoustic shocks + magnetic dissipation | Coronal heating problem (unsolved) |
Missions that mapped the chromosphere
- Skylab (1973–1979). First sustained UV observations of the chromosphere from space; established Lyα as a primary chromospheric diagnostic.
- SOHO (1995–). SUMER and CDS spectrographs measured chromosphere and transition region in EUV; provided much of the modern temperature structure.
- TRACE (1998–2010). High-resolution EUV imaging of the chromosphere-corona interface.
- Hinode (2006–). SOT, EIS, and XRT instruments — discovered type-II spicules (De Pontieu 2007) and provided high-resolution Ca II H imaging at the limb.
- SDO (2010–). AIA channel at 304 Å (He II) is the workhorse chromosphere imager. 1700 Å channel reaches the temperature minimum.
- IRIS (2013–). Mission designed entirely for the chromosphere and transition region. Mg II h/k 2796 Å and 2803 Å spectroscopy with sub-arcsecond spatial resolution and 1-second cadence. The current gold standard for chromospheric diagnostics.
- DKIST (2020–). 4-m Daniel K. Inouye Solar Telescope on Haleakala — ground-based but with 0.025-arcsec resolution. Resolves chromospheric features down to 20 km. First images of the chromospheric magnetic field at sub-arcsec scale.
Variants and related concepts
- Stellar chromospheres. Detected through Ca II H and K core emission (the Wilson-Bappu effect) in nearly every late-type star. Activity proxies like Mount Wilson S-index quantify chromospheric strength across stellar populations.
- Magnetic chromosphere. Sub-arcsec ALMA imaging in millimeter continuum has mapped magnetic flux tubes in the chromosphere directly. The DKIST and ALMA combination is currently the most precise chromospheric magnetic mapper.
- Acoustic oscillations. The 3-minute and 5-minute chromospheric oscillations are p-mode signatures that propagate up from the photosphere. Their phase and amplitude relations with photospheric Doppler signals are basic chromospheric diagnostics.
- Chromospheric heating rate. Quantitatively measured at 10⁴ W/m² (10⁷ erg/cm²/s) in the quiet Sun and 10⁵ W/m² in active plages. Direct comparisons with theoretical heating models (acoustic shocks, magnetic reconnection) underconstrain the answer.
Common misconceptions
- "The chromosphere is only visible during eclipse." Only visible to the naked eye during eclipse, but with an Hα filter on any small telescope, it's visible anytime.
- "Red color means red plasma." The chromosphere isn't a red layer of gas — it emits specifically at 6562.8 Å because of the Hα transition. The "color" is a diagnostic, not a property.
- "It's hotter at the bottom." Inverted. The temperature minimum is at the base (~4000 K). It rises to ~25000 K at the top.
- "The boundaries between layers are sharp." No. Spicules and other dynamic features blur the boundaries badly. The 1D averaging that gives nice numbered layers is a useful fiction.
- "Only the Sun has a chromosphere." Every late-type star with a convective envelope has one. Detected via Ca II H/K emission in essentially every G, K, and M dwarf observed at high enough spectral resolution.
- "Chromospheric Hα emission means hot hydrogen." Hα is at 6563 Å, in the visible. Hot plasma alone wouldn't emit there — you need the specific n=3 → n=2 transition of neutral hydrogen, which requires conditions where hydrogen is mostly neutral but the n=3 level is collisionally excited. The chromosphere is the unique part of the Sun where this works.
Frequently asked questions
Why is the chromosphere red?
It glows in Hα — the n=3→n=2 transition of neutral hydrogen at 6562.8 Å, in the deep red part of the visible spectrum. Hydrogen is by far the most abundant element in the Sun, and chromospheric conditions (T ≈ 8000–10000 K, electron density ~10¹⁷ m⁻³) populate the n=3 level enough that the n=3→n=2 transition becomes optically thick. The line is wide (about 1 Å FWHM) and strong, dominating the visible-light emission of the chromosphere. Photospheric continuum is much brighter than chromospheric Hα across most wavelengths, so the chromosphere is normally invisible — except during a total eclipse, when the photosphere is blocked and the Hα-bright chromosphere stands out as a crimson rim. Modern Hα filters with passbands of about 0.5 Å reproduce this view at any time.
Why does the temperature rise here, not fall?
This is the coronal heating problem in miniature. Below the chromosphere, temperature falls monotonically with height from the radiative zone to the photospheric temperature minimum of ~4000 K. Then the trend reverses: across the chromosphere, T rises from 4000 K to ~25000 K; across the transition region, it explodes to 10⁶ K. The chromosphere is heated by some non-radiative mechanism — acoustic waves shocking in the upper chromosphere, magneto-acoustic waves on field lines, and small-scale reconnection. The exact balance is debated but the gross energy budget is well-measured: chromospheric radiative losses are about 10⁷ erg/cm²/s (10⁴ W/m²) in quiet regions and ten times higher in active plages, with the energy coming from mechanical heating launched by photospheric convection.
How thick is the chromosphere?
Mean thickness is about 2000 km from the temperature minimum to the transition region — small compared to the Sun's 696,000 km radius. But the chromosphere is highly inhomogeneous: spicules, type-II spicules, and fibrils launch chromospheric plasma to 6000–10000 km above the photosphere on minute timescales. The 'effective' top of the chromosphere — where transition-region emission lines like C IV peak — is closer to 5000–10000 km than 2000 km. Three-dimensional models of the chromosphere are now standard; the older 1D semi-empirical models (VAL-C, FAL) are still used as reference but understood to be averages over a much more complex reality.
How is the chromosphere observed?
Ground-based: Hα filters (Daystar, Lunt, Coronado) with 0.5–0.7 Å passbands let amateur telescopes show prominences, filaments, and active plages. Professional ground-based: the Swedish 1-m Solar Telescope, the GREGOR telescope, and the new Daniel K. Inouye Solar Telescope (DKIST, first light 2020) resolve features down to 20 km on the solar surface. Space-based: SOHO's CDS and EIT, TRACE, Hinode (especially the SOT and EIS instruments), SDO/AIA channels at 304 Å (He II) and 1700 Å (continuum near the temperature minimum), and IRIS (Interface Region Imaging Spectrograph) — designed specifically for the chromosphere and transition region. During total eclipses, the chromosphere is visible to the naked eye for a few seconds at second and third contact ('flash spectrum').
What are spicules and why are they here?
Spicules are thin (~500 km wide), elongated jets of chromospheric plasma launched upward at 20–50 km/s, reaching heights of 6000–10000 km before falling back or evaporating. There are around a million spicules on the Sun at any moment, lasting about 5 minutes each. They cover the chromospheric network — the boundaries of supergranulation cells — and dominate the morphology of the upper chromosphere. Type-II spicules, identified by De Pontieu et al. (2007) on Hinode/SOT, are faster (50–150 km/s) and shorter-lived (under 100 seconds); some contain transition-region plasma at 10⁵ K, suggesting they connect chromospheric matter to the corona. Whether spicules contribute significantly to coronal heating is an active question.
What lives just above and below the chromosphere?
Below: the photosphere — the visible 'surface' at 5778 K, ~300 km thick, made of granulation cells and sunspots. Above: the transition region, a thin (~100 km) layer where temperature rises from 25000 K to over 10⁶ K, followed by the corona itself extending millions of km. The transition region is where collisional excitation gives way to coronal forbidden emission, where neutral and singly ionized atoms give way to highly stripped ions, and where the magnetic field becomes the dominant dynamical agent. Modern thinking treats the photosphere, chromosphere, transition region, and corona as a coupled magnetic system rather than discrete layers — but for descriptive purposes the boundaries by temperature remain useful.
What's the difference between Hα emission and absorption?
Hα appears in absorption against the bright photospheric continuum disk, and in emission against the dark sky above the limb. The same plasma, viewed from different angles. The visible solar disk shows Hα as a dark absorption line in the photospheric spectrum — chromospheric hydrogen absorbs continuum photons at 6562.8 Å. But at the limb, with the photospheric continuum no longer behind it, the chromosphere is the brightest thing — and emits Hα against the dark background. This is why a filament (dark, on-disk) and a prominence (bright, on the limb) are the same magnetic structure — same plasma, opposite contrast.