Astronomy 2e · The Sun: A Garden-Variety Star
Solar Activity above the Photosphere
On this page 9 sections
In 30 seconds
Above the Sun's visible surface, the action really begins. The quiet-looking disk hides a restless upper atmosphere: a thin pink Chromosphere Thin, pinkish layer above the photosphere Full entry → spiked with spicules, and above it the Corona Million-degree, low-density outer atmosphere Full entry → — a million-degree outer atmosphere that feeds the solar wind. Add magnetic fields, and you get the spectacular events of the "active Sun": prominences that hang in the corona for weeks, solar flares that release vast energy in minutes, and coronal mass ejections (CMEs) that hurl billions of tons of plasma into space.
All of these phenomena are magnetic. Sunspots (Topic 2) are where strong fields break through the photosphere; the activity above it is what those fields do to the surrounding gas — heating it, suspending it, occasionally flinging it away. This topic surveys each phenomenon, what triggers it, and how it is observed (H-alpha, coronagraphs, and space telescopes).
Why this matters
The phenomena above the photosphere are the physical link between the Sun and Earth: flares and CMEs generate space weather (Topic 4) — disrupting satellites, GPS, radio, and power grids, and posing a radiation hazard to astronauts. The coronal heating problem — why the corona (1–3 million K) is hundreds of times hotter than the photosphere beneath it — is one of the great open questions in solar physics, with answers that apply wherever magnetic energy becomes heat. The same processes occur on other stars, so the Sun is our local laboratory for stellar activity.
The college version
Core Concepts
The chromosphere: the red edge
- A thin layer (~2,000–3,000 km) between photosphere and corona, at ~10,000–20,000 K (commonly taught values).
- Seen as a pinkish-red ring at total eclipses — the color of hydrogen's H-alpha emission; routine observations use H-alpha filters.
- Spicules: small jets shooting upward at tens of km/s, reaching ~10,000 km before fading in ~5–15 minutes; they may help feed the corona and solar wind.
The transition region and corona
- Between chromosphere and corona, temperature jumps from ~20,000 K to over 1 million K in a thin Transition region Thin layer where temperature jumps from ~20,000 K to >1 million K Full entry →.
- The corona is the Sun's outer atmosphere: 1–3 million K, but so tenuous (density ~10⁻¹⁵ of the photosphere's) that it beats laboratory vacuums on Earth; it extends millions of kilometers and shows as a pearly halo at eclipses.
- The coronal heating problem: the corona is far hotter than the surface beneath it. Energy must flow up from below (sound or magnetic Alfvén waves) or be released by countless tiny reconnection events called nanoflares; observations increasingly favor magnetic processes, but the problem is unsolved.
- Coronal holes: regions where field lines open into space instead of looping back; dark in X-rays, they are the source of the fast solar wind (~800 km/s), while the slow wind (~400 km/s) comes from streamers and other structures.
Prominences and filaments
- Prominences are large, cool (~10,000 K), dense clouds of gas suspended in the corona by magnetic fields, seen as bright loops at the Sun's limb.
- Against the bright disk the same structure appears as a dark snaking line and is called a Filament The same structure as a prominence, seen dark against the disk Full entry → — same object, different viewing geometry.
- Quiescent prominences last weeks to months; eruptive prominences suddenly detach and are often linked with CMEs.
- Classic example: loop prominences arching over sunspot groups, tracing field lines like iron filings around a bar magnet.
Solar flares: magnetic explosions
- A flare is a sudden brightening caused by the rapid release of stored magnetic energy — a magnetic "short circuit" (reconnection) near sunspot groups.
- Flares are classified by their X-ray flux (GOES scale): A, B, C, M, X, each class ten times stronger than the previous; X-class flares can trigger planet-wide radio blackouts.
- Flares accelerate charged particles and emit X-rays, extreme ultraviolet, and radio, lasting minutes to about an hour.
- Because X-rays travel at light speed, flare radiation reaches Earth in ~8 minutes; flare particles and any associated CME take hours to days (Topic 4).
Coronal mass ejections (CMEs)
- A CME is a huge bubble of plasma and magnetic field ejected from the corona — up to ~10¹⁵–10¹⁶ kg, moving at hundreds to ~2,000+ km/s.
- CMEs are observed with coronagraphs (telescopes that block the Sun's disk, like LASCO on SOHO) as expanding halos.
- A fast Earth-directed CME typically arrives in 1–3 days — the main driver of geomagnetic storms (Topic 4).
- Flares and CMEs are related but distinct: flares are radiation from reconnection; CMEs are ejected matter. They often occur together (an "eruption"), but either can happen alone.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Flares and CMEs are the same event | Two related but distinct phenomena | Flare = radiation (minutes); CME = ejected matter (days to Earth); each can occur alone |
| Prominence and filament are different objects | The same structure | A prominence at the limb is a filament against the disk |
| The corona is cooler than the photosphere | It is ~100–500× hotter | 1–3 million K vs 5,800 K — the heating problem |
| Spicules and prominences are the same | Different scale/geometry | Spicules: small short-lived jets; prominences: large long-lived loops |
| The solar wind is like Earth's wind | A supersonic flow of ionized plasma | It is charged particles, not air, and interacts with Earth's magnetic field (Topic 4) |
| CMEs travel at the speed of light | They travel at hundreds–thousands of km/s | Only flare radiation arrives in ~8 min; CME plasma takes 1–3 days |
| The corona is a solid "atmosphere" boundary | A gradual, tenuous gas | No sharp edge; it fades into the solar wind |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The Sun has an invisible outer "atmosphere" far hotter than its bright surface — like air a meter from a fire being hotter than the flame! Magnetic lines from sunspots catch glowing gas in loops (prominences), and sometimes the Sun snaps those lines, sending out bursts of light (flares) or giant bubbles of gas (CMEs) that can travel to Earth.
Worked example
You watch a total solar eclipse from the path of totality. Here is what you see, mapped to the physics:
- The red ring (chromosphere). As the Moon covers the photosphere, a thin pink rim appears — H-alpha emission from the ~10,000–20,000 K chromosphere.
- Pink loops at the limb (prominences). Small bright loops hang above the rim — cool gas trapped in magnetic fields; filtered images the same day show the same structures as dark filaments.
- The pearly halo (corona). Around the black Moon, the million-degree corona shines with streamers and holes — the structures that channel the solar wind.
- Reality check. Minutes later, the disk reappears and all of it vanishes against the photosphere's glare — which is exactly why we build coronagraphs and fly X-ray telescopes into space.
Fireplace analogy. The corona is like the air above a campfire: the fire is bright, but the invisible air just above it can be far hotter than the flame's surface — energy arrives there by invisible processes (for the Sun, magnetic fields).
Key takeaways
- Chromosphere: ~2,000–3,000 km thick, ~10,000–20,000 K; home of spicules; red (H-alpha) at eclipses.
- Corona: 1–3 million K, extremely low density; the coronal heating problem is unsolved (magnetic waves/nanoflares are leading ideas).
- Coronal holes = open field lines → source of the fast solar wind (~800 km/s).
- Prominence = filament: same structure; bright at the limb, dark against the disk; quiescent (weeks–months) vs eruptive.
- Flare classes (X-ray flux): A → B → C → M → X, each step ×10; X-class can black out radio.
- CMEs: ~10¹⁵–10¹⁶ kg of plasma, reach Earth in 1–3 days; observed with coronagraphs.
- Flares = radiation; CMEs = matter — related but not the same event.
- Flare X-rays arrive in ~8 min; CME material takes days.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Describe the chromosphere and give two ways it is observed.
Show answer
The chromosphere is a thin (~2,000–3,000 km) layer at ~10,000–20,000 K above the photosphere, containing spicules; it is observed as a red ring (H-alpha) during total eclipses and routinely with H-alpha filters.
What is the coronal heating problem, and what are the leading candidate explanations?
Show answer
The corona (1–3 million K) is much hotter than the photosphere (5,800 K) despite receiving energy from below; leading ideas are magnetic waves (Alfvén waves) and nanoflares (tiny reconnection events), possibly both — the problem is not fully solved.
Explain why a Prominence Cool, dense gas suspended in the corona by magnetic fields Full entry → and a filament are the same thing.
Show answer
Both are cool, dense gas suspended by magnetic fields: at the limb it appears bright (prominence); against the bright disk, dark (filament) — same object, different viewing geometry.
How are solar flares classified, and why does classification use X-rays?
Show answer
By their X-ray flux on the GOES scale (A, B, C, M, X, ×10 per class); X-rays come from the hot flare plasma and arrive at Earth's orbit promptly, making them a practical, real-time severity measure.
What is a CME, how is it observed, and how long does it take to reach Earth?
Show answer
A CME is a bubble of plasma and magnetic field ejected from the corona (~10¹⁵–10¹⁶ kg), observed with coronagraphs; a fast Earth-directed CME typically arrives in 1–3 days.
State one way flares and CMEs are related and one way they differ.
Show answer
Related: they often occur together and both release magnetic energy; different: a flare is radiation (arrives in ~8 min) while a CME is ejected matter (arrives in days) and drives geomagnetic storms.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Chromosphere
- Thin, pinkish layer above the photosphere
- Spicule
- Small jet of gas through the chromosphere
- Corona
- Million-degree, low-density outer atmosphere
- Transition region
- Thin layer where temperature jumps from ~20,000 K to >1 million K
- Coronal hole
- Region of open magnetic field lines, dark in X-rays
- Prominence
- Cool, dense gas suspended in the corona by magnetic fields
- Filament
- The same structure as a prominence, seen dark against the disk
- Solar flare
- Sudden release of magnetic energy near sunspots
- GOES flare class
- A/B/C/M/X X-ray flux scale (×10 per class)
- Coronal mass ejection (CME)
- Huge bubble of plasma + field ejected into space
- Coronagraph
- Telescope that blocks the Sun's disk to see the corona
Sources & references
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