Photosphere
The photosphere is the visible "surface" of the Sun—the layer from which most of the sunlight we see is emitted. It is about 500 km thick and has a temperature of around 5,800 K. Sunspots appear here as darker, cooler regions where strong magnetic fields emerge. The photosphere is where solar flares often originate in the magnetic loops above sunspots.
Chromosphere
Above the photosphere lies the chromosphere, a thin layer (about 2,000 km) that appears reddish during total solar eclipses. Temperatures rise with height here (to roughly 20,000 K). The chromosphere is involved in flare and CME initiation and is observed in UV and specific spectral lines (e.g. H-alpha).
Corona
The corona is the Sun's outer atmosphere, extending millions of kilometers into space. It is much hotter (1–2 million K) than the photosphere—a long-standing puzzle explained by magnetic heating. The corona is the source of the solar wind and coronal mass ejections (CMEs). Spacecraft such as SOHO and SDO image the corona in EUV and X-rays for space weather monitoring.
Sources and further reading
- NASA SDO – Solar Dynamics Observatory, solar images
- SOHO Mission – Sun and corona observations
- NOAA SWPC – Solar cycle – Sunspots and solar cycle