The Sun, our star, is a dynamic ball of hot plasma that constantly releases energy. While its core generates immense heat through nuclear fusion, the Sun also possesses an atmosphere. Understanding these atmospheric layers is important for comprehending the Sun’s activity and its influence on our planet. The processes within these layers drive phenomena that can impact satellite communications and power grids on Earth.
The Photosphere
The innermost layer of the Sun’s atmosphere is the photosphere, the visible surface we observe. Its name, derived from the Greek word “photos” meaning light, reflects its role as the source of most sunlight reaching Earth. The photosphere has a temperature of approximately 5,500°C (9,940°F), though it can vary from about 6,125°C at its base to 4,125°C at its top.
This layer appears granular, like boiling water, due to convection cells, which are hot plasma rising to the surface and cooler plasma sinking back down. Darker, cooler regions known as sunspots, caused by intense magnetic fields, are also present. These sunspots are cooler than their surroundings, with temperatures as low as 3,000°C.
The Chromosphere
Above the photosphere lies the chromosphere, the middle layer of the Sun’s atmosphere. Named from the Greek “chroma” for color, this layer exhibits a reddish glow, typically only observable during total solar eclipses when the brighter photosphere is obscured. The chromosphere’s temperature increases with altitude, starting at around 4,500°C and rising to about 10,000°C at its upper boundary. Within this dynamic layer, various phenomena occur, including spicules, which are narrow, jet-like spikes of gas that extend upwards for thousands of kilometers. Prominences, large, loop-like structures of cooler, denser gas supported by magnetic fields, also originate in the chromosphere and extend into the layer above.
The Corona
The outermost and most expansive layer of the Sun’s atmosphere is the corona, meaning “crown” in Latin. This wispy, ethereal layer extends millions of kilometers into space. The corona is exceptionally hot, with temperatures reaching millions of degrees Celsius (1 to 2 million °C), despite its very low density.
Its extreme heat causes particles to move at very high speeds, allowing them to escape the Sun’s gravity and form the solar wind. Like the chromosphere, the corona is visible during total solar eclipses, appearing as a pearly white halo around the darkened solar disk. This vast, superheated region plays a significant role in space weather, as it is the origin of the continuous outflow of charged particles that travel throughout our solar system.