The Sun is enveloped by an atmosphere that extends far beyond its visible surface. This atmosphere is composed of plasma, a state of matter where electrons are stripped from atomic nuclei. Understanding this outer envelope is important for comprehending the flow of energy from the Sun and predicting phenomena that influence space weather. The solar atmosphere is structured into three distinct layers, each defined by differences in temperature, density, and physical characteristics.
The Photosphere
The innermost layer of the Sun’s atmosphere is the photosphere, which is what observers refer to as the star’s surface. Its name, meaning “sphere of light,” is appropriate because it is the source of nearly all the visible light energy that escapes the Sun and travels toward Earth. The temperature of this layer averages around 5,800 Kelvin. This layer is relatively thin, spanning a depth of only about 300 miles (500 kilometers).
Close examination of the photosphere reveals a mottled texture known as granulation, which resembles the surface of boiling water. These bright, bubbling cells are the tops of convection currents, where hot plasma rises at the center of the granules and cooler plasma sinks back down along the darker edges. Sunspots appear as dark patches because they are cooler than the surrounding plasma. These cooler areas are created by intense magnetic fields that suppress the flow of heat from the Sun’s interior, reducing the light emitted from that region.
The Chromosphere
Immediately above the photosphere lies the chromosphere, a layer of the atmosphere that forms a boundary between the visible surface and the outer layers. This layer derives its name, “color sphere,” from its reddish or pink hue. This coloring is caused by the strong emission of light from excited hydrogen atoms, known as hydrogen-alpha. The chromosphere is usually hidden from view by the overwhelming brightness of the photosphere, making it observable only during a total solar eclipse.
A characteristic of this layer is the reversal of the temperature gradient, where the plasma begins to heat up as it moves away from the solar interior. Temperatures in the chromosphere increase from approximately 4,000 Kelvin at its base to around 20,000 Kelvin at its upper boundary. This layer is dynamic and features narrow, jet-like eruptions of plasma called spicules, which shoot upward from the lower atmosphere. Spicules extend high into the solar atmosphere before falling back down.
The Corona
The outermost layer of the solar atmosphere is the corona, which begins high above the chromosphere and extends millions of kilometers into space. Here, the plasma continuously escapes the star’s gravity. This layer is hundreds of times hotter than the photosphere below it, reaching temperatures of one to three million Kelvin. This extreme heating requires energy to be transferred by non-thermal processes, likely involving magnetic fields, which is an active area of scientific investigation.
Despite its high temperature, the corona is extremely tenuous, making it about ten million times less dense than the photosphere. This low density is why the corona is so dim in visible light and is only seen from Earth as a faint, pearly white halo during a total solar eclipse. The continuous outward expansion of the coronal plasma generates the solar wind, a stream of charged particles that flows throughout the entire solar system. Specialized instruments called coronagraphs are used to artificially block the bright light of the photosphere, allowing scientists to study the corona at any time.