A Luminous Crown

Corona is Latin for “crown,” and it describes the region beyond the transition zone consisting of elements that have been highly ionized (stripped of their electrons) by the tremendous heat in the coronal region. Like the chromosphere, the corona is normally invisible, blotted out by the intense light of the photosphere. It is only during total solar eclipses that the corona becomes visible, at times when the disk of the moon covers the photosphere and the chromosphere. During such eclipse conditions, the significance of the Latin name becomes readily apparent: The corona appears as a luminous crown surrounding the darkened disk of the sun. When the sun is active—a cycle that peaks every 11 years—its surface becomes mottled with sunspots, and great solar flares and prominences send material far above its surface.

Not That Kind of Chrome

The sun’s lower atmosphere is called the chromosphere, normally invisible because the photosphere is far brighter. However, during a total solar eclipse, which blots out the photosphere, the chromosphere is visible as a pinkish aura around the solar disk. The strongest emission line in the hydrogen spectrum is red, and the predominance of hydrogen in the chromosphere imparts the pink hue. The chromosphere is a storm-racked region, into which spicules, jets of expelled matter thousands of miles high, intrude.
Above the chromosphere is the transition zone. As mentioned earlier, the temperature at the surface of the photosphere is 5,780 K, much cooler than the temperatures in the solar interior, which get hotter the closer one approaches the core. Yet, in the chromosphere, transition zone, and into the corona, the temperature rises sharply the farther one goes from the surface of the sun! At about 6,000 miles (10,000 km) above the photosphere, where the transition zone becomes the corona, temperatures exceed 1,000,000 K. (For detailed real-time views of the solar photosphere, chromosphere, and corona, see http://sohowww.estec.esa.nl.) How do we explain this apparent paradox? It is believed that the interaction between the sun’s strong magnetic field and the charged particles in the corona heat it to these high temperatures.

The Solar Atmosphere

The sun does not have a surface as such. What we call its surface is just the layer that emits the most light. Let’s begin our journey at the outer layers of the sun (the layers that we can actually see), and work our way in. When you look up at the sun during the day, what you are really looking at is the sun’s photosphere. The layer from which the visible photons that we see arise, the photosphere has a temperature of about 6,000 K. Lower layers are hidden behind the photosphere, and higher layers are so diffuse and faint (though very hot) that we only see them during total solar eclipses or with special satellites. Above the photosphere in the solar atmosphere are the chromosphere, the transition zone, and the corona. As we move higher in the sun’s atmosphere, the temperatures rise dramatically.