The Sun's Atmosphere
We've finally made our way to the surface. Let's trace through the atmosphere next. Just like Earth, the sun boasts an atmosphere. However, the sun's is composed of the photosphere, the chromosphere and the corona.
The photosphere is the lowest region of the sun's atmosphere and is the region that we can see. "The surface of the sun" typically refers to the photosphere, at least in lay terms. It is 180-240 miles (300-400 kilometers wide) and has an average temperature of 5,800 degrees Kelvin. It appears granulated or bubbly, much like the surface of a simmering pot of water. The bumps are the upper surfaces of the convection current cells beneath; each granulation can be 600 miles (1,000 kilometers) wide. As we pass up through the photosphere, the temperature drops and the gases, because they are cooler, do not emit as much light energy. This makes them less opaque to the human eye. Therefore, the outer edge of the photosphere looks dark, an effect called limb darkening that accounts for the clear crisp edge of the sun's surface.
The chromosphere extends above the photosphere to about 1,200 miles (2,000 kilometers). The temperature rises across the chromosphere from 4,500 degrees Kelvin to about 10,000 degrees Kelvin. The chromosphere is thought to be heated by convection within the underlying photosphere. As gases churn in the photosphere, they produce shock waves that heat the surrounding gas and send it piercing through the chromosphere in millions of tiny spikes of hot gas called spicules. Each spicule rises to approximately 3,000 miles (5,000 kilometers) above the photosphere and lasts only a few minutes. Spicules may also follow along magnetic field lines of the sun, which are made by the movements of gases inside the sun.
The corona is the final layer of the sun and extends several million miles or kilometers outward from the other spheres. It can be seen best during a solar eclipse and in X-ray images of the sun. The temperature of the corona averages 2 million degrees Kelvin. Although no one is sure why the corona is so hot, it is thought to be caused by the sun's magnetism. The corona has bright areas (hot) and dark areas called coronal holes. Coronal holes are relatively cool and are thought to be areas where particles of the solar wind escape.
Through telescope images we can see several interesting features on the sun that can have effects here on Earth. Let's take a look at three of them: sunspots, solar prominences and solar flares.