The chromosphere is the second layer of the Sun’s atmosphere, situated directly above the photosphere (the visible surface). This middle layer acts as a transitional zone between the relatively cool photosphere and the super-hot, tenuous outer corona. The chromosphere is a constantly moving layer of plasma that is difficult to observe, yet it hosts some of the Sun’s most energetic and colorful phenomena.
Defining the Chromosphere’s Place in the Sun
The chromosphere is a relatively thin shell of gas, extending upward for a few thousand kilometers above the photosphere. Its thickness ranges from about 2,000 to 10,000 kilometers, a small fraction of the Sun’s total radius. It is sandwiched between the bright photosphere and the much hotter, faint corona. The upper boundary is the transition zone, where the temperature rapidly jumps toward coronal values.
The name “chromosphere” literally means “sphere of color,” derived from the Greek word chroma. This name comes from the layer’s appearance during a total solar eclipse. When the moon blocks the brilliant light of the photosphere, the chromosphere briefly flashes into view as a beautiful, reddish-pink crescent. This striking red color is due to the intense light emitted by hydrogen atoms in the layer, specifically the Hydrogen-alpha line.
Unpacking the Chromosphere’s Unusual Temperature and Density
The temperature profile of the Sun’s atmosphere is surprisingly counterintuitive. The photosphere is about 6,000 Kelvin (K), but the temperature drops to a minimum of approximately 4,000 K at the boundary with the chromosphere. Instead of continuing to cool, the chromosphere begins to heat up dramatically.
Temperatures climb steeply from the bottom of the layer (around 4,500 K) to the top, reaching up to 20,000 K before entering the transition region. This temperature increase is not fully understood but is thought to be related to the channeling of energy by the Sun’s magnetic fields. Despite this extreme heat, the chromosphere is incredibly sparse, with a density about 10,000 times lower than the photosphere.
Because the gas is so thin, the chromosphere holds very little total thermal energy, even at high temperatures. This low density means the layer is normally overwhelmed by the light of the denser photosphere. The process that transfers energy into this layer and heats it without heating the underlying layers remains one of the major unsolved problems in solar physics. Magnetic waves and their dissipation are currently considered likely mechanisms responsible for this atmospheric heating.
Observing the Chromosphere: The H-alpha Filter
The overwhelming brilliance of the photosphere makes the fainter chromosphere nearly impossible to see. To study this layer outside of a solar eclipse, astronomers employ specialized narrow-band filters. These filters isolate a single specific wavelength of light emitted by the chromosphere’s elements, effectively blocking the continuous, bright light from the photosphere.
The most common technique uses a Hydrogen-alpha (H-alpha) filter, which transmits light at a precise wavelength of 656.3 nanometers. This ruby-red light is emitted when hydrogen atoms in the plasma transition between specific energy levels, a process strongly characteristic of the chromosphere’s conditions. By using a filter with an extremely narrow bandpass (often less than one angstrom), researchers can peer into this layer and observe its fine structure and dynamic activity.
Dynamic Features of the Chromosphere
The chromosphere is a highly turbulent and dynamic layer, characterized by numerous magnetic and plasma structures that constantly shift and evolve. One of the most distinctive features is the spicule, a short, vertical jet of plasma shooting upward from the surface. These grass-like spikes are typically about 300 kilometers wide, reach heights of up to 10,000 kilometers, and last only 10 to 15 minutes before falling back down.
Another common feature is the fibril, which is the disk counterpart of the spicule and appears as fine, thread-like structures tracing the local magnetic field lines. These features are visible in H-alpha images and are concentrated in the boundaries of the Sun’s magnetic network.
Larger, more dramatic phenomena like solar prominences and flares also have strong ties to the chromosphere. Prominences are vast, dense clouds of plasma suspended above the solar surface by magnetic loops, often extending high into the corona. Solar flares, which are sudden, intense bursts of energy, often originate in the magnetic fields that pass through the chromosphere before exploding outward.