Читать книгу Exploring the Solar System - Peter Bond - Страница 48

The chromosphere (literally “color‐sphere”) is an irregular layer, about 5,000 km thick, which lies above the photosphere. It is named after the reddish prominences which can be seen arcing above the Sun during a solar eclipse. In this region, the temperature rises from 6,000°C to about 20,000°C, possibly due to turbulence and the action of shock waves.⁶ At these higher temperatures, hydrogen emits reddish light known as H‐alpha (Hα) emission.

When the Sun is viewed through a spectrograph or a filter that isolates the hydrogen‐alpha light, many new features become visible. Huge prominences which tower above the limb are shaped by local magnetic fields, often forming loops and arcs many times bigger than Earth.

Figure 2.15 Two giant prominences rising from the chromosphere on March 18, 2003. The shape of the plasma structures is molded by the local magnetic field lines. The loops rose to an altitude equal to 20 Earth diameters in a few hours, and may have been associated with a flare and coronal mass ejection. The SOHO image was taken in extreme ultraviolet light.

(EIT consortium, ESA/NASA)

Figure 2.16 A huge solar filament erupted into space on August 31, 2012. The filament strand was stretched outward until it finally broke away. Such an erupting prominence occurs when its magnetic structure becomes unstable and bursts outward, releasing the trapped plasma. This extreme ultraviolet image was taken by NASA's Solar Dynamics Observatory.

(NASA)

When they occur above the main solar disk, these dense, gaseous tongues look very different. Instead of red eruptions, they appear as dark, thread‐like features, so they are known as filaments. Both filaments and prominences can erupt to considerable heights over the course of a few minutes or hours.

Locally concentrated magnetic fields are also associated with plage (French for “beach”), bright patches surrounding sunspots that are best seen in H‐alpha light. Plage also form part of the network of brighter regions that make up the chromospheric network. Once again, concentrated magnetic fields produced by fluid motions in the supergranules result in a patchwork that outlines the giant convectional cells. The network is best seen in H‐alpha and the ultraviolet spectral line of calcium (Ca II K).