chromosphere
/ KROH-moh-sfeer /
The chromosphere is a thin, faint layer of the Sun's atmosphere lying just above the bright photosphere. Its name means 'sphere of colour', because during a total solar eclipse, in the brief instant before the bright disk is fully covered, it flashes into view as a glowing reddish-pink rim around the black Moon.
It is a few thousand kilometres thick and far less dense than the photosphere below it. Normally it is completely drowned out by the photosphere's glare and cannot be seen, but it shines brightly in a few specific colours of light, especially the deep red glow of hydrogen, so astronomers view it through special filters that let only that colour through. Strangely, the chromosphere gets hotter with height rather than cooler — climbing from about 4,500 degrees at the bottom to perhaps 20,000 degrees near the top — which is the first hint of a deeper puzzle about how the Sun's outer atmosphere is heated.
The chromosphere matters because it is the dynamic in-between layer where the Sun's magnetic activity becomes visible. Jet-like spikes of gas called spicules, dark threads, bright patches near sunspots, and the bases of giant prominences all live here, and it is the launchpad from which energy and material rise toward the even hotter corona above.
Through a hydrogen-alpha filter, the chromosphere reveals dark filaments, bright flare ribbons, and a carpet of hair-like spicules invisible in ordinary light.
Special filters let us study the chromosphere any day, not only during eclipses.
The chromosphere's outward rise in temperature is genuine and surprising, but it is dwarfed by the corona above, where the temperature jumps to millions of degrees — the heart of the coronal heating problem.