Back when I was teaching Astronomy at Lehigh, one of the mysteries I used to impress upon my student was the question of why the solar temperature would rise so dramatically as one moved outward from the surface of the photosphere.

The photosphere – what we see as the “surface” of the sun is actually the coolest region of a star. We perceive it because the gas is finally cool enough in that region (on the order of 5-6000 K) that the atoms in the atmosphere are able to retain their associated electrons, so that the region behaves in a black-body manner.

As one moves radially outward the temperature begins to climb again and reaches well into millions of K in the solar corona (which is nearly the same temps as are found in the solar core where nuclear fusion provides the mechanism for the energy radiation in the first place.)

We understand the core’s mechanism, and the photospheric mechanism, but until just now, there’s been a great deal of controversy about how the solar corona got so darn hot. It’s totally counter-intuitive. One would expect that as the atmosphere became more rarified and further removed from the solar core, the temperature would drop, not climb.

So, that’s why this announcement is neat:

“The mystery of why temperatures in the solar corona, the sun’s outer atmosphere, soar to several million degrees Kelvin (K) —much hotter than temperatures nearer the sun’s surface—has puzzled scientists for decades. New observations made with instruments aboard Japan’s Hinode satellite reveal the culprit to be nanoflares.

This false-color temperature map shows solar active region AR10923, observed close to center of the sun’s disk. Blue regions indicate plasma near 10 million degrees K. Credit: Reale, et al. (2009)
Nanoflares are small, sudden bursts of heat and energy. ‘They occur within tiny strands that are bundled together to form a magnetic tube called a coronal loop,’ says Klimchuk. Coronal loops are the fundamental building blocks of the thin, translucent gas known as the sun’s corona.

Scientists previously thought steady heating explained the corona’s million degree temperatures. The steady heating model indicates that a coronal loop of a given length and temperature should have a specific density. However, observations showed that coronal loops have much higher density than the steady heating model predicts. Newer models based on nanoflares can explain the observed density. But no direct evidence of the nanoflares existed until now.”

Read the full article here.

The upshot is that magnetically contained small flares of explosive gas eruptions are constantly reheating the outer atmosphere. That’s not unexpected, but the news today is that the flares have now been observed.

Neat.