SOLAR physicists have puzzled for years over why the Sun’s atmosphere is so
much hotter than its surface. Now a team of researchers believe they’ve
discovered where the heat comes from even if they don’t know what generates it.
The atmosphere heats from the bottom up.
From measurements made with NASA’s Transition Region and Coronal Explorer
(TRACE) spacecraft, the team concluded that heat is generated in the bottom
layer of the Sun’s atmosphere and not uniformly throughout it, as scientists
previously thought. This insight should help explain why temperatures in the
outer layers, known as the corona, soar to millions of kelvin, while the
chromosphere—the layer just below—is around 10,000 kelvin. Their
work will appear in The Astrophysical Journal.
Markus Aschwanden, an astrophysicist at the Lockheed Martin Solar and
Astrophysics Laboratory in Palo Alto, California, and his observing team looked
at the huge loops of magnetic field that emerge from the chromosphere, arc
hundreds of thousands of kilometres into space, and plunge back into the Sun.
Hot ionised gas, or plasma, flows along these loops and clouds of electrons
swirl around the magnetic field lines and give off X-rays and ultraviolet
radiation.
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Using TRACE to study 41 different loops, they measured the intensity of
radiation at two different ultraviolet wavelengths. From this data they worked
out the density and temperature at each point along the loop. They then
calculated where heat would have to enter the loop to produce the observed
density and temperature distribution. “The surprising result is that you have to
heat only the lowest 10,000 kilometres, no matter how high the loops arc,”
Aschwanden says.
“This is a very important step toward determining what the heating
mechanisms are,” says Eric Priest, an applied mathematician at the University
of St Andrews who helped to pioneer the technique used to make the new
observations.
Now that researchers know where the heat enters the corona, the next step is
to discover what produces it. Observations so far by this team and other
researchers suggest that magnetic field lines in the chromosphere break and
reconnect to generate the heat and drive the electrons up into the loops,
Aschwanden says. Priest agrees, but adds a warning. “There is lots of evidence
that magnetic reconnection is the culprit here, but so far we haven’t proved it.”
