The sun's atmosphere is way hotter than its surface. Scientists may finally know why

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The mystery of how the sun's corona, which is its outer atmosphere, reaches millions of degrees could have a surprising explanation: cosmic dust riding the magnetic waves carrying plasma on the solar wind.

"For decades, researchers have focused mainly on how electrons, ions, magnetic fields and plasma waves transport and dissipate energy in the solar atmosphere," said lead researcher Syed Ayaz of the University of Alabama in Huntsville in a statement. "Our work adds a new ingredient to this picture: dust grains."

The finding came courtesy of NASA's Parker Solar Probe, which has flown closer to the sun than any other spacecraft, skirting the corona at a distance of 6.1 million kilometers (3.8 million miles). If you've ever witnessed a total solar eclipse, or even seen a photograph of one, then you will be familiar with the corona — the ghostly tendrils of light that surround the eclipsed sun. Those tendrils are formed from plasma, or ionized gas, at temperatures in excess of a million degrees Fahrenheit, compared to the sun's visible surface, the photosphere, which radiates at about 9,932 degrees Fahrenheit (5,500 degrees Celsius). At those temperatures, the photosphere outshines the corona only because the plasma in the corona is so sparsely distributed. This is why the only time we can see the corona is during a total solar eclipse, when the photospheric glare is blocked.

Parker does not carry a cosmic dust detector, and that's because until now dust has not been considered a serious component of the solar atmosphere. Indeed, in the high temperatures of the solar corona it had been thought that dust could not survive for very long and would therefore have no impact.

However, Parker does host a bunch of antennas and magnetometers collectively referred to as the FIELDS experiment, designed to measure the electromagnetic field and radio emissions in the solar corona. The antennas kept picking up unexpected spikes in voltage, which according to Ayaz and his team are produced by clouds of charged particles created when tiny dust grains slam into Parker at high velocity.

These dust grains have accrued an electrostatic charge, which can interact with the electromagnetic field carried by the solar wind as it leaves the sun, which in turn can influence waves of plasma reverberating through that electromagnetic field called Alfvén waves.

There are two possible, competing ways in which dust can affect the Alfvén waves, which in turn could determine how energy is dumped into the corona, heating it. On one hand, the mass of the dust can act to provide extra inertia to the plasma as it rides the solar wind, allowing the plasma energy to be transported across wider distances. On the other hand, the electric charge on the dust grains can bolster the interactions between charged particles in the plasma, the Alfvén waves and the solar electromagnetic field.

"If dust mass dominates, [Alfvén] wave energy may travel farther into the corona," said Ayaz. "If dust-charge effects dominate, the energy may be released more locally as particle heating."

The balance between these two effects can therefore control where and when energy is deposited into the corona, focusing it in areas and causing temperatures there to rise dramatically.

Future solar missions are now going to have to start taking dust into account, said Ayaz, with dedicated detectors designed to measure dust's properties close to the Sun.

"The bigger question is fascinating," said Ayaz. "Is dust simply passing through the near-Sun environment, or is it helping shape how electromagnetic energy becomes heat and solar-wind motion?"

The new discovery was reported on July 1 in The Astrophysical Journal.