First results from close to the Sun

By Richard A Lovett

The first results from NASA’s sun-diving Parker Solar Probe are in, and already they are revealing the Sun’s outer atmosphere to be even more complex than scientists had previously thought.

One of the great mysteries of the Sun is why it is that its surface temperature is only 6000 degrees Celsius, while the temperature of its corona – the upper part of its atmosphere – exceeds one million degrees. 

“This is a big puzzle in solar physics: why is the corona so hot?” says Daniel Verscharen, a space-plasma physicist at University College, London, who was not part of the Parker team but has written a commentary on its initial findings in the journal Nature. 

That’s important for a number of reasons, but one is that the hot plasma in the corona is what becomes the solar wind, which speeds away from the sun at velocities in excess of 2 million kilometres per hour.

The Parker Solar Probe’s mission, Verscharen says, is to get close enough to the Sun to answer these questions, diving ever closer in a series of encounters that will eventually bring it to just more than six million kilometres from the solar surface.

Launched in 2018, the Parker has so far completed only two such encounters and to date hasn’t gotten any closer than 24 million kilometres. But that’s less than half the distance between the Sun and Mercury, and twice as close as any prior spacecraft.

One surprise, says Verscharen, is that the Sun’s magnetic field has frequent “switchbacks” in which, for a few minutes, it suddenly reverses direction. Something similar had been seen from further away, Verscharen says, “but we had no idea that they would be so strong and would occur so often”.{%recommended 862%}

These reversals, he says, cause the velocity of the solar wind to alternately increase and decrease, and may contribute to the heating of the corona. 

Also interesting, Verscharen says, is the discovery that the plasma has turbulent fluctuations, which he refers to as “instabilities”. Such turbulence is another way in which energy can be transferred from the magnetic field to the plasma, thereby heating it.

From a practical point of view, one of the Parker’s major goals is to better understand the processes by which the Sun sometimes ejects dangerous bursts of energetic particles – called coronal mass ejections (CMEs) – which are potentially deadly to astronauts and are capable of wreaking havoc to satellites, communication systems, and power grids here on Earth.

Already, Parker has been able to observe several small CMEs, says Colin Joyce, a space physicist at Princeton University, Princeton, New Jersey, who is part of the Parker team. 

One discovery was that the dispersion of the particles – the manner in which the more and less energetic ones spread out as they travel – indicates that they hadn’t followed a straight line en route from the Sun to Parker’s detectors. This, Joyce says, indicates that the switchbacks may be affecting the paths these particles travel as they are ejected from the Sun.

Another discovery, he says, was that the Sun also produces very tiny CMEs, too small to be observed from Earth. 

If such events are common, he says, they might play a role in seeding the Sun’s atmosphere with particles that could be subsequently blasted out by larger solar eruptions—a possible aid to forecasting the conditions under which the most dangerous solar storms might occur. 

If so, that could lead to improved space-weather predictions. 

“That is very central to the Parker Solar Probe,” Verscharen says. 

The initial data from the mission are published in four papers in Nature, along with Verscharen’s commentary.

Alfvénic velocity spikes and rotational flows in the near-Sun solar wind

Highly structured slow solar wind emerging from an equatorial coronal hole

Probing the energetic particle environment near the Sun

Near-Sun observations of an F-corona decrease and K-corona fine structure