12 Jan 2015

What’s behind the hole in the Sun?

Beneath the Sun's calmly smiling face is a raging tempest. We don’t know exactly why coronal holes appear, but we do understand something of the pyrotechnics involved. Alan Duffy explains.

The Sun’s face on 1 January shows the coronal hole as a dark region in the south. These holes occur where the magnetic field fractures releasing particles into space.Credit: NASA

There were no fireworks from the Sun this New Year's. Instead NASA's Solar Dynamics Observatory spacecraft saw the exact opposite, as a gigantic dark hole appeared on the Sun’s surface. Located around the Sun’s south pole, the hole measured around 400,000 kilometres at its widest point – the equivalent of more than 30 Earths placed side-by-side.

So what caused this immense blotch on the underside of our Sun? Coronal holes were first seen by the Skylab space station in the 1970s. We still don’t know why they occur, but we do understand something of what’s going on.

The corona is the glowing outermost layer of the Sun. It’s made of plasma – super-hot material mostly composed of ionised hydrogen – with temperatures of up to three million degrees Celsius. Even though the plasma leaps millions of kilometres into space, it remains firmly tethered by the Sun’s magnetic field. But every now and again the magnetic field factures and the plasma is expelled into space at speeds of up to 800 kilometres per second. As there is less glowing plasma in the coronal hole, it looks darker compared to the surrounding corona.

It’s no surprise that we witnessed the hole around this time. At the south pole, the Sun’s magnetic field is a little vulnerable right now. The field is generated by the motion of hot plasma in the inner regions just above the nuclear furnace, somewhat akin to the way the Earth’s spinning iron core generates our own magnetic field. 

The Earth’s north-south polarity is relatively stable, but the Sun’s field gradually flips every 11 years or so. This field reversal takes months and isn’t always symmetrical – the last one, in 2013, saw the north pole flip first, while the south pole is still catching up. The transitioning field is weaker, making it more likely that material can rush out from the corona into space, offering an explanation for the enormous coronal hole we see today.

Magnetic fields in the photosphere can suddenly reconnect, triggering the release of enormous amounts of stored magnetic energy.

Some people tend to confuse coronal holes with sunspots. There are some similarities. Sunspots also appear as darker patches – not because material is escaping the Sun as in a coronal hole, but because a tangled knot of magnetic fields prevent the material from sinking back below the surface as it cools. A cool object shines less brightly than one that is hot, making it appear dark next to the rest of the Sun. Take it away from the Sun though and you would see that it is still glowing hot. These sunspots arise two layers below the corona in what is known as the photosphere. It is the layer that gives off photons of visible light and is the one we can see with our eyes.

Sandwiched between the corona and the photosphere is the chromosphere which has a temperature of 20,000ºC, while the photosphere is a relatively cool 5,800ºC. Sunspots are slightly cooler again at around 3,500ºC.

This artist concept shows the layers of the sun. Credit: NASA

Sunspots are also strongly influenced by the sun’s magnetic field. Magnetic fields in the photosphere can suddenly reconnect, triggering the release of enormous amounts of stored magnetic energy. This is seen as a solar flare, a burst of intense X-rays and light in all directions above the sunspot, which reaches the Earth eight minutes later.

Often alongside the flare is a second and potentially more dangerous event called a coronal mass ejection. Compared to the steady release of material by the coronal hole, this is a sudden titanic eruption of billions of tonnes of material from the Sun in a giant cloud of roiling plasma travelling at up to 2,000 kilometres per second.   

If the Earth is in the way of this cloud it can strike our magnetic field, heating the gases in our atmosphere to produce the Aurora Borealis and Aurora Australis (Northern and Southern lights). It’s not all pretty lights in the sky though – a particularly large event may induce currents in electrical grids and cause blackouts and destroy appliances.

One of the biggest mysteries of the Sun is how the corona sitting above the chromosphere can be heated to temperatures thousands of times hotter than the layers below – in relative terms it’s like seeing a flame burn on top of an ice cube.

Although the Sun appears to us as a calmly shining feature of our skies, events such as the coronal hole and sunspots unmask its true tempestuous nature: it is after all a hydrogen bomb larger than a million Earths. It seems that the New Year coronal hole, while not as showy as the fireworks, was an entertaining display by the Sun after all.

Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff