Our angry, stormy Sun

We know our Sun as a brilliantly bright sphere that rises in the east and sets in the west each day. That’s a simple way to describe it; what really goes on on its surface is far from the impression that it gives as it hangs, almost calmly, in the daytime sky.

While going anywhere near the Sun would be suicide with the searing heat and penetrating radiation combining to fry you alive in your spacesuit, technology has revealed this star to be an angry, bubbling cauldron of solar activity.

First up are solar flares – bursts of radiation from the sudden release of magnetic energy from active regions on the Sun’s surface, the photosphere. These regions are centred on sunspots, which are tangled knots of magnetic fields. The flares release as much as a sixth of the total amount of energy that the Sun releases every second, with much of it in X-rays or ultraviolet light. The energy of a flare can drive a cloud of charged particles to escape the solar corona in a coronal mass ejection (CME). The CME becomes a giant cloud of plasma hurtling through space and, when CMEs are pointed towards Earth, they cause solar storms.

When a CME strikes the Earth’s magnetosphere, it overloads the system and becomes a geomagnetic storm. Earth’s magnetosphere is compressed to breaking point with charged particles flooding the magnetic field lines that loop down on to the magnetic poles of the planet. The particles excite atmospheric gases (mainly oxygen and nitrogen), causing them to glow in eerie shimmering curtains of light – the aurora borealis (northern lights) and the aurora australis (southern lights). Oxygen gas glows green, while nitrogen glows purplish-red – the two primary colours seen in auroras. Usually low-level solar wind activity means that the ‘auroral arc’ is kept, in the northern hemisphere, to the Arctic Circle but the power of a geomagnetic storm can see the auroral arc extend to more southerly latitudes, over Britain and Western Europe, as far south as Spain or even, on very rare occasions, Florida in the United States. The most severe solar storm on record was the Carrington event of 1859, when auroras lit up the skies as far south as the tropics and telegraph wires began to short, sparking electricity.

Those telegraph wires remind us that auroras are only the pretty side of a geomagnetic storm. Although they are not directly harmful to people on the ground, a storm instigated by a powerful CME can destroy our technology. Satellites can short-circuit, knocking out communications. Astronauts must take shelter from the radiation in a special, shielded room onboard the International Space Station. On the ground, power lines can become swamped by raw current from the CME plasma – in 1989, a solar storm caused a large, nine-hour blackout in Quebec in Canada. In our modern world, where we rely on electronic devices, the nightmare scenario is that a powerful enough solar storm could stop everything working, wiping computers, crashing the internet, knocking out global power systems and disrupting communications. It may take months to get everything back online, in which time the world has been sent into technological, social and economic chaos.

We’re most vulnerable to solar storms at solar maximum, which is the point in the Sun’s 11-year cycle of activity when our nearest star is at its most active. Solar flares happen all the time, and CMEs strike Earth frequently, but only rarely are they as powerful as the solar activity that plunged Quebec into darkness. However, scientists are currently unable to predict solar activity or when the next big CME will be.

All of this takes place in the Sun’s heliosphere, which is the extent of its magnetic influence throughout the Solar System, where the solar wind still blows. The heliosphere goes out past the orbit of Pluto. The Voyager 1 spacecraft is currently 118 times further from the Sun than Earth is, and yet it has still to leave the heliosphere. CMEs disperse and lose power the deeper they get into the Solar System. However, solar activity can still have an effect, even on the edge of the heliosphere. Both Voyager 1 and 2 have experienced the heliosphere swelling and shrinking on gusts of the solar wind that inflate the Solar System’s magnetic bubble.

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