Sunspots overhead: solar flares are back

Giant gobs of incandescent gas are hurtling towards Earth! Satellites to fall from the sky!

Giant gobs of incandescent gas are hurtling towards Earth! Satellites to fall from the sky! Sun in giant eruption as solar flares toast astronauts!

Sadly, no. No astronauts have been harmed in the making of the latest round of breathless excitement infecting the mainstream media as the sun begins to get more active. Although astrophysicists, radio hams and climatologists are genuinely thrilled that solar cycle 24 has at long last begun, and there's been more activity on the sun's surface over the last couple of weeks than in many months previously, we're still at the low end of what will probably be a very quiet few years.

The cycles are eleven years long and are most visible as changes in the number of sunspots. At the bottom of each cycle, the sun can go for weeks without visible sunspots: at the top of a lively cycle, there can be hundreds a year.

For reasons we still don't understand, there are quite dramatic changes in the peak levels of each cycle: the sun's been going through a noisy patch for the last thousand years or so, but is now calming down faster than we expected.

Cycle 24 is late and is going to be quiet overall, but it's on its way up — and with sunspots come solar flares. We haven't had those for a while, hence the excitement recently when a couple of reasonably modest ones kicked off (and failed, unsurprisingly, to have much of an effect).

What is genuinely new is that for the first time, we've got a fleet of spacecraft watching our closest star from every angle and in unprecedented detail, right at the cycle's start. That makes every solar event potential headline news, whether or not it's objectively likely to be important. And solar flares make great pictures, as they're bright, complex and highly energetic.

Flares are caused by magnetic fields at work in the upper part of the Sun's atmosphere. Imagine two bar magnets floating on a boiling sea of custard suddenly coming together, north pole to south pole. As they snap together, a glob of superheated dessert will squirt out and up - that, in essence, is a solar flare.

If the flare is on the half of the Sun visible from the Earth, the first we'll know about it is a pulse of electromagnetic radiation across the entire spectrum from X-rays to radio. If the flare is pointing directly towards us, we'll probably then get a helping of accelerated particles - electrons, protons, ions - which is known as a coronal mass ejection (CME). There can also be intense magnetic fields associated with all of the above.

A further amplification effect is that as it can take many hours or several days for the CME to arrive after the flare, there's ample time for media speculation to build. There are huge numbers of variables in a CME, so it's not possible to tell in advance how many effects, if any, will be felt on earth when a flare kicks off.

When they do, they can be spectacular.

The initial electromagnetic pulse can directly jam radio systems, and if there's enough energy in the X-rays and ultraviolet portions the ionosphere can be radically altered for hours or days. This has various effects; shortwave transmissions that are normally reflected can be absorbed, while VHF and UHF signals that are normally line of sight only can be reflected over thousands of miles around the Earth.

But the real fun starts when a CME impacts and interacts with the Earth's magnetic field. To some extent, this is an extension of a normal, permanent process where solar plasma hits the Earth and charged particles are accelerated along the lines of flux towards the North and South Poles, or captured and flow around the equator in what's called the ring current.

When a CME hits, the ring current can increase to the point that it drastically affects the earth's magnetic field, while the increase in flow of charged particles towards the poles can induce spectacular auroras far further away from the polls than normal. Induced currents on the ground - especially in large power and telecommunication systems - can disrupt normal operations, even going as far as funnelling enough energy into transformers and cabling to permanently damage or incinerate components.

Meanwhile, away from the ground, all those fast charged particles can reset, confuse or fry satellites, make astronauts move into shielded parts of their craft and force to aircraft move to lower flight levels to avoid radiation. And while humans are much the same as they've been for hundreds of thousands of years, modern electronics has increased in density by a factor of eight since the last solar cycle - and is correspondingly more sensitive to radiation effects.

So even if cycle 24 peaks at a low level, the chances of an individual event causing problems is higher. Perhaps we should be keeping an eye out, after all.


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