Over the weekend Earth was slammed by the biggest solar storm in about a decade. Days earlier a huge x-class solar flare (the most powerful category) had blasted millions of tons of hydrogen into space, with Earth in the cross-hairs. The eruption – called a coronal mass ejection – blasted through space at more than a million miles an hour. The leading edge reached Earth on March 8 and the full strength hit Earth two days later, triggering a geomagnetic storm as well as triggering some concerns. But first a little about what happens during a solar storm and how it can affect the Earth, its inhabitants, and our society. By the way, NASA has some very nice information on solar flares and related phenomena on their heliophysics web page and the National Oceanographic and Atmosphere Administration maintains a space weather web page that nicely complements the NASA site.
Every so often – the frequency depends on solar activity, which varies over the course of the 11-year sunspot cycle – the Sun’s magnetic field twists and knots up and when the magnetic lines of force unkink (for lack of a better way to put it) the Sun burps, spraying hot gas into the interplanetary void. Most of these eruptions miss the Earth, at times we get blasted.
Much of the gas that hits us is in the form of a plasma – it’s ionized so we catch it in the form of electrons and protons with a smattering of neutrons and helium nuclei. These are also permeated with a magnetic field – when the gas reaches Earth it clams into the Earth’s magnetic field at speeds of hundreds of miles per second, compressing it towards our planet.
Normally our magnetic field does a good job of diverting charged particles away from the surface and into the van Allen radiation belts (although some charged particles always manage to funnel in along magnetic field lines at the north and south magnetic poles) but it can only do so much. A large cloud of gas from a powerful solar flare can overwhelm the Earth’s magnetic field, driving charged particles into the atmosphere to not only produce intense aurorae (the Northern and Southern lights) but also causing the magnetic field to flutter under the electromagnetic stress. Moving a magnetic field back and forth along an electrical conductor is pretty much the same thing that happens in an electrical generator – what matters is the relative motion of the magnetic field with respect to the conductor – and this motion can induce electrical currents in metals around the world. Electrical power lines, pipelines, rails, and the like can all experienced induced electrical current; this current in turn can harm satellites, can damage sensitive electronics (such as pipeline flow meters or electrical control equipment), can trip circuit breakers and overload electrical grids, and can even accelerate corrosion in pipelines that are affected.
All of this can be bad, but what gets the attention of many is the radiation dose from solar flares – all of these charged particles can make themselves felt not only in space but also on commercial airliners and even at sea level (for those who are interested you can calculate your in-flight radiation dose from galactic – not solar – cosmic rays through the FAA’s CARI software). Northern Arizona University professor Keran O’Brien, a highly respected scientist who specializes in solar and cosmic radiation, helped develop many of the calculations that went into the CARI software; using his work as well as studies of solar flares and flares on other sun-like stars we can calculate how frequently we might see high sea-level radiation doses from solar flares. It turns out that every few million years (give or take a little bit) there is a solar flare that will produce a sea-level radiation dose of about 100 rem (1 Sv) – enough to induce radiation sickness but not enough to kill an organism. Less powerful solar flares – with correspondingly lower radiation doses – are more frequent, but anything that produces a radiation dose of more than a rem happens so rarely that it is highly likely that nobody reading this will ever experience one. And neither will our children or grandchildren – such events happen only every few millennia. If – or when – we become a space-dwelling species our astronauts will have more to worry about; the solar flare that hit us recently would have been fatal to any astronaut outside the Earth’s magnetic field. But for those of us at ground level even powerful solar flares are more of an inconvenience than a direct health risk.
There has also been speculation as to the impact of solar flares that strike during one of Earth’s occasional magnetic field reversals. During these events the Earth is not totally bereft of magnetic protection, but the magnetic field is weaker and more chaotic. At present we might be heading into such a reversal – the magnetic field strength has dropped by about 10% over the last century or so – but this is still within the normal variability and geomagnetic scientists are not certain at the moment. In any event, geomagnetic reversals are frequent events (geologically speaking) that last only a few thousand years on average. The odds that a solar super-flare – something strong enough to cause short-term health problems – might hit during the relatively brief geomagnetic reversal are fairly slim. But even if the timing were to work out, a radiation dose of 100 rem (1 Sv) is hardly enough to kill anyone or to cause mass extinctions – that would take a dose of several hundred rem (several Sv).
The bottom line? Solar flares can cause problems with our increasingly-sensitive electrical and electronic infrastructure, but they are not likely to kill any of us directly. This doesn’t consider the long-term risk of getting cancer from the elevated radiation dose rates, but I’ve devoted a lot of space to cancer risks in earlier blogs and will not revisit them here. So for me – I keep flying (you’ve got to love those little bags of peanuts!), I’m continuing to live in mid-latitudes, and I’m not looking for insurance policies with solar flare coverage. But preparing for a blackout? Always a good idea!