Seen the news about the Earth's magnetic field being "about to flip"?
Well, that's not quite what is happening.
The field will "flip" at some point in the (geological) near future.
On timescales that we're used to, though, that's still a long time away – probably thousands of years.
But it doesn't happen overnight.
Instead, the field appears to fade out and come back again, reversed. This usually happens over several thousand years, but the only thing that is certain is that it's not regular in any way.
A slow fade is actually bad news, because it means that there will be a long time when we're without our protective coccon. Eek.
Earth’s magnetic field is generated deep underneath us in the Earth’s core.
It has a North and South pole, like a bar magnet.
It's thought the magnetic field reverses once every 250,000 years, on average
Humans can't use Earth's magnetic field to navigate, but some animals do.
But we do need it. Earth's magnetic field acts as a shield, protects us from charged particles streaming from the Sun.
Without it we'd be exposed to lots more solar radiation.
This is how the field has flipped throughout Earth's history:
We can see the signature of past magnetic field reversals in rock on the ocean floor.
As lava bubbles up from under mid-ocean ridges the magnetic field of Earth at the time it solidifies is captured in it – literally set in stone. Over time, as the magnetic field flips back and forth over millenia, more rock emerges and sets leaving a pattern of "magnetic stripes" that reveals Earth's magnetic history.
Since we began measuring it in 1840, Earth's magnetic field has reduced in strength by between 10 and 15%.
And we know the last reversal was 780,000 years ago, so it does seem like we're overdue.
If this gap between reversals matched the longest one we know of, that would give us up to around 200,000 years until the next one.
Some scientists studying how Earth's magnetic field is generated think it could happen much sooner – or rather that it already started 200 years ago.
Soon ESA’s SWARM mission will be able to give us a better measurement of the Earth’s magnetic field, hopefully shedding light on when exactly this reversal will happen.
Whenever it happens, there will be several (thousand) years when we're without the protection of our cosy magnetosphere.
Fortunately there's no fossil evidence that previous flips have caused species to die out.
But there hasn't been a major magnetic field reversal since modern humans have been around, so we can't say for sure how it will affect us.