Radiation and Health: Cosmic Radiation During Magnetic Field Reversals

Where does the Earth’s magnetic field come from, why do we think it’s going to reverse itself (and what in the world does that mean, anyway?), and what’s any of that got to do with me or cosmic radiation?

Image by Hans Braxmeier from Pixabay

In the last two pieces, we talked about how radiation affects the body and might affect astronauts. Now it is time to consider the Earth’s magnetic field, its effect on radiation, and how an anticipated magnetic field “reversal” might affect cosmic radiation here on Earth. Or, not to put too fine a point on it – do we have to worry that we’re all going to die? (Spoiler alert – no.) But before we can get to the biology, we really ought to start with a little bit of Earth science.

Why Earth has a magnetic field

When the Earth first coalesced about 4.6 or so billion years ago, it was a relatively homogenous piece of rock. Well…more likely a mixture of rock and magma because the final stages of formation probably involved some fairly massive impacts that would have liquified at least part, if not all, of the Earth. But the key feature is that when the Earth formed, it did not have the layers we see today – no crust, no mantle, and (most importantly) no core. During this period, it’s entirely possible that the Earth had a magnetic field of sorts, due only to whatever residual magnetism was frozen into the rocks as the Earth solidified.  That changed, rather abruptly actually, about a half-billion after the Earth formed – a little more than 4 billion years ago..........To Read More.....

Early Earth was far hotter than today, partly from energy imparted by impacting asteroids and partly due to being at least ten times as radioactive as it is today. This extra heat permeated the planet and melted and coalesced the iron and nickel that diffused through the rocks. The liquid metal, being denser than rock, sank to the planet’s center, transforming the metal’s gravitational energy into more heat, which, in turn, made it easier for the remaining iron and nickel to sink through the softened rock, releasing still more gravitational energy. We believe this process ended with the majority of Earth’s iron and nickel sinking to the center, forming a core of molten metal with a molten silicate rock floating on top of it – what later became the Earth’s mantle and crust. 

This left Earth with a core of molten metal, but possibly not with much of a magnetic field. Creating a magnetic field requires motion of some sort. This relative motion came from the molten metal's differing density (caused by temperature variations) that caused “convection cells” to sink and rise in relationship to one another. While over time, these convection cells have become more like rotating cylinders aligned with the Earth’s axis, it is still that relative motion that produces Earth’s magnetic field. According to studies of ancient rocks, it seems likely that the magnetic force formed at least 3.4 billion years ago and possibly even earlier.

Geologists studying the Earth’s magnetic field quickly realized that it has changed polarity – the direction in which the north and south magnetic poles point – repeatedly over the ages – and that the interval between these polarity reversals seems to have no apparent pattern. In the century or so since these reversals were recognized, scientists have several geodynamic theories explaining and modeling spontaneously magnetic field reversals; there is still no explanation universally accepted by the scientific community. While frustrating for scientists, luckily, it doesn’t matter much for our purposes – all we need to know is that these reversals happen. When they do, there are brief (in a geologic sense) intervals in which the magnetic field is effectively absent......To Read More.....