by Earth, our rocky, watery oasis in the cosmos, is the perfect place for life to thrive for a number of reasons.
We sit just the right distance from our home star for liquid water to exist on the planet’s surface. The gravitational pull of other large planets helps protect us from apocalyptic collisions with stray meteorites. And the planet’s magnetic field surrounds the Earth with a protective barrier that protects us from charged particles passing through space.
The Earth’s magnetic field is generated by the complex flow of molten metallic material in the planet’s outer core. The rotation of the Earth and the presence of a solid iron core affect the flow of this material, resulting in a bipolar magnetic field whose axis roughly aligns with the planet’s rotational axis.
Hidden in the chemical composition of ancient rocks are clues that the Earth’s magnetic field is a dynamic, shifting phenomenon. Cooling magma rich in iron minerals is drawn into alignment with the Earth’s magnetic field, similar to how a needle is drawn to point north on a compass. The study of ancient geomagnetic fields recorded in rocks is the subject of a discipline called “paleomagnetism.”
Paleomagnetic research has informed scientists that the Earth’s magnetic field has changed and even reversed polarity many times in the geological past. But why?
Related: The Earth’s Magnetic Field: Explained
What causes the magnetic poles to flip?
The Earth’s magnetic field varies on very short and extremely long time scales, from milliseconds to millions of years. The interaction of the magnetic field with charged particles in space can change it on short timescales, and disturbances in the magnetic field occur on longer timescales due to internal processes unfolding in the Earth’s outer liquid core.
“Secular variation in the geomagnetic field results from the influence of the magnetic field from flow in the fluid outer core and from the effects of magnetic field diffusion in the core and mantle,” geophysicist Leonardo Sagnotti told Space.com.
Due to fluctuations in the magnetic field caused by the movement of metallic material in the outer core, the polarity of the magnetic field has been completely reversed in the past on Earth. Previous palaeomagnetic studies of the states of the magnetic field have shown that there are two possible polarity states — the current ‘normal’ state, where the field lines enter towards the center of the Earth in the northern hemisphere and diverge towards the outside the Earth in the southern hemisphere. The inverse, or ‘reverse’, polarity is just as good and stable.
Paleomagnetic studies have shown that polarity reversals in the Earth’s magnetic field are not periodic and cannot be predicted. This is largely due to the behavior of the mechanisms responsible for it.
“The flow of the metallic fluid (mostly molten iron) in the outer core of the Earth is chaotic and turbulent. Polarity reversals occur during periods of low geomagnetic field intensity, in which the intensity of the dipole component is significantly reduced, and structure the field is unstable,” says Sagnotti.
The polarity reversal transition period appears to be an instantaneous geological period (ie below the geological resolution), with a which will last up to several thousand years.
How do magnetic pole reversals affect life on earth?
When the magnetic field is prone to overturning, it is in a state of reduced intensity, leaving the Earth’s atmosphere more exposed to the solar wind and cosmic rays in the form of charged particles. A recent study showed that during the Laschamps tripa recent period of low magnetic field intensity that occurred only 41,000 years ago, the global cosmic ray flux reaching the Earth’s atmosphere was up to three times higher than today’s value.
Currently, there is no significant evidence of a correlation between the mass extinction of life on Earth and geomagnetic polarity reversals. However, uncertainties in the known timescale of these magnetic ‘flips’ prevent species extinction rates and speciation from being linked to periods of low magnetic field strength.
Additionally, magnetic reversals occur frequently on geologic time scales (a few hundred times in the last 160 million years), and recorded mass extinction events occur every hundred million years or so (often much less).
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From the point of view of human civilization, the shift of the magnetic poles is not directly the cause, but the resulting period of reduced geomagnetic field intensity. Society is increasingly dependent on technology, and governments and international organizations should seriously consider the effects of reduced magnetic field strength.
“In this configuration there would be a significant increase in the penetration of charged particles into the magnetosphere at a height closer to the Earth’s surface, with important consequences for our technological world,” says Sagnotti.
The risks facing our planet and civilization could have a significant impact on civil society, the way we conduct commerce, security, communications, power infrastructure, satellites and the lives of people in low Earth orbit . Unfortunately, the random nature of magnetic variations and magnetic reversals means that we cannot predict exactly when this will happen, we just know that it will happen.
