Nature
PUBLISHED
Geologic evidence makes clear that the Earth’s magnetic poles sometimes reverse direction, so that to a compass, north becomes south and vice versa. These events don’t seem to have affected the Earth all that much in the past, but occurred back when there was no highly technological civilization to worry about. That raises the question of the effects on humanity if such a flip were to occur soon, particularly in regard to the satellites we now depend on.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.What is a geomagnetic pole reversal?
The Earth is surrounded by a magnetic field, generated by its core, which protects us from various sorts of space weather. It is widely suspected that without it, the planet would be uninhabitable. When iron-rich rocks cool from volcanic lava, they take on the direction of the field at the time, and largely maintain that once cooled. This allows us to track past changes in direction and even strength through rocks of different ages, revealing that the field sometimes reverses quite suddenly.
The reasons for these reversals, sometimes called flips, are not fully understood. The frequency of reversals has varied over time, occurring as often as once every hundred thousand years, but with none measured for 37 million years during the Cretaceous. In recent times, the rate seems to be about once every quarter of a million years, so the odds of one happening this century are very low.
We know that after a flip, things return approximately to normal, except the magnetic field points the opposite way. This must be very confusing to animals that use the geomagnetic field to navigate long journeys. We know they managed, however, as plenty of migratory species have survived many flips.
The more mysterious part of the question is what happens during a flip. It is thought that the flip can’t be instantaneous, so there must be a period where the field is weak or non-existent. Given how dependent life is on the geomagnetic field, many people get very alarmed when they learn about the flips, sometimes predicting disasters.
However, while there have been attempts to match the timing of past flips to extinction events, these are still much debated. If flips raise extinction rates, it is not by much, so the consequences for most life are probably small, suggesting the period where the field is weak isn’t very long.
Satellites during the flip
Flips may be short on geological time-scales, but we could see things very differently if one were to happen in our lives. Flips are clearly short enough that we don’t lose a significant portion of the atmosphere to solar flares, for example, but unprotected satellites might have a shorter lifespan.
Not all satellites will suffer the same effects. Small satellites known as cubesats rely on the geomagnetic field to navigate. They’re equipped with magnetometers that measure the field in three dimensions and use this to ensure they stay tilted at the right angle, although other sorts of sensors help. If the field is undetectably weak, cubesats won’t know which way to point.
On the other hand, a satellite engineer IFLScience consulted told us, “Modern satellites of any decent size will typically have a star tracker to measure their attitude with respect to the inertial frame, and anything operating below the GNSS constellations will have a GNSS for measuring Earth-relative position and velocity. Satellites in higher orbits are tracked from the ground.”
Cubesats are becoming more common as the cost of launch decreases, but – at least for the moment – it’s the larger satellites we really depend on.
That doesn’t mean everything will be alright, however. The engineer noted, “Without the magnetosphere, Low Earth Orbit satellites will see higher radiation levels.” That radiation won’t be instantly enough to cause all satellites to go dark, like in a disaster film, but it would “Lead to higher failure rates and shorter lifetimes.” Even passing over the South Atlantic Anomaly, where the Earth’s field is currently weaker, creates problems to the point non-essential operations sometimes shut down.
Satellites can be redesigned for greater robustness – after all, some space probes survive much higher solar radiation. However, once a satellite is in orbit, it’s a bit late to suddenly add some extra radiation shielding, so we could see a steady drop off of performance.
A less obvious problem, but potentially a greater one, is that satellites currently use the magnetic field to keep themselves oriented the right way.
Satellites use reaction wheels to orientate themselves, but the wheels will accumulate excess momentum, which they need to shed to keep operating.
Devices called mangetorquers (or torque rods) make their own magnetic field, and satellites use the interaction between this and the Earth’s field to get rid of excess momentum. Creating the field just requires a magnetic rod and electrical power, which solar panels can provide, so for many satellites, this is the only method available. With no field, the momentum will build up until things go badly wrong.
Other satellites use thrusters to shed the momentum, but this uses fuel, which, unlike electric power, can’t be replenished in space. A satellite using both will use up its run our of fuel more quickly in a flip, while one that has no thrusters will be in big trouble if the flip lasts for long.
Solar activity also accelerates the decay of satellites in low Earth orbit, causing the atmosphere to expand and create drag at greater heights. The current solar cycle, Solar Cycle 25, has been more active than expected, causing satellites to deorbit more quickly than usual. Without the shielding effects of the magnetic field, this atmospheric drag may increase. However, if the flip is short, how much of a problem that would be probably depends on whether it coincides with a peak or a trough of a solar cycle.
Satellites after the flip
Once the reversal has been completed and the field is restored but points the other way, there shouldn’t be much of a problem. The same protection will be provided against radiation, and satellites can use a reversed field to shed momentum just as well as the original one. Those that rely on the field to navigate might need new instructions, but flipping a positive to a negative in code should be easy to achieve.
The challenge, in other words, is for satellites to make it through the flip, rather than surviving the other side. To know how well they will do that, the major question is how long the period of low magnetic field lasts, and that is something we still don’t know, because our records from previous events are not high enough resolution to tell us.
