In the high north, everyone is already familiar with the concept of the magnetic field of the earth because it causes the beautiful phenomenon of the northern lights. We use its properties to orientate and navigate in planes, boats or just with a hand compass. The last couple of years there has been a lot of commotion about the reversal of the geomagnetic field and the consequences this can cause. To understand this better, we need to know exactly what the geomagnetic field is and how it is induced.
Text: Flore Van Maldeghem
How it works
We can divide the magnetic field in two components. The external component is called the magnetosphere. It’s the area around the earth induced by the force of the magnetic field, in which the northern lights are formed. It doesn’t have any magnetic properties from itself. It is only a consequence of the magnetic characteristics of the internal component. But that’s not what we will focus on here.
We will take a closer look at the internal component. The field has a dipolar character. You can compare it to a bar magnet with a north pole at one side and a south pole at the other side. These two fall together with the geographic North and South pole (figure 1). The magnetic field is induced through a dynamo mechanism. When a conductor moves in a primary magnetic field, an electrical current is impelled. This is also what happens with the lights on bikes that work with a dynamo. The electrical current will, in its turn, induce a secondary magnetic field that reinforces the primary field. This principle is based on the dynamo of Faraday.
In the earth, the conductor is the metallic outer core and the movement is caused by a cyclonal convection and a non-uniform rotation (figure 2). The convection is a result of the crystallisation of iron at the boundary between the outer and inner core (gravitational convection) and of the decay of radioactive elements that leads to a difference in temperature (thermal convection). The non-uniform rotation is caused by the speed difference in rotation between the upper and lower part of the outer core. It forms a north-south orientated spiral current. This process is called the magneto hydrodynamic process.
A very intriguing property of the geomagnetic field is, that it can reverse its polarity. This means that it can change the orientation of the magnetic north and south pole. In a period of normal polarity, the magnetic south pole and geographic north pole fall together. In a period of reversed polarity, the magnetic north pole and geographic north pole fall together. These periods are called chrons. Right now, we live in the Bruhnes chron with a normal polarity. A reversal happens in a couple of steps. The initial situation is a dipolar structure (figure 3A). 500 years before the peak of the inversion, the intensity of the magnetic field becomes weaker (figure 3B). At the peak we see a non-dipolar field and the intensity is ca. 10 times weaker than in the dipolar situation (figure 3C). 500 years after the peak the intensity of the field becomes stronger again (figure 3D). When the inversion is completed we see once more a strong dipolar structure, but with a reversed polarity (figure 3E). The polarity changes because the currents in the outer core that induce the magnetic field are not constant. The switch happens ca. every half a million years and takes about 1000 years.
These polarity changes happen, for geological norms, very fast. They are recorded in sediments that contain magnetic minerals. These minerals align according to the ruling magnetic field. The ocean floor gives us a constant record of these chrons and by dating them, we can set up a timescale (figure 4) which we can use to determine the age of rocks that contain magnetic minerals until the Jurassic.
The reversal will not happen in one night. It happens fast, but not that fast. We will not see the magnetic field change during our lives. When it changes it will have some consequences for our navigation systems and technology. For example, the satellites will be damaged during the weak phase of the magnetic field because they are no longer as well protected against the solar storms. Animals that rely on geomagnetism for their orientation will also have some problems with adapting to the new situation.
It will at least take a 1000 years before we find ourselves in this situation and the earth has experienced it already a lot, so we still have some time to adapt to a possible reversal.