The magnetic shield that protects Earth and makes life possible
The space around us has a magnetic structure that we have been able to explore with satellites. More importantly, it stops us from being constantly bombarded by particles borne on the solar wind
We often take the most important things for granted. For example, when was the last time you thought about Earth’s magnetic field, if ever? Besides pointing compass needles northward or directing migrating birds, does Earth’s magnetic field have any other effect on our daily lives?
Spoiler alert: every second, Earth’s magnetic field deflects about 1.5 million tons of material ejected from the Sun at high speed. If it were not there, the atmosphere would suffer direct and continuous erosion. It would not be able to avoid the direct impact of those solar particles, which would sweep everything that protects us away with them. Therefore, without Earth’s magnetic field, life as we know it would not exist on the surface of our planet. Of course, our technological societies would not be possible either, since the magnetic field also protects our electronic equipment, not just our DNA, from this same bombardment.
Earth (like Mercury, Jupiter, Saturn, Neptune, and Uranus) is surrounded by a relatively intense magnetic field that originates, for the most part, within the planet. It is believed that, at the current stage of Earth’s evolution, it is powered by the cooling and crystallization of the core. This agitates the liquid iron that surrounds it, creating powerful electrical currents that generate the magnetic field that extends into space. This type of magnetic field is known as a geodynamo, and the force field structure that deflects most of the solar wind and forms a protective shield is called the magnetosphere.
To understand how it works in more detail, let’s now travel about 80 kilometers (50 miles) above our heads. At that altitude, something fundamental happens. And a significant fraction of the gas in this region is ionized. In other words, the gas particles have an electric charge, generally because they have lost an electron in their structure due to the energetic radiation coming from our star. Charged particles behave in a very special way. They follow the magnetic field lines and, therefore, they move as if they were in lanes on a highway.
Before we continue, it is important to point out that the Sun, like all stars, ejects large amounts of material in the form of charged particles at very high speed. It does this in addition to electromagnetic energy across the entire range — our eyes are only sensitive to visible light, which is a very narrow range. This is what is known as stellar wind; or solar wind, in the case of our star. The connection between the magnetosphere and the solar wind is the heart of what is known as space weather.
If we could visualize Earth’s magnetic field, we would see that it is what we scientists call a dipolar magnetic field. This is where the lines of force leave one hemisphere and enter the other. In normal convention, the outgoing field lines are magnetic north and the incoming field lines are magnetic south. In the case of the Earth, sometimes to avoid confusion with geographic north, the convention is reversed and the magnetic north pole points south and the magnetic south pole points north. In the north, the field lines point inward, which is the opposite of what happens in magnets. The field is also inclined 11.5 degrees with respect to the planet’s axis of rotation, which is what defines the geographical north and south poles.
A fascinating structure
The Earth’s magnetic field is twice as intense at the poles as at the equator. We know this thanks to instruments on satellites that have explored both the intensity and direction of Earth’s magnetic field and confirmed its dipole-shaped nature. In addition to being complex, the form it takes is variable. Some of its components are the Van Allen radiation belts, the ring current, the magnetic tail, and the magnetopause.
Among just a few fascinating details of the structure of the magnetic field that surrounds our planet is a region that is made up of cold, dense plasma that rotates with the Earth. The Van Allen belts are also out there, where particles move with relativistic energies, in other words, close to the speed of light.
In what is known as the ring current, energetic ions move at much slower speed than in the Van Allen belts, but they have a higher density and produce an electric current that surrounds the Earth. Electrons move from the twilight zone to the zone where it is night and positively charged ions move in the opposite direction. This ring current generates a magnetic field that points in the opposite direction of the Earth’s magnetic field and that, when it becomes intense, decreases the intensity of the field measured on the surface. There are more currents that connect the ring current to the ionosphere and play an essential role in the northern lights and space weather.
To understand the global configuration of the way particles move in our space environment, we need one more fundamental ingredient. The solar wind is also magnetic. A way to simply visualize this interaction is to imagine the solar wind as the current of a river and the Earth and its magnetic field as a giant stone. Since the solar wind is supersonic we have a bow shock and behind the obstacle we have the tail. In this case, it is a magnetic tail. As for magnetic storms and where they come from, we will leave those for another occasion.
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