It often happens that the most important thing goes completely unnoticed. For example, when was the last time he thought about the Earth’s magnetic field, if he ever did? Besides directing compass needles northward or migrating birds, does the Earth’s magnetic field have any other effect on our daily lives?
Let’s start with a spoiler: The Earth’s magnetic field deflects about 1.5 million tons of material ejected from the Sun at high speed every second. If it were not there, the atmosphere would suffer direct and continuous erosion, it would not have the capacity to avoid the direct impact of those solar particles, which would drag with them everything that protects us. Therefore, without the 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 only our DNA, from this same bombardment.
The 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, now, at this 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 that 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, forming a protective shield, is called the magnetosphere.
To give some details of how it works, let’s now travel about 80 kilometers above our heads. There, at that height above the ground, something fundamental happens. And a significant fraction of the gas in this region is ionized, that is, the particles are electrically charged, generally because they have lost some electron in their structure due to the energetic radiation of our star. Charged particles behave in a very special way: they follow the magnetic field lines and, therefore, they move like on concrete highways, it is as if they were on lanes.
Before continuing, let’s point out something important: the Sun, like all stars, in addition to electromagnetic energy in the entire range (our eyes are only sensitive to visible light, which is a very narrow range), ejects large amounts of material in the form of charged particles at high speed. 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 the Earth’s magnetic field we would see that it is what we know as a dipolar magnetic field, 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 south magnetic pole points north. In the north, the field lines point inward, the opposite of magnets. It 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 placed on satellites that have explored both the intensity and direction of the Earth’s magnetic field and confirmed its dipole-shaped nature. The form it takes is, in addition to being complex, variable. Some of its components are the Van Allen radiation belts, the ring current, the magnetic tail or the magnetopause.
Let’s give just a few fascinating details of the structure of the magnetic field that surrounds the Earth. Surrounding the 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 (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 do the other way around. This ring current generates a magnetic field that points in the opposite direction of the Earth’s magnetic field and that, when intensified, 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 are missing a fundamental ingredient: the solar wind, which 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, a magnetic tail. We will leave the matter of magnetic storms and their origin for another occasion.
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