Earth's Magnetosphere: The Invisible Shield

Magnetosphere Protects Our Planet

The Earth's Magnetosphere is the space surrounding the Earth beyond the atmospheric layers, between 700 and 65,000 km from the surface. This boundary (Magnetopause) is the membrane that isolates us from interplanetary space dominated by the solar wind.

In this zone called the Van Allen Belts, similar to a cocoon surrounding the Earth, energetic particles (protons, electrons) are distributed around the Earth according to the strength of the magnetic field. The magnetic field acts as a screen deflecting the electric current of the Solar Wind, which then flows outside the magnetosphere. The solar wind is mainly composed of ions and electrons charged with considerable energy and expelled at a speed of ≈450 km/s by the thermonuclear reactions of our star.

The pressure of the solar wind compresses the part of the Earth's magnetosphere facing the Sun and stretches the region of the magnetosphere located at the rear, opposite the Sun. The Earth's magnetosphere was described in 1958 during measurements made by Geiger counters on board the first American satellite Explorer 1.

The Van Allen belt consists of two parts with different characteristics. The first, closest to the Earth, is located between 700 and 10,000 km in altitude and is mainly composed of high-energy protons. The second, the larger outer part, is located between 13,000 and 65,000 km in altitude and is composed of high-energy electrons.

The Earth's Surface is Never Directly Hit

All energetic particles circulate at high speed and are continuously brought back towards the poles of the magnetosphere. Thus, the solar wind never directly hits the Earth's surface. We, therefore, live in a magnetic bubble sheltered from deadly solar radiation.

Few humans have been on the other side of the magnetosphere facing deadly solar radiation; only the Apollo astronauts who went to the Moon crossed the Van Allen belt. Even with protective shielding, the astronauts' exposure was limited to less than an hour.

The Magnetic Shield Evolves

The Magnetic Shield is not homogeneous; it evolves continuously and changes over time due to complex physical phenomena occurring in the core.

The geomagnetic field generated more than 3,000 km below the surface emerges from the south magnetic pole, encircles the entire planet, and dives underground towards the core at the north magnetic pole. Most of the radiation flows along the magnetosphere without reaching the Earth's surface, but a small portion infiltrates at the southern and northern poles where the geomagnetic field is weakest. This corresponds to an annular zone called the "auroral zone," currently located between 65 and 75° magnetic latitude.

When solar particles infiltrate these rings, they collide with neutral gases in the atmosphere, producing colorful luminous phenomena. The more powerful the solar wind, the more spectacular these luminous veils become. These characteristic veils appear as a diffuse curtain of light that moves very rapidly and intensifies to reveal immense, impressive rays of light. The length of the aurora can measure several thousand kilometers, but its width generally does not exceed 100 meters.

South Atlantic Anomaly

There is a place on Earth that is poorly protected where the magnetic field has significantly weakened. This zone is located off the coast of Brazil and is called the South Atlantic Anomaly (SAA); it covers an area of ≈2,780 km by 2,780 km and continues to expand.

Scientists have noticed that solar radiation penetrates deeper into the upper atmosphere and is getting closer to the Earth's surface. At the altitude where airplanes fly, solar radiation is already less filtered. At 560 km altitude, certain instruments of the Hubble Space Telescope are turned off during the passage through the zone.

A study on magnetites fixed in ancient clay pottery showed that 400 years ago, the Earth's magnetic field was 10% stronger than today. Before each reversal of the Earth's magnetic field, there is a decrease in its intensity. If the SAA is a manifestation of the magnetic field reversal, we will witness a new reversal!

The evolution of the geomagnetic field over thousands of years has been analyzed in volcanic lava cores because cooled lava retains the orientation and strength of the magnetic field of the time. They thus show the polarization reversals of the field. Geophysicists have identified many over the past 28 million years, and each time the field's intensity decreases significantly. The last reversal occurred 750,000 years ago, and many scientists believe we are currently witnessing a new and abrupt reversal. We will see more and more polar auroras in places where there were none before.

Where is the Magnetic Field Generated?

The Earth's geomagnetic field is probably generated by interactions between the iron-rich inner and outer cores, creating a giant dynamo. This dynamo generates a powerful and stable magnetic field where magnetic fluxes emerge from the south pole of the core and return to the Earth's sphere through the north pole.

In the Earth's core, currents from the solid inner core heat the liquid outer core, creating particle currents around the molten metal that slowly rise and fall. Due to the Earth's rotation, the currents rotate on themselves, creating the magnetic field that emerges at the surface and extends into the surrounding space.

N.B.:
Measurements on samples of Martian meteorites have shown that Mars must have lost its magnetic field. During the first 50 million years, Mars had a powerful magnetic field. Exposed to solar winds, Mars became the sterile red planet we know today. Being twice as small as Earth, it cooled faster, and the dynamo stopped.