Earth Shield: Where Does Our Planet Have a Magnetic Field?

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

The magnetic field protects the Earth's surface from solar wind and harmful cosmic radiation. It works as a kind of shield - without its existence, the atmosphere would be destroyed. We will tell you how the Earth's magnetic field was formed and changed.

The structure and characteristics of the Earth's magnetic field

The Earth's magnetic field, or geomagnetic field, is a magnetic field generated by intra-terrestrial sources. The subject of the study of geomagnetism. Appeared 4, 2 billion years ago.

The Earth's own magnetic field (geomagnetic field) can be divided into the following main parts:

• main field,
• fields of world anomalies,
• external magnetic field.

Main field

More than 90% of it consists of a field, the source of which is inside the Earth, in the liquid outer core - this part is called the main, main or normal field.

It is approximated in the form of a series in harmonics - a Gaussian series, and in the first approximation near the Earth's surface (up to three of its radii) it is close to the magnetic dipole field, that is, it looks like the earth is a strip magnet with an axis directed approximately from north to south.

Fields of world anomalies

The real lines of force of the Earth's magnetic field, although on average close to the lines of force of the dipole, differ from them by local irregularities associated with the presence of magnetized rocks in the crust located close to the surface.

Because of this, in some places on the earth's surface, the field parameters are very different from the values in nearby areas, forming so-called magnetic anomalies. They can overlap one another if the magnetized bodies that cause them lie at different depths.

External magnetic field

It is determined by sources in the form of current systems located outside the earth's surface, in its atmosphere. In the upper part of the atmosphere (100 km and above) - the ionosphere - its molecules are ionized, forming a dense cold plasma that rises higher, therefore, a part of the Earth's magnetosphere above the ionosphere, extending to a distance of up to three of its radii, is called the plasmasphere.

Plasma is held by the Earth's magnetic field, but its state is determined by its interaction with the solar wind - the plasma flow of the solar corona.

Thus, at a greater distance from the Earth's surface, the magnetic field is asymmetric, since it is distorted under the action of the solar wind: from the Sun's side it contracts, and in the direction from the Sun it acquires a "trail" that extends for hundreds of thousands of kilometers, going beyond the Moon's orbit.

This peculiar "tailed" form arises when the plasma of the solar wind and solar corpuscular streams seems to flow around the earth's magnetosphere - the region of near-earth space, still controlled by the magnetic field of the Earth, and not the Sun and other interplanetary sources.

It is separated from interplanetary space by a magnetopause, where the dynamic pressure of the solar wind is balanced by the pressure of its own magnetic field.

Field parameters

A visual representation of the position of the lines of the magnetic induction of the Earth's field is provided by a magnetic needle, fixed in such a way that it can freely rotate both around the vertical and around the horizontal axis (for example, in a gimbal), - at each point near the surface of the Earth, it is installed in a certain way along these lines.

Since the magnetic and geographic poles do not coincide, the magnetic needle only shows an approximate north-south direction.

The vertical plane in which the magnetic needle is installed is called the plane of the magnetic meridian of the given place, and the line along which this plane intersects with the Earth's surface is called the magnetic meridian.

Thus, magnetic meridians are the projections of the lines of force of the Earth's magnetic field onto its surface, converging at the north and south magnetic poles. The angle between the directions of the magnetic and geographic meridians is called magnetic declination.

It can be western (often indicated by a "-" sign) or eastern (a "+" sign), depending on whether the north pole of the magnetic needle deviates from the vertical plane of the geographic meridian to the west or east.

Further, the lines of the Earth's magnetic field, generally speaking, are not parallel to its surface. This means that the magnetic induction of the Earth's field does not lie in the plane of the horizon of a given place, but forms a certain angle with this plane - it is called magnetic inclination. It is close to zero only at the points of the magnetic equator - the circumference of a great circle in a plane that is perpendicular to the magnetic axis.

Results of numerical modeling of the Earth's magnetic field: on the left - normal, on the right - during inversion

The nature of the earth's magnetic field

For the first time, J. Larmor tried to explain the existence of the magnetic fields of the Earth and the Sun in 1919, proposing the concept of a dynamo, according to which the maintenance of the magnetic field of a celestial body occurs under the action of the hydrodynamic motion of an electrically conductive medium.

However, in 1934, T. Cowling proved the theorem on the impossibility of maintaining an axisymmetric magnetic field by means of a hydrodynamic dynamo mechanism.

And since most of the studied celestial bodies (and even more so the Earth) were considered axially symmetric, on the basis of this it was possible to make the assumption that their field would also be axially symmetric, and then its generation according to this principle would be impossible according to this theorem.

Even Albert Einstein was skeptical about the feasibility of such a dynamo given the impossibility of the existence of simple (symmetric) solutions. Only much later was it shown that not all equations with axial symmetry describing the process of magnetic field generation will have an axially symmetric solution, even in the 1950s. asymmetrical solutions have been found.

Since then, the dynamo theory has been successfully developing, and today the generally accepted most likely explanation for the origin of the magnetic field of the Earth and other planets is a self-excited dynamo mechanism based on the generation of an electric current in a conductor when it moves in a magnetic field generated and amplified by these currents themselves.

The necessary conditions are created in the core of the Earth: in the liquid outer core, consisting mainly of iron at a temperature of about 4-6 thousand Kelvin, which perfectly conducts current, convective flows are created that remove heat from the solid inner core (generated due to the decay of radioactive elements or the release of latent heat during solidification of matter at the boundary between the inner and outer cores as the planet gradually cools).

The Coriolis forces twist these currents into characteristic spirals that form the so-called Taylor pillars. Due to the friction of the layers, they acquire an electric charge, forming loop currents. Thus, a system of currents is created that circulate along a conductive circuit in conductors moving in a (initially present, albeit very weak) magnetic field, as in a Faraday disk.

It creates a magnetic field, which, with a favorable geometry of the flows, enhances the initial field, and this, in turn, enhances the current, and the amplification process continues until the losses to Joule heat, increasing with increasing current, balance the energy inflows due to hydrodynamic movements.

It was suggested that the dynamo can be excited due to precession or tidal forces, that is, that the source of energy is the rotation of the Earth, however, the most widespread and developed hypothesis is that this is precisely thermochemical convection.

Changes in the Earth's magnetic field

Magnetic field inversion is a change in the direction of the Earth's magnetic field in the geological history of the planet (determined by the paleomagnetic method).

In an inversion, the magnetic north and magnetic south are reversed and the compass needle begins to point in the opposite direction. Inversion is a relatively rare phenomenon that has never occurred during the existence of Homo sapiens. Presumably, the last time it happened about 780 thousand years ago.

Reversals of the magnetic field occurred at intervals of time from tens of thousands of years to huge intervals of a quiet magnetic field of tens of millions of years, when the reversals did not occur.

Thus, no periodicity was found in the pole reversal, and this process is considered stochastic. Long periods of a quiet magnetic field can be followed by periods of multiple reversals with different durations and vice versa. Studies show that a change in magnetic poles can last from several hundred to several hundred thousand years.

Experts from Johns Hopkins University (USA) suggest that during reversals, the Earth's magnetosphere weakened so much that cosmic radiation could reach the Earth's surface, so this phenomenon could harm living organisms on the planet, and the next change of poles could lead to even more serious consequences for humanity up to a global catastrophe.

Scientific work in recent years has shown (including in experiment) the possibility of random changes in the direction of the magnetic field ("jumps") in a stationary turbulent dynamo. According to the head of the laboratory of geomagnetism at the Institute of Physics of the Earth, Vladimir Pavlov, inversion is a rather long process by human standards.

Geophysicists at the University of Leeds Yon Mound and Phil Livermore believe that in a couple of thousand years there will be an inversion of the Earth's magnetic field.

Displacement of the Earth's magnetic poles

For the first time, the coordinates of the magnetic pole in the Northern Hemisphere were determined in 1831, again - in 1904, then in 1948 and 1962, 1973, 1984, 1994; in the Southern Hemisphere - in 1841, again - in 1908. The displacement of the magnetic poles has been recorded since 1885. Over the past 100 years, the magnetic pole in the Southern Hemisphere has moved almost 900 km and entered the Southern Ocean.

The latest data on the state of the Arctic magnetic pole (moving towards the East Siberian world magnetic anomaly across the Arctic Ocean) showed that from 1973 to 1984 its mileage was 120 km, from 1984 to 1994 - more than 150 km. Although these figures are calculated, they are confirmed by measurements of the north magnetic pole.

After 1831, when the position of the pole was fixed for the first time, by 2019 the pole had already shifted by more than 2,300 km towards Siberia and continues to move with acceleration.

Its travel speed increased from 15 km per year in 2000 to 55 km per year in 2019. This rapid drift necessitates more frequent adjustments to navigation systems that use the Earth's magnetic field, such as compasses in smartphones or backup navigation systems for ships and aircraft.

The strength of the earth's magnetic field falls, and unevenly. Over the past 22 years, it has decreased by an average of 1.7%, and in some regions, such as the South Atlantic Ocean, by 10%. In some places, the strength of the magnetic field, contrary to the general trend, even increased.

The acceleration of the movement of the poles (by an average of 3 km / year) and their movement along the corridors of magnetic pole inversions (these corridors made it possible to reveal more than 400 paleoinversions) suggests that in this movement of the poles one should see not an excursion, but another inversion of the Earth's magnetic field.

How did the earth's magnetic field come about?

Experts at the Scripps Institute of Oceanography and the University of California have suggested that the planet's magnetic field was formed by the mantle. American scientists have developed a hypothesis proposed 13 years ago by a group of researchers from France.

It is known that for a long time, professionals argued that it was the outer core of the Earth that generated its magnetic field. But then experts from France suggested that the planet's mantle was always solid (from the moment of its birth).

This conclusion made scientists think that it was not the core that could form the magnetic field, but the liquid part of the lower mantle. The composition of the mantle is a silicate material that is considered a poor conductor.

But since the lower mantle had to remain liquid for billions of years, the movement of the liquid inside it did not produce an electric current, and in fact it was simply necessary to generate a magnetic field.

Professionals today believe that the mantle could have been a more powerful conduit than previously thought. This conclusion of specialists fully justifies the state of the early Earth. A silicate dynamo is possible only if the electrical conductivity of its liquid part was much higher and had low pressure and temperature.