Earth’s Magnetosphere

Earth is surrounded by a magnetosphere. The invisible geomagnetic lines stretch from one pole, curve far out into space, then go back to the opposite pole. The curved lines are further shaped by the electrically charged particles of the solar wind into a teardrop shape called the magnetosphere. The Magnetosphere is thus the magnetic field that prevents the solar winds, or highly energetic particles to reach Earth. Please note that the shape of magnetosphere of Earth is determined by the Earth’s internal magnetic field, the solar wind plasma, and the interplanetary magnetic field (IMF). This shape is not static but is dynamic.

Structure of the Magnetosphere

The complex structure of Earth’s magnetosphere is the result of the interplay between the charged particles originating in the upper layers of the terrestrial atmosphere, whose motion is guided by the Earth’s magnetic field, and the solar wind particles carrying the interplanetary magnetic field. The magnetosphere is basically a space filled primarily with particles from terrestrial origin.

The shape of magnetosphere keeps changing throughout the day and night, with Earth’s rotation, revolution and during solar storms and other such events which can affect it.

To understand its boundary, we take an example of a boat that moves through the sea. In front of the boat a bow wave is formed: that bow wave demarcates the region in which the boat disturbs the flow of the water. The water behind the bow wave is forced to flow smoothly around the boat’s hull. Behind the boat a wake is formed. The similar kind of interaction is the solar wind -magnetosphere interaction. The solar wind consists of particles that are mainly of solar origin. It is pervaded by the interplanetary magnetic field. A bow shock is formed in front of the Earth’s magnetosphere, which demarcates the region where the solar wind flow is impeded by the presence of the Earth. The solar wind in the magneto sheath, the region between the bow shock and the Earth’s magnetosphere, is forced to flow around the Earth’s magnetosphere and is compressed.

The impermeable outer surface of the magnetosphere, where the total pressure of the compressed solar wind precisely balances the total pressure inside the magnetosphere, is called the magnetopause. As shown in the accompanying figure, the magnetopause has a shape that is elongated and stretched out in the anti-solar direction, forming a long magnetotail, which is in a sense similar to the wake behind the boat.

Due the complex interplay, the magnetosphere becomes roughly bullet shaped and extends on the night side in the “magnetotail” or “geotail” approaching a cylinder with a radius that is around 20-25 times of the Radius of Earth. The tail stretches to around 200 times the Radius of Earth. The day side tip or sub-solar point of the magnetopause is called “nose’ of the magnetopause. It is normally located at 10 RE (Earth radii) towards the Sun.

There are two polar cusp regions above the “Geomagnetic Poles”. These are regions where solar wind can enter relatively easily into the magnetosphere. The inner magnetosphere is strongly connected to the Earth’s ionosphere. The inner region, called the plasmasphere, which consists of dense cold plasma largely of ionospheric origin, rotes more or less, along with the Earth.

Van Allen belts

In the inner region of the Earth’s magnetosphere, there are two distinct rings of electrically charged particles that encircle our planet. These are called Van Allen belts after their discover. The particles in these belts originate from different sources; some come from the solar wind, some from the Earth’s upper atmosphere, some from cosmic rays originating in the distant Universe. The belts are shaped like fat doughnuts, widest above Earth’s equator and curving downward toward Earth’s surface near the Polar Regions.

These charged particles usually come toward Earth from outer space—often from the Sun—and are trapped within these two regions of Earth’s magnetosphere. Since the particles are charged, they spiral around and along the magnetosphere’s magnetic field lines. The lines lead away from Earth’s equator, and the particles shuffle back and forth between the two magnetic poles. The closer ring is about 3,000 kilometers from Earth’s surface, and the farther belt is about 15,000 kilometers away. The highly charged particles of the Van Allen belts pose a hazard to satellites, which must protect their sensitive components with adequate shielding if their orbit spends significant time in the radiation belts.

Van Allen Belts and Impact on Apollo Mission

There are several (conspiracy) theories on impact of the Van Allen Belts on its impact on astronauts who pass through them. It is said that there is deadly radiation in the Van Allen belts, which could have killed the Apollo astronauts (Thus, claiming that actually Apollo 11 was a fake mission). The NASA claims that the nature of that radiation was known to the Apollo engineers and they were able to make suitable preparations. The principle danger of the Van Allen belts is high-energy protons, which are not that difficult to shield against. And the Apollo navigators plotted a course through the thinnest parts of the belts and arranged for the spacecraft to pass through them quickly, limiting the exposure. The Van Allen belts span only about forty degrees of earth’s latitude — twenty degrees above and below the magnetic equator. Further, The region between two to four earth radii lies between the two radiation belts and is sometimes referred to as the “safe zone”.

The diagrams of Apollo’s translunar trajectory printed in various press releases are not entirely accurate. They tend to show only a two-dimensional version of the actual trajectory. The actual trajectory was three-dimensional. The highly technical reports of Apollo, accessible to but not generally understood by the public, give the three-dimensional details of the translunar trajectory. {Source: http://www.clavius.org}

Please note that the inner belt consists mainly of energetic protons, while the outer belt consists mainly of electrons. As far as protective effects of Van Allen Belts are concerned, they have not much to credit for. Van Allen belts protect against charged particle radiation but at the same time don’t not protect against electromagnetic radiation. This protection is done by the atmosphere (Ionosphere). Thus, the statement that these belts protect earth is true in terms of particle radiation, false in terms of EM radiation. Van Allen Belts are regions of high concentrations of particle radiation. It is the Earth’s magnetic field that does the protecting, forming the belts in the process.

Chapman Ferraro Cavity

On the sunward side, the Earth’s Magnetosphere is compressed because of the solar wind, while on the other side it is elongated to around three earth radii. This creates a cavity called the Chapman Ferraro Cavity, in which the Van Allen radiation belt resides.

Magnetospheric storms

We have read above that the magnetosphere is not a static structure. Rather, it is constantly in motion, as the orientation of the Earth’s magnetic dipole varies with the Earth’s daily rotation and with its yearly revolution around the Sun, and as the solar wind is characterized by a strong time-variability on time scales ranging from seconds to years. As a consequence of this time-variability, the sizes and shapes of the regions may change with time. When material from a solar Coronal Mass Ejection travels through the interplanetary medium and hits the Earth, the dynamic pressure of the solar wind is strongly enhanced so that the bow shock and the magnetopause are pushed inward, producing a Magnetospheric storm.

Geocorona

The magnetosphere is an almost completely ionized collision less plasma. Nevertheless, a large cloud of neutral hydrogen surrounds the Earth, which is called the Geocorona. Since collisions are so rare, this neutral cloud can co-exist with the plasma in the inner regions of the magnetosphere with relatively little interference.

Other plants with magnetosphere

Other planets with intrinsic magnetic fields viz. Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter’s moon Ganymede also has a small magnetosphere, but it is situated entirely within the magnetosphere of Jupiter, leading to complex interactions.