Space Weather and Magnetospheres

C H A P T E R

N ° 10

Space Weather and Magnetospheres

In order to understand what space weather is and how it is created, we have to understand the interaction between the Sun and other celestial bodies. More specifically, the interactions between planetary magnetic fields, a planet’s atmosphere, and incoming space radiation particles (charged/ionised particles).

**Image Credit:** NASA. Showing the Sun and the solar system with the ratio of the planets compared to the Sun in kilometers.

Planetary magnetic fields

A magnetosphere is the region surrounding a planet dominated by the planet’s magnetic field. Depending on the type of magnetic field, it can either be: 1) generated by strong convective currents in a planets core, requiring complete melting of a large region of the interior and rapid rotation of the planet, or 2) from the crust of a planetary body (remanent magnetism). Planetary magnetic fields play a vital role in the protection of a planet against incoming space radiation such as solar energetic particles. Additionally, it determines the presence and density of a planet’s atmosphere. A planet’s magnetosphere, thus, plays a vital role in the determination of a planet’s potential to be habitable.

The atmosphere

The atmosphere is a mixture of gases that helps make life possible. This is done by providing a planet with air to breathe, shielding from ultraviolet (UV) radiation coming from the Sun, trapping heat to warm the planet, and preventing extreme temperature differences between day and night. Without the atmosphere, temperatures would be well below freezing. The presence of an atmosphere, thus, enables heat to get trapped and absorbed in order to keep a planet’s average surface temperature habitable. This is, for example, the case with Earth. Here, the atmosphere enables an average surface temperature at ~15°C (59°F). Atmospheric gases like carbon dioxide are particularly good at absorbing and trapping radiation. This is why small changes of these gases can directly affect the Earth’s climate. However, in order for the atmosphere to enable these things, it has to be surrounded by a strong planetary magnetic field.

Space Weather and planetary magnetic fields

Planets can either lack a magnetic field, have residual crustal magnetic fields (i.e., magnetic field of the crust of a planetary body), or a global dipole magnetic field. The presence and strength of a planets magnetic field will determine its capabilities of protecting itself. For example: If a planet lacks a magnetic field, the planet will most likely either not have an atmosphere, or it will be very thin. This means, that there will be no obstacles provided from the planet for the incoming space radiation particles to travel through in order to reach the planets surface. Thus, incoming space radiation, such as Solar Energetic Particles (SEPs) or Galactic Cosmic Rays (GCRs), will be able to directly penetrate through the planet and interact with its ground levels, causing a highly radioactive environment.

**Image Credit:** Shiba Rabiee. *Cartoon showing Mars' residual crustal magnetic fields.*

If a planet has residual crustal magnetic fields, it means that there is a presence of a magnetic field. Yet, the field does not surround the whole planet. Instead, it is only located in certain areas of the planet and most likely in the form of multiple hemispheres. Similarly, to a planet lacking a magnetic field, this creates a highly radioactive environment. Yet, residual crustal magnetic field increases the chance of potential ‘safe-zones’ or areas where particles with lower energy levels cannot interact with the ground. When discussing future exploration and habitation on other celestial bodies, the presence of residual crustal magnetic fields can, therefore, be useful in contrast to a planet with a lack of any type of a magnetic field.

In contrast to the two examples above, a global dipole magnetic field is the most ideal magnetic field to be present on a planet. This type of magnetic field is generated from the movement of molten iron in the planet’s outer core, causing the creation of powerful electric currents. This magnetic field structure refers to a magnetic field that has two poles – a north and a south pole – similar to a bar magnet. Global dipole magnetic fields are characterized by field lines that emerge from one pole and curve around to re-enter at the other pole. The direction of the field at any point is given by the direction of the field lines at that point. This is because the geomagnetic poles have the ability to trade places.

**Image Credit:** Physics Stackexchange. *Illustration of a global dipole magnetic field structure.*

A global dipole magnetic field effectively deflects most high energy particles due to three reasons: 1) the magnetic field surrounds the whole planet; 2) it has a high magnetic field strength; 3) it comprises a dense atmosphere. A planet with a global dipole magnetic field, thus, increases the chances of it being habitable.

Furthermore, in a global dipole magnetic field, the magnetosphere extends beyond the planet’s atmosphere, providing a large part of the outer space environment surrounding the planet protection from things such as Solar Energetic Particles (SEPs) and Galactic Cosmic Rays (GCRs). However, this type of magnetic field has its limits. A dipolar magnetic field does not necessarily provide an equal amount of shielding at all locations, as the strength of the field decreases with increasing distance from the planet. This means, that the further away an object is to the planet, the less protection the magnetosphere provides. This information is important when discussing space sustainability and the creation and launches of satellites and CubeSats.

Space weather and magnetic field’s strength

In addition to the existence of a planetary magnetic field, the equally important thing is the fields’ strength. The first obstacle solar particles and Galactic Cosmic Rays (GCRs) face when interacting with a planet, is its magnetic field. During this interaction one of three scenarios can occur: In the first scenario, the planet has a thin atmosphere. Additionally, the energy level of the incoming particle is higher than the strength of the magnetic field. This combination creates the ideal environment for the particle, as it can simply penetrate through the magnetic field and the atmosphere, and interact with the surface of the planet. Over time, this will create a highly radioactive, complex, and non-habitable environment.

In the second scenario, the planet has a thick atmosphere and a strong magnetic field. However, the energy level of the incoming particle is higher than the strength of the magnetic field. The particle will, therefore, penetrate through the magnetic field but get absorbed by the planet’s atmosphere before reaching the surface. This can occur if the atmosphere is dense enough to decrease the velocity (i.e., the speed of something in a given direction) of the particle. A strong enough magnetic field will naturally create a dense atmosphere, which will act as the planet’s second shield against incoming space radiation. Thus, a stronger magnetic field enables a denser atmosphere, which will increase shielding against high-energy particles. In this case, the incoming particles may interact with certain things within the planet at higher altitudes, yet the planet itself will be habitable.

In the third, and last, scenario, the magnetic field strength is higher than the energy level of the incoming particle, causing it to get trapped in the magnetic field lines. This mechanism of the magnetic field is called a ‘magnetic mirror’. Whilst trapped, the particle will travel back and forth between the two poles of the magnetic field and, eventually, deflect due to for example proton-proton collisions. The magnetic mirror is, thus, a mechanism to trap a plasma enabled by a strong enough magnetic field. This means, that the incoming radiation never reaches the planet’s atmosphere or ground levels. This is the most ideal type of scenario, as it would leave the planet completely habitable with no risks of impact from incoming space radiation.

Space weather and the Earth’s magnetic field

Earth has a global dipole magnetic field. However, the poles of the magnetic field surrounding Earth are not the same as the geographical poles. We, therefore, call the poles of the magnetic field; ‘the geomagnetic poles’. Furthermore, the magnetosphere of Earth is not a true dipolar on its surface nor in space due to small deviations and complexities in the field. These deviations are caused by changes in the Earth’s interior and crust, and the constant bombardment of solar particles from the solar wind. The changes in the Earth’s interior and crust causes localized variations in its planetary magnetic field. Whereas the solar wind distorts the field lines of the Earth’s magnetic field. The solar wind is a continues occurrence of plasma (containing charged particles) emitted from the Sun into the outer space environment. These small deviations causes the internal magnetic field of Earth to have a depression centered over the South Atlantic region (‘South Atlantic Anomaly’), causing its magnetosphere to be the weakest at its equator and the strongest at its geographical poles.

The Earth’s global dipole magnetic field has a significantly high magnetic field strength, enabling a dense atmosphere and the creation of its unique atmospheric layers. This combination gives the magnetosphere of Earth the ability to deflect most incoming particles, such as solar particles carried out by the solar wind. The combination of a strong dipolar magnetic field and Earth’s atmosphere enables the protection from most space radiation, and traps heat, helping the enablement of life on the planet. Without Earth’s magnetic field, the planet would not have such a dense atmosphere and would simply deteriorate with time, due to the constant bombardment of the solar wind.

Furthermore, as the magnetic field strength of a global dipole magnetic field decreases with increasing distance to the planet, there is a difference between the type and intensity level of radiation an object is exposed, and, thus, a difference between launching a satellite into the outer Van Allen radiation belt, the slot-region (‘safe-region’), or the region between the inner Van Allen radiation belt and Earth [To know more about the radiation belts and satellites, please read previous published articles on SR Hoplons blog].

**Image Credit:** ESA. *Showing space weather, Earth’s magnetic field, and satellites orbiting around Earth.*

Sources

NASA (n.d.): Magnetospheres. https://science.nasa.gov/heliophysics/focus-areas/magnetosphere-ionosphere/

Bonnie J. Buratti (2004): Moon. Encyclopedia of Physical Science and Technology. 3rd addition. pp. 161-172. DOI: https://doi.org/10.1016/B0-12-227410-5/00460-9.

Steven N. Shore (2003): Magnetic Fields in Astrophysics. Encyclopedia of Physical Science and Technology. 3rd addition. pp. 903-918. DOI: https://doi.org/10.1016/B0-12-227410-5/00392-6

Center For Science Education (UCAR) (n.d.): Earth’s Magnetosphere. https://scied.ucar.edu/learning-zone/sun-space-weather/earth-magnetosphere

Center For Science Education (UCAR) (n.d.): What is the Atmosphere?. https://scied.ucar.edu/learning-zone/atmosphere/what-is-atmosphere