C H A P T E R
N ° 6
Space Weather and Low Earth Orbit (LEO)
The most at risk critical space infrastructure is satellites. Depending on the chosen orbital class a satellite is launched into, the radiation exposure level and the level of vulnerability to space weather impact may differ. Modern commercial satellites often share some similar features that makes them susceptible to the same type of effects.
There are three orbital classes that space agencies commonly launch satellites into. In today’s article, SR Hoplon will introduce the first of these orbital classes: Low Earth Orbit (LEO).
Image Credit: ESA: Showing Low Earth Orbit (LEO) and altitude.
The Earth’s Global Dipole Magnetic Field
In order to understand the impact of space weather and generally space radiation on the different orbital classes, we first have to understand the Earth’s magnetic field structure and shielding capabilities, and its interaction with incoming space radiation particles.
Earth has a global dipole magnetic field. This magnetic field structure refers to a magnetic field that has two poles – a north and a south pole – similar to a bar magnet. These poles are the Earth’s geomagnetic poles and are different to the planet’s geographical poles. 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. The magnetosphere of Earth is, however, not a perfect dipole, as there are small deviations in the field. The magnetic field lines are slightly distorted by the solar wind, which is a continues occurrence of plasma (containing charged particles) emitted from the Sun into the outer space environment. Additionally, there are small localised variations in the magnetic field due to the changes in the Earth’s interior and crust.
In addition, as the Earth’s global dipole magnetic field is not truly dipolar in space or at the Earth’s surface, it causes the internal magnetic field to have a depression centered over the South Atlantic region - also known as the ‘South Atlantic Anomaly’. This causes the Earth’s magnetosphere to be the weakest at its equator and the strongest at its poles.
Furthermore, in a dipolar magnetic field, the strength of the field decreases with increasing distance from the planet. Thus, the further away an object is to the planet, the less protection the magnetosphere provides. When looking at the Van Allen Belts, we can, therefore, see a difference between the type and the intensity level of radiation an object is exposed to, and, thus, the difference between launching a satellite into the outer radiation belt, the slot-region (‘safe’-region), and the region between the inner radiation belt and Earth.
Image Credit: Unknown: Illustrating the geomagnetic poles and geographic poles of Earth.
Low Earth Orbit (LEO)
Low Earth Orbit (LEO) is located between Earth and the inner Van Allen radiation belt. The orbit is located at an altitude of anything from ~160-1000 km above Earth. This may sound low but compared to an airplane that flies at an altitude of ~14 km it is still far away from Earth. Unlike other orbital classes, Low Earth Orbit (LEO) provides different orbital pathways for agencies to launch their satellites into. One of these is Polar Earth Orbit (PEO).
As the name suggests, the Polar Earth Orbit (PEO) is a pathway that passes through the Earth’s geomagnetic polar regions from north to south in a vertical pathway, instead of the typical tilted but horizontal pathway around Earth. It is located at an altitude of 200-1000 km above Earth, making it a type of low Earth orbit. The orbital path of the satellites within this orbital path is, however, not confined to a unique radius in order for it to be called ‘polar’. The satellites simply have to pass within 20-30 degrees of the poles.
A commonly used orbital pathway within Polar Earth Orbit (PEO) is Sun-Synchronous Orbit (SSO), which is located at an altitude of 600-800 km. Satellites within this orbit is synchronized to always be in the same ‘fixed’ position relative to the Sun. This means that the satellites always visit the same area on Earth at the position’s local time.
Image Credit: ESA: Showing Sun-Synchronous Orbit (SSO) and altitude.
Space Weather and Low Earth Orbit (LEO)
The inner Van Allen Belt is dominated by Galactic Cosmic Rays (GCRs). This type of space radiation has a low flux but a high energy level that makes them extremely penetrating. Flux indicates the number of particles moving through a substance per unit area per unit time. Thus, indicating the number of particles a substance is bombarded with within a specific period of time. In addition, Galactic Cosmic Rays (GCRs) have a high rate of energy deposition measured by their Linear Energy Transfer (LET). The Linear Energy Transfer (LET) indicates the average amount of energy that is lost per unit path-length as a charged particle travels through a given object. The combination of the low flux level but high energy level, and the high rate of energy deposition, thus, makes Galactic Cosmic Rays (GCRs) highly hazardous. These particles have the ability to penetrate through objects, causing a high level of radiation exposure within an objects interior.
The energy level of Galactic Cosmic Rays (GCRs) within the inner Van Allen radiation belt is dependent on the Sun’s 11-year solar cycle that goes from solar minimum to solar maximum. During solar minimum, the least number of activities occur on the Sun, whereas the opposite happens during solar maximum. The risk of space weather impact, thus, increases during solar maximum, due to the higher frequency of occurrence of solar activities. However, this does not mean that there is no risk of space weather impact during solar minimum. During solar minimum the frequency of solar activities occurring are low, consequently reducing the likelihood of a space weather event to happen. Although, it does not completely eliminate the possibility.
During solar maximum, enhanced number of particles (Solar Energetic Particles (SEPs) originating from the Sun) and complex interplanetary magnetic field from the Sun bursts out into the outer space environment. If Earth-directed, it can penetrate through the outer and inner Van Allen radiation belts and, thus, the slot-region, and interact with the already present Galactic Cosmic Rays (GCRs) between the inner radiation belt and Earth. During this process, the Solar Energetic Particles (SEPs) enhances the overall energy level of the particles within this region. This can produce momentarily high radiation levels and exposure lasting from hours to several days, making the space environment in orbital classes like Low Earth Orbit (LEO) more hazardous.
As mentioned earlier, despite Earth having an approximate global dipole magnetic field, it has a depression center over the South Atlantic region (The South Atlantic Anomaly). This anomaly brings the Van Allen radiation belts closer to Earth at this region. The inner radiation belt – composed of high-energy protons – can approach closets to Earth’s surface in this weaker field region. This makes the region between the inner radiation belt and Earth more narrow, consequently making spacecrafts orbiting Low Earth Orbit (LEO) to encounter more intense particle flux when passing this region. This is because Solar Energetic Particles have a higher flux level than that of Galactic Cosmic Rays (GCRs).
Image Credit: NASA: Cartoon of the Van Allen Belts and orbital satellite classes.
Space Weather awareness and orbital classes
In order to understand how space weather is created and its impact on celestial bodies and their surrounding outer space environment, it is crucial to understand planetary magnetic fields and the behavior of particles in a plasma. This is because the plasma emitted from the Sun has an interplanetary magnetic field (IMF). This magnetic field is a part of the Sun’s magnetic field and is carried out into interplanetary space by different solar activities. The interplanetary magnetic field lines are said to be ‘frozen in’ to the plasma. During certain conditions, this enables Solar Energetic Particles (SEPs) to interact with planetary magnetic fields at a level we do not see the already present Galactic Cosmic Rays (GCRs) do.
Despite the risk of Solar Energetic Particles (SEPs) interacting with satellites in Low Earth Orbit (LEO), there are many advantages of launching satellites into this orbital class. Firstly, as previously mentioned, the risk of exposure to Solar Energetic Particles (SEPs) and space weather increases with the increasing distance from Earth. As Low Earth Orbit (LEO) is the closest orbital class to Earth, it is one of the safest – if not the safest - of all orbital classes to launch a satellite into in regards to exposure to Solar Energetic Particles (SEPs). Secondly, satellites within this class provides high coverage of Earth, low signal latency, and the location gives them the ability to overcome geographic conditions. This makes satellites in this orbital class a powerful addition to ground networks. The third and last advantage of this class is that satellites within this orbital class are much smaller, consequently taking up less space.
One must, however, not forget to consider the disadvantageous of Low Earth Orbit (LEO). Satellites within this orbital class move very fast across the sky. Ground stations can only see a satellite for a maximum of ~10 minute each orbit, consequently causing a need for satellite constellations in order to for example cover a specific geographical area. Satellite constellations add to the network complexity, as many ground stations are needed to communicate with all these satellites in order to avoid interference with each other’s communication. In addition, these constellations increase the risk of space contamination, and satellite colliding with other satellites or space debris which would furtherly increase space debris in the near-Earth space environment.
Furthermore, satellites within Low Earth Orbit (LEO) are more susceptible to collisions with space debris and the risk of atmospheric drag. Atmospheric drag increases the risk of satellites re-entering Earth. These risks consequently demand higher fuel consumptions in order to perform collision maneuvers, and to keep satellites within their orbital path. Thus, satellites within this orbital class can be more costly. Moreover, the orbital path within Polar Earth Orbit (PEO) changes due to the rotation of Earth, meaning that no particular spot on the planet can be viewed continuously within this pathway. In addition, satellites launched into Polar Earth Orbit (PEO) require a larger launch vehicle and fuel, as the launching vehicles cannot take advantage of Earth’s rotational velocity like it usually does with launches to other orbits.
When considering to launch a satellite into Low Earth Orbit (LEO), agencies have to consider a lot of different obstacles. Although this orbital class is the least exposed to Solar Energetic Particles (SEPs), agencies and companies should still be aware of its effect. Severe and extreme space weather events can produce particles penetrating all the way through all orbital classes - including Low Earth Orbit (LEO) -, Earth’s magnetosphere, atmosphere, and further. This means, that despite the Earth’s magnetic field being strong, it cannot withstand all levels of space weather. The risk of satellites in Low Earth Orbit (LEO) being exposed to high and complex radiation from space weather is, thus, still significant.
Sources
Rajkumar, Hajra; Bruce, T. Tsurutani (2018): ”Chapter 14 – Magnetospheric ”Killer” Relativistic Electron Dropouts (REDs) and Repopulation: A Cyclical Process”. Extreme Events in Geospace. Elsevier. PP. 373-400. DOI: https://doi.org/10.1016/B978-0-12-812700-1.00014-5
James P. McCollough et al. (2022): “Space-to-space very low frequency radio transmission in the magnetosphere using the DSX and Arase satellites”. Earth Planets Space 74. Article No. 64. DOI: https://doi.org/10.1186/s40623-022-01605-6
Krausmann, Elisabeth et al. (2016): Space Weather & Critical Infrastructures: Findings and Outlook. JRC Science for Policy Report. DOI: 10.2788/152877
Baker, D.N et al. (2004): Effect of Space Weather on Technology Infrastructure. AGU Vol. 2, Issue 2. DOI: https://doi.org/10.1029/2003SW000044
Siqi Peng et al. (2022): “An Anti-Interference On-Demand Routing Algorithm for LEO Satellite Networks”. IEEE. 95th Vehicular Technology Conference. Pp. 1-5. DOI: https://doi.org/10.1109/VTC2022-Spring54318.2022.9860908
Nagaraj G. S.; Vivek B. A. (2023): ”High-Speed Networks for Information Communication Technologies for Disaster Recovery Operations”. International Journal for Research in Applied Science & Engineering Technology (IJRASET). Vol. 11, Issue V. DOI: https://www.doi.org/10.22214/ijraset.2023.51662
D. N. Baker et al. (2017): “Space weather effects in the Earth’s radiation belts”. Space Science Reviews. Vol. 214, No. 17.
D. F. Smart; M. A. Shea (1985): “Galactic Cosmic Radiation and Solar Energetic Particles”. Handbook of geophysics and the space environment. https://www.cnofs.org/Handbook_of_Geophysics_1985/Chptr06.pdf
United States Environmental Protection Agency (EPA) (n.d.): “Cosmic Radiation”. https://www.epa.gov/radtown/cosmic-radiation
ESA (n.d.): “Types of orbits”. https://www.esa.int/Enabling_Support/Space_Transportation/Types_of_orbits#SSO
ESA (n.d.): “The polar orbit”. https://www.esa.int/Education/4._The_polar_orbit
NOAA (n.d.): “NOAA’s Geostationary and Polar-Orbiting Weather Satellites”. https://paos.colorado.edu/~fasullo/1060/resources/satellites.html
Eric Ralls (n.d.): “Earth’s Magnetic Field: Origin, Structure, and Impact on Humanity”. Earth.com, Nature, Science, Life: https://www.earth.com/earthpedia-articles/earths-magnetic-field-origin-structure-and-impact-on-humanity/
