ESA Science & Technology - Cluster shows plasmasphere interacting with Van Allen belts

The invisible bubble created by Earth's magnetic field – the magnetosphere – has been studied by space missions for more than half a century. One of the first scientific breakthroughs made by a spacecraft was the discovery of Earth's radiation belts in 1958. Two concentric, tyre-shaped belts of highly energetic (0.1–10 MeV) electrons and protons, which are trapped by the magnetic field and travel around the Earth, were revealed by an experiment on the Explorer 1 satellite. They were named after the instrument's lead scientist, James Van Allen.

The inner Van Allen belt is located typically between 6000 and 12 000 km (1 - 2 Earth radii [R_E]) above Earth's surface, although it dips much closer over the South Atlantic Ocean. The outer radiation belt covers altitudes of approximately 25 000 to 45 000 km (4 to 7 R_E). This belt is much more dynamic than the inner one, as it is readily affected by solar outbursts that impact the magnetosphere. At such times, its density can vary by several orders of magnitude.

Both belts are separated from each other by an empty "slot" region. A temporary third belt, between this slot and the outer main belt, was detected earlier this year by NASA's Van Allen Probes.

The Van Allen radiation belts partly overlap with the plasmasphere, a doughnut-shaped region of low energy charged particles (known as plasma) which co-rotates with the Earth. The cold plasma in the plasmasphere plays a crucial role in governing the dynamics of Earth's radiation belts. It does so by determining the growth and propagation of Very Low Frequency (VLF) radio waves, which are responsible for the energisation of the Van Allen radiation belts and particle loss in the belts through wave-particle interaction.

These two overlapping regions of near-Earth space have been studied many times in different ways by spacecraft. However, attempts to identify and explain how they interact have been hampered by the types of instruments flown and by the satellites' orbits. The relationship between the plasmasphere and the radiation belt boundaries is being continually investigated and much remains to be discovered.

An important new contribution has been made by an international team of physicists, led by Fabien Darrouzet, a researcher at the Belgian Institute for Space Aeronomy in Brussels. Their paper, published in the Journal of Geophysical Research, is based on data sent back by one of the quartet of Cluster spacecraft, which has been flying in formation around the Earth since 2000.

During the period 1 April 2007 to 31 March 2009, the Cluster flotilla penetrated deep inside the plasmasphere and the radiation belts, with a lowest orbital point of 2 R_E. The team decided to take this rare opportunity to analyse populations of electrons of different energies in these regions with three of the instruments on board the Cluster satellite C3.

"We wanted to study the boundaries of the two regions – the plasmasphere and the radiation belts – with instruments on board the same satellite," explains Fabien Darrouzet. "Very precise complementary data could be collected at the same time and in the same place by using three different instruments on a single Cluster spacecraft."

The positions of the outer radiation belt's boundaries were deduced by analysing background data from the CIS instrument, which is sensitive to electrons with energy > 2 MeV, while the position of the plasmapause (the edge of the plasmasphere) was obtained from the WHISPER instrument, which is able to determine the electron density inside and outside the plasmasphere. These results were then refined by comparing them with data from the RAPID instrument, which determined the locations of the radiation belts' boundaries by detecting high energy electrons between 244 and 406 keV.

Several hundred data sets were obtained over the two year period of observation, which happened to coincide with a period of low solar activity and generally quiet geomagnetic conditions.

The team's analysis of the Cluster C3 observations showed more variety in the position of the outer edge of the plasmasphere – the plasmapause – than in the position of the furthest boundary of the outer radiation belt.

How geomagnetic conditions change the relative locations of the outer boundary of the Earth's plasmasphere (the plasmapause) and the Van Allen belts. Credit: ESA - C. Carreau.

For long periods, when geomagnetic activity was low, the plasmapause was located toward the farthest reaches of the outer belt – typically around 6 R_E, but sometimes expanding outward to 8 R_E or beyond. This result contrasted with previous studies based on other spacecraft observations, which indicated a correlation between the position of the inner edge of the outer belt and the position of the plasmapause.

However, there were indications of different behaviour during the occasional periods of higher geomagnetic activity. Then, the plasmapause moved closer to the inner boundary of the outer radiation belt, at around 4.5 R_E, as observed by previous studies.

During the periods of low geomagnetic activity, the plasmasphere was more easily filled by material from the underlying ionosphere - Earth's highest atmospheric layer. During geomagnetic storms, however, the diameter of the plasmasphere was reduced and the plasmapause moved closer to Earth.

The thickness of the slot region, which separates the two main belts, was also found to follow the variations in geomagnetic activity. Particle loss in the radiation belts increased after the activity decreased and the plasmasphere expanded, causing the slot region to become wider.

"Having studied the plasmasphere and radiation belts during solar minimum, we are now intending to use Cluster data to study the links between both regions during periods of higher geomagnetic activity," says Fabien Darrouzet. "We would also like to study the wave-particle interactions in those two regions and learn more about how they influence the distribution of the particles when solar maximum occurs."

"The presence of the radiation belts is a key factor in the design of all spacecraft in low Earth orbit, as well as a natural hazard for astronauts," comments Philippe Escoubet, ESA Project Scientist for Cluster. "Forecasting the dynamics of the belts is one of our prime objectives, but this is only achievable by understanding the underlying physics."

"The Cluster mission offers the rare opportunity to analyse different regions of the inner magnetosphere with identical sensors on multiple spacecraft," he adds. "With the launch of NASA's Van Allen Probes in 2012, we look forward to an even more productive period of complementary scientific studies of near-Earth space."

Background Information

The results described in this article are reported in 'Links between the plasmapause and the radiation belt boundaries as observed by the instruments CIS, RAPID, and WHISPER onboard Cluster' by F. Darrouzet et al., published in the Journal of Geophysical Research: Space Physics, volume 118, pp 4176-4188, 2013, doi: 10.1002/jgra.50239.

The study team was led by Fabien Darrouzet (Belgian Institute for Space Aeronomy (IASB-BIRA), Brussels, Belgium) and included V. Pierrard (IASB-BIRA and Université Catholique de Louvain, Center for Space Radiations (CSR), Belgium), S. Bench (Université Catholique de Louvain, Center for Space Radiations (CSR), Belgium), G. Lointier (Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, (LPC2E), Orléans, France), J. Cabrera (Université Catholique de Louvain, Center for Space Radiations (CSR), Belgium), K. Borremans (IASB-BIRA), N. Yu Ganushkina (Department of Atmospheric, Oceanic and Space Sciences (AOSS), University of Michigan, Ann Arbor, Michigan, USA and Finnish Meteorological Institute (FMI), Earth Observations, Helsinki, Finland.) and J. De Keyser (IASB-BIRA).

Cluster is a constellation of four spacecraft flying in formation around Earth. It is the first space mission able to study, in three dimensions, the natural physical processes occurring within and in the near vicinity of the Earth's magnetosphere. Launched in 2000, it is composed of four identical spacecraft orbiting the Earth in a pyramidal configuration, along a nominal polar orbit of 4 × 19.6 Earth radii (1 Earth radius = 6380 km). Cluster's payload consists of state-of-the-art plasma instrumentation to measure electric and magnetic fields over wide frequency ranges, and key physical parameters characterising electrons and ions from energies of near 0 eV to a few MeV. The science operations are coordinated by the Joint Science Operations Centre (JSOC) at the Rutherford Appleton Laboratory, United Kingdom, and implemented by ESA's European Space Operations Centre (ESOC), in Darmstadt, Germany.

Contacts

Fabien Darrouzet
Belgian Institute for Space Aeronomy (IASB-BIRA)
Brussels, Belgium
Email: Fabien.Darrouzetaeronomie.be
Phone: +32-2-373-03-81

Johan De Keyser
Belgian Institute for Space Aeronomy (IASB-BIRA)
Brussels, Belgium
Email: Johan.DeKeyseraeronomie.be
Phone: +32-2-373-03-68

C. Philippe Escoubet
Cluster Project Scientist
Research and Scientific Support Department
Directorate of Science & Robotic Exploration
ESA, The Netherlands
Email: Philippe.Escoubetesa.int
Phone: +31-71-565-3454