Cluster Data Shows Intriguing Links Between Plasmasphere And Van Allen Belts

For more than half a century, the invisible bubble created by Earth’s magnetic field – the magnetosphere – has been studied by space missions. The discovery of Earth’s radiation belts in 1958 was one of the first scientific breakthroughs made by a spacecraft. The Explorer 1 satellite revealed two concentric, tire-shaped belts of highly energetic (0.1–10 MeV) electrons and protons, which are trapped by the magnetic field and travel around the Earth. Typically between 3700 and 7500 miles (1 – 2 Earth radii [RE]) above Earth’s surface, the inner Van Allen belt sometimes dips much closer over the South Atlantic Ocean. The altitudes of the outer belt ranges from 15,500 to 30,000 miles (4 to 7 RE). The outer belt is much more dynamic than the inner belt because it is readily affected by solar outbursts that impact the magnetosphere. During the solar outbursts, the outer belt’s density can vary by several orders of magnitude.

An empty “slot” region separates the belts from one another. NASA’s Van Allen Probes detected a third, temporary belt between the slot and the outer main belt earlier this year.

The plasmasphere is a donut shaped region of low energy charged particles that co-rotates with Earth. The Van Allen belts partly overlap this region, where the cold plasma plays a crucial role in governing the dynamics of Earth’s radiation belts. The plasmasphere determines the growth and propagation of Very Low Frequency (VLF) radio waves, which are responsible for the energization 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 in different ways by many spacecraft. The types of instruments flown and the satellites’ orbits have hampered attempts to identify and explain the mechanisms of the interactions. Scientists continually investigate the complex relationship between the plasmasphere and the radiation belt boundaries and much remains to be discovered.

Fabien Darrouzet, a researcher at the Belgian Institute for Space Aeronomy in Brussels, led a team of physicists who have made an important new contribution to the search for answers. The team based their findings on data retrieved from one of the quartet of Cluster spacecraft, which have been flying in formation around the Earth since 2000. The findings of this study were published in the Journal of Geophysical Research.

The Cluster quartet penetrated deep inside the plasmasphere and the radiation belts, with a lowest orbital point of 2 RE, from April 1, 2007, to March 31, 2009. The team analyzed populations of electrons of different energies during this rare window using 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 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.”

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