September 12, 2013
Cluster Data Shows Intriguing Links Between Plasmasphere And Van Allen Belts
[ Watch the Video: Earth’s Plasmasphere and the Van Allen Belts ]
April Flowers for redOrbit.com - Your Universe Online
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."
By analyzing background data from the CIS instrument, which is sensitive to electrons with energy > 2 MeV, the team was able to deduce the positions of the outer belt's boundaries. They obtained the position of the plasmapause (the edge of the plasmasphere) using the WHISPER instrument, which is able to determine the electron density inside and outside the plasmasphere. The team then refined their results by comparing them to data from the RAPID instrument, which determined the locations of the radiation belts' boundaries by detecting high energy electrons between 244 and 406 keV.
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 obtained several hundred data sets. The analysis of the Cluster C3 data revealed 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.
The researchers found that for long periods of low geomagnetic activity, the plasmapause was located toward the farthest reaches of the outer belt – typically around 6 RE, but sometimes expanding outward to 8 RE or beyond. Previous studies based on other spacecraft observations indicated a correlation between the position of the inner edge of the outer belt and the position of the plasmapause.
Indications of different behaviors were present, however, during the occasional periods of higher geomagnetic activity. During these periods, the plasmapause moved closer to the inner boundary of the outer radiation belt, at around 4.5 RE, as observed by previous studies.
The plasmasphere was more easily filled by material from the underlying ionosphere - Earth's highest atmospheric layer- during periods of low geomagnetic activity. However, during the geomagnetic storms the plasmasphere diameter was reduced and it moved closer to Earth.
The slot region thickness was also found to follow the variations in geomagnetic activity. After activity decreased and the plasmasphere expanded, causing the slot region to widen, particle loss in the radiation belts increased.
"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," said 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," commented 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 analyze 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."
Image 2 (below): 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 [ More Information ]