Tightening the Belt
The RBSP Mission (explained in the HOPE article) aims to increase our understanding of the Earth's radiation belts. The inner belt is marked by great stability, but the outer belt is constantly changing. Radiation belt particles are lost, e.g. by collision with the rarefied gas of the outermost atmosphere, and new ones are frequently injected from the comet-like tail of the magnetosphere (the magnetotail). The particle population of the outer belt fluctuates widely and is generally weaker in energy (less than 1 MeV or one million electronvolts), rising to energies of order 10 MeV when geomagnetic storms occur. Geomagnetic storms are temporary disturbances of the magnetosphere (the space environment around Earth) usually driven by effects which occur on the sun. These storms (usually driven by the solar wind) cause fresh particles to be injected into the radiation belts from the magnetotail. The energy of the radiation belts falls to more typical quiet time levels during the subsequent days, known as the storm recovery phase.
It is this constant variability of the radiation belts that is of most interest to scientists. There are known phenomena which give rise to these changes but the radiation belts do not always respond in the same way to the drivers. For example, there is a close, but by no means simple, relationship between storms at Earth and changes in the radiation belts. Each of these storms was preceded by similar solar conditions. Due to complex processes that can occur simultaneously during the storm period, the radiation belts can be enhanced, depressed, or essentially unchanged compared with conditions before the storm.
In addition, temporary new belts can be created during magnetic storms, sometimes within minutes of the storm's onset. Solar energetic protons, accelerated at shock waves that emanate from the sun, can provide the "seed" population for new proton belts. Although it was once thought that the behavior of the radiation belts was well-understood, observations over the last decade have given rise to new and fundamental questions about the physical processes involved in the enhancement and decay of the belts and in the formation of new ones.
Artist's view of the twin probes in the two donut-shaped regions of Earth's magnetic field.