The two van Allen twin space probes, known originally as the Radiation Belt Storm Probes (RBSP), are about to complete their two-year primary mission, as they were launched on August 30, 2012. They’re still collecting data in the Van Allen radiation belts, two rings – one internal and one external – of electrically charged particles created by the Earth’s magnetic field around the equator. An article just published on line and then on paper in “Physical Review Letters” illustrates the mechanism of electron acceleration in the Van Allen belts.
The van Allen probes were build also with the goal of understanding the mechanisms behind the acceleration and transport of electrons in the Van Allen radiation belts. In particular, the outer belt is the most unstable with its continuous changes of shape, size and composition, phenomena still not well understood.
Some of the particles in the outer band are accelerated up to speeds approaching that of light. A recent analysis of data collected by the van Allen probes by a group of scientists led by Forrest Mozer of the University of California at Berkeley allowed to better understand this process.
According to Forrest Mozer and his colleagues, the acceleration of electrons occurs in two stages. A first process raises the particles energy up to a few thousand electronvolts. This initial mechanism is powered by short pulses of the local electric field that propagate parallel to the magnetic field lines of the van Allen radiation belts. The particles trajectory is determined by the geometry of the magnetic field.
In the second stage, the interaction between electrons and plasma electromagnetic waves causes a further acceleration which cause them to reach energies of the order of the megaelectronvolts. These waves are called Whistlers and are extremely low frequency waves of the type that can also be generated in lightnings.
Forrest Mozer explained that the Van Allen probes were able to monitor this process of acceleration better than any other spacecraft because they are designed and placed in a special orbit for that purpose. Therefore their mission is a success and the data collected are allowing to better understand what happens in the van Allen radiation belts.
It’s knowledge useful also from the practical side because the activity of the belts is influenced by the Sun and the consequences can be felt on satellites but sometimes even on electronic equipment on the ground. Knowing how the processes in the van Allen radiation belts work will help us to better protect ourseleves from their effects.
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