Mind The Gap: Space Scientists Uncover Causes Of Gap In Van Allen Belts

High above the Earth's atmosphere, energetic charged particles are trapped in the Earth's magnetic field where they form the Van Allen radiation belts. Energetic electrons, travelling close to the speed of light, occupy two doughnut shaped zones, usually separated by a gap known as the slot region.

The underlying mechanism that clears the slot region of electrons has been the subject of intense scientific debate. Now, based on analysis of wave data collected over 13 months by the CRRES satellite, Dr Nigel Meredith of British Antarctic Survey and colleagues from BAS, the University of California, Los Angeles and the University of Iowa, believe that the gap is most likely formed by natural wave turbulence in space, rather than by lightning as the alternative theory suggests. Their results are published in the Journal of Geophysical Research this week.

According to lead author, Dr Nigel Meredith:

"Last year NASA scientists suggested that lightning-generated radio waves leaking out into space are responsible for the gap between the two belts by dumping particles into the atmosphere. Since lightning occurs far more often over land than water, waves in space should also occur more over land. However, after analysing satellite data we found that there is no land-ocean variation at frequencies less than 1 kiloHertz where the waves are most intense. Instead, wave activity increases during geomagnetic disturbances driven by the Sun, suggesting that natural wave turbulence is responsible for the gap."

"The results are important, because a better understanding of the radiation belts will help modellers forecast space weather more accurately, helping to protect both astronauts and satellites from radiation hazards."

Van Allen radiation belts

The Van Allen radiation belts were the foremost discovery of the space age after being detected by the first US satellite Explorer I, which was launched during the International Geophysical Year of 1957-58. They are composed of energetic charged particles trapped inside the Earth's magnetic field, which surrounds the Earth like a ring doughnut. Energetic electrons in the Earth's Van Allen radiation belts occupy two distinct regions. The inner zone, which typically extends from altitudes of 200 km to 7000 km in the equatorial plane, is relatively stable. In contrast, the outer zone, which typically lies between 13,000 km and 40,000 km in the equatorial plane, is highly dynamic. The gap between the two zones, known as the slot region, is usually devoid of energetic electrons. However, the slot can fill during strong magnetic storms, such as witnessed during the so-called Halloween storm in 2003. Particles in the slot are subsequently lost, following interactions with the radio waves, on a time-scale of days.

Professor James Van Allen died, aged 91, in Iowa in August 2006. His memorial service was held this month.

Combined Release and Radiation Effects Satellite

The Combined Release and Radiation Effects Satellite (CRRES) was a joint NASA and U.S. Department of Defense mission to study the near Earth space environment and the effects of the Earth's radiation environment on microelectronic components. The satellite was launched on 25 July 1990 and operated in a highly elliptical geosynchronous transfer orbit with a perigee of 305 km, an apogee of 35,768 km and an inclination of 18o. The orbital period was approximately 10 hours. The satellite swept through the Van Allen radiation belts on average approximately 5 times per day, providing good coverage of this important region for 13 months.

Competing theories

Radio waves in space, known as plasmaspheric hiss, are responsible for the formation of the slot region between the inner and outer radiation belt. While the details of the loss process are well known, the source of these waves has been a matter of intense debate for several decades. There are two competing theories. One theory maintains that the radio waves are generated locally via natural turbulence in space, arising from particles injected during enhanced magnetic activity driven by the Sun. The other theory suggests that the radio waves, generated by lightning activity on Earth, leak into space and evolve into hiss after multiple reflections in space.

Enhanced fluxes of energetic particles damage spacecraft and are a risk to humans in space. Improved understanding of the weather in space will help protect the satellites and astronauts operating in these regions.

Jupiter, Saturn, Uranus and Neptune all have strong magnetic fields and radiation belts. Improved understanding of the significant processes affecting the Earth's radiation belts will help astronomers to understand the radiation belts of other planets.

British Antarctic Survey is a world leader in research into global issues in an Antarctic context. It is the UK's national operator and is a component of the Natural Environment Research Council. It has an annual budget of around £40 million, runs eight research programmes and operates five research stations, two Royal Research Ships and five aircraft in and around Antarctica. More information about the work of the Survey can be found at: http://www.antarctica.ac.uk