Tightening the radiation belts

Extreme solar conditions caused the Earth’s radiation belts to shrink temporarily, yielding an opportunity to investigate particle acceleration in those regions.

In Nature this week, Richard Horne and colleagues¹ present a fresh analysis of data gathered at the time of the ‘Hallowe’en storms’ — a period of intense solar activity around the end of October 2003 — that forces a rethink of the mechanism of particle acceleration in the Van Allen belts.

The Van Allen radiation belts are torus-shaped regions surrounding the Earth, in which intense fluxes of energetic particles are trapped by the planet’s magnetic field. Horne et al. find that the acceleration mechanism generally thought to energize particles in these belts cannot account for the particles’ behaviour during the unusually active period of late 2003. Instead, they propose that interactions between high-frequency electromagnetic waves and the local plasma are responsible for filling the belts with high-energy particles.

The inner radiation belt extends from a height of 600 km to 6,000 km above the Earth’s equator. It is filled mainly with the by-products of collisions as cosmic rays hit the Earth’s atmosphere. The outer radiation belt is much larger and more variable in extent, but on average lies between 12,000 and 60,000 km in altitude and is filled by ‘injections’ of particles when magnetic storms occur. It has long been believed that large-scale oscillations in the magnetosphere — the magnetic bubble that surrounds the Earth — are responsible for exciting the radiation-belt particles to relativistic velocities.

Figure 1: The Hallowe’en storm.

In late 2003, a solar storm caused a massive distortion of the Earth’s plasmasphere, shown in these images², and of the Van Allen radiation belts. (Scale: units of Earth radius, equivalent to 6,372 km.)

Horne et al. examined data taken by both ground-based and space-borne instruments as the centre of the outer belt was displaced inwards by 6,000 km during the extreme conditions of Hallowe’en 2003 (Fig. 1). The authors show that the power held within the global oscillations is low when the particles in the radiation belts are being energized, contrary to expectation. It has been suggested that particle interactions with much higher frequency ‘chorus’ waves might be a more likely acceleration mechanism within the radiation belts, and this is borne out by Horne and colleagues’ data. Their results from numerical simulations show that high-frequency wave acceleration can indeed provide sufficient fluxes of energetic particles to explain the observations.

It seems that wave–particle interactions play an important role in both the source and sink mechanisms of the particles within the radiation belts, and also possibly for other astrophysical objects with magnetic fields.