How do electrons vanish from the Van-Allen-Belts?

Abb. 1: Visualisierung der magnetischen Umgebung der Erde mit den magnetischen Feldlinien (M. Rother, GFZ).
Abb. 2: Van-Allen-Sonden und Magnetfeldlinien (I. Michaelis/Y. Shprits, GFZ)

The Van Allen Radiation Belts –two regions of high energy particles in the near-Earths space– were the first discovery of the space age. Over half a century later the dynamics of the belts are still poorly understood. Electrons are accelerated and lost in a matter of days or hours. The difficulty in predicting the behavior of these belts lies in the competition of electron acceleration and loss mechanisms that are not yet quantified. While the acceleration mechanisms received significant attention in the recent years, the loss mechanisms remain poorly understood.

A new study published in the journal Scientific Reports clarifies arguably the most important question of the radiation belt dynamics, and the long standing questions of why and how electrons can disappear from the belts on time scales as short as only a few hours. The study has two first authers who contributed equally: Prof. Yuri Shprits from the GFZ German Center for Geosciences, Potsdam, and Xing Cao, a graduate student from Wuhan University who visited the GFZ earlier this year and conducted this research at Section 2.8 Magnetospheric Physics. It was suggested in the seventies that plasma waves which are fluctuations of magnetic and electric fields produced by ions (so-called Electromagnetic Ion Cyclotron (EMIC) waves) can scatter electrons into the atmosphere.

These waves can produce a loss of relativistic electrons into the atmosphere, similar to the loss of particles that produce aurora borealis (the ‘northern lights’) and aurora australis (the ‘southern’ lights) at higher latitudes. Particles are scattered by waves and fly along the lines of constant magnetic field (see illustration) into the atmosphere. Numerous previous theoretical calculations show that it is possible for these waves to throw relativistic electrons into the atmosphere.

However, recent observations [e.g. Shprits et al., 2013, Nature Physics, 2016 Nature Communications, 2017 GRL] show that only ultra-relativistic electrons (electrons whose kinetic energy by far exceeds the energy when electrons are at rest computed with Einstein’s famous equation E=m0c2) are subject to significant loss into the atmosphere by EMIC waves, while the relativistic electron population is almost immune to EMIC wave activity.

In this new study, the authors took into account the kinetic effects associated with a finite temperature of the plasma in so-called hot plasmas as opposed to most commonly used approximations of plasma being at zero kelvin. By taking hot plasma effects into consideration, the authors demonstrate that EMIC waves mainly contribute to the loss of the ultra-relativistic electron population. This study provides a theoretical explanation for the discrepancy between previous theoretical studies and recent observations, and reconciles theory with observations. This study will be highly relevant for the broader astrophysics community, as it provides quantitative predictions on the upper limit of the electron populations that can be trapped by magnetospheres, which will in turn help to understand the radiation environment of exoplanets and outer planets.

Original study: Cao, X., Shprits, Y. Y., Ni, B., Zhelavskaya, I. S., 2017. Scattering of Ultra-relativistic Electrons in the Van Allen Radiation Belts Accounting for Hot Plasma Effects, Scientific Reports.