Unusual stable trapping of the ultrarelativistic electrons in the Van Allen radiation belts

Abstract

Radiation in space was the first discovery of the space age. Earth's radiation belts consist of energetic particles that are trapped by the geomagnetic field and encircle the planet 1. The electron radiation belts usually form a two-zone structure with a stable inner zone and a highly variable outer zone, which forms and disappears owing to waveparticle interactions on the timescale of a day, and is strongly influenced by the very-low-frequency plasma waves. Recent observations revealed a third radiation zone at ultrarelativistic energies 2 , with the additional medium narrow belt (longlived ring) persisting for approximately 4 weeks. This new ring resulted from a combination of electron losses to the interplanetary medium and scattering by electromagnetic ion cyclotron waves to the Earth's atmosphere. Here we show that ultrarelativistic electrons can stay trapped in the outer zone and remain unaffected by the very-low-frequency plasma waves for a very long time owing to a lack of scattering into the atmosphere. The absence of scattering is explained as a result of ultrarelativistic particles being too energetic to resonantly interact with waves at low latitudes. This study shows that a different set of physical processes determines the evolution of ultrarelativistic electrons. Over half a century ago, on the basis of the observations of the first US satellite mission Explorer 1, James Van Allen and colleagues from the University of Iowa discovered the inner radiation belt 3. The inner belt is very stable and consists of electrons and protons trapped between 1.2 and 2.0 Earth radii. Later, USSR and US missions showed that the radiation belts exhibited a two-zone structure 1,4 ; there is an additional outer belt present at higher distances (∼ > 3R E). A region of relatively low electron fluxes separating the belts is usually referred to as the slot region. The outer belt consists of energetic electrons and is highly dynamic and variable. It is produced by the continuous acceleration of electrons during inward transport (second-order Fermi acceleration and betatron acceleration) 5-7 and local acceleration due to resonance with plasma waves 8-10. The energetic electrons are continuously lost to the atmosphere and also regularly depleted as a result of losses to the magnetopause (the boundary of the Earth's magnetosphere) 11-13. The two-zone structure has been observed to be altered owing to very unusual acceleration events. Observation of the so-called Halloween superstorm showed that electrons can be accelerated to relativistic energies in the slot region between the two belts. The extreme filling of the slot region in October-November 2003 was

The effects of the big storm events in the first half of 2015 on the radiation belts observed by EPT/PROBA-V

“…This new ring resulted from a combination of electron losses to the interplanetary medium and scattering by electromagnetic ion cyclotron waves to the Earth's atmosphere (Shprits et al, 2013). Associated to recent observations of Van Allen Probe spacecraft, our results show that the radiation belts are sometimes different from the general feature.…”

Prompt energization of relativistic and highly relativistic electrons during a substorm interval: Van Allen Probes observations

“…This expansion places the peak of the multi-MeV particles, now recovered in flux levels, inside the plasmasphere. Shprits et al [2013] model a similar situation in which electrons at low L shells find themselves inside the plasmasphere in a very different plasma environment, where electromagnetic ion cyclotron waves are not present and the strongest emissions are whistler mode hiss. That study points out that, in this configuration, energetic and relativistic electrons will resonate with hiss waves near the equator and will be lost to the atmosphere on a time scale of a few days.…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…Shprits et al (2013) demonstrated that EMIC waves play a crucial role for the dynamics of the ultrarelativistic populations in the radiation belts. Detached proton subauroral arcs are caused by the precipitation of ring current protons into the atmosphere produced by EMIC waves (e.g., Jordanova et al 2007;Spasojevic and Fuselier 2009;Yuan et al 2010).…”