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
[1] It has been suggested that the equilibrium structure of the slot region, which separates the inner and outer radiation belts, forms as the result of a balance between inward radial diffusion and pitch angle scattering of relativistic electrons by interactions with three types of whistler mode waves: plasmaspheric hiss, lightening-generated whistlers, and ground-based Very Low Frequency (VLF) transmitters. In this study, using the time-dependent 3D Versatile Electron Radiation Belt (VERB) code, we examine how effectively the slot can be formed by a combination of radial diffusion and pitch angle diffusion, together with Coulomb scattering, and compare the simulations with the CRRES MEA 1 MeV electron observations to examine the viability of the various scattering mechanisms. The results show that the overall time evolution of the observed two-zone structure is in a good agreement with our model simulations, which suggests a balance between inward radial diffusion due to Ultra Low Frequency (ULF) electromagnetic fluctuations and pitch angle scattering due to plasmaspheric hiss and lightning-generated whistlers. However, when inward radial diffusion due to the electrostatic fluctuations is included, agreement between the observed and simulated fluxes becomes weaker, suggesting that it is important to understand and quantify the radial diffusion rates in the slot region.Citation: Kim, K.-C., Y. Shprits, D. Subbotin, and B. Ni (2011), Understanding the dynamic evolution of the relativistic electron slot region including radial and pitch angle diffusion,
[1] Understanding the dynamics of relativistic electron acceleration, loss, and transport in the Earth's radiation belt during magnetic storms is a challenging task. The U.S. National Science Foundation's Geospace Environment Modeling (GEM) has identified five magnetic storms for in-depth study that occurred during the second half of the Combined Release and Radiation Effects Satellite (CRRES) mission in the year 1991. In this study, we show the responses of relativistic radiation belt electrons to the magnetic storms by comparing the time-dependent 3-D Versatile Electron Radiation Belt (VERB) simulations with the CRRES MEA 1 MeV electron observations in order to investigate the relative roles of the competing effects of previously proposed scattering mechanisms at different storm phases, as well as to examine the extent to which the simulations can reproduce observations. The major scattering processes in our model are radial transport due to Ultra Low Frequency (ULF) electromagnetic fluctuations, pitch angle and energy diffusion including mixed diffusion by whistler mode chorus waves outside the plasmasphere, and pitch angle scattering by plasmaspheric hiss inside the plasmasphere. The 3-D VERB simulations show that during the storm main phase and early recovery phase the estimated plasmapause is located deep in the inner region, indicating that pitch angle scattering by chorus waves can be a dominant loss process in the outer belt. We have also confirmed the important role played by mixed energy-pitch angle diffusion by chorus waves, which tends to reduce the fluxes enhanced by local acceleration, resulting in comparable levels of computed and measured fluxes. However, we cannot reproduce the more pronounced flux dropout near the boundary of our simulations during the main phase, which indicates that non-adiabatic losses may extend to L-shells lower than our simulation boundary. We also provide a detailed description of simulations for each of the GEM storm events.
We find evidence that magnetic storms are not only unnecessary for geosynchronous relativistic electron enhancements but also not directly relevant to the electron enhancements even if the enhancements are accompanied by magnetic storms. What is crucial for electron enhancements at geosynchronous orbit are sustained south‐oriented or north‐south fluctuating interplanetary magnetic field (IMF) Bz that drives sufficiently large substorm activity and small solar wind density Nsw that likely leads to low loss rate of relativistic electrons to the ionosphere and/or to the magnetopause for an extended time period. Specifically, almost all the abrupt, large electron increases in our data set took place under the condition of average AE > 235 nT and average Nsw ≤ 5 cm−3. Examination of detailed time profiles clearly shows that electron flux starts to increase quite immediately with arrival of the right IMF and solar wind conditions, regardless of a magnetic storm, leaving the accompanied magnetic storms merely coincident.
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