Reply to 'The dynamics of Van Allen belts revisited'

It is well-known that there are many wave-particle interaction processes which have the potential to affect the dynamics of the radiation belts [see e.g., the review by Mauk et al., 2013]. The issue that has continued to obstruct significant advances in our understanding of the radiation belts to the point of predictability is our ability to represent the nature of the magnetospheric processes controlling belt dynamics with sufficient accuracy to establish which dominate. In relation to the case examined here it is to determine which process or processes can act to create a third Van Allen radiation belt morphology in September 2012 as reported by Baker et al., (2013). As described in the main text of our Reply, and further expanded upon in the Supplementary Material presented here, we show that the original conclusion from Mann et al. (2016) remains valid. That is, a remnant belt and the third radiation belt morphology which arises following a subsequent flux recovery at higher L-shells, can be explained by the action of very fast outwards ULF wave radial diffusion associated with magnetopause shadowing. Contrary to the claims of the Comment by Shprits et al. (2017; hereafter S17), and the conclusions of modelling by Shprits et al. (2013; hereafter S13), the action of EMIC waves is not required

Explaining the dynamics of the ultra-relativistic third Van Allen radiation belt

Since the discovery of the Van Allen radiation belts over 50 years ago, an explanation for their complete dynamics has remained elusive. Especially challenging is understanding the recently discovered ultra-relativistic third electron radiation belt. Current theory asserts that loss in the heart of the outer belt, essential to the formation of the third belt, must be controlled by high-frequency plasma wave–particle scattering into the atmosphere, via whistler mode chorus, plasmaspheric hiss, or electromagnetic ion cyclotron waves. However, this has failed to accurately reproduce the third belt. Using a data driven, time-dependent specification of ultra-low-frequency (ULF) waves we show for the first time how the third radiation belt is established as a simple, elegant consequence of storm-time extremely fast outward ULF wave transport. High-frequency wave–particle scattering loss into the atmosphere is not needed in this case. When rapid ULF wave transport coupled to a dynamic boundary is accurately specified, the sensitive dynamics controlling the enigmatic ultra-relativistic third radiation belt are naturally explaine

Hadronic supercriticality as a trigger for γ-ray burst emission

We explore a one-zone hadronic model that may be able to reproduce γ-ray burst (GRB) prompt emission with a minimum of free parameters. Assuming only that GRBs are efficient high-energy proton accelerators and without the presence of an ab initio photon field, we investigate the conditions under which the system becomes supercritical, i.e. there is a fast, non-linear transfer of energy from protons to secondary particles initiated by the spontaneous quenching of proton-produced γ-rays. We first show analytically that the transition to supercriticality occurs whenever the proton injection compactness exceeds a critical value, which favours high proton injection luminosities and a wide range of bulk Lorentz factors. The properties of supercriticality are then studied with a time-dependent numerical code that solves concurrently the coupled equations of proton, photon, electron, neutron and neutrino distributions. For conditions that drive the system deep into the supercriticality, we find that the photon spectra obtain a Band-like shape due to Comptonization by cooled pairs and that the energy transfer efficiency from protons to γ-rays and neutrinos is high reaching ~0.3. Although some questions concerning its full adaptability to the GRB prompt emission remain open, supercriticality is found to be a promising process in that regard. © 2014 The Authors