Radiation belt electron acceleration during periods of low plasma density

Abstract

<p>Electrons in the Van Allen radiation belts can have energies in excess of 7 MeV. We present a unique way of analyzing phase space density data which demonstrates that local acceleration is capable of heating electrons up to 7 MeV. The Van Allen Probes mission not only provided unique measurements of ultra-relativistic radiation belt electrons, but also simultaneous observations of plasma waves that allowed for the routine inference of total plasma number density. Based on long-term observations, we show that the underlying plasma density has a controlling effect over local acceleration to ultra-relativistic energies, which occurs only when the plasma number density drops down to very low values (~10 cm<sup>-3</sup>). The VERB-2D model is used to simulate ultra-relativistic electron acceleration during an event which exhibits an extreme cold plasma depletion. The results show that a reduced electron plasma density allows chorus waves to efficiently resonate with electrons up to ultra-relativistic energies, producing enhancements from 100s of keV up to >7 MeV via local diffusive acceleration. We analyse statistically the observed chorus wave power during ultra-relativistic enhancement events, considering the contribution from both upper and lower band chorus waves. The PINE density model allows for the investigation of global magnetospheric density changes. We analyze the how the global cold plasma density changes during ultra-relativistic enhancement events and compare to in-situ point measurements of the plasma density.</p>

Intense chorus waves mitigate the loss of outer radiation belt relativistic electrons during storm main phase

“…3. A low background plasma density that creates a preferential condition for the local diffusive acceleration of electrons from hundreds of keV to several MeV (Allison et al, 2021). shown with solid black lines to highlight periods of significant flux variations.…”

“…3. A low background plasma density that creates a preferential condition for the local diffusive acceleration of electrons from hundreds of keV to several MeV (Allison et al, 2021). The source electron population exhibits an initial strong acceleration followed by a loss, and the seed electron population exhibits a strong acceleration followed by no/small flux variations.…”

Modeling of the cold electron plasma density for radiation belt physics

“…Machine learning models also account intrinsically for multiple dependences (e.g., Chu et al, 2017a;Zhelavskaya et al, 2021), and are undoubtedly a promising approach to combine multiple satellite observations and produce the next-generation of global empirical plasma density models. A neural network-based density model has recently served to show that the plasma density has a controlling effect over acceleration of radiation belt electrons to ultra-relativistic energies (Allison et al, 2021). Contrary to empirical fits that do not allow trustable extrapolation, machine learning techniques, such as neural networks, are extremely promising in terms of predictive capability, which is a keystone for space weather codes.…”

“…A neural-network-based upper hybrid resonance (UHR) determination algorithm (NURD) was developed to automatically determine the electron density from plasma wave measurements using Van Allen Probes data (Zhelavskaya et al, 2016(Zhelavskaya et al, , 2018. NURD is applied to Van Allen Probes EMFISIS data in Allison et al (2021) to show that the plasma density has a controlling effect over acceleration of radiation belt electrons to ultra-relativistic energies. ML-based methods for automatically determining the UHR frequency have also been applied to the Arase satellite using convolutional neural network (Hasegawa et al, 2019;Matsuda et al, 2020) and the CLUSTER mission using several automated pipelines based on neural network methods (Gilet et al, 2021).…”

Outer Radiation Belt Flux and Phase Space Density Response to Sheath Regions: Van Allen Probes and GPS Observations

“…Likely both local acceleration and inward radial diffusion are contributing to the enhancements (e.g., Allison et al, 2019;Lejosne et al, 2022). Geoeffective sheaths are typically associated with strong substorm activity (e.g., Pulkkinen et al, 2007;Kalliokoski et al, 2020), which in turn excites chorus waves that can efficiently accelerate electrons to > 1 MeV energies (Miyoshi et al, 2013;Jaynes et al, 2015;Allison et al, 2021).…”