FG: System Understanding of Radiation Belt Particle Dynamics through Multi-spacecraft and Ground-based Observations and Modeling

Focus Group Chairs

Hong Zhao, LASP, University of Colorado Boulder (hong.zhao@lasp.colorado.edu)

Lauren Blum, NASA Goddard Flight Center (lauren.w.blum@nasa.gov)

Sasha Ukhorskiy, JHU/APL (sasha.ukhorskiy@jhuapl.edu)

Xiangrong Fu, New Mexico Consortium (sfu@newmexicoconsortium.org)

Focus Group Science Topic

The Earth’s radiation belts are filled with energetic particles, which exhibit acceleration, transport, and loss processes under the influence of many physical mechanisms at timescales from minutes to days. Understanding the effectiveness and relative importance of physical mechanisms on radiation belt particles is of both scientific interest and practical needs. Since the discovery of Earth’s radiation belts 60 years ago, a lot of progress has been made on understanding the radiation belt dynamics based on in situ and ground-based observations as well as modeling efforts. Specifically, with recent missions such as Van Allen Probes, Arase, and CubeSats, many mysteries of radiation belt particles have been discovered. However, single-point measurements have limitations in revealing underlying physical mechanisms on the radiation belt particles due to spatial/temporal ambiguities and limited coverage. This focus group (FG) aims to deepen understanding of radiation belt particle dynamics on both local and global scales through coordinated measurements from multi-spacecraft as well as ground-based observations, combining with theoretical and modeling efforts. The science goals of this FG are to advance our understanding of newly explored topics which will greatly benefit from such coordinated measurements, specifically: 1) the physical mechanisms related to radiation belt electron acceleration and loss on short timescales (minutes to hours); 2) quantification of energetic electron precipitation into the atmosphere and the related physical mechanisms; 3) the properties and spatiotemporal distribution of waves in radiation belts and their effects on radiation belt particles; 4) dynamics of inner belt and slot region particles.

Timeliness

This new FG is timely because of ongoing and upcoming inner magnetosphere missions, newly revealed properties of radiation belt particle dynamics, and recently improved modeling capability in the radiation belts.

Recent and ongoing missions, including Van Allen Probes, Arase, THEMIS, MMS, LANL_GEO, GOES, POES, ground-based stations (e.g., Halley and CARISMA), CubeSats and balloons (e.g., AC6, CSSWE, FIREBIRD, and BARREL), and other space missions (e.g., GPS), provide comprehensive measurements of radiation belt particles and waves with good spatial and temporal coverage. The growing Heliophysics System Observatory, which includes a number of constellation missions, enables the multi-spacecraft and multi-mission sampling throughout the inner magnetosphere and thus provide improved observational capacities for radiation belt studies.

Upcoming missions, including the Air Force’s DSX mission and newly selected CubeSat missions focusing on the radiation belt studies (e.g., GTOSat, CIRBE, REAL, LAMP, etc.), are expected to launch within the next couple of years and will provide further radiation belt measurements at various orbits, such as LEO, MEO, GTO, etc. A number of new phenomena in the Earth’s radiation belts have been discovered in recent years, including the three-belt structure, the prevalent bump-on-tail energy spectrum inside the plasmasphere, impenetrable barrier of ultrarelativistic electrons, and cosmic ray albedo neutron decay (CRAND) as a dominant source for inner belt quasi-trapped electrons, which require further investigation through various spatiotemporal scales of observations and modeling.

Recently improved modeling capabilities, including localized models (e.g., particle-in-cell simulation, nonlinear interaction of radiation belt electrons and waves, etc) and global models (e.g., DREAM, VERB, RAM-SCB, etc), can help study the underlying physical mechanisms of observed particle dynamics in various spatial/temporal scales. Specifically, combined and improved radiation belt models by the former GEM FG, Quantitative Assessment of Radiation Belt Modeling (which has just ended in 2018), will be challenged to reproduce multi-point observations during conjunction events. The results can be used to assess the accuracy of models in various spatial/temporal scales, examine the relative importance of different mechanisms, and identify the missing mechanisms from the current models.

Goals and Deliverables

The broad goal of this new FG is to advance our understanding of radiation belt particle dynamics on both local and global scales through coordinated measurements from multi-spacecraft and ground-based observations combining with theoretical and modeling efforts. The specific science topics this FG will be focusing on, which will greatly benefit from coordinated measurements, include:

1) Improve the understanding of radiation belt particle acceleration and loss processes on short timescales (minutes to hours) and the related physical mechanisms. Various physical mechanisms have been proposed to explain the frequent acceleration and loss of radiation belt particles. Some mechanisms can cause rapid flux enhancement or depletion within minutes to hours. For example, chorus waves are able to produce orders of magnitude enhancements of radiation belt electron fluxes in a few hours; EMIC waves often scatter relativistic electrons on a timescale comparable or even shorter to their drift period; some nonlinear processes can also lead to significant changes of radiation belt electron fluxes on the timescale of electron drift period. The understanding of these rapid flux enhancements and depletions as well as the related mechanisms have been constrained by the available observations with limited spatial and/or temporal resolutions. Combining multi-point measurements from different missions will greatly improve the spatiotemporal resolution of observations and will help to identify and examine these rapid acceleration and loss processes of radiation belt particles. For example, with combined observations, the spatial/temporal ambiguities in electron phase space density evolution will be minimized, which can help identify specific physical mechanisms causing electron acceleration and loss in specific regions. Also, simultaneous measurements at different MLT during radiation belt electron fast losses can shed light on the dominant loss mechanism, e.g., magnetopause shadowing or EMIC wave scattering.

2) Quantify the radiation belt electron precipitation into the atmosphere and improve the understanding of the related physical mechanisms. Precipitation into the atmosphere is an important loss process for Earth’s radiation belt particles. Quantification of such loss as well as understanding of the physical mechanisms behind it is critical to understand the radiation belt dynamic balance. Due to the fast and patchy nature of radiation belt particle precipitation, measurements from a single spacecraft cannot accurately determine the spatial, temporal, and spectral characteristics of precipitations. Through coordinated measurements from a wide range of platforms, including satellites in GTO, GEO, MEO, and LEO, balloons, and ground observations, the spatial constraints and temporal evolutions of the precipitation events will be revealed. Magnetically conjugate observations near equator and at low altitude can be used both to identify scattering mechanisms and quantify the total loss of trapped populations; simultaneous low-altitude observations at close spatial conjunctions can provide information on the spatial properties of precipitation and help quantify local precipitation loss. Combining with modeling efforts, the underlying physical mechanisms causing radiation belt electron precipitation will gain an advanced understanding.

3) Improve the understanding of the spatiotemporal distribution, properties, and generation mechanism of waves in radiation belts and their effects on the radiation belt particles. Waves play a critical role in the radiation belt particle dynamics, while understanding spatial/temporal properties of these waves is essential to quantify the effects of various waves on the radiation belt particle acceleration and loss. Close magnetic conjunctions among spacecraft, along and/or across field lines, can provide a detailed look at wave local spatial properties; simultaneous measurements at various L and MLT can provide information of wave global distribution. These information will greatly benefit and also challenge the current wave models, stimulating the improvement of wave models. The improved wave models will be further implemented into comprehensive radiation belt models to investigate the effects of these waves on the radiation belt particles and the relative importance of individual acceleration, transport, and loss mechanisms.

4) Advance the understanding of inner belt and slot region particle dynamics. The Earth’s inner radiation belt and slot region have been little studied until recently. Recent discoveries in this low L region regarding abundant 10s – 100s of keV electrons but limited MeV electrons in the inner belt, energy- and MLT-dependent features of deep penetrations of radiation belt electrons into the low L region, and CRAND as an important source of quasi-trapped electrons in the inner belt have revealed the complexity of the particle dynamics in the inner belt and slot region which has not been well understood. Through multi-point measurements combining with theory and modeling, our understanding of inner belt and slot region particle dynamics will be greatly improved. For example, combining multi-point measurements near equator and at LEO, the dynamics and relation of trapped/quasi-trapped/precipitating electrons in the low L region can be revealed, and the roles of various mechanisms, including radial diffusion, pitch angle scattering, and CRAND, on these populations can be examined quantitatively through detailed modeling. Also, coordinated measurements in the low L region during particle deep penetration events can reveal the MLT-dependent features of energetic particle deep penetration, and with particle tracer the underlying physical mechanism causing the species- and energy-differential penetration can be examined in detail. The main deliverables from this FG will include the solutions to the above specific science questions, datasets of event-specific radiation belt particle and wave observations, and improved radiation belt local and global models for particle and waves.

Expected activities

The specific session topics and related activities include: (1) identifying the radiation belt particle enhancement and depletion events as well as wave events with conjunctive measurements of various space and ground-based missions; (2) data analysis of identified events combining multi-point measurements; (3) model improvement to include event-specific inputs and better spatial and temporal resolutions; (4) data-model comparisons for the investigation of individual mechanisms or identification of missing physical processes; (5) assessment of the role of newly discovered physical mechanisms.

The FG will organize challenges corresponding to each of the four main science questions it aims to address. We will identify a number of periods for study during which numerous conjunctive measurements are available, gather all available data, engage scientists to perform comprehensive data analysis, challenge modelers to reproduce not just single spacecraft measurements but multi-point observations to test the modeling capacity of various processes on different spatial/temporal scales, and organize the workshop-style sessions to foster more discussions on these topics.