Diffusive Particle Acceleration and Extreme Solar Energetic Particle Gradual Events
ROSES ID: NNH19ZDA001N Selection Year: 2019
Program Element: Focused Science Topic
Principal Investigator: Lingling Zhao
Affiliation(s): University Of Alabama, Huntsville
Project Member(s):
le Roux, Jakobus A. Co-I University Of Alabama, Huntsville
Zank, Gary P Co-I University Of Alabama, Huntsville
Adhikari, Laxman Postdoctoral Associate University Of Alabama, Huntsville
Li, Gang Co-I University Of Alabama, Huntsville
Summary:
Objective: Gradual solar energetic particle (SEP) events are associated generally with interplanetary shocks driven by coronal mass ejections (CME), where energetic ions are thought to be accelerated via diffusive shock acceleration (DSA). Close to the Sun, strong shocks can occasionally accelerate particles to GeV energies. Most typical CME-driven shocks tend to accelerate charged particles to quite modest energies, and the total energy contained within the accelerated SEPs is typically a small fraction (< 10%) of the kinetic energy of the shock. However, a number of extreme CME-driven shock events have been observed with energetic particle pressures not only dominating the downstream thermal and magnetic field pressures but being a significant fraction of the ram energy of the shock wave itself. These extreme shocks are fast, very broad, and do not satisfy the standard Rankine-Hugoniot conditions. For these extreme events, not only the backreaction of the energetic particles on shock structure has to be properly considered, but the turbulence generated by the SEP component streaming in the solar wind plasma needs also be taken into account as they affect the scattering and transport properties of the SEPs. Therefore, to build a complete picture of extreme SEP events, one should consider shock structure, turbulence, and energetic particles as an integrated system. The science goal of this project is to (1) model the time-dependent, multi-D mediation of shock structure by the DSA of SEPs for extreme SEP events, and (2) describe quantitatively the 2D interplanetary transport of SEPs that escape a mediated shock, stream and generate turbulence that affects the transport of the SEPs themselves.
Methodology: For Goal #1, we propose to develop a theoretical model of SEP mediated shock propagation in the inner heliosphere. The shock model will treat SEPs as a separate component from the bulk solar wind, using a diffusive transport equation formulation. To describe the scattering of particles, we will couple modern scattering theories for the spatial diffusion tensor to modern theories of turbulence transport. The PI and Co-Is have previously developed models for the structure of energetic particle mediated shock waves, charged particle scattering theories, and turbulence transport models. These models will serve as the starting point of our investigation. To accommodate the realistic solar wind, we will develop numerical solutions for 2D time-dependent shocks and solve the SEP transport equation. For Goal #2, we will solve the gyrophase-averaged particle transport equation for SEPs in the solar wind away from the shock to obtain their energy and pitch-angle distribution. The PI and Co-Is have developed numerical codes previously that solve the focused transport equation for energetic particles using a stochastic simulation method, and these will be the basis for modeling extreme SEP events. To validate our theoretical and numerical models, we will compare them against in-situ observations of plasma, magnetic fields, and SEPs from Parker Solar Probe, Helios, ACE, Wind, Ulysses, and the upcoming Solar Orbit and IMAP missions during extreme SEP events. Our existing iPATH (improved Particle Acceleration and Transport in the Heliosphere) code has successfully simulated some selected normal SEP events. However, it does not apply to extreme SEP events. We will compare the new model results i.e., the SEP-mediated extreme events, with the non-feedback (normal) models (using iPATH) to contrast the differences.
Contributions to the Focused Science Team: Our proposed investigation will contribute directly to FST #1 by focusing on the acceleration and transport of extreme SEP events, which contribute to the interplanetary radiation environment. As these extreme SEP events are often associated with CME-driven interplanetary shocks, this work may provide insight into the prediction of extreme SEP events.
Export to PDF