The Role of Three-dimensional Effects on Ion Injection and Acceleration in Perpendicular Shocks
Year: 2026
Type: article
Abstract: Understanding the conditions that enable particle acceleration at nonrelativistic collisionless shocks is essential to unveil the origin of cosmic rays. We employ 2D and 3D hybrid simulations (with kinetic ions and fluid electrons) to explore particle acceleration and magnetic field amplification in nonrelativistic perpendicular shocks, focusing on the role of shock drift acceleration and its dependence on the shock Mach number. We perform an analysis of the ion injection process and demonstrate why efficient acceleration is only observed in 3D. In particular, we show that ion injection critically depends on the “porosity” of the magnetic turbulence in the downstream region near the shock, a property describing how easily the postshock region allows particles to traverse it and return upstream without being trapped. This effect can only be properly captured in 3D. Additionally, we explore the impact of numerical resolution on ion energization, highlighting how resolving small-scale turbulence—on scales below the thermal ion gyroradius—is essential for accurately modeling particle injection. Overall, our results emphasize the necessity of high-resolution 3D simulations to capture the fundamental microphysics driving particle acceleration at perpendicular shocks.
Source: The Astrophysical Journal
Authors: Luca Orusa, Damiano Caprioli, Lorenzo Sironi, Anatoly Spitkovsky
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G2462905468, Collaborative Research: Microscale Plasma Processes at High Beta in Shock and Turbulent Environments, Collaborative Research: WoU-MMA: Multimessenger Plasma Physics Center (MPPC), Collaborative Research: WoU-MMA: Multimessenger Plasma Physics Center (MPPC), G3706614629, A Kinetic Investigation of Plasma Instabilities Driven By Energetic Particles, Collaborative Research: Cosmic Ray Feedback from Plasma to Circumgalactic Scales, Collaborative Research: A Multi-Wavelength Observational and Theoretical Study of the Fastest Evolving Stellar Explosions, SCIENCE GOALS AND OBJECTIVES COLLISIONLESS SHOCKS ARE UBIQUITOUS IN SPACE AND ASTROPHYSICAL PLASMAS. THEY ARE AN EXTREMELY IMPORTANT PHYSICAL PHENOMENON: THEY DISSIPATE KINETIC ENERGY INTO HEAT PRODUCE ENERGETIC PARTICLES AND GENERATE MAGNETIC TURBULENCE. ASTROPHYSICAL SHOCKS ARE ALSO PROMINENT SOURCES OF HIGH-ENERGY COSMIC RAYS AND PRODUCE NON-THERMAL EMISSION SPANNING FROM RADIO TO GAMMA-RAY BANDS. IN-SITU OBSERVATIONS OF NON-THERMAL PARTICLES ASSOCIATED WITH HELIOSPHERIC SHOCKS PROVIDE A UNIQUE OPPORTUNITY TO INVESTIGATE THE PROCESSES THAT LEAD TO PARTICLE ENERGIZATION ALONG WITH THE UNDERLYING ELECTROMAGNETIC STRUCTURE OF COLLISIONLESS SHOCKS. SUCH A STRUCTURE ARISES FROM THE NON-LINEAR INTERPLAY BETWEEN PARTICLES (BOTH THERMAL AND ACCELERATED) AND MAGNETIC FLUCTUATIONS WHICH CAN ONLY BE PROPERLY MODELED VIA KINETIC PLASMA SIMULATIONS. EXAMPLES OF THE QUESTIONS WE WILL ADDRESS ARE: 1) HOW DO SHOCKS GENERATE THE DISTRIBUTIONS OF ENERGETIC IONS AND ELECTRONS THEY ARE OFTEN ASSOCIATED WITH? 2) HOW ARE UPSTREAM SHOCKLETS AND SHORT LARGE-AMPLITUDE MAGNETIC STRUCTURES (SLAMS) FORMED? HOW ARE IONS AND ELECTRONS ACCELERATED IN SUCH FORESHOCK DISTURBANCES? CAN STANDARD ACCELERATION MECHANISMS ACCOUNT FOR THE RELATIVISTIC PARTICLES RECENTLY DETECTED IN SUCH ENVIRONMENTS (WILSON ET AL. PRL 2016)? 3) HOW DO NON-MAXWELLIAN TAILS IMPACT THE MACRO- AND MICROSCOPIC SHOCK DYNAMICS? 4) HOW DOES THE ACCELERATION OF ELECTRONS AND IONS DEPEND UPON THE SHOCK PARAMETERS (E.G. SHOCK SONIC AND MAGNETOSONIC MACH NUMBERS ANGLE BETWEEN SHOCK NORMAL AND BACKGROUND MAGNETIC FIELD)? 5) WHAT IS THE ROLE OF PRE-EXISTING ENERGETIC PARTICLES AND DENSITY/MAGNETIC FIELD FLUCTUATIONS? THE PROPOSED WORK WILL SIGNIFICANTLY IMPROVE OUR PRESENT UNDERSTANDING OF ENERGETIC PARTICLE PRODUCTION AND TRANSPORT IN THE HELIOSPHERE AND IN PARTICULAR AT THE TERRESTRIAL BOW SHOCK AND AT INTERPLANETARY SHOCKS NEAR 1 AU. WE WILL CONSTRUCT A QUANTITATIVE AND UNIFIED PICTURE OF COLLISIONLESS SHOCK DYNAMICS INCLUDING NON-THERMAL ELECTRONS PROTONS ALPHA PARTICLES AND HEAVIER IONS. OUR WORK WILL SIGNIFICANTLY ENHANCE OUR UNDERSTANDING OF SOLAR ENERGETIC PARTICLE GENERATION MECHANISMS WHICH IS CRITICAL TO SPACE SYSTEM DESIGNERS AND OPERATORS. PROPOSED METHODOLOGY WE PROPOSE TO EXPLOIT THE HIGH TIME ENERGY AND ANGULAR RESOLUTION OF THE WIND THEMIS AND MMS SPACECRAFT INSTRUMENT SUITES IN ORDER TO CHARACTERIZE THE POPULATIONS OF ACCELERATED PARTICLES AT HELIOSPHERIC SHOCKS AS A FUNCTION OF SHOCK AND PLASMA PARAMETERS AND COMPARE SPACECRAFT OBSERVATIONS WITH SYNTHETIC PARTICLE SPECTRA OBTAINED VIA STATE-OF-THE-ART KINETIC SIMULATIONS. WE WILL USE BOTH FULL PARTICLE-IN-CELLS (PIC) AND HYBRID SIMULATIONS (FLUID ELECTRONS/KINETIC PROTONS) TO STUDY PARTICLE THERMALIZATION INJECTION TRANSPORT AND ACCELERATION WITH PARTICULAR ATTENTION TO THE MECHANISMS RESPONSIBLE FOR THE FORMATION OF FORESHOCK DISTURBANCES AND FOR PARTICLE ENERGIZATION IN SLAMS FORESHOCK BUBBLES AND HOT-FLOW ANOMALIES.HYBRID SIMULATIONS ALLOW FOR A FULL CHARACTERIZATION OF THE ION DYNAMICS AND ON MODERN SUPERCOMPUTERS CAN COVER SCALES COMPARABLE WITH THE EARTH S BOW SHOCK. THEY CAN HANDLE MULTIPLE ION SPECIES SO THAT WE WILL INVESTIGATE THE THERMALIZATION AND ENERGIZATION OF PROTONS ALPHA PARTICLES AND HEAVIER IONS WITH ARBITRARY MASS/CHARGE RATIO. PIC SIMULATIONS CAN ALSO ACCOUNT FOR ELECTRON ACCELERATION AND WILL BE CRUCIAL ESPECIALLY FOR INVESTIGATING THE RECENT DISCOVERY OF TRANSRELATIVISTIC ELECTRONS IN FORESHOCK DISTURBANCES. PI CAPRIOLI IS AN EXPERT OF KINETIC SIMULATIONS OF NON-RELATIVISTIC SHOCKS AND WILL PERFORM THE NUMERICAL INVESTIGATION OF THE PARAMETER SPACE RELEVANT FOR HELIOSPHERIC SHOCKS. CO-I WILSON IS THE WIND SPACECRAFT PROJECT SCIENTIST HAS SIGNIFICANT EXPERTISE IN WORKING WITH THE WIND AND THEMIS SPACECRAFT AND WILL BE IN CHARGE OF SELECTING AND ANALYZING SPACECRAFT DATA TO BE COMPARED WITH SIMULATION OUTPUTS., G5895401992, THE INTERPLAY OF RECONNECTION AND TURBULENCE IN RELATIVISTIC PLASMAS: THE CASE OF BLACK HOLE ACCRETION FLOWS AND CORONAE
OpenAlex ID: W4416280642