11.5 Cosmic ray acceleration

Cosmic rays, high-energy particles from space, are key to understanding extreme energy phenomena in the universe. They originate from various sources, both galactic and extragalactic, and undergo complex acceleration processes that shape their energy spectrum and composition.

Studying cosmic rays involves detecting them through ground-based and space-based methods, analyzing their energy spectrum, and examining their composition. This research provides insights into astrophysical sources, particle acceleration mechanisms, and the properties of interstellar and intergalactic media.

Cosmic ray origins

• Cosmic rays originate from various astrophysical sources, playing a crucial role in High Energy Density Physics
• Understanding cosmic ray origins provides insights into extreme energy phenomena in the universe
• Cosmic ray studies contribute to our knowledge of particle acceleration mechanisms and high-energy astrophysics

Galactic vs extragalactic sources

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• Galactic sources primarily produce cosmic rays with energies up to 10^15 eV
• Extragalactic sources generate higher-energy cosmic rays, exceeding 10^18 eV
• Transition region between galactic and extragalactic sources occurs around 10^17 - 10^18 eV
• Magnetic field confinement influences the origin determination of cosmic rays

Supernova remnants

• Considered primary sources of galactic cosmic rays
• Shock waves from supernova explosions accelerate particles to high energies
• Observed gamma-ray emission from supernova remnants supports their role in cosmic ray production
• Acceleration efficiency depends on factors such as magnetic field amplification and shock velocity

Active galactic nuclei

• Powerful extragalactic sources of high-energy cosmic rays
• Jets from active galactic nuclei can accelerate particles to ultra-high energies
• Acceleration occurs through various mechanisms (shock acceleration, magnetic reconnection)
• Observed correlation between cosmic ray arrival directions and AGN positions supports their role as sources

Acceleration mechanisms

• Cosmic ray acceleration involves complex processes that convert kinetic and electromagnetic energy into particle energy
• Understanding these mechanisms is crucial for explaining the observed energy spectrum and composition of cosmic rays
• Acceleration mechanisms in High Energy Density Physics apply to various astrophysical environments and laboratory plasma experiments

Fermi acceleration process

• First proposed by Enrico Fermi in 1949
• Two types: first-order and second-order Fermi acceleration
• First-order Fermi acceleration occurs in shock fronts, more efficient than second-order
• Second-order Fermi acceleration involves particle interactions with moving magnetic clouds
• Energy gain per encounter in first-order Fermi acceleration: $\frac{\Delta E}{E} \propto \frac{v}{c}$

Shock acceleration

• Dominant mechanism for cosmic ray acceleration in many astrophysical sources
• Particles gain energy by repeatedly crossing shock fronts
• Diffusive shock acceleration theory explains the power-law spectrum of cosmic rays
• Acceleration efficiency depends on shock parameters (Mach number, magnetic field orientation)
• Maximum energy attainable limited by factors such as particle escape and energy losses

Magnetic reconnection

• Converts magnetic energy into kinetic energy of particles
• Occurs in regions where magnetic field lines break and reconnect
• Important in solar flares, magnetospheric substorms, and possibly in active galactic nuclei
• Can produce non-thermal particle distributions and contribute to cosmic ray acceleration
• Reconnection rate influenced by plasma parameters and geometry of magnetic field configuration

Energy spectrum

• Cosmic ray energy spectrum spans over 11 orders of magnitude in energy
• Studying the spectrum provides insights into acceleration mechanisms and source properties
• Energy spectrum measurements are crucial for understanding the highest energy phenomena in the universe

Power law distribution

• Cosmic ray flux follows a power-law distribution over most of the energy range
• Differential flux given by $\frac{dN}{dE} \propto E^{-\gamma}$
• Spectral index γ varies in different energy regions
• Power-law behavior explained by diffusive shock acceleration theory
• Deviations from power-law indicate transitions in sources or acceleration mechanisms

Knee and ankle features

• Knee occurs around 10^15 - 10^16 eV, marking a steepening of the spectrum
• Possible explanations for the knee include maximum energy of galactic accelerators or change in composition
• Ankle appears at energies around 10^18 - 10^19 eV, where the spectrum flattens
• Ankle may indicate transition from galactic to extragalactic cosmic rays
• Second knee observed around 10^17 eV, potentially related to heavy nuclei acceleration limits

Ultra-high energy cosmic rays

• Cosmic rays with energies above 10^18 eV
• Highest energy cosmic ray observed had energy of 3 × 10^20 eV
• Sources remain unknown, but likely extragalactic (active galactic nuclei, gamma-ray bursts)
• Propagation limited by interactions with cosmic microwave background (GZK cutoff)
• Study of UHECRs provides insights into extreme astrophysical environments and fundamental physics

Particle composition

• Cosmic ray composition provides information about source environments and acceleration processes
• Varies with energy, reflecting different origins and propagation effects
• Studying composition helps distinguish between different cosmic ray models and source scenarios

Protons and nuclei

• Protons dominate the cosmic ray flux at lower energies (up to ~10^15 eV)
• Heavier nuclei (helium, carbon, oxygen, iron) become more abundant at higher energies
• Relative abundances of elements differ from solar system composition
• Spallation reactions during propagation affect observed composition
• Measurements of nuclear composition help constrain cosmic ray origin and propagation models

Electrons and positrons

• Electrons constitute about 1% of cosmic rays at GeV energies
• Positron fraction increases with energy, possibly indicating dark matter annihilation or nearby pulsars
• Electron-positron pairs produced by interactions of high-energy gamma rays with matter
• Synchrotron and inverse Compton losses limit the propagation distance of high-energy electrons
• Precise measurements of electron and positron spectra provide insights into local cosmic ray sources

Antimatter in cosmic rays

• Small fraction of cosmic rays consists of antimatter particles
• Antiprotons produced mainly through cosmic ray interactions with interstellar medium
• Positron excess observed at high energies, challenging conventional cosmic ray models
• Search for antinuclei (antihelium, anticarbon) ongoing to probe primordial antimatter or exotic sources
• Antimatter measurements constrain models of cosmic ray production and propagation

Propagation and interactions

• Cosmic rays traverse vast distances before reaching Earth, interacting with various astrophysical environments
• Understanding propagation effects crucial for interpreting observed cosmic ray properties
• Propagation studies in High Energy Density Physics connect particle transport in space to laboratory plasma experiments

Galactic magnetic fields

• Confine cosmic rays within the galaxy, influencing their trajectories and residence time
• Typical strength of ~μG, with complex structure including regular and turbulent components
• Cause diffusion and energy-dependent confinement of cosmic rays
• Affect observed arrival directions, leading to near-isotropic distribution for most energies
• Synchrotron emission from cosmic ray electrons traces galactic magnetic field structure

Extragalactic magnetic fields

• Influence propagation of ultra-high energy cosmic rays over intergalactic distances
• Strength and structure poorly known, estimated to be ~nG in voids and higher in filaments
• Cause deflections in UHECR trajectories, complicating source identification
• May lead to magnetic horizon effect, limiting observable volume for highest energy cosmic rays
• Study of UHECR arrival directions provides constraints on extragalactic magnetic field properties

Interactions with cosmic microwave background

• Limit propagation of ultra-high energy cosmic rays (GZK effect)
• Protons above ~5 × 10^19 eV interact with CMB photons through pion production
• Heavier nuclei undergo photodisintegration, breaking into lighter nuclei
• Results in cosmic ray horizon of ~100 Mpc for highest energy particles
• Energy loss during propagation modifies observed energy spectrum and composition

Detection methods

• Various techniques employed to detect cosmic rays across wide energy range
• Combination of ground-based and space-based detectors provides comprehensive coverage
• Advancements in detection methods crucial for progress in cosmic ray physics and High Energy Density Physics

Ground-based observatories

• Detect secondary particles produced by cosmic ray interactions in the atmosphere
• Include extensive air shower arrays, Cherenkov telescopes, and fluorescence detectors
• Large-scale observatories (Pierre Auger Observatory, Telescope Array) study highest energy cosmic rays
• Provide large effective areas for rare ultra-high energy events
• Combine multiple detection techniques for improved energy and composition measurements

Space-based detectors

• Directly measure primary cosmic rays before atmospheric interactions
• Include magnetic spectrometers, calorimeters, and transition radiation detectors
• Examples: AMS-02 on International Space Station, PAMELA satellite
• Provide high-precision measurements of cosmic ray composition and energy spectra
• Limited by size and weight constraints, focus on lower to medium energy range

Air shower detection

• Extensive air showers initiated by high-energy cosmic rays in the atmosphere
• Secondary particles spread over large areas at ground level
• Detection methods include particle detectors, fluorescence telescopes, and radio antennas
• Shower size and lateral distribution provide information on primary particle energy and type
• Hybrid detection techniques improve reconstruction accuracy and reduce systematic uncertainties

Cosmic ray anisotropy

• Deviations from perfect isotropy in cosmic ray arrival directions
• Provides information about cosmic ray sources and magnetic field structure
• Anisotropy studies connect cosmic ray physics to broader astrophysical phenomena in High Energy Density Physics

Large-scale anisotropy

• Observed at energies below ~100 TeV
• Amplitude of ~10^-3 for TeV cosmic rays, increasing with energy
• Possible causes include solar wind modulation, galactic magnetic field effects, and nearby sources
• Dipole component dominant, with higher-order multipoles also present
• Energy dependence of anisotropy provides insights into cosmic ray propagation and source distribution

Small-scale anisotropy

• Localized excess or deficit regions in cosmic ray arrival directions
• Observed at TeV-PeV energies with angular scales of ~10°-30°
• Possible origins include turbulent magnetic fields, nearby sources, or propagation effects
• Challenging to explain within standard diffusion models of cosmic ray transport
• Study of small-scale anisotropies provides information on local interstellar medium properties

Dipole anisotropy

• Largest-scale anisotropy component, representing overall cosmic ray flow direction
• Observed at various energies, from GeV to highest energy cosmic rays
• At highest energies (>8 EeV), dipole amplitude ~6.5% pointing away from galactic center
• Supports extragalactic origin of ultra-high energy cosmic rays
• Energy dependence of dipole amplitude and direction constrains cosmic ray source scenarios

Astrophysical implications

• Cosmic ray studies provide insights into various astrophysical phenomena and fundamental physics
• Connections between cosmic ray physics and High Energy Density Physics enable cross-disciplinary research
• Understanding cosmic rays crucial for broader questions in astrophysics and cosmology

Galactic structure

• Cosmic rays interact with interstellar medium, producing gamma rays and radio emission
• Tracing cosmic ray distribution helps map galactic structure and magnetic fields
• Cosmic ray pressure contributes to galactic dynamics and star formation processes
• Galactic winds driven by cosmic rays influence galaxy evolution and chemical enrichment
• Study of cosmic ray propagation provides information on interstellar medium properties

Intergalactic medium properties

• Ultra-high energy cosmic rays probe intergalactic magnetic fields and large-scale structure
• Cosmic ray interactions with intergalactic medium produce secondary particles and radiation
• Constraints on cosmic ray flux at highest energies inform models of intergalactic medium composition
• Cosmic ray-driven intergalactic shocks may contribute to heating and magnetization of cosmic web
• Studying cosmic ray propagation in intergalactic space provides insights into cosmic void properties

Dark matter searches

• Cosmic ray antimatter measurements used to search for dark matter annihilation signals
• Positron excess and antiproton flux constrain dark matter models and particle properties
• Gamma-ray observations of galaxy clusters probe cosmic ray-induced dark matter annihilation
• Cosmic ray interactions with dark matter may produce unique signatures in energy spectrum or composition
• Combining cosmic ray data with other astrophysical observations improves dark matter constraints

Cosmic ray research

• Interdisciplinary field connecting particle physics, astrophysics, and High Energy Density Physics
• Continuous advancements in detection techniques and theoretical understanding
• Cosmic ray studies provide unique opportunities to probe extreme energy phenomena

Historical discoveries

• Cosmic rays discovered by Victor Hess in 1912 through balloon experiments
• Led to discoveries of new particles (positron, muon, pion) in early 20th century
• Fermi proposed acceleration mechanism in 1949, laying foundation for modern theories
• Discovery of extensive air showers by Pierre Auger in 1938 enabled study of highest energy cosmic rays
• Observation of GZK cutoff in cosmic ray spectrum confirmed predictions from particle physics

Current experiments

• Ground-based observatories: Pierre Auger Observatory, Telescope Array, HAWC, LHAASO
• Space-based detectors: AMS-02, PAMELA, CALET
• Neutrino observatories: IceCube, ANTARES, KM3NeT
• Multi-messenger approaches combining cosmic ray, neutrino, and gravitational wave observations
• Improvements in detector technology and analysis methods enabling precision measurements

Future prospects

• Planned upgrades to existing experiments (Auger Prime, TAx4)
• Next-generation ground-based observatories (GRAND, POEMMA)
• Space-based missions for ultra-high energy cosmic ray detection (EUSO, POEMMA)
• Advancements in machine learning techniques for improved data analysis
• Connections with other fields (gravitational wave astronomy, high-energy neutrino physics) for multi-messenger studies