During interstellar cosmic ray propagation, cosmic rays are affected by the magnetic field in the interstellar medium. This magnetic field also interacts with the gas along with radiation transport and cosmic rays in the gas layer. This article reviews anisotropy measurements and their interpretation, as well as how they show up in arrival directions when propagation effects occur.
The gamma-ray and cosmic-ray interaction is examined by referring to the information related to the interstellar medium from the flux of secondary γ-rays from the cosmic-ray and gamma-ray emission in interaction with the gas layer, the main concern being the molecular gas situation in the inner galaxy.
1. Cosmic Rays and the Interstellar Medium
The pressure of cosmic rays in the interstellar medium is approximately equal to radiation, magnetic, and gas pressure; any pressure below it is of a very similar distribution to cosmic rays. The propagation of cosmic rays and the interstellar medium is still not understood properly. A series of random walks and the mean free path between poorly known scatterings mediate the interaction between cosmic rays.
This propagation is controlled by poorly modeled local magnetic field turbulence properties. The dominant theory is that the interstellar medium mediates its turbulence by cosmic rays. One of the most significant ways to react to the dynamics of the interstellar medium and its turbulence by cosmic rays is this.
The cosmic rays have a multi-level impact on galactic interstellar medium dynamics: they generate large cold neutral fractions, inject turbulent magnetic and strongly influence the motions at intermediate scales, drive large-scale magnetic fields through Parker instability, diffuse, contribute to ionizing, and produce radio elements by spallation reaction in the young stellar system, and heat protostellar environments or molecular clouds. The fast and charged particles of cosmic rays have an impact on dust charge.
1.1. Cosmic Ray
Interstellar gas and magnetic fields share the same energy density as cosmic rays, which constitute a major component of the interstellar medium. At low energies, they participate in interstellar medium structure dynamics. Their contribution to hot gas and its ionization is significant. They exert force on the magnetized fluid by exerting a pressure gradient across the volume. Magnetic turbulence and plasma waves are generated by them.
Molecular gas and star formation depend on the turbulence that occurs in molecular gas. High-energy cosmic rays produce neutral pions, electrons, and positrons as secondary particles ( electron-positrons, gamma rays, and possibly neutrinos) by interacting with atmospheric gases.
Radiation from cosmic rays also produces light and radioactive elements. Supernova explosions are likely to produce cosmic rays. A large fraction of these supernovae explodes when the cores of massive stars collapse. Cosmic ray production seems to be largely driven by massive stars, their evolution, and their environment.
Astronomers have discovered that cosmic rays are a key component of the interstellar medium’s local and global dynamics. To model the evolution of the interstellar medium, this significant component has only recently been integrated. An international workshop on the energetic component of interstellar media aims to advance our understanding of the multifaceted effects of the energetic component.
1.1.1. Influence of Turbulence and Magnetic Field
As a key component of stellar initial mass functions and star formation rates, the turbulence of the interstellar medium is crucial. Turbulence in the interstellar medium is characterized by density variance-Mach number relative fractions, as turbulent motions in the interstellar medium are generally supersonic.
For studying this relation, several physical effects have been considered: effects due to different turbulent forcing geometries, magnetic field effects, and non-isothermal turbulence models that incorporate thermodynamic properties.
It has been determined from recent observations that the strong influence of magnetic fields and the turbulent force of geometries affect the width of the log-normal distribution fitting the probability density function. Cosmic rays also exert force over plasma along pressure gradients in addition to the above effects. This can also modify the medium statistics and dynamics.
Closer to celestial debris like massive star clusters or supernova remnants, these effects are expected to be stronger. This is because they are strongly influenced by the ISM structure surrounding the sources of cosmic rays.
1.2. Interstellar Gas
Among the gas layers between stars, most of the gas is helium and hydrogen as it is scattered between stars in other galaxies and within our galaxy. In proportion to the sun’s gases, these gases exist. The raw material for star formation is supplied through interstellar gas.
Additionally, hydrogen molecular gas layers and carbon monoxide, nitrogen, silicon, ionized oxygen, and carbon have been detected within the interstellar gas layer as remnants of previous supernova explosions. Dust particles consisting of tiny grains of iron-magnesium silicates, carbon, and iron are scattered throughout stars in the galaxy as well as in the stars in our galaxy.
In many cases, interstellar gas can be as hot as a thousand degrees or as cold as a few degrees below zero, depending on its location. In order to understand how gas behaves under different conditions, we will begin exploring the physics of the interstellar medium.
Interstellar gas near hot stars is captured in breathtaking astronomical photographs. As you can see in the hydrogen spectrum, the red line is the strongest visible line. It is called the H-alpha line by scientists. Alpha means that it is the first spectral line in the Balmer series; this accounts for glowing red in warm gas.
1.2.1. Interstellar Gas: Ionization
During this process of ionization, the electron strips are fully detached from the proton by ultraviolet radiation. Gas near the hot stars can be heated up to 10,000 K. It won’t be like this forever these protons will capture the free electron and become neutral hydrogen once more. In such a case, the atoms can absorb ultraviolet radiation again and again. When the hot gas and star are close to each other, most atoms are ionized.
In the interstellar region of space, hydrogen is characterized by its neutralized or ionized state. Researchers use Roman numerals to indicate the neutral state of an atom; the higher the Roman numeral, the more intense the stage of ionization. The H II region is a cloud of ionized hydrogen.
In hydrogen atoms, electrons are captured by their nuclei at different energy levels. The energy is given off by each transition as light. Fluorescence is a term used to describe this process. Interstellar gas also contains other elements many of which are ionized in the vicinity of hot stars; like hydrogen, they also emit light and capture electrons allowing themselves to be observed by astronomers. The H II region looks red as the red hydrogen light is the strongest.
In the same way as fluorescent H II regions of the earth, fluorescent light works on the same principle. When the current is applied, electrons collide with mercury vapor in the tube and excite it to a high level. After returning to a lower level, some of its energy is transformed into ultraviolet light. As they strike the inner wall of the light tube, they strike the phosphor-coated screen. The ultraviolet photons are absorbed by the atoms on the screen and emit visible light cascading downward energy levels.
1.3. Interstellar Medium
An interstellar medium is a region between stars that contains particles of small size and density along with diffuse clouds of gases. In the interstellar medium, there is a configuration similar to the Milky Way resulting in about 5 percent of the galaxy’s mass being made up of such matter. The ISM is filled with hydrogen gas. Formaldehyde, sodium, water, calcium, ammonia, and formaldehyde are also detected in small amounts, along with an extremely significant amount of helium. Dust particles are present in large quantities of uncertain composition. Many of the regions are covered by cosmic rays that travel through magnetic fields and interstellar space.
There is a cloud-like concentration of interplanetary matter that is condensed enough to form stars in the universe. It is estimated that these stars lose mass over time, in some cases as a result of catastrophic explosions like supernovas and other small explosions.
The mass is fed back to the interstellar medium where it is mixed with matter which has not formed the stars yet. As a result, cosmic clouds contain heavier elements. Interstellar activity is largely found in the outer parts of the Milky Way Galaxy which also has a large number of nebulae and young stars. This matter lies in the galactic disk which is a flat and plane region.
In the middle of the 20th century, all information about the interstellar medium was obtained by analyzing the effects of light from distant stars with optical telescopes. For example, neutral hydrogen atoms absorb or emit limited amounts of radio energy, or 21 cm wavelength, as they emit or absorb radio waves. Since the 1950s, researchers have been studying radio waves produced by various components of the interstellar medium. It is possible to detect hydrogen clouds by measuring them at this point and comparing them to wavelengths nearby.
Most of the information on the interstellar medium is provided by radio and optical emissions. Additionally, satellite observatory infrared telescopes have contributed to this recent change, particularly in terms of observations of the relative abundances of constituent elements.
2. An Understanding of How Cosmic Rays Affect Interstellar Medium Evolution
We explore the impact of diffusive cosmic rays (CRs) on the evolution of the interstellar medium (ISM) under varying assumptions about the supernova explosion environment. For example, in order to account for supernovae (SN) that occur in regions that have been cleared by prior supernovae, stellar winds, or radiation, we systematically vary the relative fractions of supernovae forming in high-density gas versus those occurring in random locations decoupled from star-forming gas.
When a periodic stratified gas layer is used in the simple model, the ISM structure will evolve into either a theoretical ‘peak driving’ state or a ‘thermal runaway’ state. The ISM can flip between solutions as a result of CR pressure and transport, which play an extremely powerful role in determining where the ISM goes to find a solution. The HI gas signature and gamma-ray emission are considered observable signatures.
An extensive range of model parameters is consistent with the gamma-ray luminosity of pion decay. However, there may be a large fraction of cold neutral gas in the mid-plane that is responsible for the thick layer of HI gas. It turns out both solutions get a stable volume fraction of hot gas, but they don’t get a Milky Way-like configuration, which suggests there’s more physics than we’ve discussed (e.g. cosmological circumgalactic mediums, radiation transport, and spectrally and spatially varying CR transports).
2.1. Cosmic Rays Scattering and Acceleration in A Turbulent Interstellar Medium
In MHD turbulence, cosmic ray particles scatter magnetic waves. It generates turbulence and waves as the stream accelerates faster than the local Alfvén speed. According to observations of cosmic rays at radio frequencies and earthly locations, as well as X-ray emission from cosmic rays throughout the galaxy, the ISM is characterized by particle gyroradius-scale irregularities. At 1015 eV, the distribution of cosmic ray energy can range between 1 AU and 1 PC. In contrast, the protons are between a few KeV and a few GeV.
There is no clear evidence that turbulence that scatters cosmic rays on a large scale is closely related to turbulence that scatters cosmic rays on a small scale. The energy could be generated locally, e.g., by cosmic rays themselves or by small-scale instabilities. Being part of a large-scale energy cascade. Molecular cloud turbulence presumably generates on a much larger scale, and scintillation turbulence may be similar.
One point to remember is that cosmic rays scatter diffusely rather than stream through a galaxy. The total path length is known from the secondary to primary nuclei ratio which is equivalent to 10^4 galactic disks crossing at GeV energies. The isotope ratio ranges from ~ 15 My for the leaky box model with a small halo to 10^8 years for the diffusion model with ~5 kpc halo to determine flight time. The flux of the particles scatters so widely that it is nearly isotropic at the Earth, despite only a handful of nearby sources.
2.2. Cosmic Ray Scattering in Magnetic Waves
Field line fluctuations can scatter cosmic rays as shock fronts, coherent waves, or stochastic turbulence. A common view is that the fluctuations are weaker in comparison with the mean field. This is because there is a well-defined dispersion between wave number and frequency and the wave travels much longer than the oscillation period.
Particles in such wave fields interact with crests repeatedly as they travel rapidly along the field. As their culminating pitch angle deviation increases, they begin to interact with different wave trains. Although the regularity of the turbulent medium is not obvious, the particle interaction with the wave spectrum is similar. A particle can only perform this if it sees the magnetic perturbation after each gyration with the right wavelength after each gyration. The field can be static for fast-moving particles in both cases.
2.3. Other Scattering Mechanisms
During the slow changes in the magnetic field amplitude, the energy of fast cosmic ray gyrations is adiabatically invariant. Increasing the parallel motion field puts more energy into gyromotion and removes it from the parallel motion field. Because of this, fast high-pitch angles bounce off converging field lines due to momentum conservation. The mirror force is –M || B for magnetic moment M = mv2 / 2B with particle mass m. This involves long parallel wavelengths to the field.
Clumps and molecular clouds could also serve as mirror sources.
The electromagnetic field can also scatter cosmic rays. Plasma at speed U transverse to a magnetic field generates an electric field, E = –u × B, which can be significant in the parallel direction when the magnetic field has at least a 2D structure.
Approximately parallel to the mean field, this field line occurs as an individual line. Radical diffusion has a significant impact on the spiral field because the solar wind is part of it. As well as diffusion in oblique shock fronts, it plays a significant role in vertical diffusion. As a result of anomalous diffusion, particles will skip over magnetic field lines if their irregularities are smaller than the gyroradius.
In braided field lines, the mean squared particle position is calculated rather than directly comparing time as in normal diffusion. Distillation of compounds occurs along both linear and cross-line paths. A significant perpendicular structure to the field lines is necessary for particle motion parallel to the field. Cross-field diffusion coefficients are 0.1-0.2 of parallel diffusion coefficients for ISM turbulence.
In addition, to shock, cosmic rays can be scattered by cosmic rays. As suggested by Blandford and Ostriker, the interstellar medium (ISM) consists of cavities from supernovae and also shows how the resulting shocks can accelerate and scatter cosmic rays. This model of how secondary shocks caused by supernovae arise in turbulence was extended by Bykov and Toptygin.
Stellar winds and shocks were included. An ensemble of shocks in a supersonically turbulent medium was used by Schneider to determine the composite spectrum of cosmic rays. It is more like a power law in which cosmic ray summed energy distributions are close to power laws rather than just straight lines. In contrast, a single shock energy distribution can include bumps or flat parts.
2.4. Cosmic Ray Accelerations
Cosmic rays gain energy through random scattering. The momentum diffusion principle is referred to as the second-order Fermi mechanism since energy gained by collisions depends on the second power of turbulent momentum. As light particles approach equilibrium with the heavy particles, they speed up, reminiscent of the thermalization of a star cluster. Initially, supernova remnants were used as scattering sites for this mechanism. Random motions within supernova remnants were used first as scattering sites.
The high energy density of supernovae makes their likely sources of cosmic rays. About 5% of the total power of supernovas goes into the creation of our life in the Galaxy.
Not only in supernova remnants but throughout a compressible turbulent ISM, cosmic rays are accelerated by momentum diffusion. Whenever these regions are compressed, particles trapped in weak magnetic turbulence in the ambient medium are also accelerated.
A supernova remnant’s edge is also accelerated by shock crossings. If the field is parallel to the shock direction, the outward streaming particles scatter back into the shock. This is because they encounter more turbulence. Post-shock turbulence is a compressed and amplified version of pre-shock turbulence.
Due to promotions and field line wandering, particles cycle through the shock front whenever the field is oblique to the shock direction. Each time a particle cycles through the front it gains energy from the converging flow. Because the shock velocity is proportional to the energy gained per crossing, this mechanism is called the first-order Fermi mechanism. The efficiency of injected thermal particles depends on the degree of obliqueness, with around 10-3 injected into cosmic rays. Generally, the first-order mechanism dominates in strong shocks, although the second mechanism is more influential downstream than upstream.
2.4.1 Shock Acceleration
As cosmic ray energy reaches the “knee”, shock acceleration is responsible for its spectrum. The particles that stay in the shock longest end up with the most energy. The spectrum is a distribution function of the number of shock crossings, considering the continuous loss of particles that are trapped in the downstream flow.
Observations of the edge sharpness of supernova remnants suggest the amplitude of MHD waves near the shock is 60 times the average ISM value. Observed a factor of at least 10 in the magnetic wave amplitude from Faraday rotation irregularities in a supernova remnant compared with the adjacent line of sight. The X-ray synchrotron emission from supernova remnants is direct evidence of the acceleration of relativistic electrons.
2.5. Generation of Turbulence by Cosmic Rays
Because they produce magnetic irregularities, particles of high density cannot travel along an electromagnetic field at greater speeds than the Alfvén speed. Wave damping from ion gyromotion is shown in parenthesis as the first term. As the streaming speed approaches the Alfvén speed, the second term causes instability. A cosmic ray that resonates with wavenumber k has a lower density than the background density of thermal ions, and this is what influences collective effects. An irregularity of about 2 inches in size is produced by the magnetic field.
A particle with a larger gyroradius will cause larger-scale distortions of the field (for example, 1013 cm of gyroradius for particles with Gaussian energy). Anisotropy should not be present in these distortions since they are not caused by a turbulent cascade. This could be done by cascading ISM to generate the scintillation. Cosmic rays can generate waves with energies up to 1000 Gev that can scatter enough of their energy.
A Galactic wind model can produce much higher energies. It is due to the streaming instability of Landau damping in a highly ionized Galactic halo that the boundary between diffusion and advection depends on energy. It was proposed by Hall (1980) that scintillation-scale structures are simply a consequence of mirror and firehose instabilities in the hot ISM phase in the absence of cosmic rays. As Hall notes, if the hot medium is turbulent, pressure anisotropies should arise at 0.01-0.1.
If you have parallel or perpendicular pressures, then firehose instability must be greater than, and mirror instability must be greater than. Getting this anisotropy would be possible if the mag was really big. Supernovae continuously sweep away gas and fields in the heated intercloud medium.
Concluding the article it can be seen that the CR pressure is much higher than the gas thermal pressure. If the condition for CR trapping is fulfilled, then CR pressure gradients develop within the ISM molecular clouds. In addition to propagation, CR also affects the dynamics of gas.
As a result, CRs have a significant impact on the dynamics of the interstellar medium, as well as influencing the formation of molecular clouds.
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