The Influences of Shock Thickness and Alfven Waves on the Particle Acceleration by Diffusive Shocks

The influences of the shock thickness and Alfven waves on the particle acceleration by diffusive shock waves are numerically studied through solving one-dimensional diffusive equation including the second-order Fermi effect. It is shown that the spectral index of the energetic particles strongly depends on the shock thickness. For example, the spectral index increases from 2.1 to 3.7 in the low energy range of 3—10 MeV and from 2.5 to 5.0 in the high energy range of 20—60 MeV as the thickness increases. The spectral index decreases from 4.3 to 3.1 as the particle injection energy increases. The spectral index decreases from 4.0 to 1.8 at the quasi-steady stage with the enhancement of the compression ratio from 2 to 4. The results indicate that under the influence of Alfven waves, the energetic particle spectrum at lower energy becomes flat and the spectral index decreases from 2.5 to 0.6 in the low energy range of 3—10 MeV and from 11.6 to 5.0 in the high energy range of 20—60 MeV. At the same time, the turning point energy reaches 19.6 MeV. The spectral index decreases from 5.8 to 2.9 as the energy density of Alfven waves increases. All these results are basically consistent with the theoretical models, as well as the observations of typical energetic particle events.     

Simulations of reflected radio signals from cosmic ray induced air showers

We present the calculation of coherent radio pulses emitted by extensive air showers induced by ultra-high energy cosmic rays accounting for reflection on the Earth’s surface. Results have been obtained with a simulation program that calculates the contributions from shower particles after reflection at a surface plane. The properties of the radiation are discussed in detail emphasizing the effects of reflection. The shape of the frequency spectrum is shown to be closely related to the angle of the observer with respect to shower axis, becoming hardest in the Cherenkov direction. The intensity of the flux at a fixed observation angle is shown to scale with the square of the primary particle energy to very good accuracy indicating the coherent aspect of the emission. The simulation methods of this paper provide the foundations for energy reconstruction of experiments looking at the Earth from balloons and satellites. They can also be used in dedicated studies of existing and future experimental proposals.     

A non-parametric approach to infer the energy spectrum and the mass composition of cosmic rays

The experiment KASCADE observes simultaneously the electron–photon, muon, and hadron components of high-energy extensive air showers (EAS). The analysis of EAS observables for an estimate of energy and mass of the primary particle invokes extensive Monte Carlo simulations of the EAS development for preparing reference patterns. The present studies utilize the air shower simulation code corsika with the hadronic interaction models VENUS, QGSJet and Sibyll, including simulations of the detector response and efficiency. By applying non-parametric techniques the measured data have been analyzed in an event-by-event mode and the mass and energy of the EAS inducing particles are reconstructed. Special emphasis is given to methodical limitations and the dependence of the results on the hadronic interaction model used. The results obtained from KASCADE data reproduce the knee in the primary spectrum, but reveal a strong model dependence. Owing to the systematic uncertainties introduced by the hadronic interaction models no strong change of chemical composition can be claimed in the energy range around the knee.     

Gamma-ray detectors (>10MeV)

Photons in the energy region above 10 MeV can provide unique information about the highest energy particles produced in solar flares. Measurements of the energy spectra, temporal behavior and anisotropies of the incident photons, can give valuable insights into the particle acceleration process. In this paper we review the detection techniques used at these energies, typical instruments which use these techniques, and future solar missions both actual and potential.     

The Converter Mechanism of Particle Acceleration and Its Applications to the Unidentified Egret Sources

We discuss the properties of gamma-ray radiation accompanying the acceleration of cosmic rays via the converter mechanism. The mechanism exploits multiple photon-induced conversions of high-energy particles from charged into neutral state (namely, protons to neutrons and electrons to photons) and back. Because a particle in the neutral state can freely cross the magnetic field lines, this allows to avoid both particle losses downstream and reduction in the energy gain factor, which normally takes place due to highly collimated distribution of accelerated particles. The converter mechanism efficiently operates in relativistic outflows under the conditions typical for Active Galactic Nuclei, Gamma-Ray Bursts, and microquasars, where it outperforms the standard diffusive shock acceleration. The accompanying radiation has a number of distinctive features, such as an increase of the maximum energy of synchrotron photons and peculiar radiation beam-pattern, whose opening angle is much wider at larger photon energies. This provides an opportunity to observe off-axis relativistic jets in GeV–TeV energy range. One of the implications is the possibility to explain high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars.     

Comparison of AGASA data with CORSIKA simulation

An interpretation of Akeno giant air shower array (AGASA) data by comparing the experimental results with the simulated ones by cosmic ray simulation for KASCADE (CORSIKA) has been made. General features of the electromagnetic component and low energy muons observed by AGASA can be well reproduced by CORSIKA. The form of the lateral distribution of charged particles agrees well with the experimental one between a few hundred metres and 2000 m from the core, irrespective of the hadronic interaction model studied and the primary composition (proton or iron). It does not depend on the primary energy between 10^17.5 and 10²⁰ eV as the experiment shows. If we evaluate the particle density measured by scintillators of 5 cm thickness at 600 m from the core S₀(600), suffix 0 denotes the vertically incident shower) by taking into account the similar conditions as in the experiment, the conversion relation from S₀(600) to the primary energy is expressed as E (eV)=2.15×10¹⁷S₀(600)^1.015 within 10% uncertainty among the models and composition used, which suggests the present AGASA conversion factor is the lower limit. Although the form of the muon lateral distribution fits well to the experiment within 1000 m from the core, the absolute values change with hadronic interaction model and primary composition. The slope of the ρ_μ(600) (muon density above 1 GeV at 600 m from the core) vs. S₀(600) relation in experiment is flatter than that in simulation of any hadronic model and primary composition. As the experimental slope is constant from 10¹⁵ to 10¹⁹ eV, we need to study this relation in a wide primary energy range to infer the rate of change of chemical composition with energy.     

Prospects for radio detection of ultra-high energy cosmic rays and neutrinos

The origin and nature of the highest energy cosmic ray events is currently the subject of intense investigation by giant air shower arrays and fluorescent detectors. These particles reach energies well beyond what can be achieved in ground-based particle accelerators and hence they are fundamental probes for particle physics as well as astrophysics. One of the main topics today focuses on the high energy end of the spectrum and the potential for the production of high-energy neutrinos. Above about 10²⁰ eV cosmic rays from extragalactic sources are expected to be severely attenuated by pion photoproduction interactions with photons of the cosmic microwave background. Investigating the shape of the cosmic ray spectrum near this predicted cut-off will be very important. In addition, a significant high-energy neutrino background is naturally expected as part of the pion decay chain which also contains much information.Because of the scarcity of these high-energy particles, larger and larger ground-based detectors have been built. The new generation of digital radio telescopes may play an important role in this, if properly designed. Radio detection of cosmic ray showers has a long history but was abandoned in the 1970s. Recent experimental developments together with sophisticated air shower simulations incorporating radio emission give a clearer understanding of the relationship between the air shower parameters and the radio signal, and have led to resurgence in its use. Observations of air showers by the SKA could, because of its large collecting area, contribute significantly to measuring the cosmic ray spectrum at the highest energies. Because of the large surface area of the moon, and the expected excellent angular resolution of the SKA, using the SKA to detect radio Cherenkov emission from neutrino-induced cascades in lunar regolith will be potentially the most important technique for investigating cosmic ray origin at energies above the photoproduction cut-off.     

Fusion of strings and its consequences at the highest cosmic ray energies

It is shown that the fusion of string is a source of particle production in nucleus-nucleus collisions outside the kinematical limits of nucleon-nucleon collisions. The spectrum of different particles is compared with the high energy data on p-A collisions obtaining a reasonable agreement. Results for A-B collisions at and AGeV are given and possible implications for cosmic rays are examined. Both the enhancement of the cumulative effect and the reduction of multiplicities implied by string fusion should strongly modify the first interactions and the profile of extensive air showers and should be taken into account in their simulation.     

Characterization of the atmospheric muon flux in IceCube

Muons produced in atmospheric cosmic ray showers account for the by far dominant part of the event yield in large-volume underground particle detectors. The IceCube detector, with an instrumented volume of about a cubic kilometer, has the potential to conduct unique investigations on atmospheric muons by exploiting the large collection area and the possibility to track particles over a long distance. Through detailed reconstruction of energy deposition along the tracks, the characteristics of muon bundles can be quantified, and individual particles of exceptionally high energy identified. The data can then be used to constrain the cosmic ray primary flux and the contribution to atmospheric lepton fluxes from prompt decays of short-lived hadrons.In this paper, techniques for the extraction of physical measurements from atmospheric muon events are described and first results are presented. The multiplicity spectrum of TeV muons in cosmic ray air showers for primaries in the energy range from the knee to the ankle is derived and found to be consistent with recent results from surface detectors. The single muon energy spectrum is determined up to PeV energies and shows a clear indication for the emergence of a distinct spectral component from prompt decays of short-lived hadrons. The magnitude of the prompt flux, which should include a substantial contribution from light vector meson di-muon decays, is consistent with current theoretical predictions.The variety of measurements and high event statistics can also be exploited for the evaluation of systematic effects. In the course of this study, internal inconsistencies in the zenith angle distribution of events were found which indicate the presence of an unexplained effect outside the currently applied range of detector systematics. The underlying cause could be related to the hadronic interaction models used to describe muon production in air showers.     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

Evidence for the charge-excess contribution in air shower radio emission observed by the CODALEMA experiment

CODALEMA is one of the pioneer experiments dedicated to the radio detection of ultra high energy cosmic rays (UHECR), located at the radio observatory of Nançay (France). The CODALEMA experiment uses both a particle detector array and a radio antenna array. Data from both detection systems have been used to determine the ground coordinates of the core of extensive air showers (EAS). We discuss the observed systematic shift of the core positions determined with these two detection techniques. We show that this shift is due to the charge-excess contribution to the total radio emission of air showers, using the simulation code SELFAS. The dependences of the radio core shift to the primary cosmic ray characteristics are studied in details. The observation of this systematic shift can be considered as an experimental signature of the charge excess contribution.     

Monte Carlo study of interaction between solar wind plasma and Venusian upper atmosphere

Isolated events of proton and alpha particle precipitation in the Venusian atmosphere were recorded with the use of the ASPERA-4 analyzer on board the ESA Venus Express spacecraft. Using a Monte Carlo simulation method for calculation of proton and alpha particle precipitations in the Venusian atmosphere, reflected and upward directed particle fluxes have been found. It has been found that only a vanishing percentage of protons and alpha particles are backscattered to the Venusian exosphere when neglecting the induced magnetic field and under conditions of low solar activity. Accounting for the induced field drastically changes the situation: the backscattered by the atmosphere energy fluxes increase up to 44% for the horizontal magnetic field B = 20 nT, measured for Venus, for the case of precipitating protons, and up to 64%, for alpha particles. The reflected energy fluxes increase to about 100% for both protons and alpha particles as the field grows to 40 nT, i.e., the atmosphere is protected against penetration of solar wind particles.     

Numerical modeling of ionian volcanic plumes with entrained particulates

Volcanic plumes on Jupiter's moon Io are modeled using the direct simulation Monte Carlo (DSMC) method. The modeled volcanic vent is interpreted as a “virtual” vent. A parametric study of the “virtual” vent gas temperature and velocity is performed to constrain the gas properties at the vent by observables, particularly the plume height and the surrounding condensate deposition ring radius. Also, the flow of refractory nano-size particulates entrained in the gas is modeled with “overlay” techniques which assume that the background gas flow is not altered by the particulates. The column density along the tangential line-of-sight and the shadow cast by the plume are calculated and compared with Voyager and Galileo images. The parametric study indicates that it is possible to obtain a unique solution for the vent temperature and velocity for a large plume like Pele. However, for a small Prometheus-type plume, several different possible combinations of vent temperature and velocity result in both the same shock height and peak deposition ring radius. Pele and Prometheus plume particulates are examined in detail. Encouraging matches with observations are obtained for each plume by varying both the gas and particle parameters. The calculated tangential gas column density of Pele agrees with that obtained from HST observations. An upper limit on the size of particles that track the gas flow well is found to be ∼10 nm, consistent with Voyager observations of Loki. While it is certainly possible for the plumes to contain refractory dust or pyroclastic particles, especially in the vent vicinity, we find that the conditions are favorable for SO₂ condensation into particles away from the vent vicinity for Prometheus. The shadow cast by Prometheus as seen in Galileo images is also reproduced by our simulation. A time averaged frost deposition profile is calculated for Prometheus in an effort to explain the multiple ring structure observed around the source region. However, this multiple ring structure may be better explained by the calculated deposition of entrained particles. The possibility of forming a dust cloud on Io is examined and, based on a lack of any such observed clouds, a subsolar frost temperature of less than 118 K is suggested.     

Rotation rate and velocity dispersion of planetary ring particles with size distribution II. Numerical simulation for gravitating particles

We examine rotation rates of gravitating particles in low optical depth rings, on the basis of the evolution equation of particle rotational energy derived by Ohtsuki [Ohtsuki, K., 2006. Rotation rate and velocity dispersion of planetary ring particles with size distribution. I. Formulation and analytic calculation. Icarus 183, 373-383]. We obtain the rates of evolution of particle rotation rate and velocity dispersion, using three-body orbital integration that takes into account distribution of random velocities and rotation rates. The obtained stirring and friction rates are used to calculate the evolution of velocity dispersion and rotation rate for particles in one- and two-size component rings as well as those with a narrow size distribution, and agreement with N-body simulation is confirmed. Then, we perform calculations to examine equilibrium rotation rates and velocity dispersion of gravitating ring particles with a broad size distribution, from 1 cm up to 10 m. We find that small particles spin rapidly with 〈ω²〉^1/2/Ω?¹⁰²-¹⁰³, where ω and Ω are the particle rotation rate and its orbital angular frequency, respectively, while the largest particles spin slowly, with 〈ω²〉^1/2/Ω?1. The vertical scale height of rapidly rotating small particles is much larger than that of slowly rotating large particles. Thus, rotational states of ring particles have vertical heterogeneity, which should be taken into account in modeling thermal infrared emission from Saturn's rings.     

Extremely high energy cosmic rays from relic particle decays

The expected proton and neutrino fluxes from decays of massive metastable relic particles are calculated using the HERWIG QCD event generator. The predicted proton spectrum can account for the observed flux of extremely high energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff, for a decaying particle mass of (10¹²) GeV. The lifetime required is of (10²⁰) yr if such particles constitute all of the dark matter (with a proportionally shorter lifetime for a smaller contribution). Such values are plausible if the metastable particles are hadron-like bound states from the hidden sector of supersymmetry breaking which decay through nonrenormalizable interactions. The expected ratio of the proton to neutrino flux is given as a diagonistic of the decaying particle model for the forthcoming Pierre Auger Project.     

Theoretical infrared spectrum of the ionized hydrogen between 0.8 and 4.5 microns

The problem is considered of the slow outflow of gas from a Close Binary System with subsequent formation of a shell or cloud of matter around the whole system. It appears that with a small change of velocity introduced to the most external parts of the gaseous ring around one of the components the gas particles can flow out from the binary system leaving it through the external Lagrangian point. This process can lead to the formation of a shell around the binary star. The change in kinetic energy of a gas particle corresponding to the perturbation in its motion leading to the escape through the adjacent external Lagrangian point can be smaller than 10% of the total kinetic energy of the considered particle for the case of a ring around a component with mass equal to or larger than the mass of the companion.     

Time structure of the extensive air shower front

The GREX/COVER_PLASTEX experiment has measured the temporal and spatial fine structure of the EAS disc at sea level in a new and original way, using resistive plate counter detectors for direct measurements of the arrival time of each particle crossing the detector. Data were taken at EAS core distances up to 100 m for shower size N > 10⁵ (PeV energy range). Arrival times of shower particles were measured with nanosecond accuracy. More than 450000 air shower events have been included in this analysis.     

A dynamo theory of solar flares

It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 10²⁹ erg s^#X2212;1 and the total energy of 10³² erg during a typical life time (10³ s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 10³⁷–10³⁸.     

On the escape of particles from cosmic ray modified shocks

Stationary solutions to the problem of particle acceleration at shock waves in the non-linear regime, when the dynamical reaction of the accelerated particles on the shock cannot be neglected, are known to show a prominent energy flux escaping from the shock towards upstream infinity. On physical grounds, the escape of particles from the upstream region of a shock has to be expected in all those situations in which the maximum momentum of accelerated particles,   p _max  , decreases with time, as is the case for the Sedov–Taylor phase of expansion of a shell supernova remnant, when both the shock velocity and the cosmic ray induced magnetization decrease. In this situation, at each time t , particles with momenta larger than   p _max( t )  leave the system from upstream, carrying away a large fraction of the energy if the shock is strongly modified by the presence of cosmic rays. This phenomenon is of crucial importance for explaining the cosmic ray spectrum detected at the Earth. In this paper, we discuss how this escape flux appears in the different approaches to non-linear diffusive shock acceleration, and especially in the quasi-stationary semi-analytical kinetic ones. We apply our calculations to the Sedov–Taylor phase of a typical supernova remnant, including in a self-consistent way particle acceleration, magnetic field amplification and the dynamical reaction on the shock structure of both particles and fields. Within this framework, we calculate the temporal evolution of the maximum energy reached by the accelerated particles and of the escape flux towards upstream infinity. The latter quantity is directly related to the cosmic ray spectrum detected at the Earth.     

Direct and Indirect Thermospheric Heating Sources for Solar Cycles 21–23

Solar variability is often cast in terms of radiative emission and the associated long-term climate response; however, growing societal reliance on technology is creating more interest in day-to-day solar variability. This variability is associated with both solar radiative and solar wind emissions. In this paper we explore the combined effects of radiative and solar wind fluctuations at Earth. The fluctuations in radiative and geomagnetic power create an extended interval of solar maximum for the upper atmosphere. We use a trio of empirical models to estimate, over the last three solar cycles, the relative contributions of solar extreme ultraviolet (UV) power, Joule power, and particle kinetic power to the Earth’s upper atmosphere energy budget. Daily power values are derived from three source models. The SOLAR2000 solar irradiance specification model provides estimates of the daily extreme and far UV solar power input. Geomagnetic power is derived from a combination of satellite-estimated particle precipitation power and an empirical model of Joule power from hemispherically integrated estimates of high-latitude energy deposition. During the interval 1975 to 2003, the average daily contributions were: particles – 36 GW, Joule – 95 GW and solar – 464 GW for a total of 595 GW. Solar wind-driven geomagnetic power provided 22% of the total global upper atmospheric energy. In the top 15 power events, geomagnetic power contributed two-thirds of the total power budget. In each of these events, Joule power alone exceeded solar power. With rising activity, Joule power becomes the most variable element of solar upper atmosphere interactions.