Preheating Mechanism Research Articles

Electron energization in quasi-parallel shocks

Context. In situ observations of energetic particles at the Earth’s bow-shock that are attainable by the satellite missions have fostered the opinion for a long time that electrons are most efficiently accelerated in a quasi-perpendicular shock geometry. However, shocks that are deemed to be responsible for the production of cosmic ray electrons and their radiation from sources such as supernova remnants are much more powerful and larger than the Earth’s bow-shock. Their remote observations and also in situ measurements at Saturn’s bow shock, that is, the strongest shock in the Solar System, suggest that electrons are accelerated very efficiently in the quasi-parallel shocks as well. Aims. In this paper we investigate the possibility that protons that are accelerated to high energies create sufficient wave turbulence, which is necessary for the electron preheating and subsequent injection into the diffusive shock acceleration in a quasi-parallel shock geometry. Methods. An additional test-particle-electron population, which is meant to be a low-density addition to the electron core-distribution on which the hybrid simulation operates, is introduced. Our purpose is to investigate how these electrons are energized by the “hybrid” electromagnetic field. The reduced spatial dimensionality allowed us to dramatically increase the number of macro-ions per numerical cell and achieve the converged results for the velocity distributions of test electrons. Results. We discuss the electron preheating mechanisms, which can make a significant part of thermal electrons accessible to the ion-driven waves observed in hybrid simulations. We find that the precursor wave field supplied by ions has a considerable potential to preheat the electrons before they are shocked at the subshock. Our results indicate that a downstream thermal equilibration of the hot test electrons and protons does not occur. Instead, the resulting electron-to-proton temperature ratio is a decreasing function of the shock Mach number, MA, which has a tendency for a saturation at high MA.

Modified natural inflation: A small single field model with a large tensor to scalar ratio

In this paper we explored in detail a phenomenological model of modified single field natural inflation in light of recent cosmological experiments, BICEP2. Our main goal is to construct an inflationary model which not only predicts the important cosmological quantities such as $({n}_{s},r)$ compatible with experimental observation, but also is consistent with the low energy effective theory framework. Therefore, all fundamental scales apart from ${M}_{p}$ and quantities of our interest should be within the sub-Planckian region. In order to achieve our goal, we modify the usual single field natural inflationary model by a specific form of higher derivative kinetic terms called kinetic gravity braiding. One of our guiding principles to construct such a model is the constant shift symmetry of the axion. We have chosen the form of the kinetic gravity braiding term in such a way that it predicts the required value of ${n}_{s}\ensuremath{\simeq}0.96$ and a large tensor to scalar ratio, $r>0.1$. Importantly, for a wide range of parameter space our model has sub-Planckian axion decay constant $f$ and the scale of inflation $\mathrm{\ensuremath{\Lambda}}$. However, the reheating after the end of inflation limits the value of $f$ so that we are unable to get $f$ to be significantly lower than ${M}_{p}$. Furthermore, we find sub-Planckian field excursion for the axion field $\mathrm{\ensuremath{\Delta}}\ensuremath{\phi}\ensuremath{\simeq}f$ for the sufficient number of $e$-folding, $\mathcal{N}\ensuremath{\gtrsim}50$. We also discussed in detail about the natural preheating mechanism in our model based on the recently proposed Chern-Simon coupling. We found this gravity mediated preheating is very difficult to achieve in our model. With our general analytic argument, we also would like to emphasize that Chern-Simons mediated preheating is very unlikely to happen in any slow roll inflationary model.

A FOCUSED TRANSPORT APPROACH TO THE TIME-DEPENDENT SHOCK ACCELERATION OF SOLAR ENERGETIC PARTICLES AT A FAST TRAVELING SHOCK

Some of the most sophisticated models for solar energetic particle (SEP) acceleration at coronal mass ejection driven shocks are based on standard diffusive shock acceleration theory. However, this theory, which only applies when SEP pitch-angle anisotropies are small, might have difficulty in describing first-order Fermi acceleration or the shock pre-heating and injection of SEPs into first-order Fermi acceleration accurately at lower SEP speeds where SEP pitch-angle anisotropies upstream near the shock can be large. To avoid this problem, we use a time-dependent focused transport model to reinvestigate first-order Fermi acceleration at planar parallel and quasi-parallel spherical traveling shocks between the Sun and Earth with high shock speeds associated with rare extreme gradual SEP events. The focused transport model is also used to investigate and compare three different shock pre-heating mechanisms associated with different aspects of the nonuniform cross-shock solar wind flow, namely, the convergence of the flow (adiabatic compression), the shear tensor of the flow, and the acceleration of the flow, and a fourth shock pre-heating mechanism associated with the cross-shock electric field, to determine which pre-heating mechanism contributes the most to injecting shock pre-heated source particles into the first-order Fermi acceleration process. The effects of variations in traveling shock conditions, such as increasing shock obliquity and shock slowdown, and variations in the SEP source with increasing shock distance from the Sun on the coupled processes of shock pre-heating, injection, and first-order Fermi acceleration are analyzed. Besides the finding that the cross-shock acceleration of the solar wind flow yields the dominant shock pre-heating mechanism at high shock speeds, we find that first-order Fermi acceleration at fast traveling shocks differs in a number of respects from the predictions and assumptions of standard steady-state diffusive shock acceleration theory as is discussed below.

Matrix Inflation and its String Theory Origins

Motivated by the dynamics of N coincident D3 branes in some specific flux compactifications, we construct an inflationary model in which inflation is driven by three N × N hermitian matrices Φi,i = 1,2, 3, hence the name Matrix Inflation, or M-flation for short. We show that one can consistently restrict the classical dynamics to a sector in which the Φi are proportional to the N × N irreducible representation of SU(2). In this sector our model behaves like an effective inflaton field, which takes super-Planckian field values, and 3N2 − 1 isocurvature fields. These may have the observational effects such as production of iso-curvature perturbations on cosmic microwave background. Moreover, the existence of these extra scalars provides us with a natural preheating mechanism and exit from inflation. Due to the super-Planckian excursions of the canonical effective inflaton, the model is capable of producing a considerable amount of gravity waves that can be probed by future CMB polarization experiments. Furthermore, the fine-tunings associated with unnaturally small couplings in the chaotic type inflationary scenarios are removed. We also show that even if the cutoff of the theory is lowered by the square of number of species, one can still use the effective field theory approach to justify the absence of higher dimensional operators.

VORTICITY OF INTERGALACTIC MEDIUM VELOCITY FIELD ON LARGE SCALES

We investigate the vorticity of the IGM velocity field on large scales with cosmological hydrodynamic simulation of the concordance model of LCDM. We show that the vorticity field is significantly increasing with time as it can effectively be generated by shocks and complex structures in the IGM. Therefore, the vorticity field is an effective tool to reveal the nonlinear behavior of the IGM, especially the formation and evolution of turbulence in the IGM. We find that the vorticity field does not follow the filaments and sheets structures of underlying dark matter density field and shows highly non- Gaussian and intermittent features. The power spectrum of the vorticity field is used to measure the development of turbulence in Fourier space. We show that the relation between the power spectra of vorticity and velocity fields is perfectly in agreement with the prediction of a fully developed homogeneous and isotropic turbulence from 0.2 to 3 h^{-1} Mpc at z~0. This indicates that cosmic baryonic field is in the state of fully developed turbulence on scales less than about 3 h^{-1} Mpc. The random field of the turbulent fluid yields turbulent pressure to prevent the gravitational collapsing of the IGM. The vorticity and turbulent pressure are strong inside and even outside of high density regions. In IGM regions with 10 times mean overdensity, the turbulent pressure can be on an average equivalent to the thermal pressure of the baryonic gas with a temperature of 10^5 K. The fully developed turbulence would prevent the baryons in the IGM from falling into the gravitational well of dark matter halos. Moreover, turbulent pressure is dynamical and non-thermal, which makes it different from pre-heating mechanism as it does not affect the thermal state and ionizing process of hydrogen in the IGM.

Temperature and Entropy Fields of Baryonic Gas in the Universe

(Abridged)The temperature (T) and entropy (S) fields of baryonic gas/IGM are analyzed using simulation samples by a cosmological hydro/N-body code. We show that in the nonlinear regime the dynamical similarity between the IGM and dark matter will be broken by strong shocks. The heating and entropy production by the shocks breaks the IGM into multiple phases. The mean entropy, or the cosmological entropy floor, is found to be more than 100 h^(-1/3) keV cm^2 in all regions when z 50 h^(-1/3) keV cm^2) mostly resides in areas on scales larger than 1 h^(-1) Mpc and with density $\rho_{dm}>10^2$. Therefore, gravitational shocks are an effective preheating mechanism of the IGM, and probably enough to provide the entropy excess of clusters and groups if the epoch of the gas falling in cluster cores is not earlier than z ~ 2 - 3. On the other hand, at redshifts z < 4, there is always a more than 90% volume of the low dark matter mass density regions filled by the IGM with T < 10^4.5 K. Therefore, the multiphased character and non-Gaussianity of the IGM field would explain the high-temperature and high-entropy gas observed in groups and clusters.

High resolution spectroscopy of Balmer-dominated shocks in the RCW 86, Kepler and SN 1006 supernova remnants

We report results from high resolution optical spectroscopy of three non-radiative galactic supernova remnants, RCW 86, Kepler's supernova remnant and SN 1006. We have measured the narrow component Hα line widths in Balmer-dominated filaments in RCW 86 and SN 1006, as well as the narrow component width in a Balmer-dominated knot in Kepler's SNR. The narrow component line widths measured in RCW 86 and Kepler's SNR show FWHM of km s-1, similar to what has been seen in other Balmer-dominated remnants. Of the remnants in our sample, SN 1006 is the fastest shock (~3000 km s-1). The narrow component Hα and Hβ lines in this remnant have a FWHM of merely 21 km s-1. Comparing the narrow component widths measured in our sample with those measured in other remnants shows that the width of the narrow component does not correlate in a simple way with the shock velocity. The implications for the pre-heating mechanism responsible for the observed line widths are discussed.

Ion outflow and associated perpendicular heating in the cusp observed by Interball Auroral Probe and Fast Auroral Snapshot

The spatial properties of ionospheric ion outflows associated with perpendicular heating processes in the cusp are studied using a conjunction study from two satellites and ground radar systems. Low‐energy outflowing ions are measured in a wide longitudinal range, between 13,000 and 19,000 km in altitude, over the dayside polar cap by the Hyperboloid experiment aboard the Interball Auroral Probe (AP). These observations are related to conjugate convection field measurements by the Saskatoon‐Kapuskasing pair of the Super Dual Auroral Radar Network (SuperDARN). Data analysis suggests that outflowing ions originate from a wide magnetic local time range associated with both the dayside cusp region and the dayside cleft region. A direct cusp crossing by the Fast Auroral Snapshot (FAST) satellite at 2000‐km altitude shows a correlation between transverse ion heating in a thin latitudinal region (∼1.3°) and the presence of broadband extremely low frequency (BBELF) turbulence, in addition to more intense electrostatic waves near and just above the lower hybrid (LH) frequency. This event is unusual because on the basis of the statistical survey of Freja and FAST data, most of the ion‐heating events in the midaltitude auroral zone are correlated with enhanced emissions in the BBELF range. Furthermore, the electron population has energies that are too small to drive LH waves unstable. This event offers the opportunity to analyze the contribution of cusp magnetospheric ion injections to the heating of the ambient H+ and O+ ions. Kinetic instability calculations demonstrate that LH waves are destabilized by the ring distributions, which result from injections of high‐energy magnetosheath ions. Calculations show that a preheating mechanism by BBELF turbulence near the ion gyrofrequencies is also required so that LH heating is able to occur. The altitude dependence of the LH perpendicular heating is then analyzed by modeling the transport of the injected magnetospheric ions along magnetic field lines. It is shown that LH heating acts as an additional process from ∼2000 up to 10,000 km in altitude. In addition, trajectory calculations show that the low‐energy outflowing H+ and O+ ions observed along Interball AP orbit in the polar cap are heated inside the cusp at altitudes extending up to 15,000 km. These results are then assembled to construct a possible heating scenario inside the cusp for this data set. The contribution of the different energization mechanisms to the ion heating as a function of altitude is then discussed.

Fire Front Width Effects on Fire Spread Across a Laboratory Scale Sloping Fuel Bed

In a previous study, a two-dimensional non-stationary model of fire spread across a fuel bed, which included slope effects, was proposed. Improvements of the flame radiant contribution are made in the present paper by taking into consideration long-range preheating mechanisms. Fire front is assumed to be a radiant panel and the amount of energy impinging on the unburned fuel ahead of the fire front is estimated by means of a view factor. However, computation time of this view factor is lengthy and a simplified computation is thus proposed. The predictions generated by both models are compared to data recorded during laboratory-scale experimental fires conducted across pine needle beds for slopes ranging from 0 to 30°. First, the slope effects on predicted rates of spread, temperature distribution and fire front shapes are provided. The influence of fire front width on model predictions is also presented. This effect appears to be a significant variable affecting the rate of fire spread and is qualitatively represented by the model.

The 4 February, 1986 flare and flare-filament current model

In this paper, the observational data in Hα, radio, soft X-ray, hard X-ray, and γ-ray emissions for the 3B/X3.0 solar flare on 4 February, 1986 are collected. This flare is studied in detail by using the flare-filament current model. The momentum equations and the energy equations of the filament current have been solved. The influence of the highly sheared background magnetic field on the motion of the filaments is studied through numerical calculation. The results show that the resistive tearing instability is a possible pre-heating mechanism in the preflare phase, and both the rotation of the spiral sunspots and the highly sheared background field are necessary for the energy storage of this flare. The high-energy data of the flare imply that the current-loop coalescence instability is a possible eruptive mechanism.

Heat Transfer Behavior in Solids Conveying Step of an Intermeshing Twin Screw Extruder

Particulate solids handling is the first polymer processing step taking place in twin screw extrusion, and more emphasis must be placed on understanding solids conveying and pre-heating mechanisms. Solids conveying and pre-heating of particulate resin in an extruder by heating from screws and barrel are very effective for reduction of power consumption, favorable plasticization and mixing as well as for achieving higher production rates and uniform melt temperature distribution. Hence, quantitative analysis regarding solids conveying and heat transfer behavior plays an important role in the evaluation of screw design. This study is conducted both experimentally and theoretically to obtain the solids conveying efficiency and the temperature rise characteristics in a solids conveying step of an intermeshing counter-rotating twin scerw extruder by using rigid PVC resin.As a result, the followings have become elucidated:1) The solids conveying efficiency ηe is determined by the amount of backwards leakage flow through the intermeshing sections.2) A general theoretical formula for estimating temperature increment has been induced and the temperature rise characteristics can be expressed with the heat transfer coefficient hw=70-230W/m2K depending on the filling fraction φ.3) Heat transfer between the particulate resin and barrel, screw surfaces can be grasped in a way similar to the case of the stationary packed bed and the fluidized bed.

The 1986-02-04 flare and current loop coalescence instability

In this paper observational data of the 1986-02-4 3B/3X flare in Hα, radio, soft and hard X- and γrays are collected and a comprehensive analysis made in terms of the filament current model. The rising motion of the filaments is measured. After solving the momentum and energy equations, the results show (1) that the flare was probably caused by current loop coalescence instability between two loops, (2) that resistive tearing mode instability is an efficient pre-heating mechanism and (3) that the coalescence instability is possibly an efficient mechanism for simultaneous acceleration of high-energy particles.