Resonant Transition Radiation Research Articles

Spatially Resolved Observations of Meteor Radio Afterglows With the OVRO‐LWA

AbstractWe conducted an all‐sky imaging transient search with the Owens Valley Radio Observatory Long Wavelength Array (OVRO‐LWA) data collected during the Perseid meteor shower in 2018. The data collection during the meteor shower was motivated to conduct a search for intrinsic radio emission from meteors below 60 MHz known as the meteor radio afterglows (MRAs). The data collected were calibrated and imaged using the core array to obtain lower angular resolution images of the sky. These images were input to a pre‐existing LWA transient search pipeline to search for MRAs as well as cosmic radio transients. This search detected 5 MRAs and did not find any cosmic transients. We further conducted peeling of bright sources, near‐field correction, visibility differencing and higher angular resolution imaging using the full array for these 5 MRAs. These higher angular resolution images were used to study their plasma emission structures and monitor their evolution as a function of frequency and time. With higher angular resolution imaging, we resolved the radio emission size scales to less than 1 km physical size at 100 km heights. The spectral index mapping of one of the long duration event showed signs of diffusion of plasma within the meteor trails. The unpolarized emission from the resolved radio components suggest resonant transition radiation as the possible radiation mechanism of MRAs.

Controlling Cherenkov angles with resonance transition radiation

Cherenkov radiation provides a valuable way to identify high energy particles in a wide momentum range, through the relation between the particle velocity and the Cherenkov angle. However, since the Cherenkov angle depends only on material's permittivity, the material unavoidably sets a fundamental limit to the momentum coverage and sensitivity of Cherenkov detectors. For example, Ring Imaging Cherenkov detectors must employ materials transparent to the frequency of interest as well as possessing permittivities close to unity to identify particles in the multi GeV range, and thus are often limited to large gas chambers. It would be extremely important albeit challenging to lift this fundamental limit and control Cherenkov angles as preferred. Here we propose a new mechanism that uses constructive interference of resonance transition radiation from photonic crystals to generate both forward and backward Cherenkov radiation. This mechanism can control Cherenkov angles in a flexible way with high sensitivity to any desired range of velocities. Photonic crystals thus overcome the severe material limit for Cherenkov detectors, enabling the use of transparent materials with arbitrary values of permittivity, and provide a promising option suited for identification of particles at high energy with enhanced sensitivity.

4a-B-10 多層薄膜から発生する共鳴遷移放射の単色化

4a-B-10 多層薄膜から発生する共鳴遷移放射の単色化

Enhanced emission from an ensemble of inverted atoms in the presence of another isotopic species

The emission flux from a slab of two homogeneously-broadened isotope species ensembles is analyzed. Initially one of the species is coherently nearly all inverted, the atoms of the other species are in the ground state. The slab thickness is less than the wavelength of the resonant transition radiation. In particular, in a narrow band of the slab thickness centered at 237λ/500, the emission flux from the species mixture for different detunings between the two species resonance frequencies is shown to be greatly enhanced over its value where only a single species is present.

X-ray transition radiation of electrons with energy of 300 to 900 MeV in periodic multifoil radiators

Results of experiments conducted at the Tomsk synchrotron to study resonant X-ray transition radiation generated by relativistic electrons in periodic multifoil radiators are reviewed. Both the internal synchrotron beam and the external secondary electron beam from the pair magnetic γ-spectrometer with energies ranging from 300 to 900 MeV were used in the experiments. The radiators consisted of many thin amorphous foils of various materials. The generation of X-ray radiation in a compound radiator consisting of a multifoil radiator and a crystal is also studied. In this case, the resonant X-ray transition radiation generated in the multifoil radiator is diffracted in the crystal and emitted at Bragg angles, together with the parametric X-ray radiation generated in the crystal. Spectral and angular properties of the resonant X-ray transition radiation and diffracted resonant X-ray transition radiation are investigated. The ratio between the contributions from the diffracted resonant X-ray transition radiation and other types of radiation to the total coherent X-ray radiation flux generated by electrons in periodic structures and crystals is estimated.

VUV resonant transition radiation from relativistic electrons

The modified radiator for generation of the resonant transition radiation from relativistic electrons is presented. This radiator consists of a set of small thin foils inclined with respect to the trajectory of an emitting electron. It is shown that the photoabsorption of the emitted photons is almost completely suppressed in the considered scheme. Therefore, the possibility of generating intense beams of quasimonochromatic vacuum ultraviolet (VUV) quanta by the proposed method appears.

Viewing radiation signatures of solar energetic particles in interplanetary space

A current serious limitation on the studies of solar energetic particle (SEP) events is that their properties in the inner heliosphere are studied only through in situ spacecraft observations. Our understanding of spatial distributions and temporal variations of SEP events has come through statistical studies of many such events over several solar cycles. In contrast, flare SEPs in the solar corona can be imaged through their radiative and collisional interactions with solar fields and particles. We suggest that the heliospheric SEPs may also interact with heliospheric particles and fields to produce signatures which can be remotely observed and imaged. A challenge with any such candidate signature is to separate it from that of flare SEPs. The optimum case for imaging high-energy ( E > 100 MeV) heliospheric protons may be the emission of π 0-decay γ-rays following proton collisions with solar wind (SW) ions. In the case of E > 1 MeV electrons, gyrosynchrotron radio emission may be the most readily detectible remote signal. In both cases we may already have observed one or two such events. Another radiative signature from nonthermal particles may be resonant transition radiation, which has likely already been observed from solar flare electrons. We discuss energetic neutrons as another possible remote signature, but we rule out γ-ray line and 0.511 MeV positron annihilation emission as observable signatures of heliospheric energetic ions. We are already acquiring global signatures of large inner-heliospheric SW density features and of heliosheath interactions between the SW and interstellar neutral ions. By finding an appropriate observable signature of remote heliospheric SEPs, we could supplement the in situ observations with global maps of energetic SEP events to provide a comprehensive view of SEP events.

Design of A Soft X‐ray Source with Periodic Microstructure Using Resonance Transition Radiation for Tabletop Synchrotron

AbstractA soft X‐ray source with a periodic aluminum multilayer structure with vacuum layers in between for tabletop synchrotrons with low‐energy electrons was designed, and the dependence of radiation performance on structural parameters was clarified using a theoretical calculation of the resonance transition radiation. A photon density of 1 keV pr incident electron was calculated by optimizing the thickness of the aluminum and vacuum layers. Furthermore, the designed structure was fabricated by a microfabrication technique. The effects of dimensional error on the photon density are discussed on the basis of the calculated results and the prototype structure of a 6‐MeV synchrotron. It is clarified that dimensional errors of 1.5% in Al layers and of plus or minus a few dozen nanometers in the vacuum layers are negligible in obtaining a photon density of more than 90% of the maximum value. © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Resonant Transition Radiation and Solar Radio Bursts

We calculate the resonant transition radiation (RTR) using the most general formulas and obtain approximate analytical expressions describing the RTR dependence on the fast-particle velocity and magnetic-field intensity. Dependence of the RTR on the angle between the wave vector and the magnetic-field direction and also on the thermal velocity of background-plasma electrons is considered on the basis of approximate expressions for RTR, in which the longitudinal dielectric permittivity of the plasma is written neglecting the wave vector of transverse electromagnetic waves. The obtained dependences are combined into unified expressions for RTR coefficients. Comparison with the earlier found RTR coefficients is performed. It is shown that the obtained coefficients correspond better to the nature of the resonant transition radiation. Finally, the obtained RTR coefficients are used for interpretation of the radio emission of the solar burst of December 24, 1991.

Resonant transition radiation of solar radio bursts

We calculate the resonant transition radiation (RTR) using the most general formulas and obtain approximate analytical expressions describing the RTR dependence on the fast-particle velocity and magnetic-field intensity. Dependence of the RTR on the angle between the wave vector and the magnetic-field direction and also on the thermal velocity of background-plasma electrons is considered on the basis of approximate expressions for RTR, in which the longitudinal dielectric permittivity of the plasma is written neglecting the wave vector of transverse electromagnetic waves. The obtained dependences are combined into unified expressions for RTR coefficients. Comparison with the earlier found RTR coefficients is performed. It is shown that the obtained coefficients correspond better to the nature of the resonant transition radiation. Finally, the obtained RTR coefficients are used for interpretation of the radio emission of the solar burst of December 24, 1991.

Fine structure of decametric type II radio bursts

We have performed a comparative analysis of the fine structure of two decametric type II bursts observed on July 17 and August 16, 2002, with the 1024-channel spectrograph of the UTR-2 radio telescope in the frequency range 18.5-29.5 MHz and with the IZMIRAN spectrograph in the frequency range 25-270 MHz. The August 16 burst was weak, ∼ 2-5 s.f.u., but exhibited an unusual fine structure in the form of broadband fibers (∆fe > 250-500 kHz) that drifted at a rate characteristic of type II bursts and consisted of regular narrow-band fibers (∆fe > 50-90 kHz at 24 MHz) resembling a fiber bundle. The July 17 burst was three orders of magnitude more intense (up to 4500 s.f.u. at 20 MHz) and included a similar fiber structure. The narrow fibers were irregular and shorter in duration. They differed from an ordinary fiber bundle by the absence of absorption from the low-frequency edge and by slow frequency drift (slower than that of a type II burst). Both type II bursts were also observed in interplanetary space in the WIND/WAVES RAD2 spectra, but without any direct continuation. Analysis of the corresponding coronal mass ejections (CMEs) based on SOHO/LASCO C2 data has shown that the radio source of the type II burst detected on August 16 with UTR-2 was located between the narrow CME and the shock front trailing behind that was catching up with the CME. The July 17 type II fiber burst also occurred at the time when the shock front was catching up with the CME. Under such conditions, it would be natural to assume that the emission from large fibers is related to the passage of the shock front through narrow inhomogeneities in the CME tail. Resonant transition radiation may be the main radio emission mechanism. Both events are characterized by the possible generation of whistlers between the leading CME edge and the shock front. The whistlers excited at shock fronts manifest themselves only against the background of enhanced emission from large fibers (similar to the continuum modulation in type IV bursts). The reduction in whistler group velocity inside inhomogeneities to 760 km s −1 may be responsible for the unusually low drift rate of the narrow fibers. The magnetic field inside inhomogeneities determined from fiber parameters at 24 MHz is ∼0.9 G, while the density should be increased by at least a factor of 2.

Fine Structure of Solar Radio Bursts Observed at Decametric and Hectometric Waves

The analysis of WIND/WAVES RAD2 spectra with fine structure in the form of different fibers in 14 events covering 1997 - 2005 is carried out. A splitting of broad bands of the interplanetary (IP) type II bursts into narrow band fibers of different duration is observed. The instantaneous-frequency bandwidth of fibers is stable: 200 - 300 kHz for slow-drifting fibers in type II bursts, and 700 - 1000 kHz for fast-drifting fibers in type II + IV (contin- uum). Intermediate drift bursts (IDB or fiber bursts) and zebra patterns with variable fre- quency drift of stripes, typical for the metric range, were not found. Comparison of spectra with the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO C2) images shows a connection of the generation of the fiber structures with the passage of shock fronts through narrow jets in the wake of Coronal Mass Ejec- tions (CME). Therefore the most probable emission mechanism of fibers in IP type II bursts appears to be resonance transition radiation (RTR) of fast particles at the boundary of two media with different refractive indices. The same mechanism is also valid for striae in the type III bursts. Taking into account a high-density contrast in the CME wake and the actu- ally observed small-scale inhomogeneities, the effectiveness of the RTR mechanism in IP space must be considerably higher than in the meter or decimeter wavelengths. For the most part the fibers in the type IV continuum at frequencies of 14 - 8 MHz were seen as the direct expansion of similar fine structure (as fibers or "herringbone" structure) in the decametric range observed with the Nancay and IZMIRAN spectrographs.

Multilayer interference mirrors as components for soft x‐ray WDS monochromators and tunable radiation sources

AbstractMultilayer interference mirrors (MIMs) are widely used as low‐resolution monochromators for soft x‐ray wavelength dispersive spectrometry (WDS). It is now possible to fabricate multilayer monochromators with medium resolution by etching MIMs with a large number of bilayers according to the profile of a lamellar grating. It will be shown both theoretically and experimentally that these multilayer gratings with appropriate characteristics can be used as monochromators for soft x‐rays, whose spectral resolving power is considerably improved with respect to mere interference mirrors. Moreover, tunable soft x‐ray sources based on the interaction of medium‐energy electron beams with periodic multilayer structures are presently being developed in several countries. The physical processes involved in this type of sources belong to the class of quasi‐Cerenkov emission. Among them are resonant transition radiation and parametric radiation. We present the basic concepts and the first experimental results concerning this new type of sources which should be very interesting for WDS. Copyright © 2007 John Wiley & Sons, Ltd.

Transition radiation in metal-metal multilayer nanostructures as a medical source of hard x-ray radiation

We show that a periodic metal-metal multilayer nanostructure can serve as an efficient source of hard x-ray transition radiation. Our research effort is aimed at developing an x-ray source for medical applications, which is based on using low-energy relativistic electrons. The approach toward choosing radiator-spacer couples for the generation of hard x-ray resonant transition radiation by few-MeV electrons traversing solid multilayer structures for the energies of interest to medicine (30–50keV) changes dramatically compared with that for soft x-ray radiation. We show that one of the main factors in achieving the required resonant line is the absence of the contrast of the refractive indices between the spacer and the radiator at the far wings of the radiation line; for that purpose, the optimal spacer, as a rule, should have a higher atomic number than the radiator. Having experimental goals in mind, we have considered also the unwanted effects due to bremsstrahlung radiation, absorption and scattering of radiated photons, detector-related issues, and inhibited coherence of transition radiation due to random deviation of spacing between the layers. Choosing as a model example a Mo–Ag radiator-spacer pair of materials, we demonstrate that the x-ray transition radiation line can be well resolved with the use of spatial and frequency filtering.

An optimization model of targets for resonant transition radiation in the X-ray range

Generation of resonant transition radiation (RTR) is potentially a method to obtain useable fluxes of tuneable, relatively narrow-band soft X-ray radiation from compact (<10MeV) electron accelerators. The RTR targets to be used are periodic multilayer structures of two materials with low absorption and high refractive-index contrast in the soft X-ray range. We present a model for optimizing the parameters of such targets. The model is based on an extension of the Ginzburg–Franck equation for transition radiation from a single interface. The model also accounts for absorption, elastic scattering, energy spread of the electron beam and inaccuracy in the foil thicknesses. Two examples are treated. First, a multilayer structure consisting of Mo foils and vacuum spacers is optimized for generation of 13.5nm radiation by an electron beam with energy of 9MeV. The model predicts a yield of up to 0.045 photons per electron per (eVsr). Second, a multilayer structure consisting of Al foils and vacuum spacers is optimized for generation of 0.83nm radiation by 150MeV electrons. For the latter case, the model predicts a yield of up to 3.7 photons per electron per (eVsr).

Resonant transition radiation in plamsa with magnetic inhomogeneities

We calculate the spectrum of the resonant transition radiation generated by a fast charged particle moving in plasma with small-scale random magnetic inhomogeneities. We determine the conditions under which this type of transition radiation dominates over the transition radiation on plasma density inhomogeneities. We discuss possible applications of the resonant transition radiation in astrophysical and geophysical conditions.

Spatial Evidence for Transition Radiation in a Solar Radio Burst

Microturbulence, i.e., enhanced fluctuations of plasma density and electric and magnetic fields, is of great interest in astrophysical plasmas, but occurs on spatial scales far too small to resolve by remote sensing, e.g., at ~1-100 cm in the solar corona. This Letter reports spatially resolved observations that offer strong support for the presence in solar flares of a suspected radio emission mechanism, resonant transition radiation, which is tightly coupled to the level of microturbulence and provides direct diagnostics of the existence and level of fluctuations on decimeter spatial scales. Although the level of the microturbulence derived from the radio data is not particularly high, Δn2/n2 ~ 10-5, it is large enough to affect the charged particle diffusion and give rise to effective stochastic acceleration. This finding has exceptionally broad astrophysical implications, since modern sophisticated numerical models predict generation of much stronger turbulence in relativistic objects, e.g., in gamma-ray burst sources.

Evidence for Resonant Transition Radiation in Decimetric Continuum Solar Bursts

We investigate the properties of radio bursts observed in the 1–18 GHz frequency range that display two distinct spectral components peaking at decimetric (dm) and centimetric (cm) ranges. Thedmemission is relatively smooth in both frequency and time, with timescales comparable to those of the cm component. The two spectral components display specific correlations in their temporal and spectral behavior. Through detailed analysis, we find the following: (1) A large ratio of plasma frequency to gyrofrequency is a characteristic of all of these events. (2) The dm component generally displays a high degree of o-mode polarization. (3) The dm component is produced by an incoherent emission mechanism. (4) The dm and cm components are generated by the same electron distribution. (5)Thecharacteristicenergyofthefastelectronsproducingthedmcontinuumissignificantlylowerthantheenergy of the electrons generating the microwave gyrosynchrotron component. (6) The spectral shape of the dm component is not well correlated with the fast electron distribution over energy. These findings cannot easily be explainedwithstandardmechanismsfordmsolarburstemissionbutagreewellwiththepredictionsofthetheoryof resonant transition radiation. We conclude that these two-component bursts represent a distinctive subclass of events, comprising about 10% of all bursts, with the dm continuum component generated most probably by resonant transition radiation produced by interaction of fast electrons with small-scale inhomogeneities of the background plasma. We discuss new possibilities for flaring plasma diagnostics using these two-component bursts.

The nature of decimeter-wave microburst emission

New observations of solar microbursts are reported. The spectra of microbursts can contain narrow features (Δf/f ≤ 0.03). Possible mechanisms for the generation of the microburst radio emission are analyzed, focusing on mechanisms that preferentially generate ordinary waves. Together with the well-known generation of radio emission at the fundamental plasma frequency, mechanisms associated with upper-hybrid waves and resonant-transition radiation are considered. The radio emission at the upper-hybrid frequency always corresponds to ordinary waves. Ordinary waves can also dominate in resonant-transition radiation, but the presence of well-developed small-scale turbulence in the emission region is required. Possible mechanisms for the generation of this turbulence include thermal fluctuations, Langmuir turbulence, quasi-stationary structures created by plasmawaves, and ion-acoustic waves. Probable origins of the quasi-periodicity of microbursts are analyzed.

An efficient method for calculating plural scattering of relativistic electrons in thin foils and multilayers

The radiation characteristics of soft X-ray emission by relativistic electrons, which produce (resonant) transition radiation or Cherenkov radiation inside a foil or multilayer, is determined not only by the intrinsic emission process but also by the angular distribution of the electrons due to Rutherford scattering. The number of scatterings events in these micron-thick foils or multilayers is smaller than about 20, the so-called plural scattering regime. We developed a numerical method to calculate very efficiently the angular distribution of relativistic electrons while passing through a single layer of any material and arbitrary thickness in the plural and part of the multiple scattering regime. The angular scattering distribution is a truncated weighted summation of universal distribution functions, each representing a fixed number of scattering events. These universal distribution functions have to be calculated only once and can be stored in a database for later use. This method of calculating the angular scattering distributions has also been extended to deal with multilayers and to take into account incident electron beams with an arbitrary angular distribution. The calculations have been validated by comparison with Monte Carlo simulations, which require much more computing time.