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Showing papers on "Radio wave published in 2018"



Journal ArticleDOI
TL;DR: The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase as discussed by the authors, which is consistent with the predicted behavior of the successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out.
Abstract: We present new observations of the binary neutron star merger GW170817 at $\Delta t\approx 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $\gtrsim5\sigma$ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until $\sim 1000$ days post-merger.

131 citations


Journal ArticleDOI
TL;DR: In this article, the authors study the time-dependent evolution of the temperature and ionization structure of expanding supernova or merger ejecta due to photo-ionization by a magnetar engine, in order to study the escape of X-rays and radio waves.
Abstract: Young, rapidly spinning magnetars are invoked as central engines behind a diverse set of transient astrophysical phenomena, including gamma-ray bursts (GRB), super-luminous supernovae (SLSNe), fast radio bursts (FRB), and binary neutron star (NS) mergers. However, a barrier to direct confirmation of the magnetar hypothesis is the challenge of directly observing non-thermal emission from the central engine at early times (when it is most powerful and thus detectable) due to the dense surrounding ejecta. We present CLOUDY calculations of the time-dependent evolution of the temperature and ionization structure of expanding supernova or merger ejecta due to photo-ionization by a magnetar engine, in order to study the escape of X-rays (absorbed by neutral gas) and radio waves (absorbed by ionized gas), as well as to assess the evolution of the local dispersion measure due to photo-ionization. We find that ionization breakout does not occur if the engine's ionizing luminosity decays rapidly, and that X-rays typically escape the oxygen-rich ejecta of SLSNe only on $\sim 100 \, {\rm yr}$ timescales, consistent with current X-ray non-detections. We apply these results to constrain engine-driven models for the binary NS merger GW170817 and the luminous transient ASASSN-15lh. In terms of radio transparency and dispersion measure constraints, the repeating FRB 121102 is consistent with originating from a young, $\gtrsim 30-100 \, {\rm yr}$, magnetar similar to those inferred to power SLSNe. We further show that its high rotation measure can be produced within the same nebula that is proposed to power the quiescent radio source observed co-located with FRB 121102. Our results strengthen previous work suggesting that at least some FRBs may be produced by young magnetars, and motivate further study of engine powered transients.

95 citations


Journal ArticleDOI
TL;DR: In this article, a coronal shock associated with a CME and type II radio burst was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with θ Bn ~ 70°.
Abstract: Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20–90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs.Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon.Methods. Thetype II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory(STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary.Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with θ Bn ~ 70°. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3–1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4–1.6.

66 citations


Journal ArticleDOI
01 Jun 2018-Nature
TL;DR: Observations of broadband emission from lightning on Jupiter at 600 megahertz show a lightning discharge mechanism similar to that of terrestrial lightning and indicate increased moist convection near Jupiter’s poles.
Abstract: Lightning has been detected on Jupiter by all visiting spacecraft through night-side optical imaging and whistler (lightning-generated radio waves) signatures1–6. Jovian lightning is thought to be generated in the mixed-phase (liquid–ice) region of convective water clouds through a charge-separation process between condensed liquid water and water-ice particles, similar to that of terrestrial (cloud-to-cloud) lightning7–9. Unlike terrestrial lightning, which emits broadly over the radio spectrum up to gigahertz frequencies10,11, lightning on Jupiter has been detected only at kilohertz frequencies, despite a search for signals in the megahertz range 12 . Strong ionospheric attenuation or a lightning discharge much slower than that on Earth have been suggested as possible explanations for this discrepancy13,14. Here we report observations of Jovian lightning sferics (broadband electromagnetic impulses) at 600 megahertz from the Microwave Radiometer 15 onboard the Juno spacecraft. These detections imply that Jovian lightning discharges are not distinct from terrestrial lightning, as previously thought. In the first eight orbits of Juno, we detected 377 lightning sferics from pole to pole. We found lightning to be prevalent in the polar regions, absent near the equator, and most frequent in the northern hemisphere, at latitudes higher than 40 degrees north. Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet16,17, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles9,16,18. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter. Observations of broadband emission from lightning on Jupiter at 600 megahertz show a lightning discharge mechanism similar to that of terrestrial lightning and indicate increased moist convection near Jupiter’s poles.

61 citations



Journal ArticleDOI
TL;DR: In this paper, the authors studied the high-energy gamma-ray spectrum averaged over strong flares much more accurately than before, and found it well modelled by Compton scattering of stellar radiation by relativistic electrons with the power law index of $\simeq$3.5 and a low-energy cutoff at the Lorentz factor of $\sim\!10^3$.
Abstract: We study high-energy $\gamma$-rays observed from Cyg X-3 by the Fermi Large Area Telescope and the 15-GHz emission observed by the Ryle Telescope and the Arcminute Microkelvin Imager. We determine the $\gamma$-ray spectrum averaged over strong flares much more accurately than before, and find it well modelled by Compton scattering of stellar radiation by relativistic electrons with the power law index of $\simeq$3.5 and a low-energy cutoff at the Lorentz factor of $\sim\!10^3$. We find a weaker spectrum in the soft spectral state, but only upper limits in the hard and intermediate states. We measure strong orbital modulation during the flaring state, well modelled by anisotropic Compton scattering by jet relativistic electrons. We measure weaker orbital modulation of the 15 GHz radio emission, which is well modelled by free-free absorption by the stellar wind. We find the peak of the power spectrum of the radio emission is at a period shorter by 20 s than the orbital one, which can be due to a beat of the orbital modulation with a retrograde jet precession at a period of $\simeq$170 d. A definite evidence for the jet precession is provided by a strong dependence of the amplitude and phase of the orbital modulation on the precession phase. We then study cross-correlations between radio, $\gamma$-ray and X-ray emissions. We find no measurable delay of the radio emission with respect to $\gamma$-rays, but find a radio lag of $\sim$50 d with respect to the soft X-rays in the soft state.

50 citations


Journal ArticleDOI
A. Aab1, P. Abreu2, Marco Aglietta, Ivone F. M. Albuquerque3  +391 moreInstitutions (61)
TL;DR: In this article, a comparison of the measured radio-signal amplitudes with Monte Carlo simulations of a subset of 50 events for which they reconstruct the energy using the Auger surface detector shows agreement within the uncertainties of the current analysis.
Abstract: With the Auger Engineering Radio Array (AERA) of the Pierre Auger Observatory, we have observed the radio emission from 561 extensive air showers with zenith angles between 60 and 84. In contrast to air showers with more vertical incidence, these inclined air showers illuminate large ground areas of several km2 with radio signals detectable in the 30 to 80 MHz band. A comparison of the measured radio-signal amplitudes with Monte Carlo simulations of a subset of 50 events for which we reconstruct the energy using the Auger surface detector shows agreement within the uncertainties of the current analysis. As expected for forward-beamed radio emission undergoing no significant absorption or scattering in the atmosphere, the area illuminated by radio signals grows with the zenith angle of the air shower. Inclined air showers with EeV energies are thus measurable with sparse radio-antenna arrays with grid sizes of a km or more. This is particularly attractive as radio detection provides direct access to the energy in the electromagnetic cascade of an air shower, which in case of inclined air showers is not accessible by arrays of particle detectors on the ground.

47 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and investigated the origin of the band-splitting phenomenon.
Abstract: Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with CMEs and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20-90 MHz) is now possible with LOFAR, opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon.

43 citations


Journal ArticleDOI
TL;DR: In this article, a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial Relations Observatory mission was performed, and the role of scattering due to random electron density fluctuations on time-frequency profiles of radio emissions generated in the interplanetary medium was investigated.
Abstract: Type III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time–frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time–frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations /ne in the solar wind are 0.06–0.07 over wide range of heliospheric distances.

42 citations


Journal ArticleDOI
TL;DR: In this article, an image-based method that uses two radio criteria, compactness and spectral index, to identify promising pulsar candidates among Fermi Large Area Telescope (LAT) unassociated sources.
Abstract: We describe an image-based method that uses two radio criteria, compactness and spectral index, to identify promising pulsar candidates among Fermi Large Area Telescope (LAT) unassociated sources. These criteria are applied to those radio sources from the Giant Metrewave Radio Telescope all-sky survey at 150 MHz (TGSS ADR1) found within the error ellipses of unassociated sources from the 3FGL catalog and a preliminary source list based on 7 years of LAT data. After follow-up interferometric observations to identify extended or variable sources, a list of 16 compact, steep-spectrum candidates is generated. An ongoing search for pulsations in these candidates, in gamma rays and radio, has found six millisecond pulsars and one normal pulsar. A comparison of this method with existing selection criteria based on gamma-ray spectral and variability properties suggests that the pulsar discovery space using Fermi may be larger than previously thought. Radio imaging is a hitherto underutilized source selection method that can be used, as with other multi-wavelength techniques, in the search for Fermi pulsars.

Journal ArticleDOI
TL;DR: In this article, the authors present late-time radio observations of GW170817, the first binary neutron-star (NS) merger discovered through gravitational waves (GWs) by the advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo detectors.
Abstract: We present late-time radio observations of GW170817, the first binary neutron-star (NS) merger discovered through gravitational waves (GWs) by the advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo detectors. Our observations, carried out with the Karl G. Jansky Very Large Array (VLA), were optimized to detect polarized radio emission, and thus to constrain the linear polarization fraction of GW170817. At an epoch of ≈244 days after the merger, we rule out linearly polarized emission above a fraction of ≈12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived upper limit on the radio continuum linear polarization fraction strongly constrains the magnetic field configuration in the shocked ejecta. We show that our results for GW170817 are compatible with the low level of linear polarization found in afterglows of cosmological long γ-ray bursts (GRBs). Finally, we discuss our findings in the context of future expectations for the study of radio counterparts of binary NS mergers identified by ground-based GW detectors.

Journal ArticleDOI
TL;DR: As 5G technology is expected to offer super-broadband mobile services with new features such as low latency for critical applications and low-power operation for Internet of Things (IoT) applications, it will use wireless interfaces for small or spot cells with a new RF resource, the high band.
Abstract: As fifth-generation (5G) technology is expected to offer super-broadband mobile services with new features such as low latency for critical applications and low-power operation for Internet of Things (IoT) applications, it will use wireless interfaces for small or spot cells with a new RF resource, the high band (frequencies above 6 GHz), in addition to the low band (below 6 GHz) on macro and small cells coexisting with conventional mobile technologies [1]-[4]. Another feature of 5G wireless interfaces is more precise control of radio waves in space division through the use of multi-antenna technologies, such as massive multiple-input/multiple-output (MIMO) or beamforming [5].

Journal ArticleDOI
TL;DR: In this paper, an improved method for the precise reconstruction of cosmic ray air showers above $10^{17}$ eV with sparse radio arrays is presented, based on the comparison of predictions for radio pulse shapes by CoREAS simulations to measured pulses.
Abstract: We present an improved method for the precise reconstruction of cosmic ray air showers above $10^{17}$ eV with sparse radio arrays. The method is based on the comparison of predictions for radio pulse shapes by CoREAS simulations to measured pulses. We applied our method to the data of Tunka-Rex, a 1 km$^2$ radio array in Siberia operating in the frequency band of 30-80 MHz. Tunka-Rex is triggered by the air-Cherenkov detector Tunka-133 and by scintillators (Tunka-Grande). The instrument collects air-shower data since 2012. The present paper describes updated data and signal analyses of Tunka-Rex and details of a new method applied. After efficiency cuts, when Tunka-Rex reaches its full efficiency, the energy resolution of about 10% given by the new method has reached the limit of systematic uncertainties due to the calibration uncertainty and shower-to-shower fluctuations. At the same time the shower maximum reconstruction is significantly improved up to an accuracy of 35 g/cm$^2$ compared to the previous method based on the slope of the lateral distribution. We also define and now achieved conditions of the measurements, at which the shower maximum resolution of Tunka-Rex reaches a value of 25 g/cm$^2$ and becomes competitive to optical detectors. To check and validate our reconstruction and efficiency cuts we compare individual events to the reconstruction of Tunka-133. Furthermore, we compare the mean of shower maximum as a function of primary energy to the measurements of other experiments.

Journal ArticleDOI
TL;DR: In this paper, the authors present measurements of radio transmission in the Ω(n) range through a deep region below the surface of the ice at Summit Station, Greenland, called the firn, and compare their observations to a finite-difference time-domain (FDTD) electromagnetic wave simulation.
Abstract: We present measurements of radio transmission in the $\ensuremath{\sim}100\text{ }\text{ }\mathrm{MHz}$ range through a $\ensuremath{\sim}100\text{ }\text{ }\mathrm{m}$ deep region below the surface of the ice at Summit Station, Greenland, called the firn. In the firn, the index of refraction changes due to the transition from snow at the surface to glacial ice below, affecting the propagation of radio signals in that region. We compare our observations to a finite-difference time-domain (FDTD) electromagnetic wave simulation, which supports the existence of three classes of propagation: a bulk propagation ray-bending mode that leads to so-called ``shadowed'' regions for certain geometries of transmission, a surface-wave mode induced by the ice/air interface, and an arbitrary-depth horizontal propagation mode that requires perturbations from a smooth density gradient. In the non-shadowed region, our measurements are consistent with the bulk propagation ray-bending mode both in timing and in amplitude. We also observe signals in the shadowed region, in conflict with a bulk-propagation-only ray-bending model, but consistent with FDTD simulations using a variety of firn models for Summit Station. The amplitude and timing of our measurements in all geometries are consistent with the predictions from FDTD simulations. In the shadowed region, the amplitude of the observed signals is consistent with a best-fit coupling fraction value of 2.4% (0.06% in power) or less to a surface or horizontal propagation mode from the bulk propagation mode. The relative amplitude of observable signals in the two regions is important for experiments that aim to detect radio emission from astrophysical high-energy neutrinos interacting in glacial ice, which rely on a radio propagation model to inform simulations and perform event reconstruction.

Journal ArticleDOI
TL;DR: The data provided in this article will help radio network engineers to: predict signal path loss; estimate radio coverage; efficiently reuse limited frequencies; avoid interferences; optimize handover; and adjust transmitted power level.

Journal ArticleDOI
TL;DR: A new path loss prediction model is developed based on an Adaptive Neuro-Fuzzy Inference System (ANFIS) for multi-transmitter radio propagation scenarios and applicable to the Very High Frequency (VHF) bands, offering desirable advantages in terms of simplicity, high prediction accuracy, and good generalization ability.

Journal ArticleDOI
TL;DR: A method to smoothly remove shifts and restore sources to their reference positions, in both the catalogue and image domains, is presented, generalisable to repairing any sparsely-sampled vector field distortion to some input data.

Proceedings ArticleDOI
04 Jul 2018
TL;DR: Tests of single frequency DRM radio network in the SW range with original methodology were conducted, and method for calculating the synchronism zone is proposed, showing that the most advantageous can be considered their placement at an angle of 90 degrees.
Abstract: The Digital Radio Mondiale (DRM) broadcasting standard is the only ITU approved for worldwide application in the frequency range below 30 MHz The DRM standard provides the use of single-frequency synchronous broadcasting networks Their application with competent frequency-territorial planning allows to increase the efficiency of the DRM broadcasting networks, as well as to save the frequency resource The DRM single-frequency synchronous networks construction in general differs from the DVB-T/DVB-T2 synchronous digital television networks or VHF DAB/DAB + digital broadcasting networks construction due in the presence of ionospheric propagation of radio waves These features are considered in this article Method for calculating the synchronism zone is proposed Consideration of possible scenarios for the placement of transmitters showed that the most advantageous can be considered their placement at an angle of 90 degrees Tests of single frequency DRM radio network in the SW range with original methodology were conducted Measurement results show that synchronous network gain reached the values up to 6 dB at a wire antenna and up to 11 dB at a whip antenna When using traditional for broadcasting “day” and “night” frequencies it is possible to provide broadcast quality with high reliability (ie, 98% audio decoding) within 24 hours

Journal ArticleDOI
TL;DR: For example, the data from the Waves instrument on board the Juno spacecraft indicate observations of Jovian rapid whistlers, a form of dispersed atmospherics at extremely short timescales of several milliseconds to several tens of milliseconds.
Abstract: Electrical currents in atmospheric lightning strokes generate impulsive radio waves in a broad range of frequencies, called atmospherics. These waves can be modified by their passage through the plasma environment of a planet into the form of dispersed whistlers1. In the Io plasma torus around Jupiter, Voyager 1 detected whistlers as several-seconds-long slowly falling tones at audible frequencies2. These measurements were the first evidence of lightning at Jupiter. Subsequently, Jovian lightning was observed by optical cameras on board several spacecraft in the form of localized flashes of light3–7. Here, we show measurements by the Waves instrument8 on board the Juno spacecraft9–11 that indicate observations of Jovian rapid whistlers: a form of dispersed atmospherics at extremely short timescales of several milliseconds to several tens of milliseconds. On the basis of these measurements, we report over 1,600 lightning detections, the largest set obtained to date. The data were acquired during close approaches to Jupiter between August 2016 and September 2017, at radial distances below 5 Jovian radii. We detected up to four lightning strokes per second, similar to rates in thunderstorms on Earth12 and six times the peak rates from the Voyager 1 observations13. The Waves instrument on board the Juno spacecraft has detected ~1,600 lightning strokes in roughly 1 year of close approaches to Jupiter, indicated by low-dispersion rapid whistlers much shorter than those detected by Voyager 1 in Io’s plasma torus.

Journal ArticleDOI
TL;DR: In this paper, an underwater portable radio antenna operating in the 50 MHz band and efficient for launching surface electromagnetic waves at the seawater/air interface is presented, where the antenna operation is based on the field enhancement at the antenna tip and on an impedance-matching antenna enclosure, which is filled with deionized water.
Abstract: An underwater portable radio antenna operating in the 50 MHz band and efficient for launching surface electromagnetic waves at the seawater/air interface is presented. The antenna operation is based on the field enhancement at the antenna tip and on an impedance-matching antenna enclosure, which is filled with deionized water. This enclosure allows us to reduce antenna dimensions and improve the coupling of electromagnetic energy to the surrounding salt water medium. Since surface wave propagation length far exceeds the skin depth of conventional radio waves at the same frequency, this technique is useful for broadband underwater wireless communication over several meters’ distances.

Journal ArticleDOI
TL;DR: In this article, the authors compare a large set of simulations with different primary energies and shower directions and observe differences in the radiation energy prediction for the 30-80 MHz band of 5.2%.

Journal ArticleDOI
TL;DR: In this paper, the authors obtained a robust limit on the photon mass by directly fitting a combination of the dispersion measures of radio sources, using the observed dispersion measure from two statistical samples of extragalactic pulsars, and they showed that at the 68% confidence level, the constraints on photon mass can be as low as m?≤1.51×10?48?kg8.8.
Abstract: The photon zero-mass hypothesis has been investigated for a long time using the frequency-dependent time delays of radio emissions from astrophysical sources. However, the search for a rest mass of the photon has been hindered by the similarity between the frequency-dependent dispersions due to the plasma and nonzero photon mass effects. Considering the contributions to the observed dispersion measure from both the plasma and nonzero photon mass effects, and assuming the dispersion induced by the plasma effect is an unknown constant, we obtain a robust limit on the photon mass by directly fitting a combination of the dispersion measures of radio sources. Using the observed dispersion measures from two statistical samples of extragalactic pulsars, here we show that at the 68% confidence level, the constraints on the photon mass can be as low as m?≤1.51×10?48?kg8.47×10?13?eV/c2 for the sample of 22 radio pulsars in the Large Magellanic Cloud and m?≤1.58×10?48?kg8.86×10?13?eV/c2 for the other sample of 5 radio pulsars in the Small Magellanic Cloud, which are comparable with that obtained by a single extragalactic pulsar. Furthermore, the statistical approach presented here can also be used when more fast radio bursts with known redshifts are detected in the future.

Journal ArticleDOI
TL;DR: In this article, the rotational Doppler effect induced by a radio wave carrying orbital angular momentum (OAM) is theoretically analyzed and experimentally verified, and a vector velocity detecting scheme is discussed.
Abstract: The electromagnetic beam carrying orbital angular momentum (OAM) has a twisted phase front, which can induce a rotational Doppler frequency shift when it is illuminating on a spinning object. In this paper, the rotational Doppler effect induced by a radio wave carrying OAM is theoretically analyzed and experimentally verified. Compared with an optical vortex beam, the divergence angle of a radio wave carrying OAM is typically larger than that of an optical vortex beam, which results in the radio OAM-based sensing system with short-range detection distance. In this case, the receiver's location and the deflection of the rotating plane are important factors that affect the radio rotational Doppler effect. For an off-axis receiver or a deflected spinning object, a series of extra Doppler frequency shifts (L ± N)Ω/2π are induced when the spinning object is illuminated by a wave carrying OAM with topological charge L, even for L = 0, the plane wave. Hence, the high order OAM wave should be used to distinguish the real rotational frequency shift and improve detection sensitivity of spinning velocity. Combining the rotational Doppler effect with the linear Doppler effect, a vector velocity detecting scheme is discussed.

Journal ArticleDOI
22 May 2018
TL;DR: In this paper, the authors give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the center of the shell at a distance of ∼100-200 pc.
Abstract: The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to free–free radiation from warm ionized gas. Away from the Galactic plane, the radio sky is dominated by very large (tens of degrees) loops, arcs, spurs and filaments, including the well-known North Polar Spur (NPS), which forms part of Loop I with a diameter of ∼ 120 ∘ . In polarization data, such features are often more discernible due to their high polarization fractions suggesting ordered magnetic fields, while the polarization angles suggest fields that are parallel to the filament. The exact nature of these features are poorly understood. We give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the centre of the shell at a distance of ∼100–200 pc. However, there is significant evidence for a larger distance in the range ∼500–1000 pc, while larger distances including the Galactic Centre are unlikely. We also briefly discuss other large-scale curiosities in the radio sky such as the microwave haze and anti-correlation of H α filaments and synchrotron polarized intensity.

Journal ArticleDOI
TL;DR: In this paper, the effect of the ionosphere day-night non-uniformity on the electromagnetic field amplitude at the Schumann resonance and higher frequencies was evaluated using 2DTE.

Journal ArticleDOI
TL;DR: In this paper, the authors present spatially resolved spectral radio observations of type IIIb bursts in the $30-80$ MHz range made by the Low Frequency Array (LOFAR).
Abstract: Solar radio emission features a large number of fine structures demonstrating great variability in frequency and time. We present spatially resolved spectral radio observations of type IIIb bursts in the $30-80$ MHz range made by the Low Frequency Array (LOFAR). The bursts show well-defined fine frequency structuring called "stria" bursts. The spatial characteristics of the stria sources are determined by the propagation effects of radio waves; their movement and expansion speeds are in the range of 0.1-0.6c. Analysis of the dynamic spectra reveals that both the spectral bandwidth and the frequency drift rate of the striae increase with an increase of their central frequency; the striae bandwidths are in the range of ~20-100 kHz and the striae drift rates vary from zero to ~0.3 MHz s^-1. The observed spectral characteristics of the stria bursts are consistent with the model involving modulation of the type III burst emission mechanism by small-amplitude fluctuations of the plasma density along the electron beam path. We estimate that the relative amplitude of the density fluctuations is of dn/n~10^-3, their characteristic length scale is less than 1000 km, and the characteristic propagation speed is in the range of 400-800 km/s. These parameters indicate that the observed fine spectral structures could be produced by propagating magnetohydrodynamic waves.

Journal ArticleDOI
TL;DR: In this article, the authors considered that the electron beams generating these fine structures are considered to be produced by shock drift acceleration (SDA), which excites Langmuir waves which are converted into radio waves by nonlinear wave-plasma processes.
Abstract: Context. The Sun’s activity can appear in terms of radio bursts. In the frequency range 8−33 MHz the radio telescope URAN-2 observed special fine structures appearing as a chain of stripes of enhanced radio emission in the dynamic radio spectrum. The chain drifts slowly from 26 to 23 MHz within 4 min. The individual structures consist of a “head” at the high-frequency edge and a “tail” rapidly drifting from the “head” to lower frequencies over an extent of ≈10 MHz within 8 s. Since they resemble the well-known “herring bones” in type II radio bursts, they are interpreted as shock accelerated electron beams.Aims. The electron beams generating these fine structures are considered to be produced by shock drift acceleration (SDA). The beam electrons excite Langmuir waves which are converted into radio waves by nonlinear wave-plasma processes. That is called plasma emission. The aim of this paper is to link the radio spectral data of these fine structures to the theoretical results in order to gain a better understanding of the generation of energetic electrons by shocks in the solar corona.Methods. Adopting SDA for generating energetic electrons, the accelerated electrons establish a beam-like velocity distribution. Plasma emission requires the excitation of Langmuir waves, which is efficient if the velocity of the beam electrons exceeds a few times thermal electron speed. That is the case if the angle between the shock normal and the upstream magnetic field is nearly perpendicular. Hence, the Rankine-Hugoniot relationships, which describe the shock transition in the framework of magnetohydrodynamics, are evaluated for the special case of nearly perpendicular shocks under coronal circumstances.Results. The radio data deduced from the dynamic radio spectrum can be related in the best way to the theoretical results, if the electron beams, which generate these fine structures, are generated via SDA at an almost perpendicular shock, which is traveling nearly horizontally to the surface of the Sun.

Journal ArticleDOI
06 Oct 2018-Symmetry
TL;DR: According to the result of the propagation prediction analysis, it can be stated that the proposed method’s ray tracing is capable of predicting the mmW propagation based on a raw sketch of the real environment.
Abstract: The Millimeter-Wave (mmW) technology is going to mitigate the global higher bandwidth carriers. It will dominate the future network system by the attractive advantages of the higher frequency band. Higher frequency offers a wider bandwidth spectrum. Therefore, its utilizations are rapidly increasing in the wireless communication system. In this paper, an indoor mmW propagation prediction is presented at 38 GHz based on measurements and the proposed Three-Dimensional (3-D) Ray Tracing (RT) simulation. Moreover, an additional simulation performed using 3-D Shooting Bouncing Ray (SBR) method is presented. Simulation using existing SBR and the proposed RT methods have been performed separately on a specific layout where the measurement campaign is conducted. The RT methods simulations results have been verified by comparing with actual measurement data. There is a significant agreement between the simulation and measurement with respect to path loss and received signal strength indication. The analysis result shows that the proposed RT method output has better agreement with measurement output when compared to the SBR method. According to the result of the propagation prediction analysis, it can be stated that the proposed method's ray tracing is capable of predicting the mmW propagation based on a raw sketch of the real environment.

Posted Content
TL;DR: The measurement and calibration process for polarization observations is described and how instrumental polarization can affect a measurement is illustrated to draw attention to the confusion generated by various polarization conventions.
Abstract: Modern dual-polarization receivers allow a radio telescope to characterize the full polarization state of incoming insterstellar radio waves. Many astronomers incorrectly consider a polarimeter to be the "backend" of the telescope. We go to lengths to dissuade the reader of this concept: the backend is the least complicated component of the radio telescope when it comes to measuring polarization. The feed, telescope structure, dish surface, coaxial cables, optical fibers, and electronics can each alter the polarization state of the received astronomical signal. We begin with an overview of polarized radiation, introducing Jones and Stokes vectors, and then discuss construction of digitized pseudo-Stokes vectors from the outputs of modern correlators. We describe the measurement and calibration process for polarization observations and illustrate how instrumental polarization can affect a measurement. Finally, we draw attention to the confusion generated by various polarization conventions and highlight the need for observers to state all adopted conventions when reporting polarization results.