Flux Density Variations Research Articles

Reception conditions of low frequency (LF) transmitter signals onboard DEMETER micro-satellite

We analyse the flux density variation associated to low frequency (LF) broadcasting transmitters observed by the ICE electric field experiment onboard DEMETER micro-satellite, observed from 01st Jan. to 09th Dec. 2010. We select five stations localized around the Mediterranean and the Black seas: Tipaza (252 kHz, 02°28′E, 36°33′N, Algeria), Roumoules (216 kHz, 06°08′E, 43°47′N, Monte Carlo), Polatli (180 kHz, 32°25′E, 39°45′N, Turkey), Nadour (171 kHz, 02°55′W, 35°02′N, Morocco) and Brasov (153 kHz, 25°36′E, 45°40′, Romania). The detection of the LF transmitter signals by DEMETER micro-satellite is found to depend on the radiated power, the emitted frequency, and the orbit paths with regard to the location of the stations. This leads us to characterize the reception condition of the LF signals and to define time intervals where the detection probability is high. We show that LF signal are regularly recorded, each 12 days, when the satellite is above the broadcasting station. The signal intensity levels are principally significant during the solar activity. Hence we find that the solar and the geomagnetic activities are slightly correlated to the maxima of LF signal as recorded by DEMETER. Also we note a drop of the intensity level several days before the occurrence of earthquakes in/around the Mediterranean and Black seas.

Fast iron losses model of stator taking into account the flux weakening mode for the optimal sizing of high speed permanent internal magnet synchronous machine

The optimal design of electrical machines is an important issue in automotive industry in order to reduce cost and volume of the actuator and improve its performances. In this context, the use of compact machines with high power density is preferred and consequently high speed machines where field weakening is required. In addition, a really optimal design requires to dispose sufficiently accurate models of main physical phenomena involved in electromechanical conversion. This models need to be sufficiently fast in order to be suitable with optimization process. An important limitation in the use of internal permanent magnet synchronous machine with distributed windings is temperatures insides the machine especially in the windings and thus the internal losses. At high speed with field weakening operation, iron losses, sensitive to flux densities variation in iron, could be really high because of the high electrical frequency linked to rotor speed and sharp variation of flux density waveforms inside the iron in stator due to the field weakening operation. This study, for the electric machine design, is based on the first harmonic hypothesis, i.e., without harmonic currents. An original and mathematical model has been developed and provides fast and accurate estimation of iron losses particularly in field weakening operation even with machine supplied by sinusoidal currents as described in this paper. It uses a polynomial form of iron losses in function of fundamental electrical frequency and take into account the flux density waveforms in yoke and teeth by use of nonlinear iron coefficients linked to id–iq currents. This paper will present the complete method calculating the iron coefficients from a nonlinear magnetic nodal network of the machine. A detailed study of local flux density waveform and harmonic content in yoke and teeth will be provided for two particular operating points: at maximal power without field weakening and at maximal power at maximal speed. These two points require accurate estimation in an optimal design of electrical machine. In addition, the local iron coefficients in teeth and yoke per volume unit will be provided in order to study the local evolution of iron losses in field weakening operation. It will show that iron losses do not follow the same evolution in the yoke and the teeth by the fact that the flux density distribution in teeth is more sensitive to the field weakening. An application of this model will be provided for the calculation of iron losses on whole operating space for a specific machine. A comparison will be provided between the fast model and finite elements approach.

Long-term variability of active galactic nuclei from the ”Planck” catalog

A complete sample of 104 bright active galactic nuclei (AGNs) from the “Planck” catalog (early results) were observed at 36.8 GHz with the 22-m radio telescope of the Crimean Astrophysical Observatory (CrAO).Variability indices of the sources at this frequency were determined based on data from theWMAP space observatory, theMetsa¨ hovi RadioObservatory (Finland), and the CrimeanAstrophysical Observatory. New observational results confirm that the variability of these AGNs is stronger in the millimeter than at other radio wavelengths. The variability indices probably change as a result of the systematic decrease in the AGN flux densities in the transition to the infrared. Some radio sources demonstrate significant flux-density variations, including decreases, which sometimes cause them to fall out of the analysed sample. The change of the variability index in the millimeter is consistent with the suggestion that this variability is due to intrinsic processes in binary supermassive black holes at an evolutionary stage close to coalescence. All 104 of the sources studied are well known objects that are included in various radio catalogs and have flux densities exceeding 1 Jy at 36.8 GHz.

Changes in spatial variations of sap flow in Korean pine trees due to environmental factors and their effects on estimates of stand transpiration

It is difficult to scale up measurements of the sap flux density (JS) for the characterization of tree or stand transpiration (E) due to spatial variations in JS and their temporal changes. To assess spatial variations in the sap flux density of Korean pine (Pinus koraiensis) and their effects on E estimates, we measured the JS using Granier-type sensors. Within trees, the JS decreased exponentially with the radial depth, and the JS of the east aspects were higher than those of the west aspects. Among trees, there was a positive relationship between JS and the tree diameter at breast height, and this positive relationship became stronger as the transpiration demand increased. The spatial variations that caused large errors in E estimates (i.e., up to 110.8 % when radial variation was ignored) had varied systematically with environmental factors systematic characteristics in relation to environmental factors. However, changes in these variations did not generate substantial errors in the E estimates. For our study periods, the differences in the daily E (ED) calculated by ignoring radial, azimuthal and tree-to-tree variations and the measured ED were fairly constant, especially when the daily vapor pressure deficit (D_D) was higher than 0.6 kPa. These results imply that the effect of spatial variations changes on sap flow can be a minor source of error compared with spatial variations (radial, azimuthal and tree-to-tree variations) when considering E estimates.

Observational manifestations and intrinsic properties of the RCR sources in terms of a unified model

We present a summary results of the study of radio sources showing significant variations of integral flux density using the data from the RATAN-600 surveys of 1980–1994 at a frequency of 7.6 cm. The majority of the detected variable sources have flat radio spectra, although there are also all other spectrum types found. Point and compact sources predominate, although all known morphological structures are found in the sample. Variability is detected both in quasars and galaxies. Using the catalog data, we found brightness variations in the optical and/or infrared ranges for a half of host objects of radio sources. We analyzed the properties of nonvariable and variable RCR sources. We compared the ratio of absolute magnitude to radio luminosity for sources with the active nucleus types determined from the optical data. It is found that this parameter is approximately the same for quasars with different radio luminosity. It isminimum for the strongest radio galaxies and grows up to the level characteristic of quasars with the decrease of radio luminosity. Considering that the ratio depends on obscuring properties of a dust torus, such behavior can be explained if we assume that the torus geometry and its optical depth depend on the source long. This parameter is slightly higher among variable sources than among nonvariable ones which counts in favor of the nucleus more open to an observer.

Monitoring the Galactic Centre with the Australia Telescope Compact Array

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity which is thought to arise from the innermost region of the accretion flow. We performed the monitoring observations of the Galactic Centre to study the flux-density variations at 3mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We obtain the light curves of Sgr A* by subtracting the contributions from the extended emission around it, and the elevation and time dependent gains of the telescope. We perform structure function analysis and the Bayesian blocks representation to detect flare events. The observations detect six instances of significant variability in the flux density of Sgr A* in three observations, with variations between 0.5 to 1.0 Jy, which last for 1.5 $-$ 3 hours. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, such as the source expansion speed $v_{\mathrm{exp}}$, source sizes, spectral indices, and the turnover frequency. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than $v_{\mathrm{exp}}$. No exceptional flux density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO/G2). This is consistent with its compactness and the absence of a large bow shock.

Radio follow-up of the γ-ray flaring gravitational lens JVAS B0218+357

We present results on multifrequency Very Long Baseline Array (VLBA) monitoring observations of the double-image gravitationally lensed blazar JVAS B0218+357. Multi-epoch observations started less than one month after the gamma-ray flare detected in 2012 by the Large Area Telescope on board Fermi, and spanned a 2-month interval. The radio light curves did not reveal any significant flux density variability, suggesting that no clear correlation between the high energy and low-energy emission is present. This behaviour was confirmed also by the long-term Owens Valley Radio Observatory monitoring data at 15 GHz. The milliarcsecond-scale resolution provided by the VLBA observations allowed us to resolve the two images of the lensed blazar, which have a core-jet structure. No significant morphological variation is found by the analysis of the multi-epoch data, suggesting that the region responsible for the gamma-ray variability is located in the core of the AGN, which is opaque up to the highest observing frequency of 22 GHz.

Radio spectra of High Frequency Peakers

AbstractNew radio spectra of High Frequency Peakers (HFP) obtained from the Jansky Very Large Array (JVLA) show that variability is common among this class of sources. A subsample of sources have been observed with a nearly continuous spectral sampling between 1 and 10 GHz. The observed HFP sources were previously classified as F (flat), H (HFP profile with little or no flux density variability) and V (variable, but preserving a peaked spectrum). In general, sources classified as V and H show a decrease of the flux density measured in the optically thin part of the spectrum, while there is a moderate increment in the optically thick region, resulting into a progressive shift of the spectral peak to lower frequencies. This is consistent with the idea of an expanding bubble of radio plasma. The sources with an F classification instead show substantial variability, both in spectral shape and in time evolution. In these HFP sources an irregular production of energy is best observed since the radio emission is dominated by recently generated relativistic plasma, and the contribution of mini lobes, in which old plasma accumulates, is marginal if not absent at all, given the short radiative life of electrons in strong magnetic fields (tens of mG) found in these objects. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photocatalytic activity and scale-up effect in liquid–solid mini-fluidized bed reactor

Photocatalytic degradation of methylene blue was studied in three mini-fluidized bed reactors, with Fe3+/TiO2 photocatalyst coated on the inner wall of fluidized bed (Fe3+/TiO2/quartz tube), on the surface of fluidized particles (Fe3+/TiO2/glass beads) or on both. In the reactor of Fe3+/TiO2/quartz tube with glass beads (d=3mm, initial bed height H0=15mm), due to addition and fluidization of glass beads, mass transfer coefficient increased 11–13 times and apparent reaction rate constant increased 4.9 times compared to the empty tube. In the reactor of quartz tube with Fe3+/TiO2/glass beads (d=3mm, H0=15mm), experimental results were fitted well with a simplified kinetic model. Degradation rate per unit reaction volume reached highest value at voidage of 0.75 for all inner diameter (ID) beds. In the reactor of Fe3+/TiO2/quartz tube with Fe3+/TiO2/glass beads which combined the advantages of above two forms of catalyst, the degradation ratios were 5–35% higher than that in the other two reactors. With the increase of tube ID, scale-up effect was found. Degradation rate per unit reaction volume in Fe3+/TiO2/quartz tube with glass beads (Rav,I) and that in quartz tube with Fe3+/TiO2/glass beads (Rav,II) were introduced to quantitatively describe the processing ability and scale-up effect. The values of Rav,I of 1mm ID bed were between 4.5 and 1.8 times to 3mm ID bed when glass beads increased from empty tube to H0=15mm. The values of Rav,II of 1mm ID bed were about 1.4–1.9 times to 3mm ID bed at all H0. Scale-up effect was caused by the variation of radiation flux density, specific surface area, mass transfer distance and light penetration distance.

The Study of Radio Flux Density Variations of the Quasar OJ 287 by the Wavelet and the Singular Spectrum Methods

AbstractFlux density variations of the extragalactic radio source OJ 287 are studied by applying the wavelet and the singular spectrum methods to the long-term monitoring data at 14.5, 8.0 and 4.8 GHz acquired at the University of Michigan Radio Astronomy Observatory during 40 years. This monitoring significantly supplements the episodic VLBI data. The wavelet analysis at all three frequencies revealed the presence of quasiperiods within the intervals 6.0–7.4 and 1.2–1.8 years. The singular spectrum analysis revealed the presence of quasiperiods within the intervals 6–10 and 1.6–4.0 years. For each quasiperiod the time interval of its existence was determined.

Emission-rotation correlation in pulsars: new discoveries with optimal techniques

Pulsars are known to display short-term variability. Recently, examples of longer-term emission variability have emerged that are often correlated with changes in the rotational properties of the pulsar. To further illuminate this relationship, we have developed techniques to identify emission and rotation variability in pulsar data, and determine correlations between the two. Individual observations may be too noisy to identify subtle changes in the pulse profile. We use Gaussian process (GP) regression to model noisy observations and produce a continuous map of pulse profile variability. Generally, multiple observing epochs are required to obtain the pulsar spin frequency derivative ($\dot{\nu}$). GP regression is, therefore, also used to obtain $\dot{\nu}$, under the hypothesis that pulsar timing noise is primarily caused by unmodelled changes in $\dot{\nu}$. Our techniques distinguish between two types of variability: changes in the total flux density versus changes in the pulse shape. We have applied these techniques to 168 pulsars observed by the Parkes radio telescope, and see that although variations in flux density are ubiquitous, substantial changes in the shape of the pulse profile are rare. We reproduce previously published results and present examples of profile shape changing in seven pulsars; in particular, a clear new example of correlated changes in profile shape and rotation is found in PSR~J1602$-$5100. In the shape changing pulsars, a more complex picture than the previously proposed two state model emerges. We conclude that our simple assumption that all timing noise can be interpreted as $\dot{\nu}$ variability is insufficient to explain our dataset.

EVIDENCE OF SHORT TIMESCALE FLUX DENSITY VARIATIONS OF UC H II REGIONS IN SGR B2 MAIN AND NORTH

We have recently published observations of significant flux density variations at 1.3 cm in HII regions in the star forming regions Sgr B2 Main and North (De Pree et al. 2014). To further study these variations, we have made new 7 mm continuum and recombination line observations of Sgr B2 at the highest possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA configuration (Main) and the A configuration (North). We compare these new data to archival VLA 7 mm continuum data of Sgr B2 Main observed in 2003 and Sgr B2 North observed in 2001. We find that one of the 41 known ultracompact and hypercompact HII regions in Sgr B2 (K2-North) has decreased $\sim$27% in flux density from 142$\pm$14 mJy to 103$\pm$10 mJy (2.3$\sigma$) between 2001 and 2012. A second source, F3c-Main has increased $\sim$30% in flux density from 82$\pm$8 mJy to 107 $\pm$11 mJy (1.8$\sigma$) between 2003 and 2012. F3c-Main was previously observed to increase in flux density at 1.3 cm over a longer time period between 1989 and 2012 (De Pree et al. 2014). An observation of decreasing flux density, such as that observed in K2-North, is particularly significant since such a change is not predicted by the classical hypothesis of steady expansion of HII regions during massive star accretion. Our new observations at 7 mm, along with others in the literature, suggest that the formation of massive stars occurs through time-variable and violent accretion.

ТЕПЛОВЫЕ И ДИНАМИЧЕСКИЕ ПРОЦЕССЫ В ИОНОСФЕРЕ ВО ВРЕМЯ ЧАСТНОГО ЗАТМЕНИЯ СОЛНЦА 20 МАРТА 2015 г. НАД ХАРЬКОВОМ. РЕЗУЛЬТАТЫ РАСЧЕТОВ

ТЕПЛОВЫЕ И ДИНАМИЧЕСКИЕ ПРОЦЕССЫ В ИОНОСФЕРЕ ВО ВРЕМЯ ЧАСТНОГО ЗАТМЕНИЯ СОЛНЦА 20 МАРТА 2015 г. НАД ХАРЬКОВОМ. РЕЗУЛЬТАТЫ РАСЧЕТОВ

THE EFFECT OF INTERPLANETARY SCINTILLATION ON EPOCH OF REIONIZATION POWER SPECTRA

Interplanetary Scintillation (IPS) induces intensity fluctuations in small angular size astronomical radio sources via the distortive effects of spatially and temporally varying electron density associated with outflows from the Sun. These radio sources are a potential foreground contaminant signal for redshifted HI emission from the Epoch of Reionisation (EoR) because they yield time-dependent flux density variations in bright extragalactic point sources. Contamination from foreground continuum sources complicates efforts to discriminate the cosmological signal from other sources in the sky. In IPS, at large angles from the Sun applicable to EoR observations, weak scattering induces spatially and temporally correlated fluctuations in the measured flux density of sources in the field, potentially affecting the detectability of the EoR signal by inducing non-static variations in the signal strength. In this work, we explore the impact of interplanetary weak scintillation on EoR power spectrum measurements, accounting for the instrumental spatial and temporal sampling. We use published power spectra of electron density fluctuations and parameters of EoR experiments to derive the IPS power spectrum in the wavenumber phase space of EoR power spectrum measurements. The contrast of IPS power to expected cosmological power is used as a metric to assess the impact of IPS. We show that IPS has a different spectral structure to power from foregrounds alone, but the additional leakage into the EoR observation parameter space is negligible under typical IPS conditions, unless data are used from deep within the foreground contamination region.

Assessment of exposure to MRI motion-induced fields based on the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines.

The goal of this study was to conduct an exposure assessment for workers moving through the stray stationary field of common MRI scanners, performed according to the recent International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines, which aim at avoiding annoying sensory effects. The analysis was performed through numerical simulations, using a high-resolution anatomical model that moved along realistic trajectories in proximity to a tubular and open MRI scanner. Both dosimetric indexes indicated by ICNIRP (maximum variation of the magnetic flux density vector and exposure index for the motion-induced electric field) were computed for three statures of the human model. A total of 51 exposure situations were analyzed. None of them exceeded the limit for the maximum variation of the magnetic flux density, whereas some critical cases were found when computing the induced electric field. In the latter case, the exposure indexes computed via Fourier transform and through an equivalent filter result to be consistent. The results suggest the adoption of some simple precautionary rules, useful when sensory effects experienced by an operator could reflect upon the patient's safety. Moreover, some open issues regarding the quantification of motion-induced fields are highlighted, putting in evidence the need for clarification at standardization level. Magn Reson Med 76:1291-1300, 2016. © 2015 Wiley Periodicals, Inc.

3-D Effects of Rotor Step-Skews in Permanent Magnet-Assisted Synchronous Reluctance Machines

Permanent magnet assisted synchronous reluctance machines (PMA SynRM), stemming from the combination of synchronous reluctance and interior PM machine topologies, have recently gained increasing popularity as a viable candidate for electric vehicle traction applications due to concerns regarding the price volatility of rare-earth magnet materials. Their attractiveness can be attributed to their distinct torque production mechanism, which essentially relies on the reluctance torque component enabling the use of less expensive ferrites or bonded NdFeB magnet solutions. A drawback often associated with this machine topology is high torque ripple emanating from its intrinsically high salient nature [1]. Among several machine design techniques available to mitigate the high torque ripple, rotor step-skew is known to be one of the most effective and frequently incorporated solutions. When a step-skew is performed, it causes variation of magnetic flux density along the axial (skew) direction, and thus influences the machine's electromagnetic behavior [2]. Numerous methods are available in literature to account for the effects of skew using (either) 2D or 3D numerical finite element analysis (FEA), analytical, or semi-analytical approaches, the trade-off in each case being computational time versus accuracy. Due to the prohibitive computational time and numerical memory costs of 3D FEA, the prediction of a skewed machine's electromagnetic performance typically relies on 2D domain-based techniques, such as multi-slice 2D FEA [3] or more computationally efficient semi-analytical techniques [2] exhibiting comparable accuracy. A common limitation associated with these techniques is their inability to capture certain 3D phenomena, namely the end-effects and axial interaction between skewed steps, since they explicitly rely on the 2D domain [4]. The significance of such 3D effects is influenced by the axial length to airgap diameter ratio (viz. end-effect), the number of skewed steps, the skew angle and the structure of the machine topology under consideration. The quantification, and most importantly experimental verification, of the 3D effects associated with skewed PMA SynRMs has received limited attention in literature to date.

Simulation and measurement of ferromagnetic impurities in non-magnetic aeroengine turbine disks using fluxgate magnetometers

In this paper, ferromagnetic impurities in paramagnetic aeroengine turbine disks are investigated. Because such inclusions represent a significant threat in aviation, a detailed analysis is required for impured turbine disks. For this purpose, sensitive fluxgate magnetometers are used. After a premagnetisation, this sensor is able to detect small ferromagnetic particles by recording the variation of the magnetic flux density while the disk rotates below the sensor head. This trajectory creates a unique signature. However, the measured signatures are often distorted. A main reason for these distortions is that the particles are not oriented in axial direction (in the direction of the disks axis). Up to now, it was not possible to interpret the measured signatures. Thus, a simulation tool has been developed that provides a catalogue of different magnetic flux density distributions of typical orientations, positions and various distances to the fluxgate magnetometer position. For these simulations, the particles are assumed to be dipoles. As part of impurities are not caused by concentrated particles but by elongated ones, so-called or dipole lines, the model has been expanded for these cases by using numerical integration techniques. Measurements verify the assumption to approximate impurities by dipoles.

Test of an Original Superconducting Synchronous Machine Based on Magnetic Shielding

We present a new concept for a two-pole superconducting synchronous machine. This electrical motor is composed of a classical stator yoke, and an original superconducting inductor. The inductor is composed of two coaxial circular superconducting coils separated by superconducting bulks. Both solenoids are fed by a direct current and each of them produces a magnetic field in the same direction. An inclined superconducting bulk is placed between the two solenoids. It is used as superconducting magnetic barrier. The diamagnetic behavior of a superconducting bulk cooled under zero-field is used to obtain a variation of the magnetic flux density in the air-gap. The inductor is made of low-temperature superconducting wires for the coils, NbTi, and a high-temperature superconducting YBCO bulk, cooled down to 4.2K using liquid helium for the flux barrier. The armature is wound with copper wires as a classical one. We present design and tests of this superconducting motor. We made no load, short-circuit and full load tests.

22 GHz MONITORING OF SGR A*DURING THE G2 PERI-CENTER PASSING WITH THE JAPANESE VLBI NETWORK

We have been performing daily VLBI monitoring of the flux density of Sagittarius (Sgr) <TEX>$A^{\ast}$</TEX> at 22 GHz from February 2013 to August 2014 using a sub-array of the Japanese VLBI Network (JVN). The purpose of this monitoring is to explore the flux density variability at daily time resolution for a period longer than one year with the G2 cloud approaching. The flux density of Sgr <TEX>$A^{\ast}$</TEX> is basically stable during the observational period, though there are some small variations. The average and scattering range are consistent with the previously observed values. We have observed no strong flare of Sgr <TEX>$A^{\ast}$</TEX> although it is near the expected peri-center passing.

Innovative NDT Technique Based on Ferrofluids for Detection of Surface Cracks

An innovative NDT technique is proposed for surface inspection of materials not necessarily magnetic or conductive, based on local magnetic field variations due to ferrofluid deposited in the cracks. The feasibility of the technique is assessed preliminarily, based on signal detectability without applied external magnetic field, and under applied DC fields. The signals (local magnetic flux density variations) are quantified analytically, experimentally and numerically. The model agrees well with the tests, showing that the signal increases with the applied field strength, up to the saturation magnetization of the ferrofluid, and decreases with the distance to the crack longitudinal axis, and thus it can provide useful estimations of the signal. The proposed technique, requiring application of external fields to magnetize the ferrofluid to enhance the signal, seems promising: the model suggests that signals associated to cracks significantly smaller than surface cracks in a target application like aircraft skin panel inspection NASA STD-5009 are easily detectable with commercial magnetometers.