Apparent Flux Density Research Articles

Quantifying excess power from radio frequency interference in Epoch of Reionization measurements

ABSTRACT We quantify the effect of radio frequency interference (RFI) on measurements of the 21-cm power spectrum during the Epoch of Reionization (EoR). Specifically, we investigate how the frequency structure of RFI source emission generates contamination in higher order wave modes, which is much more problematic than smooth-spectrum foreground sources. Using a relatively optimistic EoR model, we find that even a single relatively dim RFI source can overwhelm the EoR power spectrum signal of $\sim 10\, {\rm mK}^2$ for modes $0.1 \ \lt k \lt 2 \, h\, {\rm Mpc}^{-1}$. If the total apparent RFI flux density in the final power spectrum integration is kept below 1 mJy, an EoR signal resembling this optimistic model should be detectable for modes $k \lt 0.9\, h\, {\rm Mpc}^{-1}$, given no other systematic contaminants and an error tolerance as high as 10 per cent. More pessimistic models will be more restrictive. These results emphasize the need for highly effective RFI mitigation strategies for telescopes used to search for the EoR.

Determination of Desert Soil Apparent Thermal Diffusivity Using a Conduction‐Convection Algorithm

AbstractSurface soil temperatures impact land‐atmosphere interactions in desert environments. Soil apparent thermal diffusivity (k) is a crucial physical parameter affecting soil temperature. Previous studies using the conduction‐convection algorithm reported k values of desert soils for only a few days. The main objective of this study is to determine the daily and monthly variations of desert k for a range of water contents over a 10 month period. The k values were estimated with a conduction‐convection algorithm using soil temperature measured at the 0.00 m and 0.20 m depths from 1 January to 11 October 2011 at the Tazhong station in the Taklimakan desert of China. Generally, the daily values of k ranged from 1.46 × 10−7m2 s−1 to 5.88 × 10−7m2 s−1, and the 10 month average k value was 2.5(±0.8) × 10−7m2 s−1 for the 0.00 m to 0.20 m soil layer. The k values varied significantly with soil water content. The apparent convection parameter (W), which is the sum of the vertical gradient of k and apparent water flux density, was also determined. Comparison of the magnitudes of W and k gradients indicated that little water movement occurred during the dry months, some water infiltrated downward during the wet months, and some water moved upwards in response to evaporation following the wet months. These findings confirmed that the conduction‐convection algorithm described the general pattern of soil water movement. The presented daily and monthly values of k can be used as soil parameters when modeling land‐atmosphere interactions in the Taklimakan desert.

Soil Apparent Thermal Diffusivity Estimated by Conduction and by Conduction–Convection Heat Transfer Models

Abstract Soil heat transfer is complex, and conduction-alone models may not always perform well in estimating soil apparent thermal diffusivity. Soil apparent thermal diffusivity is related to soil temperature change propagation rates. Soil temperature data collected at the Tazhong station in China were used to examine the characteristics of soil apparent thermal diffusivity determined by three different algorithms and the sum of vertical gradient of soil apparent thermal diffusivity and apparent water flux density . The results showed that 1) soil apparent thermal diffusivity obtained with a conduction–convection algorithm had a better agreement with soil apparent thermal diffusivity obtained with a phase algorithm than with soil apparent thermal diffusivity obtained with an amplitude algorithm except for the case of = 0; 2) when > 0, , and when < 0, ; 3) for a given soil temperature phase shift, increased (decreased) with increasing logarithmic amplitude attenuation when the phase shift was larger (smaller) than the logarithmic amplitude attenuation, reached a maximum value when the phase shift equaled the logarithmic amplitude attenuation, and increased with increasing logarithmic amplitude attenuation; and 4) for a given logarithmic amplitude attenuation, decreased with increasing phase shift and increased (decreased) with increasing phase shift when the phase shift was larger (smaller) than times the logarithmic amplitude attenuation. These mathematical conclusions were also confirmed with field data.

Observation of Transient Behavior of Magnetic Flux in Inductive-type Fault Current Limiter with YBCO Thin Film Disc

Abstract Recently, the installation of fault current limiters (FCLs) in power systems is expected for controlling large short circuit currents. In this study, we focus on inductive-type FCLs having a YBCO superconducting thin film disc that is fabricated by metal-organic deposition. AC currents were injected into the FCL so that the periodic transient behavior of the apparent magnetic flux density around the FCL could be measured by using a pick-up coil. The magnetic flux density exhibited hysteresis when AC current was injected into the FCL. The transition between the conducting states in the YBCO layer was explained by the hysteresis relationship between the magnetic flux density and current.

RADIO AND X-RAY OBSERVATIONS OF THE TYPE Ic SN 2007gr REVEAL AN ORDINARY, NON-RELATIVISTIC EXPLOSION

We present extensive radio and X-ray observations of the nearby Type Ic SN 2007gr in NGC 1058 obtained with the Very Large Array and the Chandra X-ray Observatory and spanning 5 to 150 days after explosion. Through our detailed modeling of these data, we estimate the properties of the blastwave and the circumstellar environment. We find evidence for a freely-expanding and non-relativistic explosion with an average blastwave velocity, v~0.2c, and a total internal energy for the radio emitting material of E ~ 2 x 10^46 erg assuming equipartition of energy between electrons and magnetic fields (epsilon_e=epsilon_B=0.1). The temporal and spectral evolution of the radio emission points to a stellar wind-blown environment shaped by a steady progenitor mass loss rate of Mdot ~ 6 x 10^-7 solar masses per year (wind velocity, v_w=10^3 km/s). These parameters are fully consistent with those inferred for other SNe Ibc and are in line with the expectations for an ordinary, homologous SN explosion. Our results are at odds with those of Paragi et al. (2010) who recently reported evidence for a relativistic blastwave in SN 2007gr based on their claim that the radio emission was resolved away in a low signal-to-noise Very Long Baseline Interferometry (VLBI) observation. Here we show that the exotic physical scenarios required to explain the claimed relativistic velocity -- extreme departures from equipartition and/or a highly collimated outflow -- are excluded by our detailed Very Large Array radio observations. Moreover, we present an independent analysis of the VLBI data and propose that a modest loss of phase coherence provides a more natural explanation for the apparent flux density loss which is evident on both short and long baselines. We conclude that SN 2007gr is an ordinary Type Ibc supernova.

VECTOR MAGNETIC FIELDS OF SOLAR GRANULATION

Observations of the quiet Sun from the Solar Optical Telescope/Spectro-Polarimeter aboard the Hinode spacecraft reveal the magnetic characteristics of the solar photosphere. By making use of the deep-mode observations of three quiet regions, we have statistically studied the vector magnetic fields of solar granulation. More than 2000 normal granules are manually selected to form a sample. It is recognized that some granules are even darker than the mean photosphere in intensity, and there is a linear correlation between intensity and Doppler velocity in the granules. The distributions of longitudinal and transverse apparent magnetic flux densities, Doppler velocity, and continuum intensity of granules are obtained, and their unsigned magnetic flux measured. Two approaches are used in this study. First, we obtained the magnetic properties of granulation by averaging the measurements for all the sampling granules. Second, we reconstructed an average granular cell based on a subsample, and obtained the detailed distribution of apparent magnetic flux density within the model granular cell. All the results have been compared with those for intergranular lanes and a few typical abnormal granules. Our statistical analysis reveals the following results. (1) The unsigned magnetic flux of individual granules spans the range from 1.1 × 1015 Mx to 3.3 × 1018 Mx with a peak distribution at 1.6 × 1016 Mx. (2) The unsigned longitudinal apparent flux density of granules ranges from almost 0 to 212 Mx cm−2 with a mean longitudinal apparent flux density of 12 Mx cm−2, while the transverse apparent flux density of granules ranges from 4 to 218 Mx cm−2 with a mean transverse apparent flux density of 79 Mx cm−2. The longitudinal and transverse apparent magnetic flux densities of granules are positively correlated, and the longitudinal apparent flux density of granules is weaker than the corresponding transverse apparent flux density. (3) The magnetic inclination of granules with respect to the surface that is perpendicular to the line of sight falls in the range of 4.8–76.7 degrees with a peak distribution at 25 degrees. On average, the magnetic vectors in granules are more vertical than those in the intergranular lanes. (4) There is a strong preference that both the vertical and horizontal fields on the quiet Sun reside in the intergranular lanes. (5) The detailed distributions of apparent flux density, Doppler velocity, and continuum intensity within an average granular cell are presented. These distributions can be empirically formulated well.

The Horizontal Magnetic Flux of the Quiet‐Sun Internetwork as Observed with theHinodeSpectro‐Polarimeter

Observations of very quiet Sun using the Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) aboard the Hinode spacecraft reveal that the quiet internetwork regions are pervaded by horizontal magnetic flux. The spatial average horizontal apparent flux density derived from wavelength-integrated measures of Zeeman-induced linear polarization is -->BTapp = 55 Mx cm −2, as compared to the corresponding average vertical apparent flux density of -->| BLapp| = 11 Mx cm −2. Distributions of apparent flux density are presented. Magnetic fields are organized on mesogranular scales, with both horizontal and vertical fields showing voids of reduced flux density of a few granules spatial extent. The vertical fields are concentrated in the intergranular lanes, whereas the stronger horizontal fields are somewhat separated spatially from the vertical fields and occur most commonly at the edges of the bright granules. High-S/N observations from disk center to the limb help to constrain possible causes of the apparent imbalance between -->| BLapp| and -->BTapp, with unresolved structures of linear dimension on the surface smaller by at least a factor of 2 relative to the SOT/SP angular resolution being one likely cause of this discrepancy. Other scenarios for explaining this imbalance are discussed. The horizontal fields are likely the source of the seething fields of the quiet Sun discovered by Harvey et al. The horizontal fields may also contribute to the hidden turbulent flux suggested by studies involving Hanle effect depolarization of scattered radiation.

Hinode Observations of Horizontal Quiet Sun Magnetic Flux and the “Hidden Turbulent Magnetic Flux”

We present observations of magnetic fields of the very quiet Sun near disk center using the Spectro-Polarimeter of the Solar Optical Telescope aboard the Hinode satellite. These observations reveal for the first time the ubiquitous presence of horizontal magnetic fields in the internetwork regions. The horizontal fields are spatially distinct from the vertical fields, demonstrating that they are not arising mainly from buffeting of vertical flux tubes by the granular convection. The horizontal component has an average “apparent flux density” of 55 Mxcm$^{-2}$ (assuming the horizontal field structures are spatially resolved), in contrast to the average apparent vertical flux density of 11 Mxcm$^{-2}$. The vertical fields reside mainly in the intergranular lanes, whereas the horizontal fields occur mainly over the bright granules, with a preference to be near the outside edge of the bright granules. The large apparent imbalance of vertical and horizontal flux densities is discussed, and several scenarios are presented to explain this imbalance.

Propagating Waves and Magnetohydrodynamic Mode Coupling in the Quiet‐Sun Network

High-cadence multiwavelength optical observations were taken with the Dunn Solar Telescope at the National Solar Observatory, Sacramento Peak, accompanied by Advanced Stokes Polarimeter vector magnetograms. A total of 11 network bright points (NBPs) have been studied at different atmospheric heights using images taken in wave bands centered on Mg I b1 - 0.4 A, Hα, and Ca II K3. Wavelet analysis was used to study wave packets and identify traveling magnetohydrodynamic waves. Wave speeds were estimated through the temporal cross-correlation of signals, in selected frequency bands of wavelet power, in each wavelength. Four mode-coupling cases were identified, one in each of four of the NBPs, and the variation of the associated Fourier power with height was studied. Three of the detected mode-coupling, transverse-mode frequencies were observed in the 1.2-1.6 mHz range (mean NBP apparent flux density magnitudes over 99-111 Mx cm-2), with the final case showing 2.0-2.2 mHz (with 142 Mx cm-2). Following this, longitudinal-mode frequencies were detected in the range 2.6-3.2 mHz for three of our cases, with 3.9-4.1 mHz for the remaining case. After mode coupling, two cases displayed a decrease in longitudinal-mode Fourier power in the higher chromosphere.

Characterization of Magnetic Flux in the Quiet Sun

Observations of the quiet Sun observed with the Advanced Stokes Polarimeter are used to explore the distribution of apparent flux density (Bapp) and size scales of internetwork regions. For a typical disk-center quiet-Sun observation with excellent seeing (at a quantifiable angular resolution of 1''), average |Bapp| of 18.6 and 10.7 Mx cm-2 are found for the entire quiet region map and the internetwork region (2 σ noise < |Bapp| < 40 Mx cm-2), respectively. The weak internetwork flux appears to consist of two components: the spatially concentrated "granular" internetwork fields and a more diffuse, weaker component that has a characteristic size scale of a few arcseconds. Most of the internetwork area is occupied by measurable fields: 69% of the area has apparent flux density greater than 4.5 Mx cm-2 (3 σ), or 84% with flux density greater than 3.0 Mx cm-2 (2 σ). If the results of Hanle depolarization measurements are accepted, the internetwork flux detected here must be close (within a factor of 2-3) to being spatially resolved and must be intrinsically weak (10-30 G). Examination of Stokes polarimetry in quiet regions away from disk center indicates that the central cores of network flux, as indicated by high-polarization signals, do not have a high degree of mixed polarity. In contrast, regions immediately surrounding the network elements have stronger linear polarization than would be expected from locally vertical fields, indicating a high degree of mixed polarity. This mixed polarity may be the result of the continual "sweeping" of mixed polarity internetwork flux toward the network boundaries. When a quiet region has a significant imbalance of flux of opposite polarities, the internetwork imbalance is of the same sign and typically that of the network. The smaller imbalance in the internetwork suggests that a local dynamo produces most of the internetwork flux.

Arecibo imaging of compact high-velocity clouds

Ten isolated compact high{velocity clouds (CHVCs) of the type cataloged by Braun & Burton (1999) were imaged with the Arecibo telescope and were found to have a nested core/halo morphology. We argue that a combination of high{resolution lled{aperture and synthesis data is crucial to determining the intrinsic properties of the CHVCs. We identify the halos as Warm Neutral Medium surrounding one or more cores in the Cool Neutral Medium phase. These halos are clearly detected and resolved by the Arecibo lled{aperture imaging, which reaches a limiting sensitivity (1 )o fNHI 21 0 17 cm 2 over the typical 70 km s 1 linewidth at zero intensity. TheFWHM linewidth of the halo gas is found to be 25 km s 1 , consistent with a WNM thermal broadening within 10 4 K gas. Substantial asymmetries are found at high NHI (>10 18:5 cm 2 ) levels in 60% of our sample. A high degree of reflection-symmetry is found at low NHI (<10 18:5 cm 2 ) in all sources studied at these levels. The column{density proles of the envelopes are described well by the sky{plane projection of a spherical exponential in atomic volume density, which allows estimating the characteristic central halo column density, NHI(0) = 4:1 3: 21 0 19 cm 2 , and characteristic exponential scale{length, hB = 420 90 arcsec. For plausible values of the thermal pressure at the CNM/WNM interface, these edge proles allow distance estimates to be made for the individual CHVCs studied here which range between 150 and 850 kpc. An alternate method of distance estimation utilizing the mean exponential scale-length found in nearby low mass dwarf galaxies, hB =1 0:6 4:0 kpc, yields distances in the range 320 to 730 kpc. A consequence of having exponential edge proles is that the apparent size and total flux density of these CHVCs will be strongly dependent on the resolution as well as on the sensitivity of the data used; even a relatively deep observation with a limiting sensitivity of10 19 cm 2 over 70 km s 1 will detect only the central 30% of the source area and less than 50% of the total flux density. The exponential proles also suggest that the outer envelopes of the CHVCs are not tidally truncated. Several CHVC cores exhibit a kinematic gradient, consistent with rotation. The halos appear kinematically decoupled from the cores, in the sense that the halos do not display the velocity gradients shown by the dense cores; the gradients are therefore not likely to be due to an external cause such as tidal shear. The much higher degree of symmetry observed in the halos relative to the cores also argues against an external cause of asymmetries in the cores.

Dynamics of the Solar Chromosphere. II. CaiiH2Vand K2VGrains versus Internetwork Fields

We use the Advanced Stokes Polarimeter at the NSO/Sacramento Peak Vacuum Tower Telescope to search for spatio-temporal correlations between enhanced magnetic fields in the quiet solar internetwork photosphere and the occurrence of Ca II H2V grains in the overlying chromosphere. We address the question of whether the shocks that produce the latter are caused by magnetism-related processes, or whether they are of purely hydrodynamic nature. The observations presented here are the first in which sensitive Stokes polarimetry is combined synchronously with high-resolution Ca II H spectrometry. We pay particular attention to the nature and significance of weak polarization signals from the internetwork domain, obtaining a robust estimate of our magnetographic noise level at an apparent flux density of only 3 Mx cm-2. For the quiet Sun internetwork area analyzed here, we find no direct correlation between the presence of magnetic features with apparent flux density above this limit and the occurrence of H2V brightenings. This result contradicts the one-to-one correspondence claimed by Sivaraman & Livingston. We also find no correspondence between H2V grains and the horizontal-field internetwork features discovered by Lites et al.

Predetermination of the magnetic characteristics of stacked ferromagnetic sheets

The aim of this paper is to show the influence of the lateral magnetic anisotropy, i.e. anisotropy perpendicular to the rolling direction, on the magnetic characteristics of stacked ferromagnetic sheets. For this purpose, the characteristics of a single sheet and stacked sheets are experimentally compared. Modelling and numerical calculations using the finite element method allow us to reproduce and to explain the experimental results. In the developed model, the stacking is divided into ten slices. A variable permeability is allocated to each slice in the rolling direction and a constant permeability is assumed in the lateral direction. The results show a good agreement if the values of the apparent flux density is lower or equal to 1.2 T. The presented results concern stacks of 10, 20, 50 and 100 sheets of Fe-Si with either non-oriented or grain-oriented structures.

The cosmological deceleration parameter estimated from the angular-size/redshift relation for compact radio sources

IN cosmological models based on the standard Friedmann–Robertson–Walker geometry, the apparent flux density or angular size of standard candles or standard rods varies with redshift in a way that depends on the deceleration parameter q0. (Open universes have q0 0.5.) At low redshift, however, observational errors are much greater than the differences in q0 expected for different cosmological models, while at high redshift observational uncertainties, particularly at optical wavelengths, and apparent systematic evolutionary changes in sources obscure the expected geometrical effects. Here I show that measurements by very-long-baseline interferometry (VLBI) of compact radio sources associated with active galaxies and quasars may be largely free of evolutionary effects even at substantial redshifts. The relation between angular size and redshift for a sample of these sources indicates a value of q0 close to 0.5, corresponding to cosmological density near the critical value.

Analytical prediction of torque and inductance characteristics of identically slotted doubly-salient reluctance motors

In the paper, the numerically computed variation of normalised permeance and force with apparent tooth flux density (tooth pitch flux/tooth area) is given for identically slotted doubly-salient structures. In the computations, a commercial steel B-H characteristics is used. The data cover the range of structural variables encountered in most commercial stepping motors. Flux densities reaching heavy saturation (up to 2 T) are considered. The variables in presenting the data are chosen to be dimensionless or size independent to facilitate application to a wide range of practical motor structures. The effect of structural variables on torque under different levels of saturation is investigated from the curves and the conclusions are presented. An analytical method of static torque and inductance curve prediction for identically slotted doubly salient devices based on numerically calculated force and permeance data is also developed and applied to heavily saturated stepping motors. The results are found to be in very good agreement with the measurements. The method thus provides designers with an accurate and versatile tool.

超急冷高けい素鉄薄帯の表面粗さの制御と磁気特性

This paper describes some surface features of rapidly solidified crystal ribbons and its relation to the pupdle flow behavior with particular attention to the Nozzle-Roll gap. Magnetic properties of the rapidly quenched High Silicon Iron Alloy ribbons are strongly affected by the ribbon surface topography. Therefore, ribbon surface roughness is examined in connection with pit formation as well as fluid-flow behavior. It is shown that pit size strongly depends on the Reynolds number (Re) and that there is a Critical Reynolds number (Recrit). Below Re≤Recrit, the puddle flow becomes a laminar flow. The increasing rate of pit size is so small that a smooth ribbon can be obtained. The apparent iron loss (W0.5/50) halved and the apparent flux density (B10) is doubled resulting in an improvement of the magnetic properties of the ribbon.

FOLLOWING MINKOWSKI'S FOOTSTEPS: A SURVEY OF DISTANT RADIO SOURCES.

ABSTRACT The current status of optical identifications of 3CR radio sources is reported, and ongoing work on spectroscopy of faint radio galaxies is described. By now, about 64% of the 3CR sources have been securely identified. Statistics for those identified, which are distributed over a large range of apparent radio flux density and a large range of redshift and apparent optical magnitude, are presented. The optical luminosities of the radio galaxies are all near the top of the E cluster-galaxy sample, with Mv roughly -23.5 and Hubble constant of 50 km/sec Mpc. The spectroscopic research on faint radio galaxies has the goals of determination of redshift, measuring the galaxy continuum color and checking possible galaxy color evolution with z, and studying emission lines for insight into physics of the ionized gas near the excited source. Research has shown that for V above 19, radio galaxies of any intrinsic power generally have weak emission, with forbidden O II at 3727 A being the most common. The problems this causes in redshift determination are discussed.

A dispersion technique for measuring the bulk magnetic characteristics of magnetic particulate materials

A dispersion technique is shown which prepares powdered (particulate) materials for measurement in a 60-Hz hysteresis loop tracer ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B-H</tex> meter). The material is proportioned and wet out with linseed oil in a Hoover Automatic Muller, packed in a glass tube, and weighed. The space gradient of an ac magnetic field is used to orient the sample to a high squareness ratio. The only two physical measurements which need be made are the weight and the length of the mixture in the tube. The tube cross section need not be determined. The absolute intrinsic flux density of the material can then be calculated, using the flux determination from the hysteresis loop tracer. If the tube cross section is determined, the apparent flux density can be calculated.

Two Observational Tests: Are Quasars Superluminous Objects or Optical Effects?

view Abstract Citations (5) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Two Observational Tests: Are Quasars Superluminous Objects or Optical Effects? Barnothy, J. M. Abstract 70, 666, 1965) the author has shown that quasi-stellar sources may be images produced by gravitational lenses acting upon suitable astronomical objects. If these objects are nuclei of Seyfert galaxies, the images would be in all respects (large redshift, extreme absolute luminosity, starlike appearance, high intensity in the ultraviolet, strong and broad emission lines, secular and irregular brightness variations in optical and radio emission) similar to those of quasars and the number of images to be expected would be compatible with present observations. The number of images depends not only on the number of suitable obj ects, but also from the number and proper position of deflecting galaxies. In a nonexpanding Euclidian space the number of images brighter than a given absolute magnitude would increase with the fifth power of the distance and not with the third power as the case would be for superluminous objects. In model universes with curved and/or expanding space this difference decreases for very distant objects but remains significant. Another test which could be performed with medium-size telescopes is based on the circumstance that light flashes of a few days or few weeks duration could arise through the scanning motion of the optical axis as a consequence of the proper motion of the involved galaxies whenever the optical axis crosses a bright star. Consequently, the light curves of such flashes (as observed in 3C273) should be symmetrical, while if they are due to energy outbursts within a superluminous obj ect the light curves would not be symmetrical. The number of "image quasars versus apparent magnitude relation is a mud~ more powerful test of the cosmological model of the universe than the galaxy count versus apparent magnitude relation, or number versus flux density relation of extragalactic radio sources. Should quasars be images produced by gravitational lenses, already the observation of a few dozen quasars in the range from z=0.8 to z=l.3 would suffice to decide to which cosmological model our universe belougs. Publication: The Astronomical Journal Pub Date: April 1966 DOI: 10.1086/109875 Bibcode: 1966AJ.....71..154B full text sources ADS |