scholarly journals The Astronomical Low-Frequency Array (ALFA)

1998 ◽  
Vol 164 ◽  
pp. 393-394 ◽  
Author(s):  
D. L. Jones ◽  
K. W. Weiler ◽  
R. J. Allen ◽  
M. M. Desch ◽  
W. C. Erickson ◽  
...  
Keyword(s):  
High Resolution ◽  
Solar System ◽  
Angular Resolution ◽  
Low Frequency ◽  
Low Frequencies ◽  
Wide Range ◽  
High Resolution Images ◽  
Coherent Emission ◽  
Age Of The Universe ◽  
The Universe

AbstractThe ALFA mission is designed to map the entire sky at frequencies between approximately 0.3 and 30 MHz with angular resolution limited by interstellar and interplanetary scattering. Most of this region of the spectrum is inaccessible from the ground because of absorption and refraction by the Earth’s ionosphere. A wide range of astrophysical questions concerning solar system, galactic, and extragalactic objects could be answered with high resolution images at low frequencies, where absorption effects and coherent emission processes become important and the synchrotron lifetimes of electrons are comparable to the age of the universe.

scholarly journals Low Frequency Radio Astronomy from Above the Ionosphere

2002 ◽  
Vol 199 ◽  
pp. 488-489
Author(s):  
D. L. Jones
Keyword(s):  
High Resolution ◽  
Radio Astronomy ◽  
Coronal Mass Ejections ◽  
Solar Radio ◽  
Low Frequency ◽  
Dramatic Improvement ◽  
Radio Bursts ◽  
Solar Radio Bursts ◽  
Low Frequencies ◽  
High Resolution Images

The GMRT represents a dramatic improvement in ground-based observing capabilities for low frequency radio astronomy. At sufficiently low frequencies, however, no ground-based facility will be able to produce high resolution images while looking through the ionosphere. A space-based array will be needed to explore the objects and processes which dominate the sky at the lowest radio frequencies. An imaging radio interferometer based on a large number of small, inexpensive satellites would be able to track solar radio bursts associated with coronal mass ejections out to the distance of Earth, determine the frequency and duration of early epochs of nonthermal activity in galaxies, and provide unique information about the interstellar medium.

scholarly journals Introduction to the Joint Commission Meeting on High Resolution Imaging from the Ground

1989 ◽  
Vol 8 ◽  
pp. 545-546
Author(s):  
John Davis
Keyword(s):  
High Resolution ◽  
Angular Resolution ◽  
High Angular Resolution ◽  
High Resolution Imaging ◽  
Wide Range ◽  
High Resolution Images ◽  
Resolution Imaging ◽  
Optical Wavelengths ◽  
To Come ◽  
Very High

As a result of advances in instrumentation and techniques, from radio through to optical wavelengths, we have before us the prospect of producing very high resolution images of a wide range of objects across this entire spectral range. This prospect, and the new knowledge and discoveries that may be anticipated from it, lie behind an upsurge in interest in high resolution imaging from the ground. Several new high angular resolution instruments for radio, infrared, and optical wavelengths are expected to come into operation before the 1991 IAU General Assembly.

Prospects for High Angular Resolution at Low Frequencies

2001 ◽  
Vol 205 ◽  
pp. 418-419
Author(s):  
Dayton L. Jones
Keyword(s):  
Solar Corona ◽  
Space Weather ◽  
Angular Resolution ◽  
Interplanetary Medium ◽  
Low Frequency ◽  
High Angular Resolution ◽  
Low Frequencies ◽  
Transient Disturbances ◽  
Resolution Imaging ◽  
The Universe

High angular resolution imaging at low frequencies (below ∼ 10 MHz) requires observations from above the ionosphere. A radio interferometer array in space will be able to open new vistas in solar, terrestrial, galactic, and extragalactic astrophysics. A space-based interferometer could image and track transient disturbances in the solar corona and interplanetary medium - a new capability which is crucial for understanding many aspects of solar-terrestrial interaction and space weather. It could also produce the first sensitive, high-angular-resolution radio surveys of the entire sky at low frequencies. The radio sky will look entirely different below 10-30 MHz because new emission and absorption processes become dominant at these frequencies. As a result, low frequency surveys from space will provide a fundamentally new view of the universe and an extraordinarily large and varied science return.

Three-fold astigmatism: An important TEM aberration

1993 ◽  
Vol 51 ◽  
pp. 972-973 ◽  
Author(s):  
O.L. Krivanek ◽  
M.L. Leber
Keyword(s):  
High Resolution ◽  
Spatial Frequency ◽  
Field Emission ◽  
Azimuthal Angle ◽  
Hollow Cone ◽  
Small Probe ◽  
Wide Range ◽  
High Resolution Images ◽  
Image Position

Three-fold astigmatism resembles regular astigmatism, but it has 3-fold rather than 2-fold symmetry. Its contribution to the aberration function χ(q) can be written as:where A3 is the coefficient of 3-fold astigmatism, λ is the electron wavelength, q is the spatial frequency, ϕ the azimuthal angle (ϕ = tan-1 (qy/qx)), and ϕ3 the direction of the astigmatism.Three-fold astigmatism is responsible for the “star of Mercedes” aberration figure that one obtains from intermediate lenses once their two-fold astigmatism has been corrected. Its effects have been observed when the beam is tilted in a hollow cone over a wide range of angles, and there is evidence for it in high resolution images of a small probe obtained in a field emission gun TEM/STEM instrument. It was also expected to be a major aberration in sextupole-based Cs correctors, and ways were being developed for dealing with it on Cs-corrected STEMs.

scholarly journals The Low-Frequency Array (LOFAR): Opening a New Window on the Universe

2002 ◽  
Vol 199 ◽  
pp. 474-483
Author(s):  
Namir E. Kassim ◽  
T. Joseph W. Lazio ◽  
William C. Erickson ◽  
Patrick C. Crane ◽  
R. A. Perley ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Broad Band ◽  
Low Frequency ◽  
Electromagnetic Spectrum ◽  
Interferometric Imaging ◽  
Very Large Array ◽  
Long Wavelength ◽  
Calibration Techniques ◽  
Long Baselines ◽  
The Universe

Decametric wavelength imaging has been largely neglected in the quest for higher angular resolution because ionospheric structure limited interferometric imaging to short (< 5 km) baselines. The long wavelength (LW, 2—20 m or 15—150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. This has inspired the Low Frequency Array (LOFAR)—-a fully electronic, broad-band (15—150 MHz)antenna array which will provide an improvement of 2—3 orders of magnitude in resolution and sensitivity over the state of the art.

PIV Analysis of Dynamic Velocity Profiles in Non-Newtonian Drilling Fluids Exposed to Oscillatory Motion

2018 ◽  
Author(s):  
Maduranga Amaratunga ◽  
Roar Nybø ◽  
Rune W. Time
Keyword(s):  
Low Frequency ◽  
Velocity Profiles ◽  
Liquid Column ◽  
Oscillatory Motion ◽  
Velocity Shear ◽  
Drilling Fluids ◽  
Cross Sectional ◽  
Low Frequencies ◽  
Wide Range ◽  
Dynamic Velocity

Drilling fluids experience a wide range of shear rates and oscillatory motion while circulating through the well and also during the operations for solids control. Therefore, it is important to investigate the influence of oscillatory fields on the velocity profiles, shear rate and resulting rheological condition of non-Newtonian polymers, which are additives in drilling fluids. In this paper, we present the dynamic velocity profiles within both Newtonian (deionized water) and non-Newtonian liquids (Polyanionic Cellulose – PAC) exposed to oscillatory motion. A 15 cm × 15 cm square cross-sectional liquid column was oscillated horizontally with very low frequencies (0.75–1.75 Hz) using a laboratory made oscillating table. The dynamic velocity profiles at the bulk of the oscillating liquid column were visualized by the Particle Image Velocimetry (PIV) method, where the motion of fluid is optically visualized using light scattering “seeding” particles. Increased frequency of oscillations lead to different dynamic patterns and ranges of velocity-shear magnitudes. The experiments are part of a comprehensive study aimed at investigating the influence of low frequency oscillations on particle settling in non-Newtonian drilling fluids. It is discussed, how such motion imposed on polymeric liquids influences both flow dynamics as well as local settling velocities of cuttings particles.

scholarly journals Radio Source Environments at Redshifts > 0.5

1996 ◽  
Vol 175 ◽  
pp. 321-322
Author(s):  
M. Lacy ◽  
S. Rawlings ◽  
M. Wold ◽  
A. Bunker ◽  
K.M. Blundell ◽  
...  
Keyword(s):  
Radio Source ◽  
High Redshift ◽  
Elliptical Galaxies ◽  
Radio Sources ◽  
Powerful Radio ◽  
Host Galaxies ◽  
Local Universe ◽  
Wide Range ◽  
Age Of The Universe ◽  
The Universe

The most powerful radio sources in the local Universe are found in giant elliptical galaxies. Looking back to a redshift of 0.5 (≈ half the age of the Universe for ω = 1), we see that these host galaxies are increasingly found in moderately rich clusters. This fact gives us hope that radio sources can be used as tracers of high density environments at high redshift. By exploiting radio source samples selected over a wide range in luminosity (Blundell et al., these proceedings), we will also be able to test whether the luminosities of radio sources are correlated with their environments.

scholarly journals Solar-System Studies with Pulsar Timing Arrays

2017 ◽  
Vol 13 (S337) ◽  
pp. 154-157
Author(s):  
R. N. Caballero ◽  
Keyword(s):  
Solar System ◽  
Low Frequency ◽  
Spatial Correlations ◽  
Timing Model ◽  
Pulsar Timing ◽  
Wide Range ◽  
Sensitivity Curves ◽  
The Masses ◽  
Work Done ◽  
Pulsar Timing Array

AbstractHigh-precision pulsar timing is central to a wide range of astrophysics and fundamental physics applications. When timing an ensemble of millisecond pulsars in different sky positions, known as a pulsar timing array (PTA), one can search for ultra-low-frequency gravitational waves (GWs) through the spatial correlations that spacetime deformations by passing GWs are predicted to induce on the pulses’ times-of-arrival (TOAs). A pulsar-timing model, requires the use of a solar-system ephemeris (SSE) to properly predict the position of the solar-system barycentre, the (quasi-)inertial frame where all TOAs are referred. Here, I discuss how while errors in SSEs can introduce correlations in the TOAs that may interfere with GW searches, one can make use of PTAs to study the solar system. I discuss work done within the context of the European Pulsar Timing Array and the International Pulsar Timing Array collaborations. These include new updates on the masses of planets from PTA data, first limits on masses of the most massive asteroids, and comparisons between SSEs from independent groups. Finally, I discuss a new approach in setting limits on the masses of unknown bodies in the solar system and calculate mass sensitivity curves for PTA data.

scholarly journals Assessment of the Effects of Superior Canal Dehiscence Location and Size on Intracochlear Sound Pressures

2014 ◽  
Vol 20 (1) ◽  
pp. 62-71 ◽  
Author(s):  
Marlien E.F. Niesten ◽  
Christof Stieger ◽  
Daniel J. Lee ◽  
Julie P. Merchant ◽  
Wilko Grolman ◽  
...  
Keyword(s):  
Hearing Loss ◽  
Temporal Bone ◽  
Semicircular Canal ◽  
Low Frequency ◽  
Low Frequencies ◽  
Superior Canal Dehiscence ◽  
Superior Semicircular Canal ◽  
Wide Range ◽  
Before And After ◽  
Canal Dehiscence

Superior canal dehiscence (SCD) is a defect in the bony covering of the superior semicircular canal. Patients with SCD present with a wide range of symptoms, including hearing loss, yet it is unknown whether hearing is affected by parameters such as the location of the SCD. Our previous human cadaveric temporal bone study, utilizing intracochlear pressure measurements, generally showed that an increase in dehiscence size caused a low-frequency monotonic decrease in the cochlear drive across the partition, consistent with increased hearing loss. This previous study was limited to SCD sizes including and smaller than 2 mm long and 0.7 mm wide. However, the effects of larger SCDs (>2 mm long) were not studied, although larger SCDs are seen in many patients. Therefore, to answer the effect of parameters that have not been studied, this present study assessed the effect of SCD location and the effect of large-sized SCDs (>2 mm long) on intracochlear pressures. We used simultaneous measurements of sound pressures in the scala vestibuli and scala tympani at the base of the cochlea to determine the sound pressure difference across the cochlear partition - a measure of the cochlear drive in a temporal bone preparation - allowing for assessment of hearing loss. We measured the cochlear drive before and after SCDs were made at different locations (e.g. closer to the ampulla of the superior semicircular canal or closer to the common crus) and for different dehiscence sizes (including larger than 2 mm long and 0.7 mm wide). Our measurements suggest the following: (1) different SCD locations result in similar cochlear drive and (2) larger SCDs produce larger decreases in cochlear drive at low frequencies. However, the effect of SCD size seems to saturate as the size increases above 2-3 mm long and 0.7 mm wide. Although the monotonic effect was generally consistent across ears, the quantitative amount of change in cochlear drive due to dehiscence size varied across ears. Additionally, the size of the dehiscence above which the effect on hearing saturated varied across ears. These findings show that the location of the SCD does not generally influence the amount of hearing loss and that SCD size can help explain some of the variability of hearing loss in patients. i 2014 S. Karger AG, Basel

scholarly journals Millisecond pulsars at low frequencies

2017 ◽  
Vol 13 (S337) ◽  
pp. 311-312
Author(s):  
N. D. Ramesh Bhat ◽  
Steven E. Tremblay ◽  
Franz Kirsten
Keyword(s):  
South African ◽  
Southern Hemisphere ◽  
Low Frequency ◽  
Pulsar Emission ◽  
Millisecond Pulsars ◽  
Low Frequencies ◽  
Pulsar Timing ◽  
Wide Range ◽  
Propagation Effects ◽  
Murchison Widefield Array

AbstractLow-frequency pulsar observations are well suited for studying propagation effects caused by the interstellar medium (ISM). This is particularly important for millisecond pulsars (MSPs) that are part of high-precision timing applications such as pulsar timing arrays (PTA), which aim to detect nanoHertz gravitational waves. MSPs in the southern hemisphere will also be the prime targets for PTAs with the South African MeerKAT, and eventually with the SKA. The development of the Murchison Widefield Array (MWA) and the Engineering Development Array (EDA) brings excellent opportunities for low-frequency studies of MSPs in the southern hemisphere. They enable observations at frequencies from 50 MHz to 300 MHz, and can be exploited for a wide range of studies relating to pulsar emission physics and probing the ISM.

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