411 research outputs found
On the Anisotropic Nature of MRI-Driven Turbulence in Astrophysical Disks
The magnetorotational instability is thought to play an important role in
enabling accretion in sufficiently ionized astrophysical disks. The rate at
which MRI-driven turbulence transports angular momentum is related to both the
strength of the amplitudes of the fluctuations on various scales and the degree
of anisotropy of the underlying turbulence. This has motivated several studies
of the distribution of turbulent power in spectral space. In this paper, we
investigate the anisotropic nature of MRI-driven turbulence using a
pseudo-spectral code and introduce novel ways to robustly characterize the
underlying turbulence. We show that the general flow properties vary in a
quasi-periodic way on timescales comparable to 10 inverse angular frequencies
motivating the temporal analysis of its anisotropy. We introduce a 3D tensor
invariant analysis to quantify and classify the evolution of the anisotropic
turbulent flow. This analysis shows a continuous high level of anisotropy, with
brief sporadic transitions towards two- and three-component isotropic turbulent
flow. This temporal-dependent anisotropy renders standard shell-average,
especially when used simultaneously with long temporal averages, inadequate for
characterizing MRI-driven turbulence. We propose an alternative way to extract
spectral information from the turbulent magnetized flow, whose anisotropic
character depends strongly on time. This consists of stacking 1D Fourier
spectra along three orthogonal directions that exhibit maximum anisotropy in
Fourier space. The resulting averaged spectra show that the power along each of
the three independent directions differs by several orders of magnitude over
most scales, except the largest ones. Our results suggest that a
first-principles theory to describe fully developed MRI-driven turbulence will
likely have to consider the anisotropic nature of the flow at a fundamental
level.Comment: 13 pages, 13 figures, submitted to Ap
Multidimensional simulations of magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma collision
The energetic electromagnetic eruptions observed during the prompt phase of
gamma-ray bursts are attributed to synchrotron emissions. The internal shocks
moving through the ultrarelativistic jet, which is ejected by an imploding
supermassive star, are the likely source of this radiation. Synchrotron
emissions at the observed strength require the simultaneous presence of
powerful magnetic fields and highly relativistic electrons. We explore with one
and three-dimensional relativistic particle-in-cell simulations the transition
layer of a shock, that evolves out of the collision of two plasma clouds at a
speed 0.9c and in the presence of a quasi-parallel magnetic field. The cloud
densities vary by a factor of 10. The number densities of ions and electrons in
each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor
of the electrons is determined in the 1D simulation, as well as the orientation
and the strength of the magnetic field at the boundary of the two colliding
clouds. The relativistic masses of the electrons and ions close to the shock
transition layer are comparable as in previous work. The 3D simulation shows
rapid and strong plasma filamentation behind the transient precursor. The
magnetic field component orthogonal to the initial field direction is amplified
in both simulations to values that exceed those expected from the shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification. The simultaneous presence of
highly relativistic electrons and strong magnetic fields will give rise to
significant synchrotron emissions.Comment: 8 pages, 5 figures. This work was presented at 21st International
Conference on Numerical Simulation of Plasmas (ICNSP'09). Accepted for
publication IEEE Trans. on Plasma Scienc
Monitoring denaturation behaviour and comparative stability of DNA triple helices using oligonucleotide–gold nanoparticle conjugates
Gold nanoparticle labels, combined with UV-visible optical absorption spectroscopic methods, are employed to probe the temperature-dependent solution properties of DNA triple helices. By using oligonucleotide-nanoparticle conjugates to characterize triplex denaturation, for the first time triplex to duplex melting transitions may be sensitively monitored, with minimal signal interference from duplex to single strand melting, for both parallel and antiparallel triple helices. Further, the comparative sequence-dependent stability of DNA triple helices may also be examined using this approach. Specifically, triplex to duplex melting transitions for triplexes formed using oligonucleotides that incorporate 8-aminoguanine derivatives were successfully monitored and stabilization of both parallel and antiparallel triplexes following 8-aminoguanine substitutions is demonstrated
An Introduction to the Quasi-Optical Design of the HIFI Instrument for the Herschel Space Observatory
This paper is concerned with the quasi-optical design and analysis of the Heterodyne Instrument for the Far Infrared (HIFI) on board the European Space Agency’s Herschel Space Observatory, which is due for launch in 2007. The paper begins with an introduction to astronomy at submillimetre wavelengths followed by the science that will be carried out by HIFI. The optical layout of HIFI is presented and the quasi-optical techniques used in the analysis of band 5 of the instrument are discussed, in particular, issues associated with the design and performance of the integrated lens antenna for this band. A power coupling efficiency calculation is carried out and the overall performance of the telescope is analysed
A Qualitative Analysis of Haptic Feedback in Music Focused Exercises
We present the findings of a pilot-study that analysed the role of haptic feedback in a musical context. To examine the role of haptics in Digital Musical Instrument (DMI) design an experiment was formulated to measure the users’ perception of device usability across four separate feedback stages: fully haptic (force and tactile combined), constant force only, vibrotactile only, and no feedback. The study was piloted over extended periods with the intention of exploring the application and integration of DMIs in real-world musical contexts. Applying a music orientated analysis of this type enabled the investigative process to not only take place over a comprehensive period, but allowed for the exploration of DMI integration in everyday compositional practices. As with any investigation that involves creativity, it was important that the participants did not feel rushed or restricted. That is, they were given sufficient time to explore and assess the different feedback types without constraint. This provided an accurate and representational set of qualitative data for validating the participants’ experience with the different feedback types they were presented with
Mobile access to personal digital photograph archives
Handheld computing devices are becoming highly connected
devices with high capacity storage. This has resulted in their being able to support storage of, and access to, personal photo archives. However the only means for mobile device users to browse such archives is typically a simple one-by-one scroll through image thumbnails in the order that they were taken, or by manually organising them based on folders. In this paper we describe a system for context-based browsing of personal digital photo archives. Photos are labeled with the GPS location and time they are taken and this is used to derive other context-based metadata such as weather conditions and daylight conditions. We
present our prototype system for mobile digital photo retrieval, and an experimental evaluation illustrating the utility of location information for effective personal photo retrieval
My digital photos: where and when?
In recent years digital cameras have seen an enormous rise in popularity, leading to a huge increase in the quantity of digital photos being taken. This brings with it the challenge of organising these large collections. We preset work which organises personal digital photo collections based on date/time and GPS location, which we believe will become a key organisational methodology over the next few years as consumer digital cameras evolve to incorporate GPS and as cameras in mobile phones spread further. The accompanying video illustrates the results of our research into digital photo management tools which contains a series of screen and user interactions highlighting how a user utilises the tools we are developing to manage a personal archive of digital photos
A qualitative analysis of haptic feedback in music focused exercises new interfaces for musical expression
We present the findings of a pilot-study that analysed the role of haptic feedback in a musical context. To closely examine the role of haptics in Digital Musical Instrument (DMI) design an experiment was formulated to measure the users’ perception of device usability across four separate feedback stages: fully haptic (force and tactile combined), constant force only, vibrotactile only, and no feedback. The study was piloted over extended periods with the intention of exploring the application and integration of DMIs in real-world musical contexts. Applying a music orientated analysis of this type enabled the investigative process to not only take place over a comprehensive period, but allowed for the exploration of DMI integration in everyday compositional and explorative practices. As with any investigation that involves creativity, it was important that the participants did not feel rushed or restricted. That is, they were given sufficient time to explore and assess the different feedback types without constraint. This provided an accurate and representational set of qualitative data for validating the participants’ experience with the different feedback types they were presented with
- …