10,095 research outputs found
Developments in electromagnetic tomography instrumentation.
A new EMT sensor and instrumentation is described which combines the best features of previous systems and has a modular structure to allow for future system expansion and development
Flow field prediction and analysis study for project RAM B3 Final report
Flow field properties in shock layer surrounding Ram B3 vehicl
Disrupting Tech While Being Pro-Tech
As more libraries take on social justice concerns, pro-technology library workers have the responsibility of vetting and implementing meaningful tools that both meet patron needs and privacy. The goal of this program is for library workers to think critically how we might take a more measured approach to review and integrate technology into library services. Participants are encouraged to think beyond the glitz and glam of new toys and consider how innovation can work in favor of community goals and library ethics. BIPOC, paraprofessionals, students, recent grads, and those with reservations about technology are highly encouraged to attend
Storm‐time configuration of the inner magnetosphere: Lyon‐Fedder‐Mobarry MHD code, Tsyganenko model, and GOES observations
[1] We compare global magnetohydrodynamic (MHD) simulation results with an empirical model and observations to understand the magnetic field configuration and plasma distribution in the inner magnetosphere, especially during geomagnetic storms. The physics-based Lyon-Fedder-Mobarry (LFM) code simulates Earth\u27s magnetospheric topology and dynamics by solving the equations of ideal MHD. Quantitative comparisons of simulated events with observations reveal strengths and possible limitations and suggest ways to improve the LFM code. Here we present a case study that compares the LFM code to both a semiempirical magnetic field model and to geosynchronous measurements from GOES satellites. During a magnetic cloud event, the simulation and model predictions compare well qualitatively with observations, except during storm main phase. Quantitative statistical studies of the MHD simulation shows that MHD field lines are consistently under-stretched, especially during storm time (Dst \u3c −20 nT) on the nightside, a likely consequence of an insufficient representation of the inner magnetosphere current systems in ideal MHD. We discuss two approaches for improving the LFM result: increasing the simulation spatial resolution and coupling LFM with a ring current model based on drift physics (i.e., the Rice Convection Model (RCM)). We show that a higher spatial resolution LFM code better predicts geosynchronous magnetic fields (not only the average Bz component but also higher-frequency fluctuations driven by the solar wind). An early version of the LFM/RCM coupled code, which runs so far only for idealized events, yields a much-improved ring current, quantifiable by decreased field strengths at all local times compared to the LFM-only code
Oxygen isotopes implanted in the LDEF spacecraft
Secondary ion mass spectrometry was used to study oxygen implanted in the surface of copper from the Long Duration Exposure Facility (LDEF). Oxidation that occurred in orbit shows a characteristic oxygen isotope composition, depleted in O-18. The measured depletion is comparable to the predicted depletion (45 percent) based on a model of the gravitational separation of the oxygen isotopes. The anomalous oxygen was contained within 10nm of the surface. Tray E10 was calculated to have received 5.14 x 10(exp 21) atoms of oxygen cm(sup -2) during the LDEF mission and so there is sufficient anomalous implanted oxygen present in the surface to obtain a reliable isotopic profile
The role of the bow shock in solar wind-magnetosphere coupling
In this paper we examine the role of the bow shock in coupling solar wind
energy to the magnetosphere using global magnetohydrodynamic simulations of
the solar wind-magnetosphere interaction with southward IMF. During typical
solar wind conditions, there are two significant dynamo currents in the
magnetospheric system, one in the high-latitude mantle region tailward of
the cusp and the other in the bow shock. As the magnitude of the (southward)
IMF increases and the solar wind becomes a low Mach number flow, there is a
significant change in solar wind-magnetosphere coupling. The high-latitude
magnetopause dynamo becomes insignificant compared to the bow shock and a
large load appears right outside the magnetopause. This leaves the bow shock
current as the only substantial dynamo current in the system, and the only
place where a significant amount of mechanical energy is extracted from the
solar wind. That energy appears primarily as electromagnetic energy, and the
Poynting flux generated at the bow shock feeds energy back into the plasma,
reaccelerating it to solar wind speeds. Some small fraction of that Poynting
flux is directed into the magnetosphere, supplying the energy needed for
magnetospheric dynamics. Thus during periods when the solar wind flow has a
low Mach number, the main dynamo in the solar wind-magnetosphere system is
the bow shock
eBank UK: linking research data, scholarly communication and learning
This paper includes an overview of the changing landscape of scholarly communication and describes outcomes from the innovative eBank UK project, which seeks to build links from e-research through to e-learning. As introduction, the scholarly knowledge cycle is described and the role of digital repositories and aggregator services in linking data-sets from Grid-enabled projects to e-prints through to peer-reviewed articles as resources in portals and Learning Management Systems, are assessed. The development outcomes from the eBank UK project are presented including the distributed information architecture, requirements for common ontologies, data models, metadata schema, open linking technologies, provenance and workflows. Some emerging challenges for the future are presented in conclusion
Coherence of Spin Qubits in Silicon
Given the effectiveness of semiconductor devices for classical computation
one is naturally led to consider semiconductor systems for solid state quantum
information processing. Semiconductors are particularly suitable where local
control of electric fields and charge transport are required. Conventional
semiconductor electronics is built upon these capabilities and has demonstrated
scaling to large complicated arrays of interconnected devices. However, the
requirements for a quantum computer are very different from those for classical
computation, and it is not immediately obvious how best to build one in a
semiconductor. One possible approach is to use spins as qubits: of nuclei, of
electrons, or both in combination. Long qubit coherence times are a
prerequisite for quantum computing, and in this paper we will discuss
measurements of spin coherence in silicon. The results are encouraging - both
electrons bound to donors and the donor nuclei exhibit low decoherence under
the right circumstances. Doped silicon thus appears to pass the first test on
the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200
Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations
Low altitude field-aligned current densities ob-
tained from global magnetospheric simulations are compared
with two-dimensional distributions of Birkeland currents at
the topside ionosphere derived from magnetic field observa-
tions by the constellation of Iridium satellites. We present the
analysis of two magnetic cloud events, 17–19 August 2003
and 19–21 March 2001, where the interplanetary magnetic
field (IMF) rotates slowly (∼10◦/h) to avoid time-aliasing in
the magnetic perturbations used to calculate the Birkeland
currents. In the August 2003 event the IMF rotates from
southward to northward while maintaining a negative IMF
By during much of the interval. During the March 2001
event the IMF direction varies from dawnward to southward
to duskward. We find that the distributions of the Birkeland
current densities in the simulations agree qualitatively with
the observations for northward IMF. For southward IMF,
the dayside Region-1 currents are reproduced in the simu-
◦
the ionospheric grids in the simulations and the observations is shown to have only secondary effect on the magnitudes of the Birkeland currents. The electric potentials in the simu- lation for southward IMF periods are twice as large as those obtained from measurements of the plasma drift velocities by DMSP, implying that the reconnection rates in the simulation are too large.
Keywords. Ionosphere (Electric fields and currents; Ionosphere-magnetosphere interactions; Modeling and forecasting)
1 Introduction
Global magnetohydrodynamic (MHD) models are the most comprehensive numerical tool for studying the coupling of energy and momentum of the solar wind into the Earth’s magnetosphere and ionosphere. A particular advantage of global MHD simulations is the ability to provide continu- ous temporal and spatial coverage of key physical parame- ters over the entire simulation volume. For this reason, MHD simulations have become one of the principal tools for study- ing space weather events such as the interaction of the Earth’s magnetosphere with coronal mass ejections (CMEs) (Ridley et al., 2002) as well as magnetic storms (Slinker et al., 1998; Goodrich et al., 1998) and substorms (Lyon et al., 1998; Lopez et al., 1998; Wiltberger et al., 2000). Since the simula- tion results are frequently used to interpret physical processes in the magnetosphere–ionosphere system, assessing their ac- curacy by comparison with observations is an important task. A number of such studies have been carried out in the past us- ing space-based (Frank et al., 1995; Raeder et al., 1997) and ground-based observations (Ridley et al., 2001), or a com- bination thereof (Fedder et al., 1998; Slinker et al., 1999). However, interpreting the discrepancies between model and observations is not straightforward because the observational
lation, but appear on average 5 served location, while the nightside Region-1 currents and the Region-2 currents are largely under-represented. Com- parison of the observed and simulated Birkeland current dis- tributions, which are intimately related to the plasma drifts at the ionosphere, shows that the ionospheric convection pat- tern in the MHD model and its dependence on the IMF ori- entation is essentially correct. The Birkeland total currents in the simulations are about a factor of 2 larger than observed during southward IMF. For Bz\u3e0 the disparity in the total current is reduced and the simulations for purely northward IMF agree with the observations to within 10%. The dispar- ities in the magnitudes of the Birkeland currents between the observations and the simulation results are a combined effect of the simulation overestimating the ionospheric electric field and of the Iridium fits underestimating the magnetic pertur- bations
A low power photoemission source for electrons on liquid helium
Electrons on the surface of liquid helium are a widely studied system that
may also provide a promising method to implement a quantum computer. One
experimental challenge in these studies is to generate electrons on the helium
surface in a reliable manner without heating the cryo-system. An electron
source relying on photoemission from a zinc film has been previously described
using a high power continuous light source that heated the low temperature
system. This work has been reproduced more compactly by using a low power
pulsed lamp that avoids any heating. About 5e3 electrons are collected on 1
cm^2 of helium surface for every pulse of light. A time-resolved experiment
suggests that electrons are either emitted over or tunnel through the 1eV
barrier formed by the thin superfluid helium film on the zinc surface. No
evidence of trapping or bubble formation is seen.Comment: 9 pages, 3 figures, submitted to J. Low Temp. Phy
- …