702 research outputs found
Insights into the nature of northwest-to-southeast aligned ionospheric wavefronts from contemporaneous Very Large Array and ionosondes observations
The results of contemporaneous summer nighttime observations of midlatitude
medium scale traveling ionospheric disturbances (MSTIDs) with the Very Large
Array (VLA) in New Mexico and nearby ionosondes in Texas and Colorado are
presented. Using 132, 20-minute observations, several instances of MSTIDs were
detected, all having wavefronts aligned northwest to southeast and mostly
propagating toward the southwest, consistent with previous studies of MSTIDs.
However, some were also found to move toward the northeast. It was found that
both classes of MSTIDs were only found when sporadic-E (Es) layers of moderate
peak density (1.5<foEs<3 MHz) were present. Limited fbEs data from one
ionosonde suggests that there was a significant amount of structure with the Es
layers during observations when foEs>3 MHz that was not present when 1.5<foEs<3
MHz. No MSTIDs were observed either before midnight or when the F-region height
was increasing at a relatively high rate, even when these Es layers were
observed. Combining this result with AE indices which were relatively high at
the time (an average of about 300 nT and maximum of nearly 700 nT), it is
inferred that both the lack of MSTIDs and the increase in F-region height are
due to substorm-induced electric fields. The northeastward-directed MSTIDs were
strongest post-midnight during times when the F-region was observed to be
collapsing relatively quickly. This implies that these two occurrences are
related and likely both caused by rare shifts in F-region neutral wind
direction from southwest to northwest.Comment: Accepted for publication in the Journal of Geophysical Researc
Latitudinal variation of stochastic properties of the geomagnetic field
We explore the stochastic fractal qualities of the geomagnetic field from
210 mm ground-based magnetometers during quiet and active magnetospheric
conditions. We search through 10 yr of these data to find events that
qualify as quiet intervals, defined by Kp ≤ 1 for 1440 consecutive
minutes. Similarly, active intervals require Kp ≥ 4 for 1440 consecutive
minutes. The total for quiet intervals is ~ 4.3 x 10<sup>6</sup> and 2 x 10<sup>8</sup> min for active data points. With this large number of data we
characterize changes in the nonlinear statistics of the geomagnetic field via
measurements of a fractal scaling. A clear difference in statistical behavior
during quiet and active intervals is implied through analysis of the scaling
exponents; active intervals generally have larger values of scaling
exponents. This suggests that although 210 mm data appear
monofractal on shorter timescales, the scaling changes, with overall
variability are more likely described as a multifractional Brownian motion. We
also find that low latitudes have scaling exponents that are consistently
larger than for high latitudes
Quantum computation with Josephson-qubits by using a current-biased information bus
We propose an effective scheme for manipulating quantum information stored in
a superconducting nanocircuit. The Josephson qubits are coupled via their
separate interactions with an information bus, a large current-biased Josephson
junction treated as an oscillator with adjustable frequency. The bus is
sequentially coupled to only one qubit at a time. Distant Josephson qubits
without any direct interaction can be indirectly coupled with each other by
independently interacting with the bus sequentially, via exciting/de-exciting
vibrational quanta in the bus. This is a superconducting analog of the
successful ion trap experiments on quantum computing. Our approach differs from
previous schemes that simultaneously coupled two qubits to the bus, as opposed
to their sequential coupling considered here. The significant quantum logic
gates can be realized by using these tunable and selective couplings. The
decoherence properties of the proposed quantum system are analyzed within the
Bloch-Redfield formalism. Numerical estimations of certain important
experimental parameters are provided.Comment: 13 pages with 2 figures. submitte
Imaging an Event Horizon: Mitigation of Source Variability of Sagittarius A*
The black hole in the center of the Galaxy, associated with the compact
source Sagittarius A* (Sgr A*), is predicted to cast a shadow upon the emission
of the surrounding plasma flow, which encodes the influence of general
relativity in the strong-field regime. The Event Horizon Telescope (EHT) is a
Very Long Baseline Interferometry (VLBI) network with a goal of imaging nearby
supermassive black holes (in particular Sgr A* and M87) with angular resolution
sufficient to observe strong gravity effects near the event horizon. General
relativistic magnetohydrodynamic (GRMHD) simulations show that radio emission
from Sgr A* exhibits vari- ability on timescales of minutes, much shorter than
the duration of a typical VLBI imaging experiment, which usually takes several
hours. A changing source structure during the observations, however, violates
one of the basic assumptions needed for aperture synthesis in radio
interferometry imaging to work. By simulating realistic EHT observations of a
model movie of Sgr A*, we demonstrate that an image of the average quiescent
emission, featuring the characteristic black hole shadow and photon ring
predicted by general relativity, can nonetheless be obtained by observing over
multiple days and subsequent processing of the visibilities (scaling,
averaging, and smoothing) before imaging. Moreover, it is shown that this
procedure can be combined with an existing method to mitigate the effects of
interstellar scattering. Taken together, these techniques allow the black hole
shadow in the Galactic center to be recovered on the reconstructed image.Comment: 10 pages, 12figures, accepted for publication in Ap
Qubit Decoherence and Non-Markovian Dynamics at Low Temperatures via an Effective Spin-Boson Model
Quantum Brownian oscillator model (QBM), in the Fock-space representation,
can be viewed as a multi-level spin-boson model. At sufficiently low
temperature, the oscillator degrees of freedom are dynamically reduced to the
lowest two levels and the system behaves effectively as a two-level (E2L)
spin-boson model (SBM) in this limit. We discuss the physical mechanism of
level reduction and analyze the behavior of E2L-SBM from the QBM solutions. The
availability of close solutions for the QBM enables us to study the
non-Markovian features of decoherence and leakage in a SBM in the
non-perturbative regime (e.g. without invoking the Born approximation) in
better details than before. Our result captures very well the characteristic
non-Markovian short time low temperature behavior common in many models.Comment: 19 pages, 8 figure
Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances
Flow profiling of a surface acoustic wave nanopump
The flow profile in a capillary gap and the pumping efficiency of an acoustic
micropump employing Surface Acoustic Waves is investigated both experimentally
and theoretically. Such ultrasonic surface waves on a piezoelectric substrate
strongly couple to a thin liquid layer and generate an internal streaming
within the fluid. Such acoustic streaming can be used for controlled agitation
during, e.g., microarray hybridization. We use fluorescence correlation
spectroscopy and fluorescence microscopy as complementary tools to investigate
the resulting flow profile. The velocity was found to depend on the applied
power somewhat weaker than linearly and to decrease fast with the distance from
the ultrasound generator on the chip.Comment: 12 pages 20 figure
The geospace response to variable inputs from the lower atmosphere:a review of the progress made by Task Group 4 of CAWSES-II
The advent of new satellite missions, ground-based instrumentation networks, and the development of whole atmosphere models over the past decade resulted in a paradigm shift in understanding the variability of geospace, that is, the region of the atmosphere between the stratosphere and several thousand kilometers above ground where atmosphere-ionosphere-magnetosphere interactions occur. It has now been realized that conditions in geospace are linked strongly to terrestrial weather and climate below, contradicting previous textbook knowledge that the space weather of Earth's near space environment is driven by energy injections at high latitudes connected with magnetosphere-ionosphere coupling and solar radiation variation at extreme ultraviolet wavelengths alone. The primary mechanism through which energy and momentum are transferred from the lower atmosphere is through the generation, propagation, and dissipation of atmospheric waves over a wide range of spatial and temporal scales including electrodynamic coupling through dynamo processes and plasma bubble seeding. The main task of Task Group 4 of SCOSTEP's CAWSES-II program, 2009 to 2013, was to study the geospace response to waves generated by meteorological events, their interaction with the mean flow, and their impact on the ionosphere and their relation to competing thermospheric disturbances generated by energy inputs from above, such as auroral processes at high latitudes. This paper reviews the progress made during the CAWSES-II time period, emphasizing the role of gravity waves, planetary waves and tides, and their ionospheric impacts. Specific campaign contributions from Task Group 4 are highlighted, and future research directions are discussed
Swimming using surface acoustic waves
Microactuation of free standing objects in fluids is currently dominated by the rotary propeller, giving rise to a range of potential applications in the military, aeronautic and biomedical fields. Previously, surface acoustic waves (SAWs) have been shown to be of increasing interest in the field of microfluidics, where the refraction of a SAW into a drop of fluid creates a convective flow, a phenomenon generally known as SAW streaming. We now show how SAWs, generated at microelectronic devices, can be used as an efficient method of propulsion actuated by localised fluid streaming. The direction of the force arising from such streaming is optimal when the devices are maintained at the Rayleigh angle. The technique provides propulsion without any moving parts, and, due to the inherent design of the SAW transducer, enables simple control of the direction of travel
Climatology of Mid-latitude Ionospheric Disturbances from the Very Large Array Low-frequency Sky Survey
The results of a climatological study of ionospheric disturbances derived
from observations of cosmic sources from the Very Large Array (VLA)
Low-frequency Sky Survey (VLSS) are presented. We have used the ionospheric
corrections applied to the 74 MHz interferometric data within the VLSS imaging
process to obtain fluctuation spectra for the total electron content (TEC)
gradient on spatial scales from a few to hundreds of kilometers and temporal
scales from less than one minute to nearly an hour. The observations sample
nearly all times of day and all seasons. They also span latitudes and
longitudes from 28 deg. N to 40 deg. N and 95 deg. W to 114 deg. W,
respectively. We have binned and averaged the fluctuation spectra according to
time of day, season, and geomagnetic (Kp index) and solar (F10.7) activity.
These spectra provide a detailed, multi-scale account of seasonal and intraday
variations in ionospheric activity with wavelike structures detected at
wavelengths between about 35 and 250 km. In some cases, trends between spectral
power and Kp index and/or F10.7 are also apparent. In addition, the VLSS
observations allow for measurements of the turbulent power spectrum down to
periods of 40 seconds (scales of ~0.4 km at the height of the E-region). While
the level of turbulent activity does not appear to have a strong dependence on
either Kp index or F10.7, it does appear to be more pronounced during the
winter daytime, summer nighttime, and near dusk during the spring.Comment: accepted for publication in Radio Scienc
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