4,307 research outputs found
Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral-zone activity
Particle observations from pairs of satellites (Ogo 5, Vela 4A and 5B, Imp 3) during the recovery of plasma sheet thickness late in substorms were examined. Six of the nine events occurred within about 5 min in locations near the estimated position of the neutral sheet, but over wide ranges of east-west and radial separations. The time of occurrence and spatial extent of the recovery were related to the onset (defined by ground Pi 2 pulsations) and approximate location (estimated from ground mid-latitude magnetic signatures) of substorm expansions. It was found that the plasma sheet recovery occurred 10 - 30 min after the last in a series of Pi bursts, which were interpreted to indicate that the recovery was not due directly to a late, high latitude substorm expansion. The recovery was also observed to occur after the substorm current wedge had moved into the evening sector and to extend far to the east of the center of the last preceding substorm expansion
The LRL electron and proton spectrometer on NASA's Orbiting Geophysical Observatory 5/E/ /instrumentation and calibration/
Systems analysis of electron and proton spectrometer on OGO-
Duality Invariant Actions and Generalised Geometry
We construct the non-linear realisation of the semi-direct product of E(11)
and its first fundamental representation at lowest order and appropriate to
spacetime dimensions four to seven. This leads to a non-linear realisation of
the duality groups and introduces fields that depend on a generalised space
which possess a generalised vielbein. We focus on the part of the generalised
space on which the duality groups alone act and construct an invariant action.Comment: 59 pages (typos fixed and added comments
Two-loop Gell-Mann-Low function of N=1 supersymmetric Yang-Mills theory, regularized by higher covariant derivatives
Two-loop Gell-Mann-Low function is calculated for N=1 supersymmetric
Yang-Mills theory, regularized by higher covariant derivatives. The integrals,
which define it, are shown to be reduced to total derivatives and can be easily
calculated analytically.Comment: 17 pages, 3 eps figure
Scanning-probe spectroscopy of semiconductor donor molecules
Semiconductor devices continue to press into the nanoscale regime, and new
applications have emerged for which the quantum properties of dopant atoms act
as the functional part of the device, underscoring the necessity to probe the
quantum structure of small numbers of dopant atoms in semiconductors[1-3].
Although dopant properties are well-understood with respect to bulk
semiconductors, new questions arise in nanosystems. For example, the quantum
energy levels of dopants will be affected by the proximity of nanometer-scale
electrodes. Moreover, because shallow donors and acceptors are analogous to
hydrogen atoms, experiments on small numbers of dopants have the potential to
be a testing ground for fundamental questions of atomic and molecular physics,
such as the maximum negative ionization of a molecule with a given number of
positive ions[4,5]. Electron tunneling spectroscopy through isolated dopants
has been observed in transport studies[6,7]. In addition, Geim and coworkers
identified resonances due to two closely spaced donors, effectively forming
donor molecules[8]. Here we present capacitance spectroscopy measurements of
silicon donors in a gallium-arsenide heterostructure using a scanning probe
technique[9,10]. In contrast to the work of Geim et al., our data show
discernible peaks attributed to successive electrons entering the molecules.
Hence this work represents the first addition spectrum measurement of dopant
molecules. More generally, to the best of our knowledge, this study is the
first example of single-electron capacitance spectroscopy performed directly
with a scanning probe tip[9].Comment: In press, Nature Physics. Original manuscript posted here; 16 pages,
3 figures, 5 supplementary figure
Network Physiology reveals relations between network topology and physiological function
The human organism is an integrated network where complex physiologic
systems, each with its own regulatory mechanisms, continuously interact, and
where failure of one system can trigger a breakdown of the entire network.
Identifying and quantifying dynamical networks of diverse systems with
different types of interactions is a challenge. Here, we develop a framework to
probe interactions among diverse systems, and we identify a physiologic
network. We find that each physiologic state is characterized by a specific
network structure, demonstrating a robust interplay between network topology
and function. Across physiologic states the network undergoes topological
transitions associated with fast reorganization of physiologic interactions on
time scales of a few minutes, indicating high network flexibility in response
to perturbations. The proposed system-wide integrative approach may facilitate
the development of a new field, Network Physiology.Comment: 12 pages, 9 figure
The composition and structure of the Enceladus plume
The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn\u27s moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H(2)O gas, with a nominal line-of-sight column density of 0.90 +/- 0.23 x 10(16) cm(-2) (upper limit of 1.0 x 10(16) cm(-2)). The upper limit for N(2) is 5 x 10(13) cm(-2), or \u3c 0.5% in the plume; the lack of N(2) has significant implications for models of the geochemistry in Enceladus\u27 interior. The inferred rate of water vapor injection into Saturn\u27s magnetosphere is similar to 200 kg/s. The calculated values of H(2)O flux from three occultations observed by UVIS have a standard deviation of 30 kg/s (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume with Mach numbers of 5-8, implying vertical gas velocities that exceed 1000 m/sec. Observations at higher altitudes with the Cassini Ion Neutral Mass Spectrometer indicate correlated structure in the plume. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer
Upregulation of the cell-cycle regulator RGC-32 in Epstein-Barr virus-immortalized cells
Epstein-Barr virus (EBV) is implicated in the pathogenesis of multiple human tumours of lymphoid and epithelial origin. The virus infects and immortalizes B cells establishing a persistent latent infection characterized by varying patterns of EBV latent gene expression (latency 0, I, II and III). The CDK1 activator, Response Gene to Complement-32 (RGC-32, C13ORF15), is overexpressed in colon, breast and ovarian cancer tissues and we have detected selective high-level RGC-32 protein expression in EBV-immortalized latency III cells. Significantly, we show that overexpression of RGC-32 in B cells is sufficient to disrupt G2 cell-cycle arrest consistent with activation of CDK1, implicating RGC-32 in the EBV transformation process. Surprisingly, RGC-32 mRNA is expressed at high levels in latency I Burkitt's lymphoma (BL) cells and in some EBV-negative BL cell-lines, although RGC-32 protein expression is not detectable. We show that RGC-32 mRNA expression is elevated in latency I cells due to transcriptional activation by high levels of the differentially expressed RUNX1c transcription factor. We found that proteosomal degradation or blocked cytoplasmic export of the RGC-32 message were not responsible for the lack of RGC-32 protein expression in latency I cells. Significantly, analysis of the ribosomal association of the RGC-32 mRNA in latency I and latency III cells revealed that RGC-32 transcripts were associated with multiple ribosomes in both cell-types implicating post-initiation translational repression mechanisms in the block to RGC-32 protein production in latency I cells. In summary, our results are the first to demonstrate RGC-32 protein upregulation in cells transformed by a human tumour virus and to identify post-initiation translational mechanisms as an expression control point for this key cell-cycle regulator
Co-evolution of density and topology in a simple model of city formation
We study the influence that population density and the road network have on
each others' growth and evolution. We use a simple model of formation and
evolution of city roads which reproduces the most important empirical features
of street networks in cities. Within this framework, we explicitely introduce
the topology of the road network and analyze how it evolves and interact with
the evolution of population density. We show that accessibility issues -pushing
individuals to get closer to high centrality nodes- lead to high density
regions and the appearance of densely populated centers. In particular, this
model reproduces the empirical fact that the density profile decreases
exponentially from a core district. In this simplified model, the size of the
core district depends on the relative importance of transportation and rent
costs.Comment: 13 pages, 13 figure
Relativistic Structure of the Deuteron: 1.Electro-disintegration and y-scaling
Realistic solutions of the spinor-spinor Bethe-Salpeter equation for the
deuteron with realistic interaction kernel including the exchange of pi, sigma,
omega, rho, eta and delta mesons, are used to systematically investigate
relativistic effects in inclusive quasi-elastic electron-deuteron scattering
within the relativistic impulse approximation. Relativistic y-scaling is
considered by generalising the non relativistic scaling function to the
relativistic case, and it is shown that y-scaling does occur in the usual
relativistic scaling variable resulting from the energy conservation in the
instant form of dynamics. The present approach of y-scaling is fully covariant,
with the deuteron being described by eight components, viz. the 3S_1^{++},
3S_1^{--}, 3D_1^{++}, 3D_1^{--}, 3P_1^{+-}, 3P_1^{-+}, 1P_1^{+-}, 1P_1^{-+}
waves. It is demonstrated that if the negative relative energy states 1P_1,
3P_1 are disregarded, the concept of covariant momentum distributions N(p_0,p),
with p_0=M_D/2-\sqrt{p^2+m^2}, can be introduced, and that calculations of
lectro-disintegration cross section in terms of these distributions agree
within few percents with the exact calculations which include the 1P_1, 3P_1
states, provided the nucleon three momentum |p|\<= 1 GeV/c; in this momentum
range, the asymptotic relativistic scaling function is shown to coincide with
the longitudinal covariant momentum distribution.Comment: 32 LaTeX pages, 18 eps-figures. Final version to appear in Phys. Rev.
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