1,117 research outputs found
Vlasov-Maxwell, self-consistent electromagnetic wave emission simulations in the solar corona
1.5D Vlasov-Maxwell simulations are employed to model electromagnetic
emission generation in a fully self-consistent plasma kinetic model for the
first time in the solar physics context. The simulations mimic the plasma
emission mechanism and Larmor drift instability in a plasma thread that
connects the Sun to Earth with the spatial scales compressed appropriately. The
effects of spatial density gradients on the generation of electromagnetic
radiation are investigated. It is shown that 1.5D inhomogeneous plasma with a
uniform background magnetic field directed transverse to the density gradient
is aperiodically unstable to Larmor-drift instability. The latter results in a
novel effect of generation of electromagnetic emission at plasma frequency.
When density gradient is removed (i.e. when plasma becomes stable to
Larmor-drift instability) and a density, super-thermal, hot beam is
injected along the domain, in the direction perpendicular to the magnetic
field, plasma emission mechanism generates non-escaping Langmuir type
oscillations which in turn generate escaping electromagnetic radiation. It is
found that in the spatial location where the beam is injected, the standing
waves, oscillating at the plasma frequency, are excited. These can be used to
interpret the horizontal strips observed in some dynamical spectra. Quasilinear
theory predictions: (i) the electron free streaming and (ii) the beam long
relaxation time, in accord with the analytic expressions, are corroborated via
direct, fully-kinetic simulation. Finally, the interplay of Larmor-drift
instability and plasma emission mechanism is studied by considering
electron beam in the Larmor-drift unstable (inhomogeneous) plasma.
http://www.maths.qmul.ac.uk/~tsiklauri/movie1.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie2.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie3.mpgComment: Solar Physics (in press, the final, accepted version
Fredholm methods for billiard eigenfunctions in the coherent state representation
We obtain a semiclassical expression for the projector onto eigenfunctions by
means of the Fredholm theory. We express the projector in the coherent state
basis, thus obtaining the semiclassical Husimi representation of the stadium
eigenfunctions, which is written in terms of classical invariants: periodic
points, their monodromy matrices and Maslov indices.Comment: 12 pages, 10 figures. Submitted to Phys. Rev. E. Comments or
questions to [email protected]
Dzyaloshinskii-Moriya interaction in NaVO: a microscopic study
We present a unified account of magnetic exchange and Raman scattering in the
quasi-one-dimensional transition-metal oxide NaVO. Based on a
cluster-model approach explicit expressions for the exchange integral and the
Raman-operator are given. It is demonstrated that a combination of the
electronic-structure and the Dzyaloshinskii-Moriya interaction, allowed by
symmetry in this material, are responsible for the finite Raman cross-section
giving rise to both, one- and two-magnon scattering amplitudes.Comment: 7 pages, 1 figur
The anapole moment and nucleon weak interactions
From the recent measurement of parity nonconservation (PNC) in the Cs atom we
have extracted the constant of the nuclear spin dependent electron-nucleon PNC
interaction, ; the anapole moment constant, ; the strength of the PNC proton-nucleus potential, ; the -meson-nucleon interaction constant,
; and the strength of the neutron-nucleus potential, .Comment: Uses RevTex, 12 pages. We have added an explanation of the effect of
finite nuclear siz
The price of rapid exit in venture capital-backed IPOs
This paper proposes an explanation for two empirical puzzles surrounding initial public offerings (IPOs). Firstly, it is well documented that IPO underpricing increases during “hot issue” periods. Secondly, venture capital (VC) backed IPOs are less underpriced than non-venture capital backed IPOs during normal periods of activity, but the reverse is true during hot issue periods: VC backed IPOs are more underpriced than non-VC backed ones. This paper shows that when IPOs are driven by the initial investor’s desire to exit from an existing investment in order to finance a new venture, both the value of the new venture and the value of the existing firm to be sold in the IPO drive the investor’s choice of price and fraction of shares sold in the IPO. When this is the case, the availability of attractive new ventures increases equilibrium underpricing, which is what we observe during hot issue periods. Moreover, I show that underpricing is affected by the severity of the moral hazard problem between an investor and the firm’s manager. In the presence of a moral hazard problem the degree of equilibrium underpricing is more sensitive to changes in the value of the new venture. This can explain why venture capitalists, who often finance firms with more severe moral hazard problems, underprice IPOs less in normal periods, but underprice more strongly during hot issue periods. Further empirical implications relating the fraction of shares sold and the degree of underpricing are presented
Electroweak Radiative Corrections to Parity-Violating Electroexcitation of the
We analyze the degree to which parity-violating (PV) electroexcitation of the
resonance may be used to extract the weak neutral axial vector
transition form factors. We find that the axial vector electroweak radiative
corrections are large and theoretically uncertain, thereby modifying the
nominal interpretation of the PV asymmetry in terms of the weak neutral form
factors. We also show that, in contrast to the situation for elastic electron
scattering, the axial PV asymmetry does not vanish at the photon
point as a consequence of a new term entering the radiative corrections. We
argue that an experimental determination of these radiative corrections would
be of interest for hadron structure theory, possibly shedding light on the
violation of Hara's theorem in weak, radiative hyperon decays.Comment: RevTex, 76 page
Constraints on the Nucleon Strange Form Factors at Q^2 ~ 0.1 GeV^2
We report the most precise measurement to date of a parity-violating
asymmetry in elastic electron-proton scattering. The measurement was carried
out with a beam energy of 3.03 GeV and a scattering angle =6
degrees, with the result A_PV = -1.14 +/- 0.24 (stat) +/- 0.06 (syst) parts per
million. From this we extract, at Q^2 = 0.099 GeV^2, the strange form factor
combination G_E^s + 0.080 G_M^s = 0.030 +/- 0.025 (stat) +/- 0.006 (syst) +/-
0.012 (FF) where the first two errors are experimental and the last error is
due to the uncertainty in the neutron electromagnetic form factor. This result
significantly improves current knowledge of G_E^s and G_M^s at Q^2 ~0.1 GeV^2.
A consistent picture emerges when several measurements at about the same Q^2
value are combined: G_E^s is consistent with zero while G_M^s prefers positive
values though G_E^s=G_M^s=0 is compatible with the data at 95% C.L.Comment: minor wording changes for clarity, updated references, dropped one
figure to improve focu
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
Studies of Neutron Stars at Optical/IR Wavelengths
In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs
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