1,273 research outputs found
Closed-form expressions for particle relative velocities induced by turbulence
In this note we present complete, closed-form expressions for random relative
velocities between colliding particles of arbitrary size in nebula turbulence.
These results are exact for very small particles (those with stopping times
much shorter than the large eddy overturn time) and are also surprisingly
accurate in complete generality (that is, also apply for particles with
stopping times comparable to, or much longer than, the large eddy overturn
time). We note that some previous studies may have adopted previous simple
expressions, which we find to be in error regarding the size dependence in the
large particle regime.Comment: 8 pages, accepted as Research Note by A&
Limb imaging of the Venus O2 visible nightglow with the Venus Monitoring Camera
We investigated the Venus O2 visible nightglow with imagery from the Venus
Monitoring Camera on Venus Express. Drawing from data collected between April
2007 and January 2011, we study the global distribution of this emission,
discovered in the late 70s by the Venera 9 and 10 missions. The inferred
limb-viewing intensities are on the order of 150 kiloRayleighs at the lower
latitudes and seem to drop somewhat towards the poles. The emission is
generally stable, although there are episodes when the intensities rise up to
500 kR. We compare a set of Venus Monitoring Camera observations with
coincident measurements of the O2 nightglow at 1.27 {\mu}m made with the
Visible and Infrared Thermal Imaging Spectrometer, also on Venus Express. From
the evidence gathered in this and past works, we suggest a direct correlation
between the instantaneous emissions from the two O2 nightglow systems. Possible
implications regarding the uncertain origin of the atomic oxygen green line at
557.7 nm are noted.Comment: 7 pages, 3 figure
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
Helicopter tail rotor thrust and main rotor wake coupling in crosswind flight
The tail rotor of a helicopter with a single main rotor configuration can experience a significant reduction in thrust when the aircraft operates in crosswind flight. Brownâs vorticity transport model has been used to simulate a main rotor and tail rotor system translating at a sideslip angle that causes the tail rotor to interact with the main rotor tip vortices as they propagate downstream at the lateral extremities of the wake. The tail rotor is shown to exhibit a distinct directionally dependent mode during which tail rotors that are configured so that the blades travel forward at the top of the disk develop less thrust than tail rotors with the reverse sense of rotation. The range of flight speeds over which this mode exists is shown to vary considerably with the vertical location of the tail rotor. At low flight speeds, the directionally dependent mode occurs because the tail rotor is immersed within not only the downwash from the main rotor but also the rotational flow associated with clusters of largely disorganized vorticity within the main rotor wake. At higher flight speeds, however, the tail rotor is immersed within a coherent supervortex that strongly influences the velocity field surrounding the tail rotor
Electronic susceptibilities in systems with anisotropic Fermi surfaces
The low temperature dependence of the spin and charge susceptibilities of an
anisotropic electron system in two dimensions is analyzed. It is shown that the
presence of inflection points at the Fermi surface leads, generically, to a dependence, and a more singular behavior, ,
is also possible. Applications to quasi two-dimensional materials are
discussed.Comment: 8 pages, 5 figures, revtex 4 styl
Limb observations of the Martian atmosphere with Mars Expressâ High Resolution Stereo Camera
Introduction:
Good knowledge about the aerosol distribution and
compositions is essential for the understanding of
thermodynamic processes in the Martian atmosphere,
which in turn is important for the understanding of the
Martian climate and the altitude of the upper boundary
of the atmosphere. The last point is of special interest
for spacecraft aerobreaking manoeuvres.
The Martian atmosphere often shows horizontal
layers of haze up to altitudes of about 80 km. These
have been described and analysed e.g. by Jaquin et al.,
1986, usingViking Orbiter images and by Montmessin
et al., 2006, who used SPICAM stellar occultation
data. Both showed seasonal and latitudinal changes
in the vertical structure of the aerosol distribution and
composition. Apart from SPICAM, the High Resolution
Stereo Camera (HRSC) is also on board ESAâs
robotic spacecraft Mars Express. HRSC was build and
is operated by the German Aerospace Center (Neukum
et al. 2004; Jaumann et al. 2007). Mars Express is orbiting
Mars in an elliptical orbit, with HRSC scanning
the surface of Mars, primarily for geological research.
In addition to that, HRSC has been used to sample the
planetary limb.
We examine the HRSC planetary limb data and
analyse the seasonal and latitudinal variations of the
maximum altitude of the haze layer and of the occurrence
of high altitude detached hazes. We make
some comparisons with earlier work. In contrast to the
SPICAM instrument, HRSC observes the atmosphere
during daytime, which makes it possible to compare
night and daytime observations.
The HRSC Limb Data:
HRSC is a push broom scanner with nine line sensors
pointing in different directions to facilitate stereoscopic
imaging. Four of the sensors have colour filters
at 440 nm, 530 nm, 750 nm and 970 nm, respectively.
The five other sensors all have filters centred at 650
nm. These panchromatic filters have a much wider
bandpass than the four colour filters.
The surface observations which are HRSCâs main purpose,
are usually take while the spacecraft is nadirtracking
near pericentre. Limb observations, however,
are mostly made with a pointing of the spacecraft being
inertially fixxed in celestial space. This leaves
only a small time window to make observations of
the limb during descent or ascent. Therefore, usually
only a few of the nine sensors can be used for the
limb observation. Due to the motion of the spacecraft,
the individual image lines are taken at different
geographical locations and altitudes. The position of
each image pixel above the limb has to be calculated
from the spacecraft positioning information (Scholten,
pers. comm.). The typical difference in altitude between
two neighbouring pixels is between a couple
of dozen metres and 150 m. HRSC has been observing
the limb occasionally throughout the mission since
2004. So far the northern hemisphere and especially
the north polar region, were particularly well covered
(Figure 1 and 2).
In Figure 2, we give an overview of the available
data, sorted by season (LS) and latitude. The
channel in which the observations have been made is colourcoded.
Most observations were made with the
panchromatic channels. There are also many observations
with the blue and green sensors and only a
few were made in the red and infra red channels. We
find the best data coverage in northern spring in the
northern most latitudes. For obvious reasons, we do
not have any data during polar nights. For most of
our actual analysis we sample the five central pixels of
the sensor lines. This allows for minimal horizontal
averaging.
Analysis:
As an example, Fig. 3 shows images and profiles for
the blue, nadir, and green channels from orbit 6104.
Al three images show a continuously bright limb haze
until an altitude of about 20 km. At higher altitudes
the limb haze becomes darker and stratified consistent
with the limb profiles described by Jacquin et al., 1986.
As Mars Express progresses along its orbit, the limb
observations are made at different locations above the
surface. The locations of the three profiles in Fig. 3
are still in close proximity of each other, in fact they
overlap, but none the less they show different vertical
aerosol distributions. Beginning above the North Polar
cap and going southward, we observe less reflectivity
above 20 km and more reflectivity below 20 km, hinting
at different compositions or amounts of aerosols.
It is not possible to obtain and compare profiles at
the same location and at the same time with different
sensors, but still, averages of profiles over place and
season can provide us with information about typical
atmospheric conditions. In Fig. 4 we show spectra
from the average profiles at three different latitudinal
bands between 70ïżœNâ90ïżœN, 30ïżœSâ30ïżœN, and 90ïżœSâ
70ïżœS, on the left, centre, and right, respectively. The
different symbols and colours represent the different
altitudes at which the spectra were sampled. The size
of the symbol increases with the number of averaged
profiles. There are very few observations above the
South Polar region (compare Fig. 1). In the North (and
South) Polar region there is almost no signal above
30 km altitude, while around the equator the limb haze
remains bright until altitudes of about 60 km. At the
poles, the spectrum at 10 km is reddish. At higher
altitudes the spectrum gets whiter, indicating smaller
particles or higher ice content. At the low latitudes
the spectra are reddish up to 40 km. At 60 km we see
a more or less white spectrum. Figure 5 shows the
maximum altitude of the aerosols as seen by HRSC,
depending on season. During aphelion (LS ïżœ 70ïżœ)
the maximum altitude of the aerosols that are visible
with HRSC is around 40 km. During perihelion (LS ïżœ
250ïżœ) the maximum altitude is around 70 km.
Discussion:
Figure 1 and 2 show that there are plenty of visual
and near infra red HRSC observations of the Martian
limb available. These show aerosol distributions that
change with season and latitude (Fig. 3 and 4). The
plots in Fig. 4 show the spectra of the average limb
profiles at several altitudes for three latitudinal bands.
Two important distinctions can be made between the
equatorial and the polar regions. First, the altitude at
which aerosol occur is higher in the equatorial region
and second, the composition of the aerosols at different
altitudes is different. While the spectrum is white
around 20 km altitude above the north pole, it is red at
the low latitudes.
The seasonal variations of maximum altitude of
the aerosols is in good agreement with Jaquin et al.
(1986) and with Montmessin et al. (2006). The similarity
between Montmessinâs results and ours is likely
to be due to the large annual variation of atmospheric
dust load compared to the diurnal cycle. A much
closer look at the data, is forseen to analyse the daily
variation of aerosols in the Martian atmosphere. The
CO2 and waterice aerosols are more likely to change
their vertical distribution (above the planetary boundary
layer) between day and night than the mineral
(dust) aerosols. Spectral information would help to
discriminate between these components. HRSC can
not provide it, because the observation for the different
filters take place at different locations and times
(see Fig. 3). An alternative is to fit aerosol models to
the inverted profiles. Currently, we are preparing this
work.
Mars Expressâ HRSC limb data present a valuable
opportunity to analyse Mars daytime atmospheric dust
at a high vertical resolution. This work gives a short
overview of the available data and analyses some seasonal
and latitudinal properties
Kinematics of electrons near a Van Hove singularity
A two dimensional electronic system, where the Fermi surface is close to a
Van Hove singularity, shows a variety of weak coupling instabilities, and it is
a convenient model to study the interplay between antiferromagnetism and
anisotropic superconductivity. We present a detailed analysis of the kinematics
of the electron scattering in this model. The similitudes, and differences,
between a standard Renormalization Group approach and previous work based on
parquet summations of log divergences are analyzed, with emphasis on the
underlying physical processes. General properties of the phase diagram are
discussed.Comment: 5 pages, 3 postscript figure
Superconducting and pseudogap phases from scaling near a Van Hove singularity
We study the quantum corrections to the Fermi energy of a two-dimensional
electron system, showing that it is attracted towards the Van Hove singularity
for a certain range of doping levels. The scaling of the Fermi level allows to
cure the infrared singularities left in the BCS channel after renormalization
of the leading logarithm near the divergent density of states. A phase of
d-wave superconductivity arises beyond the point of optimal doping
corresponding to the peak of the superconducting instability. For lower doping
levels, the condensation of particle-hole pairs due to the nesting of the
saddle points takes over, leading to the opening of a gap for quasiparticles in
the neighborhood of the singular points.Comment: 4 pages, 6 Postscript figures, the physical discussion of the results
has been clarifie
Diffuse Gamma-Ray Emission from Starburst Galaxies and M31
We present a search for high energy gamma-ray emission from 9 nearby
starburst galaxies and M31 with the EGRET instrument aboard CGRO. Though the
diffuse gamma-ray emission from starburst galaxies was suspected to be
detectable, we find no emission from NGC 253, M82 nor from the average of all 9
galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the
averaged survey observations is 1.8 x 10-8 ph cm-2 s-1. From a model of the
expected radio and gamma-ray emission, we find that the magnetic field in the
nuclei of these galaxies is > 25 micro Gauss, and the ratio of proton and
electron densities is < 400. The EGRET limits indicate that the rate of massive
star formation in the survey galaxies is only about an order of magnitude
higher than in the Milky Way. The upper limit to the gamma-ray flux above 100
MeV for M31 is 1.6 x 10-8 ph cm-2 s-1. At the distance of M31, the Milky Way
flux would be over twice this value, indicating higher gamma-ray emissivities
in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher
than in M31, our null detection of M31 supports the theory of the supernova
origin of cosmic rays in galaxies.Comment: 17 pages, plus 1 Postscript figure, AAS Latex macros v4.0, accepted
for publication in ApJ Main Journa
Deformation of the Fermi surface in the extended Hubbard model
The deformation of the Fermi surface induced by Coulomb interactions is
investigated in the t-t'-Hubbard model. The interplay of the local U and
extended V interactions is analyzed. It is found that exchange interactions V
enhance small anisotropies producing deformations of the Fermi surface which
break the point group symmetry of the square lattice at the Van Hove filling.
This Pomeranchuck instability competes with ferromagnetism and is suppressed at
a critical value of U(V). The interaction V renormalizes the t' parameter to
smaller values what favours nesting. It also induces changes on the topology of
the Fermi surface which can go from hole to electron-like what may explain
recent ARPES experiments.Comment: 5 pages, 4 ps figure
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