1,624 research outputs found
DC Electric Fields and Associated Plasma Drifts Observed with the C/NOFS Satellite
Initial DC electric field observations and associated plasma drifts are presented from the Vector Electric Field Investigation (VEFI) on the Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite. We present statistical averages of the vector fields for the first year of operations that include both the zonal and radial components of the resulting E x B plasma flows at low latitudes. Magnetic field data from the VEFI science magnetometer are used to compute the plasma flows. The DC electric field detector reveals zonal and radial electric fields that undergo strong diurnal variations, typically displaying eastward and outward-directed fields during the day and westward and downward-directed fields at night. There is considerable variation in the large scale DC electric field data, in both the daytime and nighttime cases, with enhanced structures typically observed at night. In general, the measured zonal DC electric field amplitudes include excursions that extend within the 0.4 - 2 m V/m range, corresponding to E x B drifts of the order of 30-150 m/s. The average vertical or radial electric fields may exceed the zonal fields in amplitude by a factor of 1.5 to 2. Although the data compare well, in a general sense, with previous satellite observations and statistical patterns of vertical ion drifts, the E x B drifts we report from C/NOFS rarely show a pronounced pre-reversal enhancement after sunset. We attribute this to a combination of extreme solar minimum conditions and the fact that the C/NOFS orbit of 401 by 867 km carries the probes essentially above the lower altitude regions where the wind-driven dynamo might be expected to create enhanced upwards drifts in the early evening. Evidence for wavenumber 4 tidal effects and other longitudinal signatures have been detected and will be presented. We also discuss off-equatorial electric fields and their relation to the ambient plasma density
Warp signatures of the Galactic disk as seen in mid infrared from Midcourse Space Experiment
The gross features in the distribution of stars as well as warm (T >~ 100 K)
interstellar dust in the Galactic disk have been investigated using the recent
mid infrared survey by Midcourse Space Experiment (MSX) at 8, 12, 14 & 21
micron bands. An attempt has been made to determine the location of the
Galactic mid-plane at various longitudes, using two approaches : (i) fitting
exponential functions to the latitude profiles and (ii) statistical
indicators.The former method is successful for the inner Galaxy (-90 < l < 90),
and quantifies characteristic angular scales along latitude, which have been
translated to linear scale heights (z_h) and radial length scales (R_l) using
geometric description of the Galactic disk. The distribution of warm dust in
the Galactic disk is found to be characterised by R_l < 6 kpc and 60 < z_h <~
100 pc, in agreement with other studies. The location of the Galactic mid-plane
as a function of longitude, for stars as well as warm dust, has been searched
for signatures of warp-like feature in their distribution, by fitting sinusoid
with phase and amplitude as parameters. In every case, the warp signature has
been detected. An identical analysis of the DIRBE/COBE data in all its ten
bands covering the entire infrared spectrum (1.25-240 micron), also leads to
detection of warp signatures with very similar phase as found from the MSX
data. Our results have been compared with those from other studies.Comment: To be published in 'Astronomy and Astrophysics' (12 pages including 9
figures & 4 tables
Initial Results of DC Electric Fields, Associated Plasma Drifts, Magnetic Fields, and Plasma Waves Observed on the C/NOFS Satellite
Initial results are presented from the Vector Electric Field Investigation (VEFI) on the Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite, a mission designed to understand, model, and forecast the presence of equatorial ionospheric irregularities. The VEFI instrument includes a vector DC electric field detector, a fixed-bias Langmuir probe operating in the ion saturation regime, a flux gate magnetometer, an optical lightning detector, and associated electronics including a burst memory. Compared to data obtained during more active solar conditions, the ambient DC electric fields and their associated E x B drifts are variable and somewhat weak, typically < 1 mV/m. Although average drift directions show similarities to those previously reported, eastward/outward during day and westward/downward at night, this pattern varies significantly with longitude and is not always present. Daytime vertical drifts near the magnetic equator are largest after sunrise, with smaller average velocities after noon. Little or no pre-reversal enhancement in the vertical drift near sunset is observed, attributable to the solar minimum conditions creating a much reduced neutral dynamo at the satellite altitude. The nighttime ionosphere is characterized by larger amplitude, structured electric fields, even where the plasma density appears nearly quiescent. Data from successive orbits reveal that the vertical drifts and plasma density are both clearly organized with longitude. The spread-F density depletions and corresponding electric fields that have been detected thus far have displayed a preponderance to appear between midnight and dawn. Associated with the narrow plasma depletions that are detected are broad spectra of electric field and plasma density irregularities for which a full vector set of measurements is available for detailed study. Finally, the data set includes a wide range of ELF/VLF/HF oscillations corresponding to a variety of plasma waves, in particular banded ELF hiss, whistlers, and lower hybrid wave turbulence triggered by lightning-induced sferics. The VEFI data represents a new set of measurements that are germane to numerous fundamental aspects of the electrodynamics and irregularities inherent to the Earth's low latitude ionosphere
Gas flow and dark matter in the inner parts of early-type barred galaxies
This paper presents the dynamical simulations run in the potential derived
from the light distribution of 5 late-type barred spiral galaxies. The aim is
to determine whether the mass distribution together with the hydrodynamical
simulations can reproduce the observed line-of-sight velocity curves and the
gas morphology in the inner regions of the sample barred galaxies. The light
distribution is obtained from the -band and the -band combined together.
The M/L is determined using population synthesis models. The observations and
the methodology of the mass distribution modelling are presented in a companion
paper. The SPH models using the stellar mass models obtained directly from the
-band light distributions give a good representation of the gas distribution
and dynamics of the modelled galaxies, supporting the maximum disk assumption.
This result indicates that the gravitational field in the inner region is
mostly provided by the stellar luminous component. When 40% of the total mass
is transferred to an axisymmetric dark halo, the modelled kinematics clearly
depart from the observed kinematics, whereas the departures are negligible for
dark mass halos of 5% and 20% of the total mass. This result sets a lower limit
for the contribution of the luminous component of about 80%, which is in
agreement with the maximum disk definition of the stellar mass contribution to
the rotation curve (about 85%10).Comment: 28 pages, 30 figures. Accepted for publication in A&A on 17/05/2004.
High resolution figures on publicatio
Persistent Longitudinal Variations of Plasma Density and DC Electric Fields in the Low Latitude Ionosphere Observed with Probes on the C/NOFS Satellite
Continuous measurements using in situ probes on consecutive orbits of the C/N0FS satellite reveal that the plasma density is persistently organized by longitude, in both day and night conditions and at all locations within the satellite orbit, defined by its perigee and apogee of 401 km and 867 km, respectively, and its inclination of 13 degrees. Typical variations are a factor of 2 or 3 compared to mean values. Furthermore, simultaneous observations of DC electric fields and their associated E x B drifts in the low latitude ionosphere also reveal that their amplitudes are also strongly organized by longitude in a similar fashion. The drift variations with longitude are particularly pronounced in the meridional component perpendicular to the magnetic field although they are also present in the zonal component as well. The longitudes of the peak meridional drift and density values are significantly out of phase with respect to each other. Time constants for the plasma accumulation at higher altitudes with respect to the vertical drift velocity must be taken into account in order to properly interpret the detailed comparisons of the phase relationship of the plasma density and plasma velocity variations. Although for a given period corresponding to that of several days, typically one longitude region dominates the structuring of the plasma density and plasma drift data, there is also evidence for variations organized about multiple longitudes at the same time. Statistical averages will be shown that suggest a tidal "wave 4" structuring is present in both the plasma drift and plasma density data. We interpret the apparent association of the modulation of the E x B drifts with longitude as well as that of the ambient plasma density as a manifestation of tidal forces at work in the low latitude upper atmosphere. The observations demonstrate how the high duty cycle of the C/NOFS observations and its unique orbit expose fundamental processes at work in the low latitude, inner regions of geospace
Study of muons near shower cores at sea level using the E594 neutrino detector
The E594 neutrino detector has been used to study the lateral distribution of muons of energy 3 GeV near shower cores. The detector consists of a 340 ton fine grain calorimeter with 400,000 cells of flash chamber and dimensions of 3.7 m x 20 m x 3.7 m (height). The average density in the calorimeter is 1.4 gm/sq cm, and the average Z is 21. The detector was triggered by four 0.6 sq m scintillators placed immediately on the top of the calorimeter. The trigger required at least two of these four counters. The accompanying extensive air showers (EAS) was sampled by 14 scintillation counters located up to 15 m from the calorimeter. Several off line cuts have been applied to the data. Demanding five particles in at least two of the trigger detectors, a total of 20 particles in all of them together, and an arrival angle for the shower 450 deg reduced the data sample to 11053 events. Of these in 4869 cases, a computer algorithm found at least three muons in the calorimeter
What repeat expansion disorders can teach us about the Central Dogma
Pathogenic repeat sequences underlie several human disorders, including amyotrophic lateral sclerosis, Huntington's disease, and myotonic dystrophy. Here, we speak to several researchers about how repeat sequences have been implicated in affecting all aspects of the Central Dogma of molecular biology through their effects on DNA, RNA, and protein
Structured DC Electric Fields With and Without Associated Plasma Density Gradients Observed with the C/NOFS Satellite
DC electric field observations and associated plasma drifts gathered with the Vector Electric Field Investigation on the Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite typically reveal considerable variation at large scales (approximately 100's of km), in both daytime and nighttime cases, with enhanced structures usually confined to the nightside. Although such electric field structures are typically associated with plasma density depletions and structures, as observed by the Planar Langmuir Probe on C/NOFS, what is surprising is the number of cases in which large amplitude, structured DC electric fields are observed without a significant plasma density counterpart structure, including their appearance at times when the ambient plasma density appears relatively quiescent. We investigate the relationship of such structured DC electric fields and the ambient plasma density in the C/NOFS satellite measurements observed thus far, taking into account both plasma density depletions and enhancements. We investigate the mapping of the electric fields along magnetic field lines from distant altitudes and latitudes to locations where the density structures, which presumably formed the original seat of the electric fields, are no longer discernible in the observations. In some cases, the electric field structures and spectral characteristics appear to mimic those associated with equatorial spread-F processes, providing important clues to their origins. We examine altitude, seasonal, and longitudinal effects in an effort to establish the origin of such structured DC electric fields observed both with, and without, associated plasma density gradient
The composition of cosmic rays near the Bend (10 to the 15th power eV) from a study of muons in air showers at sea level
The distribution of muons near shower cores was studied at sea level at Fermilab using the E594 neutrino detector to sample the muon with E testing 3 GeV. These data are compared with detailed Monte Carlo simulations to derive conclusions about the composition of cosmic rays near the bend in the all particle spectrum. Monte Carlo simulations generating extensive air showers (EAS) with primary energy in excess of 50 TeV are described. Each shower record contains details of the electron lateral distribution and the muon and hadron lateral distributions as a function of energy, at the observation level of 100g/cm. The number of detected electrons and muons in each case was determined by a Poisson fluctuation of the number incident. The resultant predicted distribution of muons, electrons, the rate events are compared to those observed. Preliminary results on the rate favor a heavy primary dominated cosmic ray spectrum in energy range 50 to 1000 TeV
Meson Structure Functions in Valon Model
Parton distributions in a {\it{valon}} in the next-to-leading order is used
to determine the patron distributions in pion and kaon. The validity of the
valon model is tested and shown that the partonic content of the valon is
universal and independent of the valon type. We have evaluated the valon
distribution in pion and kaon, and in particular it is shown that the results
are in good agreement with the experimental data on pion structure in a wide
range of Comment: 13 pages with 7 figures included, The manuscript is revised, figures
are added and some errors are corrected. Accepted for publication in Physical
Review
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