3,027 research outputs found
Medium and large-scale variations of dynamo-induced electric fields from AE ion drift measurements
Current models of the low latitude electric field are largely based on data from incoherent scatter radars. These observations are extended through the addition of the rather extensive high quality electric field measurements from the Ion Drift Meter (IDM) aboard the Atmosphere Explorer (AE) spacecraft. Some preliminary results obtained from the Unified Abstract files of satellite AE-E are presented. This satellite was active from the end of 1975 through June 1981 in various elliptical and circular orbits having an inclination near 20 deg. The resulting data can be examined for the variation of ion drift with latitude, longitude, season, solar cycle, altitude, and magnetic activity. The results presented deal primarily with latitudinal variations of the drift features. Diagrams of data are given and briefly interpreted. The preliminary results presented here indicate that IDM data from the AE and the more recent Dynamics Explorer B spacecraft should continue to disclose some interesting and previously unobserved dynamical features of the low latitude F region
Description and catalog of ionospheric F-region data, Jicamarca Radar Observatory, November 1966 - April 1969
Equatorial ionospheric F-region data reduced from the Jicamarca Radar Observatory (JRO) incoherent scatter observations for particular periods is described. It lists in catalog form the times of the observations made during those periods. These F-region data include the electron concentration and the electron and ion temperatures. The data were inferred from the incoherent scatter observations of JRO
Constraints on the distance to SGR 1806-20 from HI absorption
The giant flare detected from the magnetar SGR 1806-20 on 2004 December 27
had a fluence more than 100 times higher than the only two other SGR flares
ever recorded. Whereas the fluence is independent of distance, an estimate for
the luminosity of the burst depends on the source's distance, which has
previously been argued to be ~15 kpc. The burst produced a bright radio
afterglow, against which Cameron et al. (2005) have measured an HI absorption
spectrum. This has been used to propose a revised distance to SGR 1806-20 of
between 6.4 and 9.8 kpc. Here we analyze this absorption spectrum, and compare
it both to HI emission data from the Southern Galactic Plane Survey and to
archival 12-CO survey data. We confirm ~6 kpc, as a likely lower limit on the
distance to SGR 1806-20, but argue that it is difficult to place an upper limit
on the distance to SGR 1806-20 from the HI data currently available. The
previous value of ~15 kpc thus remains the best estimate of the distance to the
source.Comment: 3 pages, 1 embedded EPS figure. Added sentences to end of Abstract
and Conclusion, clarifying that most likely distance is 15 kpc. ApJ Letters,
in pres
Fitting Together the HI Absorption and Emission in the SGPS
In this paper we study 21-cm absorption spectra and the corresponding
emission spectra toward bright continuum sources in the test region (326deg< l
< 333 deg) of the Southern Galactic Plane Survey. This survey combines the high
resolution of the Australia Telescope Compact Array with the full brightness
temperature information of the Parkes single dish telescope. In particular, we
focus on the abundance and temperature of the cool atomic clouds in the inner
galaxy. The resulting mean opacity of the HI, , is measured as a
function of Galactic radius; it increases going in from the solar circle, to a
peak in the molecular ring of about four times its local value. This suggests
that the cool phase is more abundant there, and colder, than it is locally.
The distribution of cool phase temperatures is derived in three different
ways. The naive, ``spin temperature'' technique overestimates the cloud
temperatures, as expected. Using two alternative approaches we get good
agreement on a histogram of the cloud temperatures, T(cool), corrected for
blending with warm phase gas. The median temperature is about 65 K, but there
is a long tail reaching down to temperatures below 20 K. Clouds with
temperatures below 40 K are common, though not as common as warmer clouds (40
to 100 K).
Using these results we discuss two related quantities, the peak brightness
temperature seen in emission surveys, and the incidence of clouds seen in HI
self-absorption. Both phenomena match what would be expected based on our
measurements of and T(cool).Comment: 50 pages, 20 figure
The VLA Galactic Plane Survey
The VLA Galactic Plane Survey (VGPS) is a survey of HI and 21-cm continuum
emission in the Galactic plane between longitude 18 degrees 67 degr. with
latitude coverage from |b| < 1.3 degr. to |b| < 2.3 degr. The survey area was
observed with the Very Large Array (VLA) in 990 pointings. Short-spacing
information for the HI line emission was obtained by additional observations
with the Green Bank Telescope (GBT). HI spectral line images are presented with
a resolution of 1 arcmin x 1 arcmin x 1.56 km/s (FWHM) and rms noise of 2 K per
0.824 km/s channel. Continuum images made from channels without HI line
emission have 1 arcmin (FWHM) resolution. VGPS images are compared with images
from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane
Survey (SGPS). In general, the agreement between these surveys is impressive,
considering the differences in instrumentation and image processing techniques
used for each survey. The differences between VGPS and CGPS images are small, <
6 K (rms) in channels where the mean HI brightness temperature in the field
exceeds 80 K. A similar degree of consistency is found between the VGPS and
SGPS. The agreement we find between arcminute resolution surveys of the
Galactic plane is a crucial step towards combining these surveys into a single
uniform dataset which covers 90% of the Galactic disk: the International
Galactic Plane Survey (IGPS). The VGPS data will be made available on the World
Wide Web through the Canadian Astronomy Data Centre (CADC).Comment: Accepted for publication in The Astronomical Journal. 41 pages, 13
figures. For information on data release, colour images etc. see
http://www.ras.ucalgary.ca/VGP
The Effelsberg-Bonn HI Survey: Data reduction
Starting in winter 2008/2009 an L-band 7-Feed-Array receiver is used for a
21-cm line survey performed with the 100-m telescope, the Effelsberg-Bonn HI
survey (EBHIS). The EBHIS will cover the whole northern hemisphere for decl.>-5
deg comprising both the galactic and extragalactic sky out to a distance of
about 230 Mpc. Using state-of-the-art FPGA-based digital fast Fourier transform
spectrometers, superior in dynamic range and temporal resolution to
conventional correlators, allows us to apply sophisticated radio frequency
interference (RFI) mitigation schemes.
In this paper, the EBHIS data reduction package and first results are
presented. The reduction software consists of RFI detection schemes, flux and
gain-curve calibration, stray-radiation removal, baseline fitting, and finally
the gridding to produce data cubes. The whole software chain is successfully
tested using multi-feed data toward many smaller test fields (1--100 square
degrees) and recently applied for the first time to data of two large sky
areas, each covering about 2000 square degrees. The first large area is toward
the northern galactic pole and the second one toward the northern tip of the
Magellanic Leading Arm. Here, we demonstrate the data quality of EBHIS Milky
Way data and give a first impression on the first data release in 2011.Comment: 17 pages, 14 figures; to be published in ApJ
GSH23.0-0.7+117, a neutral hydrogen shell in the inner Galaxy
GSH23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in the
VLA Galactic Plane Survey (VGPS). Only the blueshifted side of the shell was
detected. The expansion velocity and systemic velocity were determined through
the systematic behavior of the HI emission with velocity. The center of the
shell is at (l,b,v)=(23.05,-0.77,+117 km/s). The angular radius of the shell is
6.8', or 15 pc at a distance of 7.8 kpc. The HI mass divided by the volume of
the half-shell implies an average density n_H = 11 +/- 4 cm^{-3} for the medium
in which the shell expanded. The estimated age of GSH23.0-0.7+117 is 1 Myr,
with an upper limit of 2 Myr. The modest expansion energy of 2 * 10^{48} erg
can be provided by the stellar wind of a single O4 to O8 star over the age of
the shell. The 3 sigma upper limit to the 1.4 GHz continuum flux density
(S_{1.4} < 248 mJy) is used to derive an upper limit to the Lyman continuum
luminosity generated inside the shell. This upper limit implies a maximum of
one O9 star (O8 to O9.5 taking into account the error in the distance) inside
the HI shell, unless most of the incident ionizing flux leaks through the HI
shell. To allow this, the shell should be fragmented on scales smaller than the
beam (2.3 pc). If the stellar wind bubble is not adiabatic, or the bubble has
burst (as suggested by the HI channel maps), agreement between the energy and
ionization requirements is even less likely. The limit set by the non-detection
in the continuum provides a significant challenge for the interpretation of
GSH23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicable
to other Galactic HI shells that have not been detected in the continuum.Comment: 18 pages, 6 figures. Figures 1 and 4 separately in GIF format.
Accepted for publication in Astrophysical Journa
Band-structure topologies of graphene: spin-orbit coupling effects from first principles
The electronic band structure of graphene in the presence of spin-orbit
coupling and transverse electric field is investigated from first principles
using the linearized augmented plane-wave method. The spin-orbit coupling opens
a gap at the -point of the magnitude of 24 eV (0.28 K). This
intrinsic splitting comes 96% from the usually neglected and higher
orbitals. The electric field induces an additional (extrinsic)
Bychkov-Rashba-type splitting of 10 eV (0.11 K) per V/nm, coming from the
- mixing. A 'mini-ripple' configuration with every other atom is
shifted out of the sheet by less than 1% differs little from the intrinsic
case.Comment: 4 pages, 4 figure
Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers
Two distinct types of magnetoresistance oscillations are observed in two
electronic Fabry-Perot interferometers of different sizes in the integer
quantum Hall regime. Measuring these oscillations as a function of magnetic
field and gate voltages, we observe three signatures that distinguish the two
types. The oscillations observed in a 2.0 square micron device are understood
to arise from the Coulomb blockade mechanism, and those observed in an 18
square micron device from the Aharonov-Bohm mechanism. This work clarifies,
provides ways to distinguish, and demonstrates control over, these distinct
physical origins of resistance oscillations seen in electronic Fabry-Perot
interferometers.Comment: related papers at http://marcuslab.harvard.ed
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