36,932 research outputs found
Gravity field determination and error assessment techniques
Linear estimation theory, along with a new technique to compute relative data weights, was applied to the determination of the Earth's geopotential field and other geophysical model parameters using a combination of satellite ground-based tracking data, satellite altimetry data, and the surface gravimetry data. The relative data weights for the inhomogeneous data sets are estimated simultaneously with the gravity field and other geophysical and orbit parameters in a least squares approach to produce the University of Texas gravity field models. New techniques to perform calibration of the formal covariance matrix for the geopotential solution were developed to obtain a reliable gravity field error estimate. Different techniques, which include orbit residual analysis, surface gravity anomaly residual analysis, subset gravity solution comparisons and consider covariance analysis, were applied to investigate the reliability of the calibration
Non-damping oscillations at flaring loops
Context. QPPs are usually detected as spatial displacements of coronal loops
in imaging observations or as periodic shifts of line properties in
spectroscopic observations. They are often applied for remote diagnostics of
magnetic fields and plasma properties on the Sun. Aims. We combine imaging and
spectroscopic measurements of available space missions, and investigate the
properties of non-damping oscillations at flaring loops. Methods. We used the
IRIS to measure the spectrum over a narrow slit. The double-component Gaussian
fitting method was used to extract the line profile of Fe XXI 1354.08 A at "O
I" window. The quasi-periodicity of loop oscillations were identified in the
Fourier and wavelet spectra. Results. A periodicity at about 40 s is detected
in the line properties of Fe XXI, HXR emissions in GOES 1-8 A derivative, and
Fermi 26-50 keV. The Doppler velocity and line width oscillate in phase, while
a phase shift of about Pi/2 is detected between the Doppler velocity and peak
intensity. The amplitudes of Doppler velocity and line width oscillation are
about 2.2 km/s and 1.9 km/s, respectively, while peak intensity oscillate with
amplitude at about 3.6% of the background emission. Meanwhile, a quasi-period
of about 155 s is identified in the Doppler velocity and peak intensity of Fe
XXI, and AIA 131 A intensity. Conclusions. The oscillations at about 40 s are
not damped significantly during the observation, it might be linked to the
global kink modes of flaring loops. The periodicity at about 155 s is most
likely a signature of recurring downflows after chromospheric evaporation along
flaring loops. The magnetic field strengths of the flaring loops are estimated
to be about 120-170 G using the MHD seismology diagnostics, which are
consistent with the magnetic field modeling results using the flux rope
insertion method.Comment: 9 pages, 9 figures, 1 table, accepted by A&
The Influences of Outflow on the Dynamics of Inflow
Both numerical simulations and observations indicate that in an
advection-dominated accretion flow most of the accretion material supplied at
the outer boundary will not reach the inner boundary. Rather, they are lost via
outflow. Previously, the influence of outflow on the dynamics of inflow is
taken into account only by adopting a radius-dependent mass accretion rate
with . In this paper, based on a 1.5
dimensional description to the accretion flow, we investigate this problem in
more detail by considering the interchange of mass, radial and azimuthal
momentum, and the energy between the outflow and inflow. The physical
quantities of the outflow is parameterized based on our current understandings
to the properties of outflow mainly from numerical simulations of accretion
flows. Our results indicate that under reasonable assumptions to the properties
of outflow, the main influence of outflow has been properly included by
adopting .Comment: 16 pages, 5 figures. accepted for publication in Ap
An improved model for the Earth's gravity field
An improved model for the Earth's gravity field, TEG-1, was determined using data sets from fourteen satellites, spanning the inclination ranges from 15 to 115 deg, and global surface gravity anomaly data. The satellite measurements include laser ranging data, Doppler range-rate data, and satellite-to-ocean radar altimeter data measurements, which include the direct height measurement and the differenced measurements at ground track crossings (crossover measurements). Also determined was another gravity field model, TEG-1S, which included all the data sets in TEG-1 with the exception of direct altimeter data. The effort has included an intense scrutiny of the gravity field solution methodology. The estimated parameters included geopotential coefficients complete to degree and order 50 with selected higher order coefficients, ocean and solid Earth tide parameters, Doppler tracking station coordinates and the quasi-stationary sea surface topography. Extensive error analysis and calibration of the formal covariance matrix indicate that the gravity field model is a significant improvement over previous models and can be used for general applications in geodesy
Ground states of hard-core bosons in one dimensional periodic potentials
With Girardeau's Fermi-Bose mapping, we find the exact ground states of
hard-core bosons residing in a one dimensional periodic potential. The analysis
of these ground states shows that when the number of bosons is commensurate
with the number of wells in the periodic potential, the boson system is a
Mott insulator whose energy gap, however, is given by the single-particle band
gap of the periodic potential; when is not commensurate with , the
system is a metal (not a superfluid). In fact, we argue that there may be no
superfluid phase for any one-dimensional boson system in terms of Landau's
criterion of superfluidity. The Kronig-Penney potential is used to illustrate
our results.Comment: 6 pages, 6 figure
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