5,900 research outputs found
Measurements of ocean wave spectra and modulation transfer function with the airborne two frequency scatterometer
The directional spectrum and the microwave modulation transfer function of ocean waves can be measured with the airborne two frequency scatterometer technique. Similar to tower based observations, the aircraft measurements of the Modulation Transfer Function (MTF) show that it is strongly affected by both wind speed and sea state. Also detected are small differences in the magnitudes of the MTF between downwind and upwind radar look directions, and variations with ocean wavenumber. The MTF inferred from the two frequency radar is larger than that measured using single frequency, wave orbital velocity techniques such as tower based radars or ROWS measurements from low altitude aircraft. Possible reasons for this are discussed. The ability to measure the ocean directional spectrum with the two frequency scatterometer, with supporting MTF data, is demonstrated
Advanced 2-frequency ocean sensing radar using high resolution antenna beams
The opportunity to use a large space antenna system for remote sensing applications permits the creation of an advanced ocean sensing radar that combines the abilities of previously developed techniques. The 15 meter antenna will permit much higher angular and spatial resolution at the surface that will lead to techniques of observing ocean wave heights and the directional spectrum that had not previously been feasible from space. At the same time, sensors to measure ocean surface winds can be in operation and the data from both can be combined to increase the accuracy of each individual sensor. The existing capabilities and sensor techniques with typical data characteristics for the individual measurement of sea surface quantities are outlined
Two-step phase changes in cubic relaxor ferroelectrics
The field-driven conversion between the zero-field-cooled frozen relaxor
state and a ferroelectric state of several cubic relaxors is found to occur in
at least two distinct steps, after a period of creep, as a function of time.
The relaxation of this state back to a relaxor state under warming in zero
field also occurs via two or more sharp steps, in contrast to a one-step
relaxation of the ferroelectric state formed by field-cooling. An intermediate
state can be trapped by interrupting the polarization. Giant pyroelectric noise
appears in some of the non-equilibrium regimes. It is suggested that two
coupled types of order, one ferroelectric and the other glassy, may be required
to account for these data.Comment: 27 pages with 8 figures to appear in Phys. Rev.
Keck Pencil-Beam Survey for Faint Kuiper Belt Objects
We present the results of a pencil-beam survey of the Kuiper Belt using the
Keck 10-m telescope. A single 0.01 square degree field is imaged 29 times for a
total integration time of 4.8 hr. Combining exposures in software allows the
detection of Kuiper Belt Objects (KBOs) having visual magnitude V < 27.9. Two
new KBOs are discovered. One object having V = 25.5 lies at a probable
heliocentric distance d = 33 AU. The second object at V = 27.2 is located at d
= 44 AU. Both KBOs have diameters of about 50 km, assuming comet-like albedos
of 4%.
Data from all surveys are pooled to construct the luminosity function from
red magnitude R = 20 to 27. The cumulative number of objects per square degree,
N (< R), is fitted to a power law of the form log_(10) N = 0.52 (R - 23.5).
Differences between power laws reported in the literature are due mainly to
which survey data are incorporated, and not to the method of fitting. The
luminosity function is consistent with a power-law size distribution for
objects having diameters s = 50 to 500 km; dn ~ s^(-q) ds, where the
differential size index q = 3.6 +/- 0.1. The distribution is such that the
smallest objects possess most of the surface area, but the largest bodies
contain the bulk of the mass. Though our inferred size index nearly matches
that derived by Dohnanyi (1969), it is unknown whether catastrophic collisions
are responsible for shaping the size distribution. Implications of the absence
of detections of classical KBOs beyond 50 AU are discussed.Comment: Accepted to AJ. Final proof-edited version: references added,
discussion of G98 revised in sections 4.3 and 5.
Aging in the Relaxor Ferroelectric PMN/PT
The relaxor ferroelectric
(PbMnNbO)(PbTiO), ,
(PMN/PT(90/10)) is found to exhibit several regimes of complicated aging
behavior. Just below the susceptibility peak there is a regime exhibiting
rejuvenation but little memory. At lower temperature, there is a regime with
mainly cumulative aging, expected for simple domain-growth. At still lower
temperature, there is a regime with both rejuvenation and memory, reminiscent
of spin glasses. PMN/PT (88/12) is also found to exhibit some of these aging
regimes. This qualitative aging behavior is reminiscent of that seen in
reentrant ferromagnets, which exhibit a crossover from a domain-growth
ferromagnetic regime into a reentrant spin glass regime at lower temperatures.
These striking parallels suggest a picture of competition in PMN/PT (90/10)
between ferroelectric correlations formed in the domain-growth regime with
glassy correlations formed in the spin glass regime. PMN/PT (90/10) is also
found to exhibit frequency-aging time scaling of the time-dependent part of the
out-of-phase susceptibility for temperatures 260 K and below. The stability of
aging effects to thermal cycles and field perturbations is also reported.Comment: 8 pages RevTeX4, 11 figures; submitted to Phys. Rev.
Are there compact heavy four-quark bound states?
We present an exact method to study four-quark systems based on the
hyperspherical harmonics formalism. We apply it to several physical systems of
interest containing two heavy and two light quarks using different quark-quark
potentials. Our conclusions mark the boundaries for the possible existence of
compact, non-molecular, four-quark bound states. While states
may be stable in nature, the stability of states would imply
the existence of quark correlations not taken into account by simple quark
dynamical modelsComment: 10 pages, 1 figure. Accepted for publication in Phys. Rev.
Barkhausen Noise in a Relaxor Ferroelectric
Barkhausen noise, including both periodic and aperiodic components, is found
in and near the relaxor regime of a familiar relaxor ferroelectric,
PbMgNbO, driven by a periodic electric field. The
temperature dependences of both the amplitude and spectral form show that the
size of the coherent dipole moment changes shrink as the relaxor regime is
entered, contrary to expectations based on some simple models.Comment: 4 pages RevTeX4, 5 figures; submitted to Phys Rev Let
Identification of a lineage of multipotent hematopoietic progenitors
All multipotent hematopoietic progenitors in C57BL-Thy-1.1 bone marrow are divided among three subpopulations of Thy-1.1^(lo) Sca-1^+ Lin^(-/lo) c-kit^+ cells: long-term reconstituting Mac-1^-CD4^-c-kit^+ cells and transiently reconstituting Mac-1^(lo)CD4^-or Mac-1^(lo) CD4^(lo) cells. This study shows that the same populations, with similar functional activities, exist in mice whose hematopoietic systems were reconstituted by hematopoietic stem cells after lethal irradiation. We demonstrate that these populations form a lineage of multipotent progenitors from long-term self-renewing stem cells to the most mature multipotent progenitor population. In reconstituted mice, Mac-1- CD4^-c-kit^+ cells gave rise to Mac-1^(lo)CD4^- cells, which gave rise to Mac-1^(lo)CD4^(lo) cells. Mac-1^- CD4^-c-kit^+ cells had long-term self-renewal potential, with each cell being capable of giving rise to more than 10^4 functionally similar Mac-1^-CD4^-c-kit^+ cells. At least half of Mac-1^(lo)CD4^- cells had transient self-renewal potential, detected in the spleen 7 days after reconstitution. Mac-1^(lo)CD4^(lo) cells did not have detectable self-renewal potential. The identification of a lineage of multipotent progenitors provides an important tool for identifying genes that regulate self-renewal and lineage commitment
A Titan exploration study: Science, technology, and mission planning options, volume 2
For abstract, see Vol.
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