2,607 research outputs found
A Theory on the Convective Origins of Active Longitudes on Solar-like Stars
Using a thin flux tube model in a rotating spherical shell of turbulent,
solar-like convective flows, we find that the distribution of emerging flux
tubes in our simulation is inhomogeneous in longitude, with properties similar
to those of active longitudes on the Sun and other solar-like stars. The
large-scale pattern of flux emergence our simulations produce exhibits
preferred longitudinal modes of low order, drift with respect to a fixed
reference system, and alignment across the Equator at low latitudes between 15
degrees. We suggest that these active-longitude-like emergence patterns are the
result of columnar, rotationally aligned giant cells present in our convection
simulation at low latitudes. If giant convecting cells exist in the bulk of the
solar convection zone, this phenomenon, along with differential rotation, could
in part provide an explanation for the behavior of active longitudes.Comment: This paper was accepted to The Astrophysical Journal on May 6, 201
Evolving, complex topography from combining centers of Gaussian curvature.
Liquid crystal elastomers and glasses can have significant shape change determined by their director patterns. Cones deformed from circular director patterns have nontrivial Gaussian curvature localized at tips, curved interfaces, and intersections of interfaces. We employ a generalized metric compatibility condition to characterize two families of interfaces between circular director patterns, hyperbolic and elliptical interfaces, and find that the deformed interfaces are geometrically compatible. We focus on hyperbolic interfaces to design complex topographies and nonisometric origami, including n-fold intersections, symmetric and irregular tilings. The large design space of threefold and fourfold tiling is utilized to quantitatively inverse design an array of pixels to display target images. Taken together, our findings provide comprehensive design principles for the design of actuators, displays, and soft robotics in liquid crystal elastomers and glasses
Terahertz electron-hole recollisions in GaAs/AlGaAs quantum wells: robustness to scattering by optical phonons and thermal fluctuations
Electron-hole recollisions are induced by resonantly injecting excitons with
a near-IR laser at frequency into quantum wells driven by a
~10 kV/cm field oscillating at THz. At K, up to
18 sidebands are observed at frequencies , with . Electrons and holes recollide with
total kinetic energies up to 57 meV, well above the meV
threshold for longitudinal optical (LO) phonon emission. Sidebands with order
up to persist up to room temperature. A simple model shows that LO
phonon scattering suppresses but does not eliminate sidebands associated with
kinetic energies above .Comment: 5 pages, 4 figure
Structural Design and Analysis of a Light-Weight Laminated Composite Heat Sink for Spaceflight PWBs
In order to reduce the overall weight in spaceborne electronic systems, a conventional metallic heat sink typically used for double-sided printed wiring boards was suggested to be replaced by light-weight and high-strength laminated composite materials. Through technology validation assurance (TVA) approach, it has been successfully demonstrated that using laminated composite heat sink can not only reduce the weight of the heat sink by nearly 50%, but also significantly lower the internal thermally-induced stresses that are largely responsible for potential delamination under cyclic temperature variations. With composite heat sink, both thermal and dynamic performance of the double-sided printed wiring board (PWB) exceeds that of its counterpart with metallic heat sink. Also included in this work is the original contribution to the understanding of creep behavior of the worst-case leadless chip carrier (LCC) surface mount solder joint. This was identified as the interconnection most susceptible to thermal fatigue damage in the PWB assembly
Comparing Simulations of Rising Flux Tubes Through the Solar Convection Zone with Observations of Solar Active Regions: Constraining the Dynamo Field Strength
We study how active-region-scale flux tubes rise buoyantly from the base of
the convection zone to near the solar surface by embedding a thin flux tube
model in a rotating spherical shell of solar-like turbulent convection. These
toroidal flux tubes that we simulate range in magnetic field strength from 15
kG to 100 kG at initial latitudes of 1 degree to 40 degrees in both
hemispheres. This article expands upon Weber, Fan, and Miesch (Astrophys. J.,
741, 11, 2011) (Article 1) with the inclusion of tubes with magnetic flux of
10^20 Mx and 10^21 Mx, and more simulations of the previously investigated case
of 10^22 Mx, sampling more convective flows than the previous article, greatly
improving statistics. Observed properties of active regions are compared to
properties of the simulated emerging flux tubes, including: the tilt of active
regions in accordance with Joy's Law as in Article 1, and in addition the
scatter of tilt angles about the Joy's Law trend, the most commonly occurring
tilt angle, the rotation rate of the emerging loops with respect to the
surrounding plasma, and the nature of the magnetic field at the flux tube apex.
We discuss how these diagnostic properties constrain the initial field strength
of the active region flux tubes at the bottom of the solar convection zone, and
suggest that flux tubes of initial magnetic field strengths of \geq 40 kG are
good candidates for the progenitors of large (10^21 Mx to 10^22 Mx) solar
active regions, which agrees with the results from Article 1 for flux tubes of
10^22 Mx. With the addition of more magnetic flux values and more simulations,
we find that for all magnetic field strengths, the emerging tubes show a
positive Joy's Law trend, and that this trend does not show a statistically
significant dependence on the magnetic flux.Comment: Accepted to Solar Physics Topical Issue: Solar Dynamics and Magnetism
from the Interior to the Atmospher
Automated inspection of solder joints for surface mount technology
Researchers at NASA/GSFC evaluated various automated inspection systems (AIS) technologies using test boards with known defects in surface mount solder joints. These boards were complex and included almost every type of surface mount device typical of critical assemblies used for space flight applications: X-ray radiography; X-ray laminography; Ultrasonic Imaging; Optical Imaging; Laser Imaging; and Infrared Inspection. Vendors, representative of the different technologies, inspected the test boards with their particular machine. The results of the evaluation showed limitations of AIS. Furthermore, none of the AIS technologies evaluated proved to meet all of the inspection criteria for use in high-reliability applications. It was found that certain inspection systems could supplement but not replace manual inspection for low-volume, high-reliability, surface mount solder joints
Spatially Resolved Stellar Populations of Eight GOODS-South AGN at z~1
We present a pilot study of the stellar populations of 8 AGN hosts at z~1 and
compare to (1) lower redshift samples and (2) a sample of nonactive galaxies of
similar redshift. We utilize K' images in the GOODS South field obtained with
the laser guide star adaptive optics (LGSAO) system at Keck Observatory. We
combine this K' data with B, V, i, and z imaging from the ACS on HST to give
multi-color photometry at a matched spatial resolution better than 100 mas in
all bands. The hosts harbor AGN as inferred from their high X-ray luminosities
(L_X > 10^42 ergs/s) or mid-IR colors. We find a correlation between the
presence of younger stellar populations and the strength of the AGN, as
measured with [OIII] line luminosity or X-ray (2-10 keV) luminosity. This
finding is consistent with similar studies at lower redshift. Of the three Type
II galaxies, two are disk galaxies and one is of irregular type, while in the
Type I sample there only one disk-like source and four sources with smooth,
elliptical/spheroidal morphologies. In addition, the mid-IR SEDs of the strong
Type II AGN indicate that they are excited to LIRG (Luminous InfraRed Galaxy)
status via galactic starbursting, while the strong Type I AGN are excited to
LIRG status via hot dust surrounding the central AGN. This supports the notion
that the obscured nature of Type II AGN at z~1 is connected with global
starbursting and that they may be extincted by kpc-scale dusty features that
are byproducts of this starbursting.Comment: 56 pages, 39 figures, accepted to A
The Rise of Active Region Flux Tubes in the Turbulent Solar Convective Envelope
We use a thin flux tube model in a rotating spherical shell of turbulent
convective flows to study how active region scale flux tubes rise buoyantly
from the bottom of the convection zone to near the solar surface. We
investigate toroidal flux tubes at the base of the convection zone with field
strengths ranging from 15 kG to 100 kG at initial latitudes ranging from 1
degree to 40 degrees with a total flux of 10^22 Mx. We find that the dynamic
evolution of the flux tube changes from convection dominated to magnetic
buoyancy dominated as the initial field strength increases from 15 kG to 100
kG. At 100 kG, the development of Omega-shaped rising loops is mainly
controlled by the growth of the magnetic buoyancy instability. However, at low
field strengths of 15 kG, the development of rising Omega-shaped loops is
largely controlled by convective flows, and properties of the emerging loops
are significantly changed compared to previous results in the absence of
convection. With convection, rise times are drastically reduced (from years to
a few months), loops are able to emerge at low latitudes, and tilt angles of
emerging loops are consistent with Joy's Law for initial field strengths of
greater than or equal to 40 kG. We also examine other asymmetries that develop
between the leading and following legs of the emerging loops. Taking all the
results together, we find that mid-range field strengths of approximately 40 -
50 kG produce emerging loops that best match the observed properties of solar
active regions.Comment: Accepted for publication in The Astrophysical Journa
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