876 research outputs found
Predicting the Sun's Polar Magnetic Fields with a Surface Flux Transport Model
The Sun's polar magnetic fields are directly related to solar cycle
variability. The strength of the polar fields at the start (minimum) of a cycle
determine the subsequent amplitude of that cycle. In addition, the polar field
reversals at cycle maximum alter the propagation of galactic cosmic rays
throughout the heliosphere in fundamental ways. We describe a surface magnetic
flux transport model that advects the magnetic flux emerging in active regions
(sunspots) using detailed observations of the near-surface flows that transport
the magnetic elements. These flows include the axisymmetric differential
rotation and meridional flow and the non-axisymmetric cellular convective flows
(supergranules) all of which vary in time in the model as indicated by direct
observations. We use this model with data assimilated from full-disk
magnetograms to produce full surface maps of the Sun's magnetic field at
15-minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the
rise to maximum of cycle 24). We tested the predictability of this model using
these maps as initial conditions, but with daily sunspot area data used to give
the sources of new magnetic flux. We find that the strength of the polar fields
at cycle minimum and the polar field reversals at cycle maximum can be reliably
predicted up to three years in advance. We include a prediction for the cycle
24 polar field reversal.Comment: 12 pages, 9 figures, ApJ accepte
Measurements of the Sun's High Latitude Meridional Circulation
The meridional circulation at high latitudes is crucial to the build-up and
reversal of the Sun's polar magnetic fields. Here we characterize the
axisymmetric flows by applying a magnetic feature cross-correlation procedure
to high resolution magnetograms obtained by the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We focus on
Carrington Rotations 2096-2107 (April 2010 to March 2011) - the overlap
interval between HMI and the Michelson Doppler Investigation (MDI). HMI
magnetograms averaged over 720 seconds are first mapped into heliographic
coordinates. Strips from these maps are then cross-correlated to determine the
distances in latitude and longitude that the magnetic element pattern has
moved, thus providing meridional flow and differential rotation velocities for
each rotation of the Sun. Flow velocities were averaged for the overlap
interval and compared to results obtained from MDI data. This comparison
indicates that these HMI images are rotated counter-clockwise by 0.075 degrees
with respect to the Sun's rotation axis. The profiles indicate that HMI data
can be used to reliably measure these axisymmetric flow velocities to at least
within 5 degrees of the poles. Unlike the noisier MDI measurements, no evidence
of a meridional flow counter-cell is seen in either hemisphere with the HMI
measurements: poleward flow continues all the way to the poles. Slight
North-South asymmetries are observed in the meridional flow. These asymmetries
should contribute to the observed asymmetries in the polar fields and the
timing of their reversals.Comment: 6 pages, 3 color figures, accepted for publication in The
Astrophysical Journal Lette
Experiments on transient natural convection in inclined enclosures using calibrated multichannel electronic interferometry and digital particle image velocimetry
Simultaneous quantitative measurements are made of both the temperature and velocity fields for two-dimensional transient natural convection in an inclined rectangular enclosure. The fluid in the enclosure is initially stationary and isothermal. The transient boundary conditions are initiated by instantaneously heating and cooling two opposing walls. All other walls are insulated and nonconducting. The evolution of the flow to steady state is determined for a Prandtl number of 6.38, a Rayleigh number of 1.5 10\sp5, and an aspect ratio of 1.0, at angles of inclination of /4, /2, and 3/4. The temperature is measured using calibrated multichannel electronic interferometry, and the velocity is measured using digital particle image velocimetry. This allows accurate measurement of the temperature and velocity fields throughout the enclosure and visualization of the boundary layers, intrusion layers, and recirculation zones. The angle of inclination is shown to have a significant effect on the flow and heat transfer. However, the expected transient oscillations are not evident. This is probably due to differences in the boundary conditions at startup between the current experiments and the numerical simulations
An automated monitoring system for the production and measurement of metal fatigue
Includes bibliography.New equipment for producing and following the fatigue process, in metals, has been developed. The specimens, which must be in the shape of tuning forks, are resonated at high amplitudes. This is achieved by attaching small, powerful, samarium-cobalt magnets to the ends of the tynes, enabling them to be driven efficiently by a "U"-core electromagnet. A small, piezoceramic strain gauge provides a method of picking up the vibrations. To maintain resonance, the signal is used in a positive feedback loop, which incorporates an analogue multiplier to provide AGC. This also keeps the amplitude constant at any desired level, throughout the duration of an experiment
Carboniferous and Permian magmatism in Scotland
Extensional tectonics to the north of the Variscan Front during the Early Carboniferous generated fault-controlled basins across the British Isles, with accompanying basaltic magmatism. In Scotland Dinantian magmatism was dominantly mildly alkaline-transitional in composition. Tournaisian activity was followed by widespread Visean eruptions largely concentrated within the Scottish Midland Valley where the lava successions, dominantly of basaltic-hawaiitic composition, attained thicknesses of up to 1000 m. Changing stress fields in the late Visean coincided with a change in the nature of the igneous activity; subsequently, wholly basic magmatism persisted into the Silesian. As sedimentary basin fills increased, sill intrusion tended to dominate over lava extrusion. In the Late Carboniferous (Stephanian) a major melting episode, producing large volumes of tholeiitic magma, gave rise to a major dyke swarm and sills across northern England and Scotland. Alkali basaltic magmatism persisted into the Permian, possibly until as late as 250 Ma in Orkney. Geochemical data suggest that the Carboniferous-Permian magmas were dominantly of asthenospheric origin, derived from variable degrees of partial melting of a heterogeneous mantle source; varying degrees of interaction with the lithosphere are indicated. Peridotite, pyroxenite and granulite-facies basic meta-igneous rocks entrained as xenoliths within the most primitive magmas provide evidence for metasomatism of the lithospheric mantle and high-pressure crystal fractionation
Reproducing the Photospheric Magnetic Field Evolution During the Rise of Cycle 24 with Flux Transport by Supergranules
We simulate the transport of magnetic flux in the Sun s photosphere by an evolving pattern of cellular horizontal flows (supergranules). Characteristics of the simulated flow pattern match observed characteristics including the velocity power spectrum, cell lifetimes, and cell pattern motion in longitude and latitude. Simulations using an average, and north-south symmetric, meridional motion of the cellular pattern produce polar magnetic fields that are too weak in the North and too strong in the South. Simulations using cellular patterns with meridional motions that evolve with the observed changes in strength and north-south asymmetry will be analyzed to see if they reproduce the polar field evolution observed during the rise of Cycle 24
Taxonomy of North American fish Eimeriidae
Taxonomic descriptions, line drawings, and references are given for the 30 named and 5 unnamed species of North American fish Eimeriidae. In addition, a key was developed based on available morphologic data to distinguish between similar species. Taxa are divided into two genera: Eimeria (27 species) which are tetr&sporocystic with dizoic, nonbivalved sporocysts, and Goussia (3 species) which are tetrasporocystic with dizoic, bivalved sporocysts that lack Stleda bodies and have sporocyst walls composed of two longitudinal valves. (PDF file contains 24 pages.
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