6,274 research outputs found
Detection of the transitional layer between laminar and turbulent flow areas on a wing surface
A system is disclosed for detecting the laminar to turbulent boundary layer transition on a surface while simultaneously taking pressure measurements. The system uses an accelerometer for producing electrical signals proportional to the noise levels along the surface and a transducer for producing electrical signals proportional to pressure along the surface. The signals generated by the accelerometer and transducer are sent to a data reduction system for interpretation and storage
Tests show that aluminum welds are improved by bead removal
Tests with 2218-T87 aluminum alloy plate indicate improvements in strength, ductility, fatigue properties, and burst pressure result when one or both of the top and bottom weld beads are removed. There is, however, a drop in yield strength. The consistency of test data is considerably improved by weld bead removal
The stability of coronal and prominence magnetic fields
The significance of the localized instabilities is not yet fully understood. The nonlinear coupling of these modes may give rise to an explosive instability, with the modes coupling to longer wavelengths. On the other hand, if the modes saturate early, than the main effect of the instability may be an enhancement of transport coefficients. Nonetheless, the equation provides a simple test for the stability of cylindrical magnetic fields
A numerical model of standard to blowout jets
We report on three-dimensional (3D) MHD simulations of the formation of jets produced during the emergence and eruption of solar magnetic fields. The interaction between an emerging and an ambient magnetic field in the solar atmosphere leads to (external) reconnection and the formation of "standard" jets with an inverse Y-shaped configuration. Eventually, low-atmosphere (internal) reconnection of sheared fieldlines in the emerging flux region produces an erupting magnetic flux rope and a reconnection jet underneath it. The erupting plasma blows out the ambient field and, moreover, it unwinds as it is ejected into the outer solar atmosphere. The fast emission of the cool material that erupts together with the hot outflows due to external/internal reconnection form a wider "blowout" jet. We show the transition from "standard" to "blowout" jets and report on their 3D structure. The physical plasma properties of the jets are consistent with observational studies.Peer reviewe
MHD mode conversion in a stratified atmosphere
Mode conversion in the region where the sound and Alfven speeds are equal is
a complex process, which has been studied both analytically and numerically,
and has been seen in observations. In order to further the understanding of
this process we set up a simple, one-dimensional model, and examine wave
propagation through this system using a combination of analytical and numerical
techniques. Simulations are carried out in a gravitationally stratified
atmosphere with a uniform, vertical magnetic field for both isothermal and
non-isothermal cases. For the non-isothermal case a temperature profile is
chosen to mimic the steep temperature gradient encountered at the transition
region. In all simulations, a slow wave is driven on the upper boundary, thus
propagating down from low-beta to high-beta plasma across the mode-conversion
region. In addition, a detailed analytical study is carried out where we
predict the amplitude and phase of the transmitted and converted components of
the incident wave as it passes through the mode-conversion region. A comparison
of these analytical predictions with the numerical results shows good
agreement, giving us confidence in both techniques. This knowledge may be used
to help determine wave types observed and give insight into which modes may be
involved in coronal heating.Comment: 7 pages, 5 figure
AMPR/SSMI data comparisons
The AMPR (Advanced Microwave Precipitation Radiometer) was flown for the first time with successful data collection over precipitation targets in Florida and off the Oregon coast. The AMPR met its expected performance levels with very low noise and relatively troublefree operation. Numerous rain cloud systems over land and ocean were overflown and the measurements at 10.7, 19.35, 37.1, and 85.5 GHz reveal a wide variety of microphysical conditions which exist within rain cloud systems. Although predicted by radiative transfer model calculations from cloud model simulations, this diversity was not observed before due to the poor spatial resolution of spaceborne microwave radiometers. Saturation of the 19.35 GHz rain emission signal was frequently observed in the oceanic rain systems, supporting the desirability of a 10 GHz channel on the TRMM (Tropical Rain Measuring Mission) microwave radiometer for sensitivity to the higher rain rates
The nucleotide sequence of a human immnnoglobulin C-gamma-1 gene
We report the nucleotide sequence of a gene encoding the constant region of a human immnnoglobulin γ1 heavy chain (Cγ1). A comparison of this sequence with those of the Cγ2 and Cγ4 genes reveals that these three human Cγ genes share considerable homology in both coding and noncoding regions. The nucleotide sequence differences indicate that these genes diverged from one another approximately 6–8 million years ago. An examination of hinge exons shows that these coding regions have evolved more rapidly than any other areas of the Cγ genes in terms of both base substitution and deletion–insertion events. Coding sequence diversity also is observed in areas of CH domains which border the hinge
MHD Mode Conversion around a 2D Magnetic Null Point
Mode conversion occurs when a wave passes through a region where the sound
and Alfven speeds are equal. At this point there is a resonance, which allows
some of the incident wave to be converted into a different mode. We study this
phenomenon in the vicinity of a two-dimensional, coronal null point. As a wave
approaches the null it passes from low- to high-beta plasma, allowing
conversion to take place. We simulate this numerically by sending in a slow
magnetoacoustic wave from the upper boundary; as this passes through the
conversion layer a fast wave can clearly be seen propagating ahead. Numerical
simulations combined with an analytical WKB investigation allow us to determine
and track both the incident and converted waves throughout the domain.Comment: 4 pages, 2 figure
Solar Magnetic Fields
This review provides an introduction to the generation and evolution of the
Sun's magnetic field, summarising both observational evidence and theoretical
models. The eleven year solar cycle, which is well known from a variety of
observed quantities, strongly supports the idea of a large-scale solar dynamo.
Current theoretical ideas on the location and mechanism of this dynamo are
presented.
The solar cycle influences the behaviour of the global coronal magnetic field
and it is the eruptions of this field that can impact on the Earth's
environment. These global coronal variations can be modelled to a surprising
degree of accuracy. Recent high resolution observations of the Sun's magnetic
field in quiet regions, away from sunspots, show that there is a continual
evolution of a small-scale magnetic field, presumably produced by small-scale
dynamo action in the solar interior.
Sunspots, a natural consequence of the large-scale dynamo, emerge, evolve and
disperse over a period of several days. Numerical simulations can help to
determine the physical processes governing the emergence of sunspots. We
discuss the interaction of these emerging fields with the pre-existing coronal
field, resulting in a variety of dynamic phenomena.Comment: 44 pages, 8 figures. To appear shortly in a special issue of Physics
of the Earth and Planetary Interior
Sunspot rotation. I. A consequence of flux emergence
Context. Solar eruptions and high flare activity often accompany the rapid
rotation of sunspots. The study of sunspot rotation and the mechanisms driving
this motion are therefore key to our understanding of how the solar atmosphere
attains the conditions necessary for large energy release.
Aims. We aim to demonstrate and investigate the rotation of sunspots in a 3D
numerical experiment of the emergence of a magnetic flux tube as it rises
through the solar interior and emerges into the atmosphere. Furthermore, we
seek to show that the sub-photospheric twist stored in the interior is injected
into the solar atmosphere by means of a definitive rotation of the sunspots.
Methods. A numerical experiment is performed to solve the 3D resistive
magnetohydrodynamic (MHD) equations using a Lagrangian-Remap code. We track the
emergence of a toroidal flux tube as it rises through the solar interior and
emerges into the atmosphere investigating various quantities related to both
the magnetic field and plasma.
Results. Through detailed analysis of the numerical experiment, we find clear
evidence that the photospheric footprints or sunspots of the flux tube undergo
a rotation. Significant vertical vortical motions are found to develop within
the two polarity sources after the field emerges. These rotational motions are
found to leave the interior portion of the field untwisted and twist up the
atmospheric portion of the field. This is shown by our analysis of the relative
magnetic helicity as a significant portion of the interior helicity is
transported to the atmosphere. In addition, there is a substantial transport of
magnetic energy to the atmosphere. Rotation angles are also calculated by
tracing selected fieldlines; the fieldlines threading through the sunspot are
found to rotate through angles of up to 353 degrees over the course of the
experiment
- …