931 research outputs found
Study of the Source Regions of Coronal Mass Ejections Using Yohkoh SXT Data
The scientific objective of the program was to better understand how CMEs (Coronal Mass Ejections) are initiated at the sun by examining structures on the disk which are related to the origins of CMEs. CMEs represent important disruptions of large-scale structures of closed magnetic fields in the corona, and result in significant disturbances of the interplanetary medium and near-Earth space. The program pertained to NASA's objectives of understanding the physics of solar activity and the structured and evolution of the corona, and the results are being applied to understanding CMEs currently being observed by SOHO near the sun and by WIND and Ulysses in the heliosphere. Three general areas of research were pursued in the program. One was to use Yohkoh soft X-ray telescope (SXT) images of eruptive events visible against the solar disk to examine the coronal structures and the boundaries of the large-scale magnetic fields considered to be involved in coronal mass ejections (CMEs). The second area involved a survey and study of SXT X-ray arcade events which exhibit dimming, or the possible depletion of coronal material above and possibly before onset of the bright long-duration event (LDE). Finally, we studied the SXT data during periods when white light CMEs were observed the HAO Mauna Loa K-coronameter and, conversely, we examined the white light data during periods when expanding X-ray loops were observed at the limb
Identification of Flexural Rigidity in a Kirchhoff Plates Model Using a Convex Objective and Continuous Newton Method
This work provides a detailed theoretical and numerical study of the inverse problem of identifying flexural rigidity in Kirchhoff plate models. From a mathematical standpoint, this inverse problem requires estimating a variable coefficient in a fourth-order boundary value problem.This inverse problem and related estimation problems associated with general plates and shellmodels have been investigated by numerous researchers through an optimization framework using the output least-squares (OLSs) formulation. OLS yields a nonconvex framework and hence it is suitable for investigating only the local behavior of the solution. In this work, we propose a new convex framework for the inverse problem of identifying a variable parameter in a fourth-order inverse problem. Existence results, optimality conditions, and discretization issues are discussed in detail. The discrete inverse problem is solved by using a continuous Newton method. Numerical results show the feasibility of the proposed framework
Coronal radiation belts
The magnetic field of the solar corona has a large-scale dipole character,
which maps into the bipolar field in the solar wind. Using standard
representations of the coronal field, we show that high-energy ions can be
trapped stably in these large-scale closed fields. The drift shells that
describe the conservation of the third adiabatic invariant may have complicated
geometries. Particles trapped in these zones would resemble the Van Allen Belts
and could have detectable consequences. We discuss potential sources of trapped
particles
PCV1 DOES ROUTE OF ADMINISTRATION FOR ESTROGEN HORMONE THERAPY IMPACT RISK OF VENOUS THROMBOEMBOLISM: ESTRADIOL TRANSDERMAL SYSTEM VERSUS ORAL ESTROGEN-ONLY HORMONE THERAPY
Variations of Heavy Ion Abundances Relative to Proton Abundances in Large Solar Energetic (E \u3e 10 MeV) Particle Events
The elemental composition of heavy ions (with atomic number Z \u3e 2) (hi-Z) in large gradual E \u3e 10 MeV nuc-1 SEP events has been extensively studied in the 2-15 MeV nuc-1 range to determine the acceleration processes and transport properties of SEPs. These studies invariably are based on abundances relative to those of a single element such as C or O and often neglect H and He, the elements of primary interest for space weather. The total radiation of an SEP event is determined not only by the H and He properties but also by those of hi-Z ions whose abundances and variations relative to H from one event to another are unknown. We report a study to determine those variations in a group of 15 large SEP events over the period 2000 to 2015. Five hi-Z ions (He, C, O, Mg, & Fe) were selected to determine variations of their fluences relative to those of H in the 13.5-50.7 MeV nuc-1 energy range for each SEP event. Our average hi-Z abundance ratios slightly exceed those reported by [1] at lower energies, with the Fe event abundances showing the largest standard deviation of an order of magnitude. The event abundances were weakly correlated with H fluences and strongly correlated with speeds Vcme of associated coronal mass ejections (CMEs). These correlations may be evidence of streaming limits in the shock regions of H in the largest events
Streaming flow by oscillating bubbles: Quantitative diagnostics via particle tracking velocimetry
Oscillating microbubbles can be used as microscopic agents. Using external acoustic fields they are able to set the surrounding fluid into motion, Erode surfaces and even to carry particles attached to their interfaces. Although the acoustic streaming flow that the bubble generates in its vicinity has been often observed, it has never been measured and quantitatively compared with the available theoretical models. The scarcity of quantitative data is partially due to the strong three-dimensional character of bubble-induced streaming flows, which demands advanced velocimetry techniques. In this work, we present quantitative measurements of the flow generated by single and pairs of acoustically excited sessile microbubbles using a three-dimensional particle tracking technique. Using this novel experimental approach we are able to obtain the bubble's resonant oscillating frequency, study the boundaries of the linear oscillation regime, give predictions on the flow strength and the shear in the surrounding surface and study the flow and the stability of a two-bubble system. Our results show that velocimetry techniques are a suitable tool to make diagnostics on the dynamics of acoustically excited microbubbles
A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster
We present the results of a high-resolution imaging survey for brown dwarf
binaries in the Pleiades open cluster. The observations were carried out with
the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample
consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their
orbital motion, but we did not resolve any new binary candidates in the
separation range between 5.4AU and 1700AU and masses in the range
0.035--0.065~Msun. Together with the results of our previous study (Martin et
al., 2003), we can derive a visual binary frequency of 13.3\%
for separations greater than 7~AU masses between 0.055--0.065~M_{\sun} and
mass ratios between 0.45--0.91.0. The other observed properties of
Pleiades brown dwarf binaries (distributions of separation and mass ratio)
appear to be similar to their older counterparts in the field.Comment: 29 pages, 7 figures, 6 tables, accepted for publication in Ap
Determining the Magnetic Field Orientation of Coronal Mass Ejections from Faraday Rotation
We describe a method to measure the magnetic field orientation of coronal
mass ejections (CMEs) using Faraday rotation (FR). Two basic FR profiles,
Gaussian-shaped with a single polarity or "N"-like with polarity reversals, are
produced by a radio source occulted by a moving flux rope depending on its
orientation. These curves are consistent with the Helios observations,
providing evidence for the flux-rope geometry of CMEs. Many background radio
sources can map CMEs in FR onto the sky. We demonstrate with a simple flux rope
that the magnetic field orientation and helicity of the flux rope can be
determined 2-3 days before it reaches Earth, which is of crucial importance for
space weather forecasting. An FR calculation based on global
magnetohydrodynamic (MHD) simulations of CMEs in a background heliosphere shows
that FR mapping can also resolve a CME geometry curved back to the Sun. We
discuss implementation of the method using data from the Mileura Widefield
Array (MWA).Comment: 22 pages with 9 figures, accepted for publication in Astrophys.
Evolution of plasma turbulence excited with particle beams
Particles ejected from the Sun that stream through the surrounding plasma of
the solar wind are causing instabilities. These generate wavemodes in a certain
frequency range especially within shock regions, where particles are
accelerated. The aim of this paper is to investigate of amplified Alfvenic
wavemodes in driven incompressible magnetohydrodynamic turbulence. Results of
different heliospheric scenarios from isotropic and anisotropic plasmas, as
well as turbulence near the critical balance are shown. The energy transport of
the amplified wavemode is governed by the mechanisms of diffusion, convection
and dissipation of energy in wavenumber space. The strength of these effects
varies with energy and wavenumber of the mode in question. Two-dimensional
energy spectra of spherical k-space integration that permit detailed insight
into the parallel and perpendicular development are presented. The evolution of
energy injected through driving shows a strong energy transfer to perpendicular
wavemodes. The main process at parallel wavemodes is the dissipation of energy
in wavenumber space. The generation of higher harmonics along the parallel
wavenumber axis is observed. We find evidence for a critical balance in our
simulations.Comment: Accepted for publication in A&
Contact Discontinuities in Models of Contact Binaries Undergoing Thermal Relaxation Oscillations
In this paper we pursue the suggestion by Shu, Lubow & Anderson (1979) and
Wang (1995) that contact discontinuity (DSC) may exist in the secondary in the
expansion TRO (thermal relaxation oscillation) state. It is demonstrated that
there is a mass exchange instability in some range of mass ratio for the two
components. We show that the assumption of {\it constant} volume of the
secondary should be relaxed in DSC model. For {\it all} mass ratio the
secondary alway satisfies the condition that no mass flow returns to the
primary through the inner Lagrangian point. The secondary will expand in order
to equilibrate the interaction between the common convective envelope and the
secondary. The contact discontinuity in contact binary undergoing thermal
relaxation does not violate the second law of thermodynamics. The maintaining
condition of contact discontinuity is derived in the time-dependent model. It
is desired to improve the TRO model with the advanced contact discontinuity
layer in future detailed calculations.Comment: 5 pages in emulateapj, 1 figur
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