1,228 research outputs found
The magnetic RayleighâTaylor instability in solar prominences
This is the author accepted manuscript. The final version is available from Springer via the DOI in this record.The magnetic RayleighâTaylor instability is a fundamental instability
of many astrophysical systems and recent observations are consistent
with this instability developing in solar prominences. Prominences are cool,
dense clouds of plasma that form in the solar corona that display a wide range
of dynamics of a multitude of spatial and temporal scales, and two different
phenomena that have been discovered to occur in prominences can be understood
as resulting from the RayleighâTaylor instability. The first is that of
plumes that rise through quiescent prominences from low density bubbles that
form below them. The second is that of a prominence eruption that fragments
as the material falls back to the solar surface. To identify these events as the
magnetic RayleighâTaylor instability, a wide range of theoretical work, both
numerical and analytical, has been performed, though alternative explanations
do exist. For both of these sets of observations, determining that they are created
by the magnetic RayleighâTaylor instability has meant that the linear
instability conditions and nonlinear dynamics can be used to make estimates
of the magnetic field strength. There are strong connections between these
phenomena and those in a number of other astro, space and plasma systems,
making these observations very important for our understanding of the role of
the RayleighâTaylor instability in magnetised systems
Quiescent Prominence Dynamics Observed with the Hinode Solar Optical Telescope. II. Prominence Bubble Boundary Layer Characteristics and the Onset of a Coupled KelvinâHelmholtz RayleighâTaylor Instability
This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this record.We analyze solar quiescent prominence bubble characteristics and instability dynamics using Hinode/Solar
Optical Telescope (SOT) data. We measure bubble expansion rate, prominence downflows,
and the profile of the boundary layer brightness and thickness as a function of time. The largest
bubble analyzed rises into the prominence with a speed of about 1.3 km sâ1 until it is destabilized
by a localized shear flow on the boundary. Boundary layer thickness grows gradually as prominence
downflows deposit plasma onto the bubble with characteristic speeds of 20 â 35 km sâ1
. Lateral
downflows initiate from the thickened boundary layer with characteristic speeds of 25 â 50 km sâ1
,
âdrainingâ the layer of plasma. Strong shear flow across one bubble boundary leads to an apparent
coupled Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) instability. We measure shear flow speeds above
the bubble of 10 km sâ1 and infer interior bubble flow speeds on the order of 100 km sâ1
. Comparing
the measured growth rate of the instability to analytic expressions, we infer a magnetic flux density
across the bubble boundary of ⌠10â3 T (10 gauss) at an angle of ⌠70âŠ
to the prominence plane.
The results are consistent with the hypothesis that prominence bubbles are caused by magnetic flux
that emerges below a prominence, setting up the conditions for RT, or combined KH-RT, instability
flows that transport flux, helicity, and hot plasma upward into the overlying coronal magnetic flux
ropeTEB was supported by NASA contracts NNM07AA01C (Solar-B FPP), NNG04EA00C (SDO/AIA) while at the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL), and by The National Weather Service (NWS) Office of Science and Technology Integration (OSTI) while at the National Oceanic and Atmospheric Administration (NOAA). A.H. was supported by his STFC Ernest Rutherford Fellowship grant number ST/L00397X/2. W.L. was supported by NASA HGI grant NNX15AR15G and NASA contract NNG09FA40C (IRIS) at LMSAL
Differences between Doppler velocities of ions and neutral atoms in a solar prominence
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.Context. In astrophysical systems with partially ionized plasma the motion of ions is governed by the magnetic field while the neutral
particles can only feel the magnetic fieldâs Lorentz force indirectly through collisions with ions. The drift in the velocity between
ionized and neutral species plays a key role in modifying important physical processes like magnetic reconnection, damping of
magnetohydrodynamic waves, transport of angular momentum in plasma through the magnetic field, and heating.
Aims. This paper investigates the differences between Doppler velocities of calcium ions and neutral hydrogen in a solar prominence
to look for velocity differences between the neutral and ionized species.
Methods. We simultaneously observed spectra of a prominence over an active region in H I 397 nm, H I 434 nm, Ca II 397 nm, and
Ca II 854 nm using a high dispersion spectrograph of the Domeless Solar Telescope at Hida observatory, and compared the Doppler
velocities, derived from the shift of the peak of the spectral lines presumably emitted from optically-thin plasma.
Results. There are instances when the difference in velocities between neutral atoms and ions is significant, e.g. 1433 events (⌠3 %
of sets of compared profiles) with a difference in velocity between neutral hydrogen atoms and calcium ions greater than 3Ï of the
measurement error. However, we also found significant differences between the Doppler velocities of two spectral lines emitted from
the same species, and the probability density functions of velocity difference between the same species is not significantly different
from those between neutral atoms and ions.
Conclusions. We interpreted the difference of Doppler velocities as a result of motions of different components in the prominence
along the line of sight, rather than the decoupling of neutral atoms from plasma.This work was supported by a Grant-in-Aid for Scientific
Research (No. 22244013, P.I. K. Ichimoto; No. 15K17609, P.I. T. Anan; No.
16H01177, P.I. T. Anan) from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. A.H. is supported by his STFC Emest Rutherford
Fellowship grant number ST/L00397X/2
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Incentive compensation vs SOX: evidence from corporate acquisition decisions
In this paper, we use the introduction of the Sarbanes-Oxley Act in 2002 to assess the impact of executive option and stock grants on corporate acquisition decisions. Amongst its many innovations, the Sarbanes-Oxley Act (SOX) has limited the value and effect of equity-related compensation. We find strong evidence of a shift in the factors driving acquisitions post-SOX. Specifically, while bid premiums have fell irrespectively of the type of acquirer, highly incentivised managers have become more risk-averse after the passage of the Act. Investors also appear to have recognised the effect of a change in equity-related pay. Both market response to acquisition announcements and post-acquisition performance have been improved after the introduction of SOX but these cannot be attributed to firms that grant high levels of incentive compensation to their managers. Our results are robust to a number of explanatory factors and confounding events in the post-SOX period
Kelvin-Helmholtz induced mixing in multi-fluid partially-ionised plasmas
This is the final version. Available on open access from the Royal Society via the DOI in this recordData accessibility: This article has no additional data.Turbulence is a fundamental process that drives
mixing and energy redistribution across a wide
range of astrophysical systems. For warm (T â 104 K)
plasma, the material is partially ionized, consisting
of both ionized and neutral species. The interactions
between ionized and neutral species are thought to
play a key role in heating (or cooling) of partially
ionized plasmas. Here, mixing is studied in a two fluid partially ionized plasma undergoing the shear driven KelvinâHelmholtz instability to evaluate the
thermal processes within the mixing layer. Two dimensional numerical simulations are performed
using the open-source (PIP) code that solves for a two fluid plasma consisting of a charge-neutral plasma
and multiple excited states of neutral hydrogen. Both
collisional and radiative ionization and recombination
are included. In the mixing layer, a complex array of
ionization and recombination processes occur as the
cooler layer joins the hotter layer, and vice versa. In
localized areas of the mixing layer, the temperature
exceeds the initial temperatures of either layer with
heating dominated by collisional recombinations
over turbulent dissipation. The mixing layer is in
approximate ionization-recombination equilibrium,
however the obtained equilibrium is different to
the SahaâBoltzmann local thermal equilibrium. The
dynamic mixing processes may be important in
determining the ionization states, and with that
intensities of spectral lines, of observed mixing layers.
This article is part of the theme issue âPartially
ionized plasma of the solar atmosphere: recent
advances and future pathwaysâScience and Technology Facilities Council (STFC
Partially ionized plasma of the solar atmosphere: recent advances and future pathways
This is the final version. Available from the Royal Society via the DOI in this record.âŻData accessibility. This article has no additional dat
Multi-centre parallel arm randomised controlled trial to assess the effectiveness and cost-effectiveness of a group-based cognitive behavioural approach to managing fatigue in people with multiple sclerosis
Abstract (provisional)
Background
Fatigue is one of the most commonly reported and debilitating symptoms of multiple sclerosis (MS); approximately two-thirds of people with MS consider it to be one of their three most troubling symptoms. It may limit or prevent participation in everyday activities, work, leisure, and social pursuits, reduce psychological well-being and is one of the key precipitants of early retirement. Energy effectiveness approaches have been shown to be effective in reducing MS-fatigue, increasing self-efficacy and improving quality of life. Cognitive behavioural approaches have been found to be effective for managing fatigue in other conditions, such as chronic fatigue syndrome, and more recently, in MS. The aim of this pragmatic trial is to evaluate the clinical and cost-effectiveness of a recently developed group-based fatigue management intervention (that blends cognitive behavioural and energy effectiveness approaches) compared with current local practice.
Methods
This is a multi-centre parallel arm block-randomised controlled trial (RCT) of a six session group-based fatigue management intervention, delivered by health professionals, compared with current local practice. 180 consenting adults with a confirmed diagnosis of MS and significant fatigue levels, recruited via secondary/primary care or newsletters/websites, will be randomised to receive the fatigue management intervention or current local practice. An economic evaluation will be undertaken alongside the trial. Primary outcomes are fatigue severity, self-efficacy and disease-specific quality of life. Secondary outcomes include fatigue impact, general quality of life, mood, activity patterns, and cost-effectiveness. Outcomes in those receiving the fatigue management intervention will be measured 1 week prior to, and 1, 4, and 12 months after the intervention (and at equivalent times in those receiving current local practice). A qualitative component will examine what aspects of the fatigue management intervention participants found helpful/unhelpful and barriers to change.
Discussion
This trial is the fourth stage of a research programme that has followed the Medical Research Council guidance for developing and evaluating complex interventions. What makes the intervention unique is that it blends cognitive behavioural and energy effectiveness approaches. A potential strength of the intervention is that it could be integrated into existing service delivery models as it has been designed to be delivered by staff already working with people with MS. Service users will be involved throughout this research. Trial registration: Current Controlled Trials ISRCTN7651747
An upper limit to the masses of stars
There is no accepted upper mass limit for stars. Such a basic quantity
escapes both theory, because of incomplete understanding of star formation, and
observation, because of incompleteness in surveying the Galaxy. The Arches
cluster is ideal for such a test, being massive enough to expect stars at least
as massive as 400 solar masses, and young enough for its most massive members
to still be visible. It is old enough to be free of its natal molecular cloud,
and close enough, and at a well-established distance, for us to discern its
individual stars. Here I report an absence of stars with initial masses greater
than 130 M_Sun in the Arches cluster, where the typical mass function predicts
18. I conclude that this indicates a firm limit of 150 M_Sun for stars as the
probability that the observations are consistent with no limit is 10^-8.Comment: To appear in Nature, March 10, 2005, Vol. 34, No. 7030, 192 (ST ScI
Eprint #1645). More files can be found at http://www.stsci.edu/~fige
The Formulated Microbicide RC-101 Was Safe and Antivirally Active Following Intravaginal Application in Pigtailed Macaques
Background: RC-101 is a congener of the antiretroviral peptide retrocyclin, which we and others have reported is active against clinical HIV-1 isolates from all major clades, does not hemagglutinate, and is non-toxic and non-inflammatory in cervicovaginal cell culture. Herein, film-formulated RC-101 was assessed for its antiviral activity in vitro, safety in vivo, retention in the cervix and vagina, and ability to remain active against HIV-1 and SHIV after intravaginal application in macaques. Methodology/Principal Findings: RC-101 was formulated as a quick-dissolving film (2000 Όg/film), retained complete activity in vitro as compared to unformulated peptide, and was applied intravaginally in six pigtailed macaques daily for four days. At one and four days following the final application, the presence of RC-101 was assessed in peripheral blood, cervicovaginal lavage, cytobrushed cervicovaginal cells, and biopsied cervical and vaginal tissues by quantitative western blots. One day following the last film application, cervical biopsies from RC-101-exposed and placebo-controlled macaques were collected and were subjected to challenge with RT-SHIV in an ex vivo organ culture model. RC-101 peptide was detected primarily in the cytobrush and biopsied cervical and vaginal tissues, with little to no peptide detected in lavage samples, suggesting that the peptide was associated with the cervicovaginal epithelia. RC-101 remained in the tissues and cytobrush samples up to four days post-application, yet was not detected in any sera or plasma samples. RC-101, extracted from cytobrushes obtained one day post-application, remained active against HIV-1 BaL. Importantly, cervical biopsies from RC-101-treated animals reduced RT-SHIV replication in ex vivo organ culture as compared to placebo-treated animals. Conclusions/Significance:Formulated RC-101 was stable in vivo and was retained in the mucosa. The presence of antivirally active RC-101 after five days in vivo suggests that RC-101 would be an important molecule to develop further as a topical microbicide to prevent HIV-1 transmission. © 2010 Cole et al
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