122 research outputs found
An overview of flux braiding experiments
Parker has hypothesised that, in a perfectly ideal environment, complex
photospheric motions acting on a continuous magnetic field will result in the
formation of tangential discontinuities corresponding to singular currents. We
review direct numerical simulations of the problem and find the evidence points
to a tendency for thin but finite thickness current layers to form, with
thickness exponentially decreasing in time. Given a finite resistivity these
layers will eventually become important and cause the dynamical process of
energy release. Accordingly, a body of work focusses on evolution under
continual boundary driving. The coronal volume evolves into a highly dynamic
but statistically steady state where quantities have a temporally and spatially
intermittent nature and where the Poynting flux and dissipation are decoupled
on short timescales. Although magnetic braiding is found to be a promising
coronal heating mechanism much work remains to determine its true viability.
Some suggestions for future study are offered.Comment: 11 figures, 23 pages. To be published in Philosophical Transactions A
(2015
Three-Dimensional Magnetic Reconnection
The importance of magnetic reconnection as an energy release mechanism in
many solar, stellar, magnetospheric and astrophysical phenomena has long been
recognised. Reconnection is the only mechanism by which magnetic fields can
globally restructure, enabling them to access a lower energy state. Over the
past decade, there have been some major advances in our understanding of
three-dimensional reconnection. In particular, the key characteristics of 3D
magnetohydrodynamic (MHD) reconnection have been determined. For instance, 3D
reconnection (i) occurs with or without nulls, (ii) occurs continuously and
continually throughout a diffusion region and (iii) is driven by counter
rotating flows.
Furthermore, analysis of resistive 3D MHD magnetic experiments have revealed
some intriguing effects relating to where and how reconnection occurs. To
illustrate these new features, a series of constant-resistivity experiments,
involving the interaction of two opposite-polarity magnetic sources in an
overlying field, are considered. Such a simple interaction represents a typical
building block of the Sun's magnetic atmosphere. By following the evolution of
the magnetic topology, we are able to explain where, how and at what rate the
reconnection occurs. Remarkably there can be up to five energy release sites at
anyone time (compared to one in the potential case) and the duration of the
interaction increases (more than doubles) as the resistivity decreases (by a
factor of 16). The decreased resistivity also leads to a higher peak ohmic
dissipation and more energy being released in total, as a result of a greater
injection of Poynting flux.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail
Magnetic reconnection is one of the most important processes in
astrophysical, space and laboratory plasmas. Identifying the structure around
the point at which the magnetic field lines break and subsequently reform,
known as the magnetic null point, is crucial to improving our understanding
reconnection. But owing to the inherently three-dimensional nature of this
process, magnetic nulls are only detectable through measurements obtained
simultaneously from at least four points in space. Using data collected by the
four spacecraft of the Cluster constellation as they traversed a diffusion
region in the Earth's magnetotail on 15 September, 2001, we report here the
first in situ evidence for the structure of an isolated magnetic null. The
results indicate that it has a positive-spiral structure whose spatial extent
is of the same order as the local ion inertial length scale, suggesting that
the Hall effect could play an important role in 3D reconnection dynamics.Comment: 14 pages, 4 figure
Football fans in training: the development and optimization of an intervention delivered through professional sports clubs to help men lose weight, become more active and adopt healthier eating habits
<p>Background: The prevalence of obesity in men is rising, but they are less likely than women to engage in existing weight management programmes. The potential of professional sports club settings to engage men in health promotion activities is being increasingly recognised. This paper describes the development and optimization of the Football Fans in Training (FFIT) programme, which aims to help overweight men (many of them football supporters) lose weight through becoming more active and adopting healthier eating habits.</p>
<p>Methods: The MRC Framework for the design and evaluation of complex interventions was used to guide programme development in two phases. In Phase 1, a multidisciplinary working group developed the pilot programme (p-FFIT) and used a scoping review to summarize previous research and identify the target population. Phase 2 involved a process evaluation of p-FFIT in 11 Scottish Premier League (SPL) clubs. Participant and coach feedback, focus group discussions and interviews explored the utility/acceptability of programme components and suggestions for changes. Programme session observations identified examples of good practice and problems/issues with delivery. Together, these findings informed redevelopment of the optimized programme (FFIT), whose components were mapped onto specific behaviour change techniques using an evidence-based taxonomy.</p>
<p>Results: p-FFIT comprised 12, weekly, gender-sensitised, group-based weight management classroom and ‘pitch-side’ physical activity sessions. These in-stadia sessions were complemented by an incremental, pedometer-based walking programme. p-FFIT was targeted at men aged 35-65 years with body mass index ≥ 27 kg/m2. Phase 2 demonstrated that participants in p-FFIT were enthusiastic about both the classroom and physical activity components, and valued the camaraderie and peer-support offered by the programme. Coaches appreciated the simplicity of the key healthy eating and physical activity messages. Suggestions for improvements that were incorporated into the optimized FFIT programme included: more varied in-stadia physical activity with football-related components; post-programme weight management support (emails and a reunion session); and additional training for coaches in SMART goal setting and the pedometer-based walking programme.</p>
<p>Conclusions: The Football Fans in Training programme is highly acceptable to participants and SPL coaches, and is appropriate for evaluation in a randomised controlled trial.</p>
Ultra-high-resolution observations of persistent null-point reconnection in the solar corona
MMagnetic reconnection is a key mechanism involved in solar eruptions and is also a prime possibility to heat the low corona to millions of degrees. Here, we present ultra-high-resolution extreme ultraviolet observations of persistent null-point reconnection in the corona at a scale of about 390 km over one hour observations of the Extreme-Ultraviolet Imager on board Solar Orbiter spacecraft. The observations show formation of a null-point configuration above a minor positive polarity embedded within a region of dominant negative polarity near a sunspot. The gentle phase of the persistent null-point reconnection is evidenced by sustained point-like high-temperature plasma (about 10 MK) near the null-point and constant outflow blobs not only along the outer spine but also along the fan surface. The blobs appear at a higher frequency than previously observed with an average velocity of about 80 km s−1 and life-times of about 40 s. The null-point reconnection also occurs explosively but only for 4 minutes, its coupling with a mini-filament eruption generates a spiral jet. These results suggest that magnetic reconnection, at previously unresolved scales, proceeds continually in a gentle and/or explosive way to persistently transfer mass and energy to the overlying coron
Satellite Observations of Separator Line Geometry of Three-Dimensional Magnetic Reconnection
Detection of a separator line that connects magnetic nulls and the
determination of the dynamics and plasma environment of such a structure can
improve our understanding of the three-dimensional (3D) magnetic reconnection
process. However, this type of field and particle configuration has not been
directly observed in space plasmas. Here we report the identification of a pair
of nulls, the null-null line that connects them, and associated fans and spines
in the magnetotail of Earth using data from the four Cluster spacecraft. With
di and de designating the ion and electron inertial lengths, respectively, the
separation between the nulls is found to be ~0.7di and an associated
oscillation is identified as a lower hybrid wave with wavelength ~ de. This in
situ evidence of the full 3D reconnection geometry and associated dynamics
provides an important step toward to establishing an observational framework of
3D reconnection.Comment: 10 pages, 3 figures and 1 tabl
Strongly magnetized pulsars: explosive events and evolution
Well before the radio discovery of pulsars offered the first observational
confirmation for their existence (Hewish et al., 1968), it had been suggested
that neutron stars might be endowed with very strong magnetic fields of
-G (Hoyle et al., 1964; Pacini, 1967). It is because of their
magnetic fields that these otherwise small ed inert, cooling dead stars emit
radio pulses and shine in various part of the electromagnetic spectrum. But the
presence of a strong magnetic field has more subtle and sometimes dramatic
consequences: In the last decades of observations indeed, evidence mounted that
it is likely the magnetic field that makes of an isolated neutron star what it
is among the different observational manifestations in which they come. The
contribution of the magnetic field to the energy budget of the neutron star can
be comparable or even exceed the available kinetic energy. The most magnetised
neutron stars in particular, the magnetars, exhibit an amazing assortment of
explosive events, underlining the importance of their magnetic field in their
lives. In this chapter we review the recent observational and theoretical
achievements, which not only confirmed the importance of the magnetic field in
the evolution of neutron stars, but also provide a promising unification scheme
for the different observational manifestations in which they appear. We focus
on the role of their magnetic field as an energy source behind their persistent
emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of
"NewCompStar" European COST Action MP1304, 43 pages, 8 figure
The tearing instability of resistive magnetohydrodynamics
In this chapter we explore the linear onset of one of the most important instabilities of resistive magnetohydrodynamics, the tearing instability. In particular, we focus on two important aspects of the onset of tearing: asymptotic (modal) stability and transient (non-modal) stability. We discuss the theory required to understand these two aspects of stability, both of which have undergone significant development in recent years
Spider-Venom Peptides as Therapeutics
Spiders are the most successful venomous animals and the most abundant terrestrial predators. Their remarkable success is due in large part to their ingenious exploitation of silk and the evolution of pharmacologically complex venoms that ensure rapid subjugation of prey. Most spider venoms are dominated by disulfide-rich peptides that typically have high affinity and specificity for particular subtypes of ion channels and receptors. Spider venoms are conservatively predicted to contain more than 10 million bioactive peptides, making them a valuable resource for drug discovery. Here we review the structure and pharmacology of spider-venom peptides that are being used as leads for the development of therapeutics against a wide range of pathophysiological conditions including cardiovascular disorders, chronic pain, inflammation, and erectile dysfunction
Dynamism in the solar core
Recent results of a mixed shell model heated asymmetrically by transient
increases in nuclear burning indicate the transient generation of small hot
spots inside the Sun somewhere between 0.1 and 0.2 solar radii. These hot
bubbles are followed by a nonlinear differential equation system with finite
amplitude non-homologous perturbations which is solved in a solar model. Our
results show the possibility of a direct connection between the dynamic
phenomena of the solar core and the atmospheric activity. Namely, an initial
heating about DQ_0 ~ 10^{31}-10^{37} ergs can be enough for a bubble to reach
the outer convective zone. Our calculations show that a hot bubble can arrive
into subphotospheric regions with DQ_final ~ 10^{28} - 10^{34} ergs with a high
speed, up to 10 km s-1, approaching the local sound speed. We point out that
the developing sonic boom transforms the shock front into accelerated particle
beam injected upwards into the top of loop carried out by the hot bubble above
its forefront traveling from the solar interior. As a result, a new perspective
arises to explain flare energetics. We show that the particle beams generated
by energetic deep-origin hot bubbles in the subphotospheric layers have masses,
energies, and chemical compositions in the observed range of solar
chromospheric and coronal flares. It is shown how the emergence of a hot bubble
into subphotospheric regions offers a natural mechanism that can generate both
the eruption leading to the flare and the observed coronal magnetic topology
for reconnection. We show a list of long-standing problems of solar physics
that our model explains. We present some predictions for observations, some of
which are planned to be realized in the near future.Comment: 44 pages, 20 figure
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