4,450 research outputs found
Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result
It is shown that if a current-carrying magnetic flux tube is bulged at its
axial midpoint z=0 and constricted at its axial endpoints z=+h,-h, then plasma
will be accelerated from z=+h,-h towards z=0 resulting in a situation similar
to two water jets pointed at each other. The ingested plasma convects embedded,
frozen-in toroidal magnetic flux from z=+h,-h to z=0. The counter-directed
flows collide and stagnate at z=0 and in so doing (i) convert their
translational kinetic energy into heat, (ii) increase the plasma density at
z~0, and (iii) increase the embedded toroidal flux density at z~0. The increase
in toroidal flux density at z~0 increases the toroidal field Bphi and hence
increases the magnetic pinch force at z~0 and so causes a reduction of the flux
tube radius at z~0. Thus, the flux tube develops an axially uniform
cross-section, a decreased volume, an increased density, and an increased
temperature. This model is proposed as a likely hypothesis for the
long-standing mystery of why solar coronal loops are observed to be axially
uniform, hot, and bright.Comment: to appear in Physics of Plasmas 24 pages, 5 figure
Unstable coronal loops : numerical simulations with predicted observational signatures
We present numerical studies of the nonlinear, resistive magnetohydrodynamic
(MHD) evolution of coronal loops. For these simulations we assume that the
loops carry no net current, as might be expected if the loop had evolved due to
vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical
flux tube with line-tied ends. For a given amount of twist in the magnetic
field it is well known that once such a loop exceeds a critical length it
becomes unstableto ideal MHD instabilities. The early evolution of these
instabilities generates large current concentrations. Firstly we show that
these current concentrations are consistent with the formation of a current
sheet. Magnetic reconnection can only occur in the vicinity of these current
concentrations and we therefore couple the resistivity to the local current
density. This has the advantage of avoiding resistive diffusion in regions
where it should be negligible. We demonstrate the importance of this procedure
by comparison with simulations based on a uniform resistivity. From our
numerical experiments we are able to estimate some observational signatures for
unstable coronal loops. These signatures include: the timescale of the loop
brightening; the temperature increase; the energy released and the predicted
observable flow speeds. Finally we discuss to what extent these observational
signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure
Symptom complexes in patients with seropositive arthralgia and in patients newly diagnosed with rheumatoid arthritis: a qualitative exploration of symptom development
Objective: The aim of this study was to explore symptoms and symptom development during the earliest phases of rheumatoid arthritis (RA) in patients with seropositive arthralgia and patients newly diagnosed with RA
Numerical simulations of kink instability in line-tied coronal loops
The results from numerical simulations carried out using a new shock-capturing, Lagrangian-remap, 3D MHD code, Lare3d are presented. We study the evolution of the m=1 kink mode instability in a photospherically line-tied coronal loop that has no net axial current. During the non-linear evolution of the kink instability, large current concentrations develop in the neighbourhood of the infinite length mode rational surface. We investigate whether this strong current saturates at a finite value or whether scaling indicates current sheet formation. In particular, we consider the effect of the shear, defined by where is the fieldline twist of the loop, on the current concentration. We also include a non-uniform resistivity in the simulations and observe the amount of free magnetic energy released by magnetic reconnection
Ideal Internal Kink Modes in a Line-tied Screw Pinch
It is well known that the radial displacement of the internal kink mode
in a periodic screw pinch has a steep jump at the resonant surface where
. In a line-tied system, relevant to solar and
astrophysical plasmas, the resonant surface is no longer a valid concept. It is
then of interest to see how line-tying alters the aforementioned result for a
periodic system. If the line-tied kink also produces a steep gradient,
corresponding to a thin current layer, it may lead to strong resistive effects
even with weak dissipation. Numerical solution of the eigenmode equations shows
that the fastest growing kink mode in a line-tied system still possesses a jump
in the radial displacement at the location coincident with the resonant surface
of the fastest growing mode in the periodic counterpart. However, line-tying
thickens the inner layer and slows down the growth rate. As the system length
approaches infinity, both the inner layer thickness and the growth rate
approach the periodic values. In the limit of small , the critical length for instability .
The relative increase in the inner layer thickness due to line-tying scales as
.Comment: To appear in Physics of Plasma
Automated LASCO CME catalog for solar cycle 23: are CMEs scale invariant?
In this paper we present the first automatically constructed LASCO CME
catalog, a result of the application of the Computer Aided CME Tracking
software (CACTus) on the LASCO archive during the interval September 1997 -
January 2007. We have studied the CME characteristics and have compared them
with similar results obtained by manual detection (CDAW CME catalog). On
average CACTus detects less than 2 events per day during solar minimum up to 8
events during maximum, nearly half of them being narrow (< 20 degrees).
Assuming a correction factor, we find that the CACTus CME rate is surprisingly
consistent with CME rates found during the past 30 years. The CACTus statistics
show that small scale outflow is ubiquitously observed in the outer corona. The
majority of CACTus-only events are narrow transients related to previous CME
activity or to intensity variations in the slow solar wind, reflecting its
turbulent nature. A significant fraction (about 15%) of CACTus-{\it only}
events were identified as independent events, thus not related to other CME
activity. The CACTus CME width distribution is essentially scale invariant in
angular span over a range of scales from 20 to 120 degrees while previous
catalogues present a broad maximum around 30 degrees. The possibility that the
size of coronal mass outflows follow a power law distribution could indicate
that no typical CME size exists, i.e. that the narrow transients are not
different from the larger well-defined CMEs.Comment: 13 pages. ApJ, accepte
Frequency division multiplexing readout of 60 low-noise transition-edge sensor bolometers
We demonstrate multiplexing readout of 60 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing readout chain with bias frequencies ranging from 1 to 3.5 MHz and with a typical frequency spacing of 32 kHz. The readout chain starts with a two-stage SQUID amplifier and has a noise level of 9.5 pA/ â{ Hz } . We compare current-voltage curves and noise spectra of TESs measured in a single-pixel mode and in a multiplexing mode. We also map the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where there are 43 pixels that do not show more than 10% difference in NEP and 5% in saturation power when measured in single pixel and multiplex modes. We have read out a TES with an NEP of 0.45 aW/ â{ Hz } in the multiplexing-mode, which demonstrates the capability of reading out ultra-low noise TES bolometer arrays for space applications
Electrical cross talk of a frequency division multiplexing readout for a transition edge sensor bolometer array
We have characterized and mapped the electrical cross talk (ECT) of a frequency division multiplexing (FDM) system with a transition edge sensor (TES) bolometer array, which is intended for space applications. By adding a small modulation at 120 Hz to the AC bias voltage of one bolometer and measuring the cross talk response in the current noise spectra of the others simultaneously, we have for the first time mapped the ECT level of 61 pixels with a nominal frequency spacing of 32 kHz in a 61 Ă 61 matrix and a carrier frequency ranging from 1 MHz to 4 MHz. We find that about 94% of the pixels show an ECT level of less than 0.4%. Only the adjacent pixels reach this level, and the ECT for the rest of the pixels is less than 0.1%. We also observe higher ECT levels, up to 10%, between some of the pixels, which have bundled long, parallel coplanar wires connecting TES bolometers to inductor-capacitor filters. In this case, the high mutual inductances dominate. To mitigate this source of ECT, the coplanar wires should be replaced by microstrip wires in the array. Our study suggests that an FDM system can have a relatively low ECT level, e.g., around 0.4% if the frequency spacing is 30 kHz. Our results successfully demonstrate a low electrical cross talk for a space FDM technology
The effect of angular momentum conservation in the phase transitions of collapsing systems
The effect of angular momentum conservation in microcanonical thermodynamics
is considered. This is relevant in gravitating systems, where angular momentum
is conserved and the collapsing nature of the forces makes the microcanonical
ensemble the proper statistical description of the physical processes. The
microcanonical distribution function with non-vanishing angular momentum is
obtained as a function of the coordinates of the particles. As an example, a
simple model of gravitating particles, introduced by Thirring long ago, is
worked out. The phase diagram contains three phases: for low values of the
angular momentum the system behaves as the original model, showing a
complete collapse at low energies and an entropy with a convex intruder. For
intermediate values of the collapse at low energies is not complete and the
entropy still has a convex intruder. For large there is neither collapse
nor anomalies in the thermodynamical quantities. A short discussion of the
extension of these results to more realistic situations is exposed.Comment: Latex, 21 pages, 5 figures. Corrected misprints in some equations and
a few clarifying remarks adde
Preferences and beliefs of Dutch orthopaedic surgeons and patients reduce the implementation of "Choosing Wisely" recommendations in degenerative knee disease
Purpose: The purpose of this study was to assess which factors were associated with the implementation of âChoosing Wiselyâ recommendations to refrain from routine MRI and arthroscopy use in degenerative knee disease. Methods: Cross-sectional surveys were sent to 123 patients (response rate 95%) and 413 orthopaedic surgeons (response rate 62%) fulfilling the inclusion criteria. Univariate and multivariate logistic regression analyses were used to identify factors associated with implementation of âChoosing Wiselyâ recommendations. Results: Factors reducing implementation of the MRI recommendation among patients included explanation of added value by an orthopaedic surgeon [OR 0.18 (95% CI 0.07â0.47)] and patient preference for MRI [OR 0.27 (95% CI 0.08â0.92)]. Factors reducing implementation among orthopaedic surgeons were higher valuation of own MRI experience than existing evidence [OR 0.41 (95% CI 0.19â0.88)] and higher estimated patientsâ knowledge to participate in shared decision-making [OR 0.38 (95% CI 0.17â0.88)]. Factors reducing implementation of the arthroscopy recommendation among patients were orthopaedic surgeonsâ preferences for an arthroscopy [OR 0.03 (95% CI 0.00â0.22)] and positive experiences with arthroscopy of friends/family [OR 0.03 (95% CI 0.00â0.39)]. Factors reducing implementation among orthopaedic surgeons were higher valuation of own arthroscopy experience than existing evidence [OR 0.17 (95% CI 0.07â0.46)] and belief in the added value [OR 0.28 (95% CI 0.10â0.81)]. Conclusions: Implementation of âChoosing Wiselyâ recommendations in degenerative knee disease can be improved by strategies to change clinician beliefs about the added value of MRIs and arthroscopies, and by patient-directed strategies addressing patient preferences and underlying beliefs for added value of MRI and arthroscopies resulting from experiences of people in their environment. Level of evidence: IV
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