24,043 research outputs found
Ultra-violet footpoints as tracers of coronal magnetic connectivity and restructuring during a solar flare
<p><b>Context:</b> The bright, compact ultraviolet sources that appear in flare ribbons are interpreted as sites of energisation of the chromosphere, most likely by electron beams from the corona. Previously we have developed an algorithm to track these compact sources in observations by the Transition Region and Coronal Explorer (TRACE), recording position and intensity. We now exploit this further.</p>
<p><b>Aims:</b> We aim at identifying conjugate footpoint pairs by cross-correlating the TRACE 1600 Å lightcurves in one particular event – the 2002-July-17 M 8.5 flare. We also seek the spatial relationship between the magnetic flux transfer (reconnection) rate, well-connected locations, and energy input by electrons.</p>
<p><b>Methods:</b> We performed wavelet à trous filtering on the UV light curves, followed by a linear cross-correlation, to identify well-correlated pairs. We used RHESSI data to determine the locations of strong electron beam input.</p>
<p><b>Results:</b> Maps of footpoint pairs were produced in which we can identify well-separated locations that have well-correlated 1600 Å light curves. The time lag between credible conjugate footpoint brightenings can be a few seconds. The flare magnetic connectivity is found to evolve with time. RHESSI hard X-ray sources are found where the flux transfer rate is highest.</p>
<p><b>Conclusions:</b> We propose that the correlated footpoints are in fact conjugate pairs that are magnetically linked. In some instances, this linkage may be via a coronal null. The time lag in many cases is consistent with excitation by relativistic particles, but correlations with a longer time lag may suggest excitation by waves.</p>
A digitally controlled system for effecting and presenting a selected electrical resistance
A digitally controlled resistance generator is described, in which resistors having values selected according to an expression 2(sup N-1)(R), where N is equal to the number of terms in the expression, and R is equal to the lowest value of resistance, are electrically inserted into a resistive circuit in accordance with a parallel binary signal provided by an analog-to-digital converter or a programmable computer. This binary signal is coupled via optical isolators which, when activated by a logical 1, provides a negative potential to some or all of the gate inputs of the normally on field effect transistors which, when on, shorts out the associated resistor. This applied negative potential turns the field effect transistors off and electrically inserts the resistor coupled between the source terminal and the drain terminal of that field effect transistor into the resistive circuit between the terminals
The limitations of using only CAD and DHM in design relating to high value manufacturing
The ergonomics suites available within computer aided design and digital human modelling programs are increasingly being used to predict and prevent ergonomic and human factors risk due to poor design. To further aid the reduction in poor design, it is of importance to understand the need for user input and the limitations of these software programs. These limitations include: the small number of available anthropometric population samples; and the disconnect between what a designer perceives as possible, and what is possible within a manufacturing environment. A method of mitigating these limitations is the use of user input using virtual reality suites, mock-ups and motion capture technology
Mathematical morphology and applications in automated sunspot detection
This presentation discusses the mathematical morphology and applications in automated sunspot detection
Unambiguous evidence of coronal implosions during solar eruptions and flares
In the implosion conjecture, coronal loops contract as the result of magnetic energy release in solar eruptions and flares. However, after almost two decades, observations of this phenomenon are still rare and most previous reports are plagued by projection effects so that loop contraction could be either true implosion or just a change in loop inclination. In this paper, to demonstrate the reality of loop contractions in the global coronal dynamics, we present four events with the continuously contracting loops in an almost edge-on geometry from the perspective of SDO/AIA, which are free from the ambiguity caused by the projection effects, also supplemented by contemporary observations from STEREO for examination. In the wider context of observations, simulations and theories, we argue that the implosion conjecture is valid in interpreting these events. Furthermore, distinct properties of the events allow us to identify two physical categories of implosion. One type demonstrates a rapid contraction at the beginning of the flare impulsive phase, as magnetic free energy is removed rapidly by a filament eruption. The other type, which has no visible eruption, shows a continuous loop shrinkage during the entire flare impulsive phase, which we suggest shows the ongoing conversion of magnetic free energy in a coronal volume. Corresponding scenarios are described that can provide reasonable explanations for the observations. We also point out that implosions may be suppressed in cases when a heavily mass-loaded filament is involved, possibly serving as an alternative account for their observational rarity
On the Spatial Distribution of Hard X-Rays from Solar Flare Loops
The aim of this paper is to investigate the spatial structure of the
impulsive phase hard X-ray emission from solar flares. This work is motivated
by the YOHKOH and the forthcoming HESSI observations. Summarizing past results,
it is shown that the transport effects can account for the observations by
inhomogeneous loops where there is a strong field convergence and/or density
enhancement at the top of the flaring loop. Scattering by plasma turbulence at
the acceleration site or pancake type pitch angle distribution of the
accelerated electrons can also give rise to enhanced emission at the loop tops.
These could be a natural consequence of acceleration by plasma waves. This
paper considers a general case of stochastic scattering and acceleration that
leads to an isotropic pitch angle distribution and an enhanced emission from
the loop tops or the acceleration site.
Following the formalism developed in earlier papers the strength and the
spectrum of the radiation expected from the acceleration site and the foot
points are evaluated and their dependence on the parameters describing the
acceleration process and the flare plasma are determined. The theoretical ratio
of these two intensities and relative values of their spectral indices are
compared with the YOHKOH observations, demonstrating that the above mentioned
parameters can be constrained with such observations. It is shown that future
high spatial and spectral resolution observations, for example those expected
from HESSI, can begin to distinguish between different models and constrain
their parameters.Comment: 37 pages with 20 figures. Accepted for publication in ApJ
http://www.astronomy.stanford.ed
Onset of Electron Acceleration in a Flare Loop
We carried out detailed analysis of X-ray and radio observations of a simple
flare loop that occurred on 12th August 2002, with the impulsive hard X-ray
(HXR) light curves dominated by a single pulse. The emission spectra of the
early impulsive phase are consistent with an isothermal model in the coronal
loop with a temperature reaching several keVs. A power-law high-energy spectral
tail is evident near the HXR peak time, in accordance with the appearance of
footpoints at high energies, and is well correlated with the radio emission.
The energy content of the thermal component keeps increasing gradually after
the disappearance of this nonthermal component. These results suggest that
electron acceleration only covers a central period of a longer and more gradual
energy dissipation process and that the electron transport within the loop
plays a crucial role in the formation of the inferred power-law electron
distribution. The spectral index of power-law photons shows a very gradual
evolution indicating a quasi-steady state of the electron accelerator, which is
confirmed by radio observations. These results are consistent with the theory
of stochastic electron acceleration from a thermal background. Advanced
modeling with coupled electron acceleration and spatial transport processes is
needed to explain these observations more quantitatively, which may reveal the
dependence of the electron acceleration on the spatial structure of the
acceleration region
Development of a flight-qualified whole-body dosimeter system Final report
Whole-body dosimeter system for monitoring radiation exposure to crew during space mission
Child Mental Health and Human Capital Accumulation: The Case of ADHD Revisited
Recently, Currie and Stabile (2006) made a significant contribution to our understanding of the influence of ADHD symptoms on a variety of school outcomes including participation in special education, grade repetition and test scores. Their contributions include using a broad sample of children and estimating sibling fixed effects models to control for unobserved family effects. In this paper we look at a sample of older children and confirm and extend many of the JCMS findings in terms of a broader set of measures of human capital and additional specifications.
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