591 research outputs found
The flaring and quiescent components of the solar corona
The solar corona is a template to understand stellar activity. The Sun is a
moderately active star, and its corona differs from active stars: active
stellar coronae have a double-peaked EM(T) with the hot peak at 8-20 MK, while
the non flaring solar corona has one peak at 1-2 MK. We study the average
contribution of flares to the solar EM(T) to investigate indirectly the
hypothesis that the hot peak of the EM(T) of active stellar coronae is due to a
large number of unresolved solar-like flares, and to infer properties on the
flare distribution from nano- to macro-flares. We measure the disk-integrated
time-averaged emission measure, EM_F(T), of an unbiased sample of solar flares
analyzing uninterrupted GOES/XRS light curves over time intervals of one month.
We obtain the EM_Q(T) of quiescent corona for the same time intervals from the
Yohkoh/SXT data. To investigate how EM_F(T) and EM_Q(T) vary with the solar
cycle, we evaluate them at different phases of the cycle (from Dec. 1991 to
Apr. 1998). Irrespective of the solar cycle phase, EM_F(T) appears like a peak
of the distribution significantly larger than the values of EM_Q(T) for T~5-10
MK. As a result the time-averaged EM(T) of the whole solar corona is
double-peaked, with the hot peak, due to time-averaged flares, located at
temperature similar of that of active stars, but less enhanced. The EM_F(T)
shape supports the hypothesis that the hot EM(T) peak of active coronae is due
to unresolved solar-like flares. If this is the case, quiescent and flare
components should follow different scaling laws for increasing stellar
activity. In the assumption that the heating of the corona is entirely due to
flares, from nano- to macro-flares, then either the flare distribution or the
confined plasma response to flares, or both, are bimodal.Comment: 8 pages, 7 postscript figures, accepted for publication in Astronomy
and Astrophysic
Signatures of impulsive localized heating in the temperature distribution of multi-stranded coronal loops
We study the signatures of different coronal heating regimes on the
differential emission measure (DEM) of multi-stranded coronal loops by means of
hydrodynamic simulations. We consider heating either uniformly distributed
along the loops or localized close to the chromospheric footpoints, in both
steady and impulsive conditions. Our simulations show that condensation at the
top of the loop forms when the localized heating is impulsive with a pulse
cadence time shorter than the plasma cooling time, and the pulse energy is
below a certain threshold. A condensation does not produce observable
signatures in the global DEM structure. Conversely, the DEM coronal peak is
found sensitive to the pulse cadence time. Our simulations can also give an
explanation of the warm overdense and hot underdense loops observed by TRACE,
SOHO and Yohkoh. However, they are unable to reproduce both the transition
region and the coronal DEM structure with a unique set of parameters, which
outlines the need for a more realistic description of the transition region.Comment: 31 pages, 7 figure
Multi-wavelength analysis of high energy electrons in solar flares: a case study of August 20, 2002 flare
A multi-wavelength spatial and temporal analysis of solar high energy
electrons is conducted using the August 20, 2002 flare of an unusually flat
(gamma=1.8) hard X-ray spectrum. The flare is studied using RHESSI, Halpha,
radio, TRACE, and MDI observations with advanced methods and techniques never
previously applied in the solar flare context. A new method to account for
X-ray Compton backscattering in the photosphere (photospheric albedo) has been
used to deduce the primary X-ray flare spectra. The mean electron flux
distribution has been analysed using both forward fitting and model independent
inversion methods of spectral analysis. We show that the contribution of the
photospheric albedo to the photon spectrum modifies the calculated mean
electron flux distribution, mainly at energies below 100 keV. The positions of
the Halpha emission and hard X-ray sources with respect to the current-free
extrapolation of the MDI photospheric magnetic field and the characteristics of
the radio emission provide evidence of the closed geometry of the magnetic
field structure and the flare process in low altitude magnetic loops. In
agreement with the predictions of some solar flare models, the hard X-ray
sources are located on the external edges of the Halpha emission and show
chromospheric plasma heated by the non-thermal electrons. The fast changes of
Halpha intensities are located not only inside the hard X-ray sources, as
expected if they are the signatures of the chromospheric response to the
electron bombardment, but also away from them.Comment: 26 pages, 9 figures, accepted to Solar Physic
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Passive sampling methods for contaminated sediments: Risk assessment and management
This paper details how activity-based passive sampling methods (PSMs), which provide information on bioavailability in terms of freely dissolved contaminant concentrations (Cfree), can be used to better inform risk management decision making at multiple points in the process of assessing and managing contaminated sediment sites. PSMs can increase certainty in site investigation and management, because Cfree is a better predictor of bioavailability than total bulk sediment concentration (Ctotal) for 4 key endpoints included in conceptual site models (benthic organism toxicity, bioaccumulation, sediment flux, and water column exposures). The use of passive sampling devices (PSDs) presents challenges with respect to representative sampling for estimating average concentrations and other metrics relevant for exposure and risk assessment. These challenges can be addressed by designing studies that account for sources of variation associated with PSMs and considering appropriate spatial scales to meet study objectives. Possible applications of PSMs include: quantifying spatial and temporal trends in bioavailable contaminants, identifying and evaluating contaminant source contributions, calibrating site-specific models, and, improving weight-of-evidence based decision frameworks. PSM data can be used to assist in delineating sediment management zones based on likelihood of exposure effects, monitor remedy effectiveness, and, evaluate risk reduction after sediment treatment, disposal, or beneficial reuse after management actions. Examples are provided illustrating why PSMs and freely dissolved contaminant concentrations (Cfree) should be incorporated into contaminated sediment investigations and study designs to better focus on and understand contaminant bioavailability, more accurately estimate exposure to sediment-associated contaminants, and better inform risk management decisions. Research and communication needs for encouraging broader use are discussed. Integr Environ Assess Manag 2014;10:224â236. © 2014 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of SETAC
Dynamics and Radiation of Young Type-Ia Supernova Remnants: Important Physical Processes
We examine and analyze the physical processes that should be taken into
account when modeling young type-Ia SNRs, with ages of several hundred years.
It is shown, that energy losses in the metal-rich ejecta can be essential for
remnants already at this stage of evolution. The influence of electron thermal
conduction and the rate of the energy exchange between electrons and ions on
the temperature distribution and the X-radiation from such remnants is studied.
The data for Tycho SNR from the XMM-Newton X-ray telescope have been employed
for the comparison of calculations with observations.Comment: 19 pages, 8 figure
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Keeping Steady as She Goes: A Negotiated Order Perspective on Technological Evolution
A central idea in the theory of technology cycles is that social and political mechanisms are most important during the selection of a dominant design, and that eras of incremental change are socially uninteresting periods in which innovation is driven by technological momentum and elaboration of the dominant design. In this essay, we overturn the ontological assumption that social order is inherently stable, drawing on Anselm Strauss's concept of negotiated order to analyze the persistence of a dominant design as a social accomplishment: an outcome of ongoing processes that reinforce or challenge a socially negotiated order. Thus, we shift focus from battles over standards to periods of normal innovation. We extend the technology cycles model to explain social dynamics in periods of incremental change, and to make predictions specifying how contextual conditions in standards-setting organizations affect social interaction, leading to reinforcement or challenge to a socio-technical order
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Coordinated Cluster/Double Star observations of dayside reconnection signatures
The recent launch of the equatorial spacecraft of the Double Star mission, TC-1, has provided an unprecedented opportunity to monitor the southern hemisphere dayside magnetopause boundary layer in conjunction with northern hemisphere observations by the quartet of Cluster spacecraft. We present first results of one such situation where, on 6 April 2004, both Cluster and the Double Star TC-1 spacecraft were on outbound transits through the dawnside magnetosphere. The observations are consistent with ongoing reconnection on the dayside magnetopause, resulting in a series of flux transfer events (FTEs) seen both at Cluster and TC-1, which appear to lie north and south of the reconnection line, respectively. In fact, the observed polarity and motion of each FTE signature advocates the existence of an active reconnection region consistently located between the positions of Cluster and TC-1, with Cluster observing northward moving FTEs with +/â polarity, whereas TC-1 sees â/+ polarity FTEs. This assertion is further supported by the application of a model designed to track flux tube motion for the prevailing interplanetary conditions. The results from this model show, in addition, that the low-latitude FTE dynamics are sensitive to changes in convected upstream conditions. In particular, changing the interplanetary magnetic field (IMF) clock angle in the model suggests that TC-1 should miss the resulting FTEs more often than Cluster and this is borne out by the observations
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
A coronal explosion on the flare star CN Leonis
We present simultaneous high-temporal and high-spectral resolution
observations at optical and soft X-ray wavelengths of the nearby flare star CN
Leo. During our observing campaign a major flare occurred, raising the star's
instantaneous energy output by almost three orders of magnitude. The flare
shows the often observed impulsive behavior, with a rapid rise and slow decay
in the optical and a broad soft X-ray maximum about 200 seconds after the
optical flare peak. However, in addition to this usually encountered flare
phenomenology we find an extremely short (~2 sec) soft X-ray peak, which is
very likely of thermal, rather than non-thermal nature and temporally coincides
with the optical flare peak. While at hard X-ray energies non-thermal bursts
are routinely observed on the Sun at flare onset, thermal soft X-ray bursts on
time scales of seconds have never been observed in a solar nor stellar context.
Time-dependent, one-dimensional hydrodynamic modeling of this event requires an
extremely short energy deposition time scale of a few seconds to reconcile
theory with observations, thus suggesting that we are witnessing the results of
a coronal explosion on CN Leo. Thus the flare on CN Leo provides the
opportunity to observationally study the physics of the long-sought
"micro-flares" thought to be responsible for coronal heating.Comment: 7 pages, accepted by A&
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