111 research outputs found
The Cooling of Coronal Plasmas. iv: Catastrophic Cooling of Loops
We examine the radiative cooling of coronal loops and demonstrate that the
recently identified catastrophic cooling (Reale and Landi, 2012) is due to the
inability of a loop to sustain radiative / enthalpy cooling below a critical
temperature, which can be > 1 MK in flares, 0.5 - 1 MK in active regions and
0.1 MK in long tenuous loops. Catastrophic cooling is characterised by a rapid
fall in coronal temperature while the coronal density changes by a small
amount. Analytic expressions for the critical temperature are derived and show
good agreement with numerical results. This effect limits very considerably the
lifetime of coronal plasmas below the critical temperature
Enthalpy-based Thermal Evolution of Loops: II. Improvements to the Model
This paper develops the zero-dimensional (0D) hydrodynamic coronal loop model
"Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by Klimchuk et al
(2008), which studies the plasma response to evolving coronal heating,
especially impulsive heating events. The basis of EBTEL is the modelling of
mass exchange between the corona and transition region and chromosphere in
response to heating variations, with the key parameter being the ratio of
transition region to coronal radiation. We develop new models for this
parameter that now include gravitational stratification and a physically
motivated approach to radiative cooling. A number of examples are presented,
including nanoflares in short and long loops, and a small flare. The new
features in EBTEL are important for accurate tracking of, in particular, the
density. The 0D results are compared to a 1D hydro code (Hydrad) with generally
good agreement. EBTEL is suitable for general use as a tool for (a) quick-look
results of loop evolution in response to a given heating function, (b)
extensive parameter surveys and (c) situations where the modelling of hundreds
or thousands of elemental loops is needed. A single run takes a few seconds on
a contemporary laptop
Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity
Context: The sharp temperature and density gradients in the coronal
transition region are a challenge for models and observations.
Aims: We set out to get linearly- and quadratically-weighted average electron
densities in the region emitting the S VI lines, using the observed opacity and
the emission measure of these lines.
Methods: We analyze SoHO/SUMER spectroscopic observations of the S VI lines,
using the center-to-limb variations and radiance ratios to derive the opacity.
We also use the Emission Measure derived from radiance at disk center.
Results: We get an opacity at S VI line center of the order of 0.05. The
resulting average electron density is 2.4 10^16 m^-3 at T = 2 10^5 K. This
value is higher than the values obtained from radiance measurements.
Conversely, taking a classical value for the density leads to a too high value
of the thickness of the emitting layer.
Conclusions: The pressure derived from the Emission Measure method compares
well with previous determinations and implies a low opacity of 5 10^-3 to
10^-2. The fact that a direct derivation leads to a much higher opacity remains
unexplained, despite tentative modeling of observational biases. Further
measurements need to be done, and more realistic models of the transition
region need to be used.Comment: 11 pages, 9 figure
Structures in the outer solar atmosphere
The structure and dynamics of the outer solar atmosphere are reviewed with
emphasis on the role played by the magnetic field. Contemporary observations
that focus on high resolution imaging over a range of temperatures, as well as
UV, EUV and hard X-ray spectroscopy, demonstrate the presence of a vast range
of temporal and spatial scales, mass motions, and particle energies present. By
focussing on recent developments in the chromosphere, corona and solar wind, it
is shown that small scale processes, in particular magnetic reconnection, play
a central role in determining the large-scale structure and properties of all
regions. This coupling of scales is central to understanding the atmosphere,
yet poses formidable challenges for theoretical models.Comment: 41 Pages, 15 Figures, Accepted for publication in Space Science
Review
Modeling the Radiative Signatures of Turbulent Heating in Coronal Loops
The statistical properties of the radiative signature of a coronal loop subject to turbulent heating obtained from a three-dimensional (3D) magnetohydrodynamics (MHD) model are studied. The heating and cooling of a multistrand loop is modeled and synthetic spectra for Fe XII 195.12, Fe XV 284.163, and Fe XIX 1118.06 ? are calculated, covering a wide temperature range. The results show that the statistical properties of the thermal and radiative energies partially reflect those of the heating function in that power-law distributions are transmitted, but with very significant changes in the power-law indices. There is a strong dependence on the subloop geometry. Only high-temperature radiation (?107 K) preserves reasonably precise information on the heating function
The gamma-ray giant flare from SGR1806-20: Evidence for crustal cracking via initial timescales
We report here on serendipitous observations of the intense gamma-ray flare
from SGR 1806-20 that occured on 27 December 2004. Unique data from the Cluster
and Double Star-2 satellites, designed to study the Earth's magnetosphere,
provide the first observational evidence of three separate timescales within
the early (first 100ms) phases of this class of events. These observations
reveal that, in addition to the initial very steep (<0.25ms) X-ray onset, there
is firstly a 4.9ms exponential rise timescale followed by a continued
exponential rise in intensity on a timescale of 70ms. These three timescales
are a prominent feature of current theoretical models including the timescale
(several ms) for fracture propagation in the crust of the neutron star.Comment: 10 pages including 2 figures Ap J Letters in press, May 200
Critical Issues For Understanding Particle Acceleration in Impulsive Solar Flares
This paper, a review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Observatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, timing, and efficiency of particle acceleration in flares. These are summarized in the first part of the review. Particle acceleration processes are then discussed, with;particular emphasis on new developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration by both sub- and super-Dreicer electric fields. Finally, issues that arise when these mechanisms are incorporated into the large-scale flare structure are considered. Stochastic and super-Dreicer acceleration may occur either in a single large coronal reconnection site or at multiple \u27\u27fragmented\u27\u27 energy release sites. Sub-Dreicer acceleration requires a highly filamented coronal current pattern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare energy release region
Reconstructing the 3-D Trajectories of CMEs in the Inner Heliosphere
A method for the full three-dimensional (3-D) reconstruction of the
trajectories of coronal mass ejections (CMEs) using Solar TErrestrial RElations
Observatory (STEREO) data is presented. Four CMEs that were simultaneously
observed by the inner and outer coronagraphs (COR1 and 2) of the Ahead and
Behind STEREO satellites were analysed. These observations were used to derive
CME trajectories in 3-D out to ~15Rsun. The reconstructions using COR1/2 data
support a radial propagation model. Assuming pseudo-radial propagation at large
distances from the Sun (15-240Rsun), the CME positions were extrapolated into
the Heliospheric Imager (HI) field-of-view. We estimated the CME velocities in
the different fields-of-view. It was found that CMEs slower than the solar wind
were accelerated, while CMEs faster than the solar wind were decelerated, with
both tending to the solar wind velocity.Comment: 17 pages, 10 figures, 1 appendi
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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;10:224–236. © 2014 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of SETAC
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