763 research outputs found
The energetics of the gradual phase
Reseachers compare results with those in the chapter by Moore et al. (1980), who reached five main conclusions about the gradual phase: (1) the typical density of the soft X-ray emitting plasma is between 10 to the 11th power and 10 to the 12th power cm-3 for compact flares and between 10 to the 10th power and 10 to the 11th power cm-3 for a large-area flare; (2) cooling is by conduction and radiation in roughly equal proportions; (3) continual heating is needed in the decay phase of two-ribbon flares; (4) continual heating is probably not needed in compact events; (5) most of the soft-X-ray-emitting plasma results from chromospheric evaporation. The goal was to reexamine these problems with the data from the Solar Maximum Mission (SMM) and other supporting instruments as well as to take advantage of recent theoretical advances. SMM is capable of measuring coronal temperatures more accurately and with a better cadence than has been possible before. The SMM data set is also unique in that the complete transit of an active region was observed, with soft X-ray and UV images being taken every few minutes. Researcher's were therefore able to establish the pre-flare conditions of the region and see whether anything has changed as a result of the flare. The assumptions made in attempting to determine the required plasma parameters are described. The derived parameters for the five prime flares are presented, and the role of numerical simulations is discussed
Multi-wavelength observations and modelling of a canonical solar flare
This paper investigates the temporal evolution of temperature, emission
measure, energy loss and velocity in a C-class solar flare from both an
observational and theoretical perspective. The properties of the flare were
derived by following the systematic cooling of the plasma through the response
functions of a number of instruments -- RHESSI (>5 MK), GOES-12 (5-30 MK),
TRACE 171 A (1 MK) and SOHO/CDS (~0.03-8 MK). These measurements were studied
in combination with simulations from the 0-D EBTEL model. At the flare on-set,
upflows of ~90 km s-1 and low level emission were observed in Fe XIX,
consistent with pre-flare heating and gentle chromospheric evaporation. During
the impulsive phase, upflows of ~80 km s-1 in Fe XIX and simultaneous downflows
of 20 km s-1 in He I and O V were observed, indicating explosive chromospheric
evaporation. The plasma was subsequently found to reach a peak temperature of
~13 MK in approximately 10 minutes. Using EBTEL, conduction was found to be the
dominant loss mechanism during the initial ~300s of the decay phase. It was
also found to be responsible for driving gentle chromospheric evaporation
during this period. As the temperature fell below ~8 MK, and for the next
~4,000s, radiative losses were determined to dominate over conductive losses.
The radiative loss phase was accompanied by significant downflows of <40 km s-1
in O V. This is the first extensive study of the evolution of a canonical solar
flare using both spectroscopic and broad-band instruments in conjunction with a
hydrodynamic model. While our results are in broad agreement with the standard
flare model, the simulations suggest that both conductive and non-thermal beam
heating play important roles in heating the flare plasma during the impulsive
phase of at least this event.Comment: 10 pages, 7 figures, 2 tables. Accepted for publication in A&
Flare energetics
In this investigation of flare energetics, researchers sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energics of these flares. The role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model is discussed. Finally, a critique of the current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested
Using a Differential Emission Measure and Density Measurements in an Active Region Core to Test a Steady Heating Model
The frequency of heating events in the corona is an important constraint on
the coronal heating mechanisms. Observations indicate that the intensities and
velocities measured in active region cores are effectively steady, suggesting
that heating events occur rapidly enough to keep high temperature active region
loops close to equilibrium. In this paper, we couple observations of Active
Region 10955 made with XRT and EIS on \textit{Hinode} to test a simple steady
heating model. First we calculate the differential emission measure of the apex
region of the loops in the active region core. We find the DEM to be broad and
peaked around 3\,MK. We then determine the densities in the corresponding
footpoint regions. Using potential field extrapolations to approximate the loop
lengths and the density-sensitive line ratios to infer the magnitude of the
heating, we build a steady heating model for the active region core and find
that we can match the general properties of the observed DEM for the
temperature range of 6.3 Log T 6.7. This model, for the first time,
accounts for the base pressure, loop length, and distribution of apex
temperatures of the core loops. We find that the density-sensitive spectral
line intensities and the bulk of the hot emission in the active region core are
consistent with steady heating. We also find, however, that the steady heating
model cannot address the emission observed at lower temperatures. This emission
may be due to foreground or background structures, or may indicate that the
heating in the core is more complicated. Different heating scenarios must be
tested to determine if they have the same level of agreement.Comment: 16 pages, 9 figures, accepted to Ap
Thermalisation of self-interacting solar flare fast electrons
Most theoretical descriptions of the production of solar flare bremsstrahlung
radiation assume the collision of dilute accelerated particles with a cold,
dense target plasma, neglecting interactions of the fast particles with each
other. This is inadequate for situations where collisions with this background
plasma are not completely dominant, as may be the case in, for example,
low-density coronal sources. We aim to formulate a model of a self-interacting,
entirely fast electron population in the absence of a dense background plasma,
to investigate its implications for observed bremsstrahlung spectra and the
flare energy budget. We derive approximate expressions for the time-dependent
distribution function of the fast electrons using a Fokker-Planck approach. We
use these expressions to generate synthetic bremsstrahlung X-ray spectra as
would be seen from a corresponding coronal source. We find that our model
qualitatively reproduces the observed behaviour of some flares. As the flare
progresses, the model's initial power-law spectrum is joined by a lower energy,
thermal component. The power-law component diminishes, and the growing thermal
component proceeds to dominate the total emission over timescales consistent
with flare observations. The power-law exhibits progressive spectral hardening,
as is seen in some flare coronal sources. We also find that our model requires
a factor of 7 - 10 fewer accelerated electrons than the cold, thick target
model to generate an equivalent hard X-ray flux. This model forms the basis of
a treatment of self-interactions among flare fast electrons, a process which
affords a more efficient means to produce bremsstrahlung photons and so may
reduce the efficiency requirements placed on the particle acceleration
mechanism. It also provides a useful description of the thermalisation of fast
electrons in coronal sources.Comment: 9 pages, 7 figures, accepted for Astronomy & Astrophysics; this
version clarifies arguments around Eqs. (11) and (20
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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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
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
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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