13,433 research outputs found
Are Chromospheric Nanoflares a Primary Source of Coronal Plasma?
It has been suggested that the hot plasma of the solar corona comes primarily
from impulsive heating events, or nanoflares, that occur in the lower
atmosphere, either in the upper part of the ordinary chromosphere or at the
tips of type II spicules. We test this idea with a series of hydrodynamic
simulations. We find that synthetic Fe XII (195) and Fe XIV (274) line profiles
generated from the simulations disagree dramatically with actual observations.
The integrated line intensities are much too faint; the blue shifts are much
too fast; the blue-red asymmetries are much too large; and the emission is
confined to low altitudes. We conclude that chromospheric nanoflares are not a
primary source of hot coronal plasma. Such events may play an important role in
producing the chromosphere and powering its intense radiation, but they do not,
in general, raise the temperature of the plasma to coronal values. Those cases
where coronal temperatures are reached must be relatively uncommon. The
observed profiles of Fe XII and Fe XIV come primarily from plasma that is
heated in the corona itself, either by coronal nanoflares or a quasi-steady
coronal heating process. Chromospheric nanoflares might play a role in
generating waves that provide this coronal heating.Comment: 14 pages, 6 figures, accepted by Astrophysical Journa
Vortex/boundary-layer interactions: Data report, volume 1
This report summarizes the work done under NASA Grant NAGw-581, Vortex/Boundary Layer Interactions. The experimental methods are discussed in detail and numerical results are presented, but are not fully interpreted. This report should be useful to anyone who wishes to make further use of the data (available on floppy disc or magnetic tape) for the development of turbulence models or the validation of predictive methods. Journal papers are in course of preparation
Promoting Emotional and Behavioral Health in Preteens: Benchmarks of Success and Challenges Among Programs in Santa Clara and San Mateo Counties
P/PV conducted a two-year study for The Lucile Packard Foundation for Children's Health to assess the effectiveness of the foundation's youth development grantmaking program and to offer lessons for future grantmaking endeavors. The resulting report describes benchmarks of quality programs for youth and strategies for addressing common program challenges
Diagnosing the time-dependence of active region core heating from the emission measure: II. Nanoflare trains
The time-dependence of heating in solar active regions can be studied by
analyzing the slope of the emission measure distribution cool-ward of the peak.
In a previous study we showed that low-frequency heating can account for 0% to
77% of active region core emission measures. We now turn our attention to
heating by a finite succession of impulsive events for which the timescale
between events on a single magnetic strand is shorter than the cooling
timescale. We refer to this scenario as a "nanoflare train" and explore a
parameter space of heating and coronal loop properties with a hydrodynamic
model. Our conclusions are: (1) nanoflare trains are consistent with 86% to
100% of observed active region cores when uncertainties in the atomic data are
properly accounted for; (2) steeper slopes are found for larger values of the
ratio of the train duration to the post-train cooling and draining
timescale , where depends on the number of heating events,
the event duration and the time interval between successive events ();
(3) may be diagnosed from the width of the hot component of the
emission measure provided that the temperature bins are much smaller than 0.1
dex; (4) the slope of the emission measure alone is not sufficient to provide
information about any timescale associated with heating - the length and
density of the heated structure must be measured for to be uniquely
extracted from the ratio
Child Well-being in the Pacific Rim
This study extends previous efforts to compare the well-being of children using multi-dimensional indicators derived from sample survey and administrative series to thirteen countries in the Pacific Rim. The framework for the analysis of child well-being is to organise 46 indicators into 21 components and organise the components into 6 domains: material situation, health, education, subjective well-being, living environment, as well as risk and safety. Overall, Japan, Singapore and Taiwan have the highest child well-being and Thailand, Indonesia and the Philippines the lowest. However, there are substantial variations between the domains. Japan and Korea perform best on the material well-being of children and also do well on health and education but they have the lowest subjective well-being among their children by some margin. There is a relationship between child well-being and GDP per capita but children in China have higher well-being than you would expect given their GDP and children in Australia have lower well-being. The analysis is constrained by missing data particularly that the Health Behaviour of School-Aged Children Survey is not undertaken in any of these countries
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
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