18 research outputs found
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
Selective serotonin reuptake inhibitors in childhood and adolescent depression: recent controversies
Metronidazole: a method for its determination in biological fluids and its disposition kinetics in the dog
Continent-wide declines in shallow reef life over a decade of ocean warming.
Human society is dependent on nature , but whether our ecological foundations are at risk remains unknown in the absence of systematic monitoring of species’ populations . Knowledge of species fuctuations is particularly inadequate in the marine realm . Here we assess the population trends of 1,057 common shallow reef species from multiple phyla at 1,636 sites around Australia over the past decade. Most populations decreased over this period, including many tropical fshes, temperate invertebrates (particularly echinoderms) and southwestern Australian macroalgae, whereas coral populations remained relatively stable. Population declines typically followed heatwave years, when local water temperatures were more than 0.5 °C above temperatures in 2008. Following heatwaves , species abundances generally tended to decline near warm range edges, and increase near cool range edges. More than 30% of shallow invertebrate species in cool latitudes exhibited high extinction risk, with rapidly declining populations trapped by deep ocean barriers, preventing poleward retreat as temperatures rise. Greater conservation effort is needed to safeguard temperate marine ecosystems, which are disproportionately threatened and include species with deep evolutionary roots. Fundamental among such efforts, and broader societal needs to efficiently adapt to interacting anthropogenic and natural pressures, is greatly expanded monitoring of species’ population trends .Graham J. Edgar ... Camille Mellin ... et al