505 research outputs found
Ejection of cool plasma into the hot corona
We investigate the processes that lead to the formation, ejection and fall of
a confined plasma ejection that was observed in a numerical experiment of the
solar corona. By quantifying physical parameters such as mass, velocity, and
orientation of the plasma ejection relative to the magnetic field, we provide a
description of the nature of this particular phenomenon. The time-dependent
three-dimensional magnetohydrodynamic (3D MHD) equations are solved in a box
extending from the chromosphere to the lower corona. The plasma is heated by
currents that are induced through field line braiding as a consequence of
photospheric motions. Spectra of optically thin emission lines in the extreme
ultraviolet range are synthesized, and magnetic field lines are traced over
time. Following strong heating just above the chromosphere, the pressure
rapidly increases, leading to a hydrodynamic explosion above the upper
chromosphere in the low transition region. The explosion drives the plasma,
which needs to follow the magnetic field lines. The ejection is then moving
more or less ballistically along the loop-like field lines and eventually drops
down onto the surface of the Sun. The speed of the ejection is in the range of
the sound speed, well below the Alfven velocity. The plasma ejection is
basically a hydrodynamic phenomenon, whereas the rise of the heating rate is of
magnetic nature. The granular motions in the photosphere lead (by chance) to a
strong braiding of the magnetic field lines at the location of the explosion
that in turn is causing strong currents which are dissipated. Future studies
need to determine if this process is a ubiquitous phenomenon on the Sun on
small scales. Data from the Atmospheric Imaging Assembly on the Solar Dynamics
Observatory (AIA/SDO) might provide the relevant information.Comment: 12 pages, 10 figure
Constant cross section of loops in the solar corona
The corona of the Sun is dominated by emission from loop-like structures.
When observed in X-ray or extreme ultraviolet emission, these million K hot
coronal loops show a more or less constant cross section. In this study we show
how the interplay of heating, radiative cooling, and heat conduction in an
expanding magnetic structure can explain the observed constant cross section.
We employ a three-dimensional magnetohydrodynamics (3D MHD) model of the
corona. The heating of the coronal plasma is the result of braiding of the
magnetic field lines through footpoint motions and subsequent dissipation of
the induced currents. From the model we synthesize the coronal emission, which
is directly comparable to observations from, e.g., the Atmospheric Imaging
Assembly on the Solar Dynamics Observatory (AIA/SDO). We find that the
synthesized observation of a coronal loop seen in the 3D data cube does match
actually observed loops in count rate and that the cross section is roughly
constant, as observed. The magnetic field in the loop is expanding and the
plasma density is concentrated in this expanding loop; however, the temperature
is not constant perpendicular to the plasma loop. The higher temperature in the
upper outer parts of the loop is so high that this part of the loop is outside
the contribution function of the respective emission line(s). In effect, the
upper part of the plasma loop is not bright and thus the loop actually seen in
coronal emission appears to have a constant width. From this we can conclude
that the underlying field-line-braiding heating mechanism provides the proper
spatial and temporal distribution of the energy input into the corona --- at
least on the observable scales.Comment: 8 pages, 9 figures, accepted for publication in A&
Markets or Monopolies? Considerations for Addressing Health Care Consolidation in California
Over the past three decades, markets for health insurers and providers have gone through waves of consolidation. As of 2018, 95% of metropolitan areas in the United States had highly concentrated hospital markets. Markets for health insurers are also highly concentrated. Between 2006 and 2014, the combined market share of the top four insurers climbed from 74% to 83%. The coronavirus pandemic appears to be fueling another round of consolidation — especially acquisition of providers by private equity firms.Markets or Monopolies? Considerations for Addressing Health Care Consolidation in California compiles the latest research and data on California's health care systems and shows that consolidation is not limited to any one system, market segment, or geographic region in the state: Most markets across California are highly concentrated. Hospital markets, in particular, are now approaching "monopoly levels" in many California counties. In addition, there is mounting evidence that mergers of health care companies are resulting in increased prices for health care services, with little or no improvement in quality for consumers.The report highlights several actions policymakers could consider, given significant consolidation. For example, California's attorney general has the authority to block transactions that transfer a "material amount of the assets" only for nonprofit health facilities. To increase scrutiny of provider mergers in California, policymakers could require all health care providers — not just nonprofit ones — to provide written notice to, and obtain the written consent of, the attorney general. Policymakers could also expand the authority of state regulatory agencies to include "affordability standards" when they review health insurance plans for sale in California
Magnetic field diagnostics and spatio-temporal variability of the solar transition region
Magnetic field diagnostics of the transition region from the chromosphere to
the corona faces us with the problem that one has to apply extreme UV
spectro-polarimetry. While for coronal diagnostic techniques already exist
through infrared coronagraphy above the limb and radio observations on the
disk, for the transition region one has to investigate extreme UV observations.
However, so far the success of such observations has been limited, but there
are various projects to get spectro-polarimetric data in the extreme UV in the
near future. Therefore it is timely to study the polarimetric signals we can
expect for such observations through realistic forward modeling.
We employ a 3D MHD forward model of the solar corona and synthesize the
Stokes I and Stokes V profiles of C IV 1548 A. A signal well above 0.001 in
Stokes V can be expected, even when integrating for several minutes in order to
reach the required signal-to-noise ratio, despite the fact that the intensity
in the model is rapidly changing (just as in observations). Often this
variability of the intensity is used as an argument against transition region
magnetic diagnostics which requires exposure times of minutes. However, the
magnetic field is evolving much slower than the intensity, and thus when
integrating in time the degree of (circular) polarization remains rather
constant. Our study shows the feasibility to measure the transition region
magnetic field, if a polarimetric accuracy on the order of 0.001 can be
reached, which we can expect from planned instrumentation.Comment: Accepted for publication in Solar Physics (4.Mar.2013), 19 pages, 9
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Electrically Driven Light Emission from Individual CdSe Nanowires
We report electroluminescence (EL) measurements carried out on three-terminal
devices incorporating individual n-type CdSe nanowires. Simultaneous optical
and electrical measurements reveal that EL occurs near the contact between the
nanowire and a positively biased electrode or drain. The surface potential
profile, obtained by using Kelvin probe microscopy, shows an abrupt potential
drop near the position of the EL spot, while the band profile obtained from
scanning photocurrent microscopy indicates the existence of an n-type Schottky
barrier at the interface. These observations indicate that light emission
occurs through a hole leakage or an inelastic scattering induced by the rapid
potential drop at the nanowire-electrode interface.Comment: 12 pages, 4 figure
Growth and Transport Properties of Complementary Germanium Nanowire Field Effect Transistors
n- and p-type Ge nanowires were synthesized by a multistep process in which axial elongation, via vapor–liquid–solid (VLS) growth, and doping were accomplished in separate chemical vapor deposition steps. Intrinsic, single-crystal, Ge nanowires prepared by Au nanocluster-mediated VLS growth were surface-doped in situ using diborane or phosphine, and then radial growth of an epitaxial Ge shell was used to cap the dopant layer. Field-effect transistors prepared from these Ge nanowires exhibited on currents and transconductances up to 850 µA/µm and 4.9 µA/V, respectively, with device yields of \u3e85%
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