186 research outputs found
Multiple transonic solutions and a new class of shock transitions in solar and stellar winds
The steady isothermal solar wind equations are shown to admit, under certain circumstances, mutliple transonic solutions when, for example, momentum deposition gives rise to multiplee critical points in the flow. These multiple solutions consist of a continuous solution and solutions which involve shock transitions between critical solutions. The ambiguity arising from the multiplicity of the solutions can be resolved by following the time evolution of a wind profile with one critical point. Results of the numerical integration of the time-dependent equations with momentum addition show that each of these multiple solutions is physically accessible and depends on the rate of change of momentum deposition. These results suggest that standing shocks are likely to be present in the inner solar wind flow
Formation of standing shocks in stellar winds and related astrophysical flows
Stellar winds and other analogous astrophysical flows can be described, to lowest order, by the familiar one dimensional hydrodynamic equations which, being nonlinear, admit in some instances discontinuous as well as continuous transonic solutions for identical inner boundary conditions. The characteristics of the time dependent differential equations of motion are described to show how a perturbation changes profile in time and, under well defined conditions, develops into a stationary shock discontinuity. The formation of standing shocks in wind type astrophysical flows depends on the fulfillment of appropriate necessary conditions, which are determined by the conservation of mass, momentum and energy across the discontinuity, and certain sufficient conditions, which are determined by the flow's history
Coronal plasma diagnostics from ground‐based observations
In this paper we discuss the potential of ground‐based visible observations of the solar corona to address the key open problems in the physics of the solar atmosphere and of solar activity. We first compare the diagnostic potential of visible observations with those of high‐resolution spectrometers and narrowband imagers working in the EUV and X‐ray wavelength ranges. We then review the main diagnostic techniques (and introduce a few new ones) that can be applied to line and continuum emission in the solar atmosphere, and the physical problems that they enable us to address. Finally, we briefly review the main features of ground‐based coronographic instrumentation currently being developed and planned.Key PointsWe compare the characteristics of visible coronal observations with those in the EUV and X‐ray wavelength rangesWe review the magnetic field and plasma diagnostic techniques from coronal spectral lines in the visibleWe describe the future ground‐based coronographic instrumentation for coronal observations in the visible wavelength rangePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134492/1/jgra52907_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134492/2/jgra52907.pd
Explosive Events and the Evolution of the Photospheric Magnetic Field
Transition region explosive events have long been suggested as direct
signatures of magnetic reconnection in the solar atmosphere. In seeking further
observational evidence to support this interpretation, we study the relation
between explosive events and the evolution of the solar magnetic field as seen
in line-of-sight photospheric magnetograms. We find that about 38% of events
show changes of the magnetic structure in the photosphere at the location of an
explosive event over a time period of 1 h. We also discuss potential
ambiguities in the analysis of high sensitivity magnetograms
Energy Distribution of Micro-events in the Quiet Solar Corona
Recent imaging observations of EUV line emissions have shown evidence for
frequent flare-like events in a majority of the pixels in quiet regions of the
solar corona. The changes in coronal emission measure indicate impulsive
heating of new material to coronal temperatures. These heating or evaporation
events are candidate signatures of "nanoflares" or "microflares" proposed to
interpret the high temperature and the very existence of the corona. The energy
distribution of these micro-events reported in the literature differ widely,
and so do the estimates of their total energy input into the corona. Here we
analyze the assumptions of the different methods, compare them by using the
same data set and discuss their results.
We also estimate the different forms of energy input and output, keeping in
mind that the observed brightenings are most likely secondary phenomena. A
rough estimate of the energy input observed by EIT on the SoHO satellite is of
the order of 10% of the total radiative output in the same region. It is
considerably smaller for the two reported TRACE observations. The discrepancy
can be explained partially by different thresholds for flare detection. There
is agreement on the slope and the absolute value of the distribution if the
same method were used and a numerical error corrected. The extrapolation of the
power law to unobserved energies that are many orders of magnitude smaller
remains questionable. Nevertheless, these micro-events and unresolved smaller
events are currently the best source of information on the heating process of
the corona
A new look at a polar crown cavity as observed by SDO/AIA
Context.
The Solar Dynamics Observatory (SDO) was launched in February 2010 and is now providing an unprecedented view of the solar activity at high spatial resolution and high cadence covering a broad range of temperature layers of the atmosphere.
Aims.
We aim at defining the structure of a polar crown cavity and describing its evolution during the erupting process.
Methods.
We use the high-cadence time series of SDO/AIA observations at 304 Å (50 000 K) and 171 Å (0.6 MK) to determine the structure of the polar crown cavity and its associated plasma, as well as the evolution of the cavity during the different phases of the eruption. We report on the observations recorded on 13 June 2010 located on the north-west limb.
Results.
We observe coronal plasma shaped by magnetic field lines with a negative curvature (U-shape) sitting at the bottom of a cavity. The cavity is located just above the polar crown filament material. We thus observe the inner part of the cavity above the filament as depicted in the classical three part coronal mass ejection (CME) model composed of a filament, a cavity, and a CME front. The filament (in this case a polar crown filament) is part of the cavity, and it makes a continuous structuring from the filament to the CME front depicted by concentric ellipses (in a 2D cartoon).
Conclusions.
We propose to define a polar crown cavity as a density depletion sitting above denser polar crown filament plasma drained down the cavity by gravity. As part of the polar crown filament, plasma at different temperatures (ranging from 50 000 K to 0.6 MK) is observed at the same location on the cavity dips and sustained by a competition between the gravity and the curvature of magnetic field lines. The eruption of the polar crown cavity as a solid body can be decomposed into two phases: a slow rise at a speed of 0.6 km s-1 and an acceleration phase at a mean speed of 25 km s-1
Space-time localization of inner heliospheric plasma turbulence using multiple spacecraft radio links
Radio remote sensing of the heliosphere using spacecraft radio signals has
been used to study the near-sun plasma in and out of the ecliptic, close to the
sun, and on spatial and temporal scales not accessible with other techniques.
Studies of space-time variations in the inner solar wind are particularly
timely because of the desire to understand and predict space weather, which can
disturb satellites and systems at 1AU and affect human space exploration. Here
we demonstrate proof-of-concept of a new radio science application for
spacecraft radio science links. The differing transfer functions of plasma
irregularities to spacecraft radio up- and downlinks can be exploited to
localize plasma scattering along the line of sight. We demonstrate the utility
of this idea using Cassini radio data taken in 2001-2002. Under favorable
circumstances we demonstrate how this technique, unlike other remote sensing
methods, can determine center-of-scattering position to within a few
thousandths of an AU and thickness of scattering region to less than about 0.02
AU. This method, applied to large data sets and used in conjunction with other
solar remote sensing data such as white light data, has space weather
application in studies of inhomogeneity and nonstationarity in the near-sun
solar wind.Comment: 28 Pages including 14 Figures (7 unique figures in both inline format
and full-page format)
From Forbidden Coronal Lines to Meaningful Coronal Magnetic Fields
We review methods to measure magnetic fields within the corona using the
polarized light in magnetic-dipole (M1) lines. We are particularly interested
in both the global magnetic-field evolution over a solar cycle, and the local
storage of magnetic free energy within coronal plasmas. We address commonly
held skepticisms concerning angular ambiguities and line-of-sight confusion. We
argue that ambiguities are in principle no worse than more familiar remotely
sensed photospheric vector-fields, and that the diagnosis of M1 line data would
benefit from simultaneous observations of EUV lines. Based on calculations and
data from eclipses, we discuss the most promising lines and different
approaches that might be used. We point to the S-like [Fe {\sc XI}] line (J=2
to J=1) at 789.2nm as a prime target line (for ATST for example) to augment the
hotter 1074.7 and 1079.8 nm Si-like lines of [Fe {\sc XIII}] currently observed
by the Coronal Multi-channel Polarimeter (CoMP). Significant breakthroughs will
be made possible with the new generation of coronagraphs, in three distinct
ways: (i) through single point inversions (which encompasses also the analysis
of MHD wave modes), (ii) using direct comparisons of synthetic MHD or
force-free models with polarization data, and (iii) using tomographic
techniques.Comment: Accepted by Solar Physics, April 201
Properties of solar polar coronal plumes constrained by Ultraviolet Coronagraph Spectrometer data
We investigate the plasma dynamics (outflow speed and turbulence) inside
polar plumes. We compare line profiles (mainly of \ion{O}{6}) observed by the
UVCS instrument on SOHO at the minimum of solar cycle 22-23 with model
calculations. We consider Maxwellian velocity distributions with different
widths in plume and inter-plume regions. Electron densities are assumed to be
enhanced in plumes and to approach inter-plume values with increasing height.
Different combinations of the outflow and turbulence velocity in the plume
regions are considered. We compute line profiles and total intensities of the
\ion{H}{1} Ly and the \ion{O}{6} doublets. The observed profile shapes
and intensities are reproduced best by a small solar wind speed at low
altitudes in plumes that increases with height to reach ambient inter-plume
values above roughly 3-4 R_\sun combined with a similar variation of the
width of the velocity distribution of the scattering atoms/ions. We also find
that plumes very close to the pole give narrow profiles at heights above 2.5
R_\sun, which are not observed. This suggests a tendency for plumes to be
located away from the pole. We find that the inclusion of plumes in the model
computations provides an improved correspondence with the observations and
confirms previous results showing that published UVCS observations in polar
coronal holes can be roughly reproduced without the need for large temperature
anisotropy. The latitude distributions of plumes and magnetic flux
distributions are studied by analyzing data from different instruments on SOHO
and with SOLIS.Comment: 11 figure
Dynamic mitral regurgitation and acute pulmonary edema
peer reviewedWe report the case of a 61-year old patient with signs and symptoms of heart failure with mid-range left ventricular ejection fraction and moderate mitral regurgitation of mixed etiology (rheumatic heart disease, toxic and ischemic). The dynamic behaviour of the mitral regurgitation was revealed by an acute episode of pulmonary edema in the context of an abrupt elevation of blood pressure inducing an increase in left ventricular afterload. Dynamic mitral regurgitation must be considered in any patient with exercise dyspnea who has a moderate mitral regurgitation in resting conditions or in patients with repeated acute pulmonary edema without an obvious cause. Exercise stress echocardiography is the best diagnostic test to explore the dynamic behaviour of the mitral regurgitation. Surgery or percutaneous treatment may be proposed in severe cases
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