4,930 research outputs found
Effects of magnetic fields on radiatively overstable shock waves
We discuss high-resolution simulations of one-dimensional, plane-parallel
shock waves with mean speeds between 150 and 240 km/s propagating into gas with
Alfven velocities up to 40 km/s and outline the conditions under which these
radiative shocks experience an oscillatory instability in the cooling length,
shock velocity, and position of the shock front. We investigate two forms of
postshock cooling: a truncated single power law and a more realistic piecewise
power law. The degree of nonlinearity of the instability depends strongly on
the cooling power law and the Alfven Mach number: for power-law indices \alpha
< 0 typical magnetic field strengths may be insufficient either to stabilize
the fundamental oscillatory mode or to prevent the oscillations from reaching
nonlinear amplitudes.Comment: 11 text pages, LaTeX/AASTeX (aaspp4); 5 figures; accepted by Ap
The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution
The stability properties of one-dimensional radiative shocks with a power-law
cooling function of the form are the main
subject of this work. The linear analysis originally presented by Chevalier &
Imamura, is thoroughfully reviewed for several values of the cooling index
and higher overtone modes. Consistently with previous results, it is
shown that the spectrum of the linear operator consists in a series of modes
with increasing oscillation frequency. For each mode a critical value of the
cooling index, , can be defined so that modes with are unstable, while modes with
are stable. The perturbative analysis is complemented by several numerical
simulations to follow the time-dependent evolution of the system for different
values of . Particular attention is given to the comparison between
numerical and analytical results (during the early phases of the evolution) and
to the role played by different boundary conditions. It is shown that an
appropriate treatment of the lower boundary yields results that closely follow
the predicted linear behavior. During the nonlinear regime, the shock
oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle.
The modes of oscillations during this phase do not necessarily coincide with
those predicted by linear theory, but may be accounted for by mode-mode
coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical
Journa
International Product Differentiation Through a Country Brand: An Economic Analysis of National Branding as a Marketing Strategy for Agricultural Products
branding, marketing, Agribusiness, Agricultural and Food Policy, Food Consumption/Nutrition/Food Safety,
Secondary Rayleigh-Taylor type Instabilities in the Reconnection Exhaust Jet as a Mechanism for Supra-Arcade Downflows
Supra-arcade downflows (hereafter referred to as SADs) are low-emission,
elongated, finger-like features usually observed in active-region coronae above
post-eruption flare arcades. Observations exhibit downward moving SADs
intertwined with bright upward moving spikes. Whereas SADs are dark voids,
spikes are brighter, denser structures. Although SADs have been observed for
decades, the mechanism of formation of SADs remains an open issue. In our
three-dimensional resistive magnetohydrodynamic simulations, we demonstrate
that secondary Rayleigh-Taylor type instabilities develop in the downstream
region of a reconnecting current sheet. The instability results in the
formation of low-density coherent structures that resemble SADs, and
high-density structures that appear to be spike-like. Comparison between the
simulation results and observations suggests that secondary Rayleigh-Taylor
type instabilities in the exhaust of reconnecting current sheets provide a
plausible mechanism for observed SADs and spikes
STEREO quadrature observations of coronal dimming at the onset of mini-CMEs
Context: Using unique quadrature observations with the two STEREO spacecraft,
we investigate coronal dimmings at the onset of small-scale eruptions. In CMEs
they are believed to indicate the opening up of the coronal magnetic fields at
the start of the eruption. Aims: It is to determine whether coronal dimming
seen in small-scale eruptions starts before or after chromospheric plasma
ejection. Methods: One STEREO spacecraft obtained high cadence, 75 s, images in
the He II 304A channel, and the other simultaneous images in the Fe IX/FeX 171A
channel. We concentrate on two well-positioned chromospheric eruptions that
occurred at disk center in the 171A images, and on the limb in 304A. One was in
the quiet Sun and the other was in an equatorial coronal hole. We compare the
timing of chromospheric eruption seen in the 304A limb images with the
brightenings and dimmings seen on disk in the 171A images. Further we use
off-limb images of the low frequency 171A power to infer the coronal structure
near the eruptions. Results: In both the quiet Sun and the coronal hole
eruption, on disk 171A dimming was seen before the chromospheric eruption, and
in both cases it extends beyond the site of the chromospheric eruption. The
quiet Sun eruption occurred on the outer edge of the enclosing magnetic field
of a prominence and may be related to a small disruption of the prominence just
before the 171A dimming. Conclusions: These small-scale chromospheric eruptions
started with a dimming in coronal emission just like their larger counterparts.
We therefore suggest that a fundamental step in triggering them was the removal
of overlying coronal field.Comment: 4 pages, 8 figures. To appear A&A Letters. Movies accompanying this
Letter are at http://www.mps.mpg.de/data/outgoing/innes/dims
International Product Differentiation through a Country Brand: An Economic Analysis of National Branding as a Marketing Strategy for Agricultural Products
Trade policy initiatives of developed country governments are in flux. Governmentsâ need for new trade policy measures has arisen partly because of constraints imposed on the use of export subsidies by the Agreement on Agriculture reached as part of the Uruguay Round of General Agreement on Tariffs and Trade in 1994. Further disciplines on export subsidies and other policy measures may be agreed on in the Doha Round of World Trade Organization (WTO) negotiations, accentuating the need for new policy measures. While the Doha Round may not successfully reach an agreement, the current modalities show provisional agreement on the elimination of multiple forms of export subsidies. There is provisional agreement on more stringent restrictions on the use of export credit programs. Controls on exporting state trading agenciesâ ability to subsidize exports are tentatively agreed (Furtan, 2005). Food aid, which can also be used to circumvent disciplines on export subsidies, is also likely to be subject to WTO disciplines.International Relations/Trade,
Radiative instabilities in simulations of spherically symmetric supernova blast waves
High-resolution simulations of the cooling regions of spherically symmetric
supernova remnants demonstrate a strong radiative instability. This
instability, whose presence is dependent on the shock velocity, causes
large-amplitude fluctuations in the shock velocity. The fluctuations begin
almost immediately after the radiative phase begins (upon shell formation) if
the shock velocity lies in the unstable range; they last until the shock slows
to speeds less than approximately 130 km/s. We find that shock-velocity
fluctuations from the reverberations of waves within the remnant are small
compared to those due to the instability. Further, we find (in plane-parallel
simulations) that advected inhomogeneities from the external medium do not
interfere with the qualitative nature of the instability-driven fluctuations.
Large-amplitude inhomogeneities may alter the phases of shock-velocity
fluctuations, but do not substantially reduce their amplitudes.Comment: 18 pages text, LaTeX/AASTeX (aaspp4); 10 figures; accepted by Ap
Multi-Spacecraft Observations of Recurrent 3He-Rich Solar Energetic Particles
We study the origin of 3He-rich solar energetic particles (<1 MeV/nucleon)
that are observed consecutively on STEREO-B, ACE, and STEREO-A spacecraft when
they are separated in heliolongitude by more than 90{\deg}. The 3He-rich period
on STEREO-B and STEREO-A commences on 2011 July 1 and 2011 July 16,
respectively. The ACE 3He-rich period consists of two sub-events starting on
2011 July 7 and 2011 July 9. We associate the STEREO-B July 1 and ACE July 7
3He-rich events with the same sizeable active region producing X-ray flares
accompanied by prompt electron events, when it was near the west solar limb as
seen from the respective spacecraft. The ACE July 9 and STEREO-A July 16 events
were dispersionless with enormous 3He enrichment, lacking solar energetic
electrons and occurring in corotating interaction regions. We associate these
events with a small, recently emerged active region near the border of a
low-latitude coronal hole that produced numerous jet-like emissions temporally
correlated with type III radio bursts. For the first time we present
observations of 1) solar regions with long-lasting conditions for 3He
acceleration and 2) solar energetic 3He that is temporary
confined/re-accelerated in interplanetary space.Comment: accepted for publication in The Astrophysical Journa
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