4,956 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
Energy spectra of 3He-rich solar energetic particles associated with coronal waves
In addition to their anomalous abundances, 3He-rich solar energetic particles
(SEPs) show puzzling energy spectral shapes varying from rounded forms to power
laws where the later are characteristics of shock acceleration. Solar sources
of these particles have been often associated with jets and narrow CMEs, which
are the signatures of magnetic reconnection involving open field. Recent
reports on new associations with large-scale EUV waves bring new insights on
acceleration and transport of 3He-rich SEPs in the corona. We examined energy
spectra for 32 3He-rich SEP events observed by ACE at L1 near solar minimum in
2007-2010 and compared the spectral shapes with solar flare signatures obtained
from STEREO EUV images. We found the events with jets or brightenings tend to
be associated with rounded spectra and the events with coronal waves with power
laws. This suggests that coronal waves may be related to the unknown second
stage mechanism commonly used to interpret spectral forms of 3He-rich SEPs.Comment: Presented at 15th Annual International Astrophysics Conference "The
Science of Ed Stone". Accepted for publication in Journal of Physics:
Conference Serie
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
EIS/Hinode observations of Doppler flow seen through the 40 arcsec wide slit
The Extreme ultraviolet Imaging Spectrometer (EIS) on board Hinode is the
first solar telescope to obtain wide slit spectral images that can be used for
detecting Doppler flows in transition region and coronal lines on the Sun and
to relate them to their surrounding small scale dynamics. We select EIS lines
covering the temperature range 6x10^4 K to 2x10^6 K that give spectrally pure
images of the Sun with the 40 arcsec slit. In these images Doppler shifts are
seen as horizontal brightenings. Inside the image it is difficult to
distinguish shifts from horizontal structures but emission beyond the image
edge can be unambiguously identified as a line shift in several lines separated
from others on their blue or red side by more than the width of the
spectrometer slit (40 pixels). In the blue wing of He II, we find a large
number of events with properties (size and lifetime) similar to the
well-studied explosive events seen in the ultraviolet spectral range.
Comparison with X-Ray Telescope (XRT) images shows many Doppler shift events at
the footpoints of small X-ray loops. The most spectacular event observed showed
a strong blue shift in transition region and lower corona lines from a small
X-ray spot that lasted less than 7 min. The emission appears to be near a cool
coronal loop connecting an X-ray bright point to an adjacent region of quiet
Sun. The width of the emission implies a line-of-sight velocity of 220 km/s. In
addition, we show an example of an Fe XV shift with a velocity about 120 km/s,
coming from what looks like a narrow loop leg connecting a small X-ray
brightening to a larger region of X-ray emission.Comment: 12 pages, 8 figures, to be published in Solar Physic
Shocking Clouds in the Cygnus Loop
With Hubble Space Telescope Wide-Field Planetary Camera 2 observations of the
Cygnus Loop supernova remnant, we examine the interaction of an interstellar
cloud with the blast wave on physical scales of 10^15 cm. The shock front is
distorted, revealing both edge-on and face-on views of filaments and diffuse
emission, similar to those observed on larger scales at lower resolution. We
identify individual shocks in the cloud of density n~15 cm^-3 having velocity
v_s~170 km/s. We also find the morphologically unusual diffuse Balmer-dominated
emission of faster shocks in a lower-density region. The obstacle diffracts
these shocks, so they propagate at oblique angles with respect to the primary
blast wave. The intricate network of diffuse and filamentary H alpha emission
arises during the early stage of interaction between the cloud and blast wave,
demonstrating that complex shock propagation and emission morphology occur
before the onset of instabilities that destroy clouds completely.Comment: 7 pages including 5 figures; 1 color figure; to appear in the ApJ,
Oct. 10, 2001; full-resolution figures available at
http://www.pha.jhu.edu/~levenson/preprints/cyglhst.p
An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere
We present an analysis of the X-ray spectrum of the Local Bubble, obtained by
simultaneously analyzing spectra from two XMM-Newton pointings on and off an
absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use
the difference in the Galactic column density in these two directions to deduce
the contributions of the unabsorbed foreground emission due to the Local
Bubble, and the absorbed emission from the Galactic halo and the extragalactic
background. We find the Local Bubble emission is consistent with emission from
a plasma in collisional ionization equilibrium with a temperature and an emission measure of 0.018 cm^{-6} pc. Our
measured temperature is in good agreement with values obtained from ROSAT
All-Sky Survey data, but is lower than that measured by other recent XMM-Newton
observations of the Local Bubble, which find
(although for some of these observations it is possible that the foreground
emission is contaminated by non-Local Bubble emission from Loop I). The higher
temperature observed towards other directions is inconsistent with our data,
when combined with a FUSE measurement of the Galactic halo O VI intensity. This
therefore suggests that the Local Bubble is thermally anisotropic.
Our data are unable to rule out a non-equilibrium model in which the plasma
is underionized. However, an overionized recombining plasma model, while
observationally acceptable for certain densities and temperatures, generally
gives an implausibly young age for the Local Bubble (\la 6 \times 10^5 yr).Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 9
figure
The Queensland Mango Genomics Initiative
Mango is an important industry for Queensland, Australia, with an annual value exceeding $80 million. The Kensington Pride cultivar, prized by consumers for desirable taste and colour characteristics, commands 60% of the domestic market though this market share has declined in recent years as new varieties, such as Calypso™, get established with consumers. In 2005, the Queensland Government's Department of Agriculture and Fisheries commenced the Mango Genomics Initiative. This project brought together multidisciplinary teams of breeders, pathologists, sensory scientists, flavour chemists and molecular biologists to develop a suite of tools and inter-related data sets to support the accelerated development of new commercial mango varieties. An overview of the Mango Genomics Initiative will be presented here culminating in the generation of a draft Kensington Pride mango genome sequence
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