191 research outputs found
The Response of Antarctic Sea Ice Algae to Changes in pH and CO2
Ocean acidification substantially alters ocean carbon chemistry and hence pH but the effects on sea ice formation and the
CO2 concentration in the enclosed brine channels are unknown. Microbial communities inhabiting sea ice ecosystems
currently contribute 10–50% of the annual primary production of polar seas, supporting overwintering zooplankton species,
especially Antarctic krill, and seeding spring phytoplankton blooms. Ocean acidification is occurring in all surface waters but
the strongest effects will be experienced in polar ecosystems with significant effects on all trophic levels. Brine algae
collected from McMurdo Sound (Antarctica) sea ice was incubated in situ under various carbonate chemistry conditions. The
carbon chemistry was manipulated with acid, bicarbonate and bases to produce a pCO2 and pH range from 238 to
6066 matm and 7.19 to 8.66, respectively. Elevated pCO2 positively affected the growth rate of the brine algal community,
dominated by the unique ice dinoflagellate, Polarella glacialis. Growth rates were significantly reduced when pH dropped
below 7.6. However, when the pH was held constant and the pCO2 increased, growth rates of the brine algae increased by
more than 20% and showed no decline at pCO2 values more than five times current ambient levels. We suggest that
projected increases in seawater pCO2, associated with OA, will not adversely impact brine algal communities
Formation and Structure of a Current Sheet in Pulsed-Power Driven Magnetic Reconnection Experiments
We describe magnetic reconnection experiments using a new, pulsed-power
driven experimental platform in which the inflows are super-sonic but
sub-Alfv\'enic.The intrinsically magnetised plasma flows are long lasting,
producing a well-defined reconnection layer that persists over many
hydrodynamic time scales.The layer is diagnosed using a suite of high
resolution laser based diagnostics which provide measurements of the electron
density, reconnecting magnetic field, inflow and outflow velocities and the
electron and ion temperatures.Using these measurements we observe a balance
between the power flow into and out of the layer, and we find that the heating
rates for the electrons and ions are significantly in excess of the classical
predictions. The formation of plasmoids is observed in laser interferometry and
optical self-emission, and the magnetic O-point structure of these plasmoids is
confirmed using magnetic probes.Comment: 14 pages, 12 figures. Accepted for publication in Physics of Plasma
Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors
We present results from new experiments to study the dynamics of radiative
shocks, reverse shocks and radiative precursors. Laser ablation of a solid
piston by the Orion high-power laser at AWE Aldermaston UK was used to drive
radiative shocks into a gas cell initially pressurised between and $1.0 \
bar with different noble gases. Shocks propagated at {80 \pm 10 \ km/s} and
experienced strong radiative cooling resulting in post-shock compressions of {
\times 25 \pm 2}. A combination of X-ray backlighting, optical self-emission
streak imaging and interferometry (multi-frame and streak imaging) were used to
simultaneously study both the shock front and the radiative precursor. These
experiments present a new configuration to produce counter-propagating
radiative shocks, allowing for the study of reverse shocks and providing a
unique platform for numerical validation. In addition, the radiative shocks
were able to expand freely into a large gas volume without being confined by
the walls of the gas cell. This allows for 3-D effects of the shocks to be
studied which, in principle, could lead to a more direct comparison to
astrophysical phenomena. By maintaining a constant mass density between
different gas fills the shocks evolved with similar hydrodynamics but the
radiative precursor was found to extend significantly further in higher atomic
number gases (\sim4$ times further in xenon than neon). Finally, 1-D and 2-D
radiative-hydrodynamic simulations are presented showing good agreement with
the experimental data.Comment: HEDLA 2016 conference proceeding
BOW SHOCK FRAGMENTATION DRIVEN BY A THERMAL INSTABILITY IN LABORATORY ASTROPHYSICS EXPERIMENTS
The role of radiative cooling during the evolution of a bow shock was studied
in laboratory-astrophysics experiments that are scalable to bow shocks present
in jets from young stellar objects. The laboratory bow shock is formed during
the collision of two counter-streaming, supersonic plasma jets produced by an
opposing pair of radial foil Z-pinches driven by the current pulse from the
MAGPIE pulsed-power generator. The jets have different flow velocities in the
laboratory frame and the experiments are driven over many times the
characteristic cooling time-scale. The initially smooth bow shock rapidly
develops small-scale non-uniformities over temporal and spatial scales that are
consistent with a thermal instability triggered by strong radiative cooling in
the shock. The growth of these perturbations eventually results in a global
fragmentation of the bow shock front. The formation of a thermal instability is
supported by analysis of the plasma cooling function calculated for the
experimental conditions with the radiative packages ABAKO/RAPCAL.Comment: 9 pages, 5 figures, Accepted for publication in The Astrophysical
Journal on 5th November 201
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