516 research outputs found
Some notes on worms in sheep in southern Western Australia
DURING the last fifty or more years several surveys have been made of the worm parasites which occur in southern Western Australia.
In the \u27fifties, thanks to the stimulus of Mr. H. McL. Gordon, supported by Dr. H. W. Bennetts, observations were made on the sequence of the infestations in autumn-born weaners in the country stretching from Moora to Cranbrook and Bridgetown, and later near Geraldton and Esperance
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The combustion characteristics and stable carbon isotopic compositions of irradiated organic matter: implications for terrestrial and extraterrestrial sample analysis
Exposure to ionizing radiation causes the mean combustion temperature of naturally occurring, solid, terrestrial organic matter, derived from the radiation-induced polymerization of methane, to increase
Nonlinear wave propagation and reconnection at magnetic X-points in the Hall MHD regime
The highly dynamical, complex nature of the solar atmosphere naturally
implies the presence of waves in a topologically varied magnetic environment.
Here, the interaction of waves with topological features such as null points is
inevitable and potentially important for energetics. The low resistivity of the
solar coronal plasma implies that non-MHD effects should be considered in
studies of magnetic energy release in this environment. This paper investigates
the role of the Hall term in the propagation and dissipation of waves, their
interaction with 2D magnetic X-points and the nature of the resulting
reconnection. A Lagrangian remap shock-capturing code (Lare2d) is used to study
the evolution of an initial fast magnetoacoustic wave annulus for a range of
values of the ion skin depth in resistive Hall MHD. A magnetic null-point
finding algorithm is also used to locate and track the evolution of the
multiple null-points that are formed in the system. Depending on the ratio of
ion skin depth to system size, our model demonstrates that Hall effects can
play a key role in the wave-null interaction. In particular, the initial
fast-wave pulse now consists of whistler and ion-cyclotron components; the
dispersive nature of the whistler wave leads to (i) earlier interaction with
the null, (ii) the creation of multiple additional, transient nulls and, hence,
an increased number of energy release sites. In the Hall regime, the relevant
timescales (such as the onset of reconnection and the period of the oscillatory
relaxation) of the system are reduced significantly, and the reconnection rate
is enhanced.Comment: 13 pages, 10 figure
Three-dimensional shielding effects on charged particle fluences measured in the P0006 experiment of LDEF
Three-dimensional shielding effects on cosmic ray charged particle fluences were measured with plastic nuclear track detectors in the P0006 experiment on Long Duration Exposure Facility (LDEF). The azimuthal and polar angle distributions of the galactic cosmic ray particles (mostly relativistic iron) were measured in the main stack and in four side stacks of the P0006 experiment, located on the west end of the LDEF satellite. A shadowing effect of the shielding of the LDEF satellite is found. Total fluence of stopping protons was measured as a function of the position in the main and side stacks of the P0006 experiment. Location dependence of total track density is explained by the three-dimensional shielding model of the P0006 stack. These results can be used to validate 3D mass model and transport code calculations and also for predictions of the outer radiation environment for the Space Station Freedom
Radiation exposure of LDEF: Initial results
Initial results from LDEF include radiation detector measurements from four experiments, P0006, P0004, M0004, and A0015. The detectors were located on both the leading and trailing edges of the orbiter and also on the Earthside end. This allowed the directional dependence of the incoming radiation to be measured. Total absorbed doses from thermoluminescent detectors (TLDs) verified the predicted spatial east-west dose ratio dependence of a factor approx. 2.5, due to trapped proton anisotropy in the South Atlantic Anomaly. On the trailing edge of the orbiter a range of doses from 6.64 to 2.91 Gy were measured under Al equivalent shielding of 0.42 to 1.11 g/sq cm. A second set of detectors near this location yielded doses of 6.48 to 2.66 Gy under Al equivalent shielding of 0.48 to 15.4 g/sq cm. On the leading edge, doses of 2.58 to 2.10 Gy were found under Al equivalent shielding of 1.37 to 2.90 g/sq cm. Initial charged particle LET (linear energy transfer) spectra, fluxes, doses and dose equivalents, for LET in H2O greater than or = 8 keV/micron, were measured with plastic nuclear track detectors (PNTDs) located in two experiments. Also preliminary data on low energy neutrons were obtained from detectors containing (6)LiF foils
Charged particle LET-spectra measurements aboard LDEF
The linear energy transfer (LET) spectra of charged particles was measured in the 5 to 250 keV/micron (water) interval with CR-39 and in the 500 to 1500 keV/micron (water) interval with polycarbonate plastic nuclear track detectors (PNTDs) under different shielding depths in the P0006 experiment. The optimal processing conditions were determined for both PNTDs in relation to the relatively high track densities due to the long term exposure in space. The total track density was measured over the selected samples, and tracks in coincidence on the facing surfaces of two detector sheets were selected for measuring at the same position on each sheet. The short range (SR) and Galactic Cosmic Ray (GCR) components were measured separately with CR-39 PNTDs and the integral dose and dose rate spectra of charged particles were also determined. The high LET portion of the LET spectra was measured with polycarbonate PNTDs with high statistical accuracy. This is a unique result of this exposure due to the low flux of these types of particles for typical spaceflight durations. The directional dependence of the charged particles at the position of the P0006 experiment was also studied by four small side stacks which surrounded the main stack and by analyzing the dip angle and polar angle distributions of the measured SR and GCR particle tracks in the main stack
LET spectra measurements of charged particles in the P0006 experiment on LDEF
Measurements are under way of the charged particle radiation environment of the Long Duration Exposure Facility (LDEF) satellite using stacks of plastic nuclear track detectors (PNTD's) placed in different locations of the satellite. In the initial work the charge, energy, and linear energy transfer (LET) spectra of charged particles were measured with CR-39 double layer PNTD's located on the west side of the satellite (Experiment P0006). Primary and secondary stopping heavy ions were measured separately from the more energetic particles. Both trapped and galactic cosmic ray (GCR) particles are included, with the latter component being dominated by relativistic iron particles. The results from the P0006 experiment will be compared with similar measurements in other locations on LDEF with different orientation and shielding conditions. The remarkably detailed investigation of the charged particle radiation environment of the LDEF satellite will lead to a better understanding of the radiation environment of the Space Station Freedom. It will enable more accurate prediction of single event upsets (SEU's) in microelectronics and, especially, more accurate assessment of the risk - contributed by different components of the radiation field (GCR's, trapped protons, secondaries and heavy recoils, etc.) - to the health and safety of crew members
The onset of Impulsive Bursty reconnection at a two-dimensional current layer
The sudden reconnection of a non-force free 2D current layer, embedded in a
low-beta plasma, triggered by the onset of an anomalous resistivity, is studied
in detail. The resulting behaviour consists of two main phases. Firstly, a
transient reconnection phase, in which the current in the layer is rapidly
dispersed and some flux is reconnected. This dispersal of current launches a
family of small amplitude magnetic and plasma perturbations, which propagate
away from the null at the local fast and slow magnetosonic speeds. The vast
majority of the magnetic energy released in this phase goes into internal
energy of the plasma, and only a tiny amount is converted into kinetic energy.
In the wake of the outwards propagating pulses, an imbalance of Lorentz and
pressure forces creates a stagnation flow which drives a regime of impulsive
bursty reconnection, in which fast reconnection is turned on and off in a
turbulent manner as the current density exceeds and falls below a critical
value. During this phase, the null current density is continuously built up
above a certain critical level, then dissipated very rapidly, and built up
again, in a stochastic manner. Interestingly, the magnetic energy converted
during this quasi-steady phase is greater than that converted during the
initial transient reconnection phase. Again essentially all the energy
converted during this phase goes directly to internal energy. These results are
of potential importance for solar flares and coronal heating, and set a
conceptually important reference for future 3D studies
Is null-point reconnection important for solar flux emergence?
The role of null-point reconnection in a 3D numerical MHD model of solar
emerging flux is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to interact and
reconnect with a horizontal overlying magnetic field in the atmosphere. Null
points appear as the reconnection begins and persist throughout the rest of the
emergence, where they can be found mostly in the model photosphere and
transition region, forming two loose clusters on either side of the emerging
flux tube. Up to 26 nulls are present at any one time, and tracking in time
shows that there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality of the nulls
in terms of their methods of creation and destruction, their balance of signs,
their long lifetimes, and their geometrical stability. We then show that due to
the low parallel electric fields associated with the nulls, null-point
reconnection is not the main type of magnetic reconnection involved in the
interaction of the newly emerged flux with the overlying field. However, the
large number of nulls implies that the topological structure of the magnetic
field must be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc
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