834 research outputs found
First record of freshwater fish on the Cape Verdean archipelago
The Cape Verdean islands form a distinct aquatic freshwater ecoregion characterized mainly by temporal water bodies with an adapted invertebrate community. Freshwater fish were not previously recorded from the archipelago. During a non-exhaustive survey of freshwater bodies on five islands of the archipelago, the first presence of a freshwater fish was recorded. Using barcoding sequences, the species was identified as the guppy (Poecilia reticulata), a highly invasive species alien to the Cape Verdean Islands
Drivers of linkage disequilibrium across a species' geographic range
While linkage disequilibrium (LD) is an important parameter in genetics and evolutionary biology, the drivers of LD remain elusive. Using whole-genome sequences from across a species' range, we assessed the impact of demographic history and mating system on LD. Both range expansion and a shift from outcrossing to selfing in North American Arabidopsis lyrata were associated with increased average genome-wide LD. Our results indicate that range expansion increases short-distance LD at the farthest range edges by about the same amount as a shift to selfing. However, the extent over which LD in genic regions unfolds was shorter for range expansion compared to selfing. Linkage among putatively neutral variants and between neutral and deleterious variants increased to a similar degree with range expansion, providing support that genome-wide LD was positively associated with mutational load. As a consequence, LD combined with mutational load may decelerate range expansions and set range limits. Finally, a small number of genes were identified as LD outliers, suggesting that they experience selection by either of the two demographic processes. These included genes involved in flowering and photoperiod for range expansion, and the self-incompatibility locus for mating system
Cluster PEACE observations of electron pressure tensor divergence in the magnetotail
Cluster crossed the magnetotail neutral sheet on four occasions between 16: 38 and 16: 43 UT on 08/17/2003. The four-spacecraft capabilities of Cluster are used to determine spatial gradients from the magnetic field vectors and, for the first time, full electron pressure tensors. We find that the contribution to the electric field from the Hall term (max of similar to 6 mV/m) pointed towards the neutral sheet, whereas that from the electron pressure divergence ( max of similar to 1 mV/m) pointed away from the neutral sheet. The electric field contributions in this direction were closely anti-correlated. During this period Clusters 1 and 4 were sometimes above and below the neutral sheet respectively. This allowed the simultaneous observation of magnetic fields that are interpreted as two quadrants of the Hall magnetic field system. An associated field-aligned current system was detected using the curlometer and moments of the particle distributions
Magnetocentrifugal Winds in 3D: Nonaxisymmetric Steady State
Outflows can be loaded and accelerated to high speeds along rapidly rotating,
open magnetic field lines by centrifugal forces. Whether such
magnetocentrifugally driven winds are stable is a longstanding theoretical
problem. As a step towards addressing this problem, we perform the first
large-scale 3D MHD simulations that extend to a distance times
beyond the launching region, starting from steady 2D (axisymmetric) solutions.
In an attempt to drive the wind unstable, we increase the mass loading on one
half of the launching surface by a factor of , and reduce it by the
same factor on the other half. The evolution of the perturbed wind is followed
numerically. We find no evidence for any rapidly growing instability that could
disrupt the wind during the launching and initial phase of propagation, even
when the magnetic field of the magnetocentrifugal wind is toroidally dominated
all the way to the launching surface. The strongly perturbed wind settles into
a new steady state, with a highly asymmetric mass distribution. The
distribution of magnetic field strength is, in contrast, much more symmetric.
We discuss possible reasons for the apparent stability, including stabilization
by an axial poloidal magnetic field, which is required to bend field lines away
from the vertical direction and produce a magnetocentrifugal wind in the first
place.Comment: 10 pages, 2 figures, accepted for publication in ApJ
Supermagnetosonic jets behind a collisionless quasi-parallel shock
The downstream region of a collisionless quasi-parallel shock is structured
containing bulk flows with high kinetic energy density from a previously
unidentified source. We present Cluster multi-spacecraft measurements of this
type of supermagnetosonic jet as well as of a weak secondary shock front within
the sheath, that allow us to propose the following generation mechanism for the
jets: The local curvature variations inherent to quasi-parallel shocks can
create fast, deflected jets accompanied by density variations in the downstream
region. If the speed of the jet is super(magneto)sonic in the reference frame
of the obstacle, a second shock front forms in the sheath closer to the
obstacle. Our results can be applied to collisionless quasi-parallel shocks in
many plasma environments.Comment: accepted to Phys. Rev. Lett. (Nov 5, 2009
Shocklets, SLAMS, and field-aligned ion beams in the terrestrial foreshock
We present Wind spacecraft observations of ion distributions showing
field-aligned beams (FABs) and large-amplitude magnetic fluctuations composed
of a series of shocklets and short large-amplitude magnetic structures (SLAMS).
We show that the SLAMS are acting like a local quasi-perpendicular shock
reflecting ions to produce the FABs. Previous FAB observations reported the
source as the quasi-perpendicular bow shock. The SLAMS exhibit a foot-like
magnetic enhancement with a leading magnetosonic whistler train, consistent
with previous observations. The FABs are found to have T_b ~ 80-850 eV,
V_b/V_sw ~ 1-2, T_{b,perp}/T{b,para} ~ 1-10, and n_b/n_i ~ 0.2-14%. Strong ion
and electron heating are observed within the series of shocklets and SLAMS
increasing by factors \geq 5 and \geq 3, respectively. Both the core and halo
electron components show strong perpendicular heating inside the feature.Comment: 11 pages, 3 EPS figures, submitted to Geophysical Research Letter
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