354 research outputs found
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
Global and local disturbances in the magnetotail during reconnection
We examine Cluster observations of a reconnection event at <I>x</I><sub>GSM</sub>=&minus;15.7 <I>R<sub>E</sub></I> in the magnetotail on 11 October 2001, when Cluster recorded the current sheet for an extended period including the entire duration of the reconnection event. The onset of reconnection is associated with a sudden orientation change of the ambient magnetic field, which is also observed simultaneously by Goes-8 at geostationary orbit. Current sheet oscillations are observed both before reconnection and during it. The speed of the flapping motions is found to increase when the current sheet undergoes the transition from quiet to active state, as suggested by an earlier statistical result and now confirmed within one single event. Within the diffusion region both the tailward and earthward parts of the quadrupolar magnetic Hall structure are recorded as an x-line passes Cluster. We report the first observations of the Hall structure conforming to the kinks in the current sheet. This results in relatively strong fluctuations in <I>B<sub>z</sub></I>, which are shown to be the Hall signature tilted in the <I>yz</I> plane with the current sheet
Cluster observations of structures at quasi-parallel bow shocks
International audienceCollisionless quasi-parallel shocks are thought to be composed of a patchwork of short, large-amplitude magnetic structures (SLAMS) which act to thermalise the plasma, giving rise to a spatially extended and time varying shock transition. With the launch of Cluster, new observations of the three-dimensional shape and size of shock structures are available. In this paper we present SLAMS observations made when the Cluster tetrahedron scale size was ~100km. The SLAMS magnetic field enhancement is typically well correlated between spacecraft on this scale, although small differences are observed. The statistical characteristics of these differences contain information on the typical gradients of magnetic field changes within the SLAM structure which, in the case studied here, occur on scales of 100-150km, comparable with the upstream ion inertial length
Relating near-Earth observations of an interplanetary coronal mass ejection to the conditions at its site of origin in the solar corona
A halo coronal mass ejection (CME) was detected on January 20, 2004. We use solar remote sensing data (SOHO, Culgoora) and near-Earth in situ data (Cluster) to identify the CME source event and show that it was a long duration flare in which a magnetic flux rope was ejected, carrying overlying coronal arcade material along with it. We demonstrate that signatures of both the arcade material and the flux rope material are clearly identifiable in the Cluster and ACE data, indicating that the magnetic field orientations changed little as the material traveled to the Earth, and that the methods we used to infer coronal magnetic field configurations are effective
Nonlinear growth of firehose and mirror fluctuations in turbulent galaxy-cluster plasmas
In turbulent high-beta astrophysical plasmas (exemplified by the galaxy
cluster plasmas), pressure-anisotropy-driven firehose and mirror fluctuations
grow nonlinearly to large amplitudes, dB/B ~ 1, on a timescale comparable to
the turnover time of the turbulent motions. The principle of their nonlinear
evolution is to generate secularly growing small-scale magnetic fluctuations
that on average cancel the temporal change in the large-scale magnetic field
responsible for the pressure anisotropies. The presence of small-scale magnetic
fluctuations may dramatically affect the transport properties and, thereby, the
large-scale dynamics of the high-beta astrophysical plasmas.Comment: revtex, 4 pages, 1 figure; replaced to match published versio
Nonlinear theory of mirror instability near threshold
An asymptotic model based on a reductive perturbative expansion of the drift
kinetic and the Maxwell equations is used to demonstrate that, near the
instability threshold, the nonlinear dynamics of mirror modes in a magnetized
plasma with anisotropic ion temperatures involves a subcritical
bifurcation,leading to the formation of small-scale structures with amplitudes
comparable with the ambient magnetic field
Cryptic invasion drives phenotypic changes in central European threespine stickleback
Cryptic invasions are commonly associated with genetic changes of the native species or genetic lineage that the invaders replace. Phenotypic shifts resulting from cryptic invasions are less commonly reported given the relative paucity of historical specimens that document such phenotypic changes. Here, I study such a case in two populations of threespine stickleback from central Europe, comparing contemporary patterns of gene flow with phenotypic changes between historical and contemporary population samples. I find gene flow from an invasive lineage to be associated with significant phenotypic changes, where the degree of phenotypic change corresponds with the level of gene flow that a population receives. These findings underline the utility of combining genetic approaches with phenotypic data to estimate the impact of gene flow in systems where anthropogenic alterations have removed former geographic barriers promoting cryptic invasions
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