6,222 research outputs found
The Effect of Large Scale Magnetic Turbulence on the Acceleration of Electrons by Perpendicular Collisionless Shocks
We study the physics of electron acceleration at collisionless shocks that
move through a plasma containing large-scale magnetic fluctuations. We
numerically integrate the trajectories of a large number of electrons, which
are treated as test particles moving in the time dependent electric and
magnetic fields determined from 2-D hybrid simulations (kinetic ions, fluid
electron). The large-scale magnetic fluctuations effect the electrons in a
number of ways and lead to efficient and rapid energization at the shock front.
Since the electrons mainly follow along magnetic lines of force, the
large-scale braiding of field lines in space allows the fast-moving electrons
to cross the shock front several times, leading to efficient acceleration.
Ripples in the shock front occuring at various scales will also contribute to
the acceleration by mirroring the electrons. Our calculation shows that this
process favors electron acceleration at perpendicular shocks. The current study
is also helpful in understanding the injection problem for electron
acceleration by collisionless shocks. It is also shown that the spatial
distribution of energetic electrons is similar to in-situ observations (e.g.,
Bale et al. 1999; Simnett et al. 2005). The process may be important to our
understanding of energetic electrons in planetary bow shocks and interplanetary
shocks, and explaining herringbone structures seen in some type II solar radio
bursts.Comment: 23 pages, 6 figures, accepted by Ap
Early-time velocity autocorrelation for charged particles diffusion and drift in static magnetic turbulence
Using test-particle simulations, we investigate the temporal dependence of
the two-point velocity correlation function for charged particles scattering in
a time-independent spatially fluctuating magnetic field derived from a
three-dimensional isotropic turbulence power spectrum. Such a correlation
function allowed us to compute the spatial coefficients of diffusion both
parallel and perpendicular to the average magnetic field. Our simulations
confirm the dependence of the perpendicular diffusion coefficient on turbulence
energy density and particle energy predicted previously by a model for
early-time charged particle transport. Using the computed diffusion
coefficients, we exploit the particle velocity autocorrelation to investigate
the time-scale over which the particles "decorrelate" from the solution to the
unperturbed equation of motion. Decorrelation time-scales are evaluated for
parallel and perpendicular motions, including the drift of the particles from
the local magnetic field line. The regimes of strong and weak magnetic
turbulence are compared for various values of the ratio of the particle
gyroradius to the correlation length of the magnetic turbulence. Our simulation
parameters can be applied to energetic particles in the interplanetary space,
cosmic rays at the supernova shocks, and cosmic-rays transport in the
intergalactic medium.Comment: 10 pages, 11 figures, The Astrophyical Journal in pres
Is European Inter Regionalism a Relevant Approach for the World or Just for Europe? Jean Monnet/Robert Schuman Paper Series Vol. 7 No. 14 September 2007
[Introduction]. Most of the literature on inter regionalism seems to accept that, as a region, the European Union (EU) stimulates interregional cooperation in such a way that it may be responsible for adding a new category of relevant actors to world politics. This development is usually viewed by academics, mostly Europeans, positively, as a situation born from the affirmation of a European “soft power” vis-à-vis other alternatives of world order. Additionally, it is often assumed that the successful integration model of the EU is responsible for creating the conditions for emulation in other regional integration projects elsewhere, and implicitly that it could be the cornerstone of future world order. This paper explores this argument in order to provide a more accurate reading of European actions in the international landscape. Our starting point is the fact that the characteristics of the power that a nation state, or a group of nations such as the EU, exercises can be extrapolated from its internal make-up but should not be divorced from its actual behavior in the international arena. So far the behavior of the EU in that arena has been patterned along traditional lines of behavior of aspiring powers within historical settings dominated by a nation who faces problems in maintaining its hegemony. Thus, we argue that most of the literature on European “soft power” exceptionality is based on a reading of the EU internal make-up that is not paying attention to its actual behavior in the international arena. We accept that internal factors weigh on the foreign policy decisions made by a nation state or a group of nations, but also that the feeling of exceptionality – that a nation or group can develop because of its internal make-up – does not necessarily “spill over” to its international relations. So, we are going to test the proposition that, in order to protect its exceptionality, a nation state or group of nations applies different mechanisms to its international dealings, but those mechanisms are not different from the ones employed in similar international situations by nation states whose internal make-ups differ. In other words, the participation of a group of nations in the world order does not translate into a different behavior than that of previously isolated nation states, exemplified by the realist proposition that nation states and groups of nations still “value survival above all else.” And, in the case of the EU, this implies the survival of an internal make-up that has been successful for its members. As a consequence, inter regionalism can be seen as a European centered approach aimed at defending the permanence of the gains achieved by means of regional integration, but with little relevance in the medium and long term for a more democratic or more meaningful world order for developing nations. This paper contends that hegemony and emulation are just two faces of the power exercised by an actor within the international system. When a nation or group of nations has enough power to do so, it imposes its hegemony. When it does not have enough power, especially enough power to compete with an existing hegemon1 -- or it is unwilling to do so-- it would tend to emphasize other elements (its value system, for example) as an emulation horizon for other nations. Both behaviors translate into active foreign policies toward the rest of the world, and activism in foreign policy almost always is born out of the need to defend itself. In the case of the EU, its behavior towards Eastern European countries is that of a hegemon, according to the definition of hegemony in “Theorizing Regional Integration and Inter-Regional Relations (2)” (2006) – the EU establishes the goals, monitors the course of action, and supports the instruments required to carry out the undertakings agreed upon.(3) However, beyond Eastern Europe and Turkey, the EU lacks the power to impose its hegemony and is limited to resorting to an alleged “soft power” deal –i.e., inter regionalism. In the first section of this paper, we provide historical examples of the U.S., a nation state which has employed both hegemony and emulation as defense mechanisms at different stages in its history. This will demonstrate that, though much is made out of the European Union emulation, it is a mechanism to assert economic and political power that has been used before by individual nation states. And, in the second section, we discuss aspects of EU foreign policy towards developing nations that, when taken together, suggest that the adding of new relevant actors to world order by way of inter regionalism may help create a new balance of power under European tutelage, but this balance will not necessarily lead to a more democratic or lasting world order
Particle acceleration by collisionless shocks containing large-scale magnetic-field variations
Diffusive shock acceleration at collisionless shocks is thought to be the
source of many of the energetic particles observed in space. Large-scale
spatial variations of the magnetic field has been shown to be important in
understanding observations. The effects are complex, so here we consider a
simple, illustrative model. Here, we solve numerically the Parker transport
equation for a shock in the presence of large-scale sinusoidal magnetic-field
variations. We demonstrate that the familiar planar-shock results can be
significantly altered as a consequence of large-scale, meandering magnetic
lines of force. Because perpendicular diffusion coefficient is
generally much smaller than parallel diffusion coefficient ,
the energetic charged particles are trapped and preferentially accelerated
along the shock front in the regions where the connection points of magnetic
field lines intersecting the shock surface converge, and thus create the "hot
spots" of the accelerated particles. For the regions where the connection
points separate from each other, the acceleration to high energies will be
suppressed. Further, the particles diffuse away from the "hot spot" regions and
modify the spectra of downstream particle distribution. These features are
qualitatively similar to the recent Voyager's observation in the Heliosheath.
These results are potentially important for particle acceleration at shocks
propagating in turbulent magnetized plasmas as well as those which contain
large-scale nonplanar structures. Examples include anomalous cosmic rays
accelerated by the solar wind termination shock, energetic particles observed
in propagating heliospheric shocks, and galactic cosmic rays accelerated by
supernova blast waves, etc.Comment: accepted to Ap
Evidence of Confinement of Solar-energetic Particles to Interplanetary Magnetic Field Lines
We present new observations of solar-energetic particles (SEPs) associated with impulsive solar flares that show
evidence for their confinement to interplanetary magnetic field lines. Some SEP events exhibit intermittent intensity
dropouts becausemagnetic field lines filledwith and empty of particle flux mix together. The edges of these dropouts
are observed to be very sharp, suggesting that particles cannot easily move from a filled to an empty field line in
the time available during their transport from the Sun. In this paper, we perform high time-resolution observations
of intensity fall-off at the edges of observed SEP dropouts in order to look for signatures of particle motion off
field lines. However, the statistical study is dominated by one particularly intense event. The inferred length scale
of the intensity decay is comparable to the gyroradii of the particles, suggesting that particles only rarely scatter off magnetic field lines during interplanetary transport
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