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 $\kappa_\perp$ is generally much smaller than parallel diffusion coefficient $\kappa_\parallel$, 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