Parametric analysis of pitch angle scattering and losses of relativistic electrons by oblique EMIC waves

This study analyzes the effects of electromagnetic ion cyclotron (EMIC) waves on relativistic electron scattering and losses in the Earth’s outer radiation belt. EMIC emissions are commonly observed in the inner magnetosphere and are known to reach high amplitudes, causing significant pitch angle changes in primarily >1 MeV electrons via cyclotron resonance interactions. We run test-particle simulations of electrons streaming through helium band waves with different amplitudes and wave normal angles and assess the sensitivity of advective and diffusive scattering behaviors to these two parameters, including the possibility of very oblique propagation. The numerical analysis confirms the importance of harmonic resonances for oblique waves, and the very oblique waves are observed to efficiently scatter both co-streaming and counter-streaming electrons. However, strong finite Larmor radius effects limit the scattering efficiency at high pitch angles. Recently discussed force-bunching effects and associated strong positive advection at low pitch angles are, surprisingly, shown to cause no decrease in the phase space density of precipitating electrons, and it is demonstrated that the transport of electrons into the loss cone balances out the scattering out of the loss cone. In the case of high-amplitude obliquely propagating waves, weak but non-negligible losses are detected well below the minimum resonance energy, and we identify them as the result of non-linear fractional resonances. Simulations and theoretical analysis suggest that these resonances might contribute to subrelativistic electron precipitation but are likely to be overshadowed by non-resonant effects

Hybridsimulationen der Sternwindwechselwirkung mit close-in extrasolaren Planeten

A large fraction of exoplanets orbit their host stars on distances much smaller than any planet in the Solar System. This opens up for the possibility of qualitatively new kinds of stellar wind interaction. The understanding of these interactions may be important both for future detection methods and for mass loss estimates. In this work we investigate such close-in stellar wind interaction using primarily hybrid simulations. These describe plasmas by modeling electrons as a fluid and ions as particles. Two scenarios of stellar wind interaction with unmagnetized, Earth-sized, close-in exoplanets in orbit around a Sun-like star are investigated: 1.) interaction with an extremely hydrodynamically expanding atmosphere, and 2.) quasiparallel stellar wind interaction. In the expanding ionosphere study we can see how the bow shock, magnetic draping and ion composition boundary are pushed upstream, increasing the size of the interaction region. In the process it creates a significant wake behind the planet, largely void of electromagnetic fields and dominated only by the expanding ionosphere. An attempt is also made to analytically estimate the standoff distance and compare these estimates with simulations. In the quasiparallel interaction study we observe how generic features of quasiperpendicular interaction are modified. The dayside bow shock surface is replaced by a vaguely defined parallel shock that destroys the strict division between magnetosheath and stellar wind. The stellar wind also penetrates deeper into the ionosphere.Ein großer Anteil der Exoplaneten umkreist ihre Sterne in viel kleineren Entfernungen als die Planeten in unserem Sonnensystem. Dies eröffnet die Möglichkeit von qualitativ neuen Arten von Sternwindwechselwirkungen. Das Verständnis dieser Wechselwirkungen kann sowohl für zukünftige Nachweismethoden als auch für die Massenverlustschätzungen wichtig sein. In dieser Arbeit untersuchen wir diese close-in Sternwindwechselwirkungen mit Hybridsimulationen. Diese beschreiben Plasmen durch die Modellierung von Elektronen als Flüssigkeit und Ionen als Teilchen. Zwei Szenarien der Sternwindwechselwirkung mit unmagnetisierten, erdgroßen, close-in Exoplaneten im Orbit um einen sonnenähnlichen Stern werden untersucht: 1.) Wechselwirkung mit einer extrem hydrodynamisch expandierenden Atmosphäre, und 2.) Quasi-parallele Sternwindwechselwirkung. In der Studie der expandierenden Ionosphäre können wir sehen, wie die Bugstoßwelle, magnetische Drapierung und Grenze der Ionenzusammensetzung entgegen der Flussrichtung geschoben werden, was die Wechselwirkungsregion vergrößert. In dem Prozess entsteht ein deutlicher Wake hinter dem Planeten, der größtenteils frei von elektromagnetischen Feldern ist und nur von der Ionosphäre dominiert wird. Es wird außerdem versucht, auf analytische Weise den Abstand zur Ionopause zu bestimmen und diese Schätzungen mit Simulationen zu vergleichen. In der Studie der quasi-parallelen Wechselwirkung beobachten wir, wie generische Eigenschaften der quasi-senkrechten Wechselwirkung verändert werden. Die tagseitige Bugstoßwelle wird durch einen undeutlich definierten parallelen Schock, der die strikte Trennung zwischen Magnetoschicht und Sternwind zerstört, ersetzt. Der Sternwind dringt auch tiefer in die Ionosphäre ein

Collisionless electrons in a thin high Mach number shock: dependence on angle and </b><b><i>b</i></b>

International audienceIt is widely believed that electron dynamics in the shock front is essentially collisionless and determined by the quasistationary magnetic and electric fields in the shock. In thick shocks the electron motion is adiabatic: the magnetic moment is conserved throughout the shock and v2^ ? B. In very thin shocks with large cross-shock potential (the last feature is typical for shocks with strong electron heating), electrons may become demagnetized (the magnetic moment is no longer conserved) and their motion may become nonadiabatic. We consider the case of substantial demagnetization in the shock profile with the small-scale internal structure. The dependence of electron dynamics and downstream distributions on the angle between the shock normal and upstream magnetic field and on the upstream electron temperature is analyzed. We show that demagnetization becomes significantly stronger with the increase of obliquity (decrease of the angle) which is related to the more substantial influence of the inhomogeneous parallel electric field. We also show that the demagnetization is stronger for lower upstream electron temperatures and becomes less noticeable for higher temperatures, in agreement with observations. We also show that demagnetization results, in general, in non-gyrotropic down-stream distributions

Quantification of hydraulic redistribution in maize roots using neutron radiography

Abstract Plants redistribute water from wet to dry soil layers through their roots, in the process called hydraulic redistribution. Although the relevance and occurrence of this process are well accepted, resolving the spatial distribution of hydraulic redistribution remains challenging. Here, we show how to use neutron radiography to quantify the rate of water efflux from the roots to the soil. Maize (Zea mays L.) plants were grown in a sandy substrate 40 cm deep. Deuterated water (D2O) was injected in the bottom wet compartment, and its transport through the roots to the top dry soil was imaged using neutron radiography. A diffusion–convection model was used to simulate the transport of D2O in soil and root and inversely estimate the convective fluxes. Overnight, D2O appeared in nodal and lateral roots in the top compartment. By inverse modeling, we estimated an efflux from lateral roots into the dry soil equal to jr = 2.35 × 10−7 cm−1. A significant fraction of the redistributed water flew toward the tips of nodal roots (3.85 × 10−8 cm3 s−1 per root) to sustain their growth. The efflux from nodal roots depended on the roots’ length and growth rate. In summary, neutron imaging was successfully used to quantify hydraulic redistribution. A numerical model was needed to differentiate the effects of diffusion and convection. The highly resolved images showed the spatial heterogeneity of hydraulic redistribution

Development of a computational model for predicting solar wind flows past nonmagnetic terrestrial planets

A computational model for the determination of the detailed plasma and magnetic field properties of the global interaction of the solar wind with nonmagnetic terrestrial planetary obstacles is described. The theoretical method is based on an established single fluid, steady, dissipationless, magnetohydrodynamic continuum model, and is appropriate for the calculation of supersonic, super-Alfvenic solar wind flow past terrestrial ionospheres

Lagrangian description of warm plasmas

Efforts are described to extend the averaged Lagrangian method of describing small signal wave propagation and nonlinear wave interaction, developed by earlier workers for cold plasmas, to the more general conditions of warm collisionless plasmas, and to demonstrate particularly the effectiveness of the method in analyzing wave-wave interactions. The theory is developed for both the microscopic description and the hydrodynamic approximation to plasma behavior. First, a microscopic Lagrangian is formulated rigorously, and expanded in terms of perturbations about equilibrium. Two methods are then described for deriving a hydrodynamic Lagrangian. In the first of these, the Lagrangian is obtained by velocity integration of the exact microscopic Lagrangian. In the second, the expanded hydrodynamic Lagrangian is obtained directly from the expanded microscopic Lagrangian. As applications of the microscopic Lagrangian, the small-signal dispersion relations and the coupled mode equations are derived for all possible waves in a warm infinite, weakly inhomogeneous magnetoplasma, and their interactions are examined

Individual-Based Modelling Potentials and Limitations

Individual-based modelling (IBM) is an important option in ecology for the study of specific properties of complex ecological interaction networks. The main application of this model type is the analysis of population characteristics at high resolution. IBM also contributes to the advancement of ecological theory. One of the remarkable potentials of the approach is the possibility of studying self-organization and emergent properties that arise from individual actions on higher integration levels, especially on the population level

Absolute-Convective Instabilities and Their Associated Wave Packets in a Compressible Reacting Mixing Layer

In this paper the transition from convective to absolute instability in a reacting compressible mixing layer with finite rate chemistry is examined. The reaction is assumed to be one step, irreversible, and of Arrhenius type. It is shown that absolute instability can exist for moderate heat release without backflow. The effects of the temperature ratio, heat release parameter, Zeldovich number, equivalence ratio, direction of propagation of the disturbances, and the Mach number on the transition value of the velocity ratio are given. The present results are compared to those obtained from the flame sheet model for the temperature using the Lock similarity solution for the velocity profile. Finally, the structure of the wave packets produced by an impulse in the absolutely unstable flow is examined. © 1993 American Institute of Physics

Jets in the magnetosheath: IMF control of where they occur

Magnetosheath jets are localized regions of plasma that move faster towards the Earth than the surrounding magnetosheath plasma. Due to their high velocities, they can cause indentations when colliding into the magnetopause and trigger processes such as magnetic reconnection and magnetopause surface waves. We statistically study the occurrence of these jets in the subsolar magnetosheath using measurements from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and OMNI solar wind data from 2008 to 2011. We present the observations in the B-IMF-v(SW) plane and study the spatial distribution of jets during different interplanetary magnetic field (IMF) orientations. Jets occur downstream of the quasi-parallel bow shock approximately 9 times as often as downstream of the quasi-perpendicular shock, suggesting that foreshock processes are responsible for most jets. For an oblique IMF, with 30-60 degrees cone angle, the occurrence increases monotonically from the quasi-perpendicular side to the quasi-parallel side. This study offers predictability for the numbers, locations, and magnetopause impact rates of jets observed during different IMF orientations, allowing us to better forecast the formation of these jets and their impact on the magnetosphere