M. Strumik

THE EFFECTS OF LOCAL INTERSTELLAR MAGNETIC FIELD ON ENERGETIC NEUTRAL ATOM SKY MAPS

We study the effects of the strength and direction of the local interstellar magnetic field (ISMF) on the heliosphere geometry that generates the locus of points associated with the position of the IBEX ribbon of energetic neutral atoms. MHD heliosphere models are run for a variety of ISMF parameters to specifically study the correlation between locations of maxima of the ISMF magnitude along the field lines and places where the local ISMF B is perpendicular to radial vectors r from the Sun, i.e., B ? r = 0. The study confirms the existence of a strong physical relationship between the ribbon and the ISMF.

IBEX RIBBON: WHAT COULD IT TELL ABOUT THE LOCAL INTERSTELLAR MAGNETIC FIELD?

We show that the shape of the IBEX Ribbon can be reproduced assuming energetic neutral atoms originate in regions beyond the heliopause where the interstellar magnetic field is the strongest and perpendicular to radial directions from the Sun. The best fit to the observed ribbon was obtained for the local interstellar magnetic field B ? = 3.0 ? 1.0 ?G pointing from ecliptic/galactic coordinates (?, ?)/(l, b) = (225? ? 5?, 35? ? 5?)/(27? ? 5?, 51? ? 5?) close to the apparent ribbon center (?, ?)/(l, b) = (221?, 39?)/(33?, 55?). The geometrical considerations presented below should prove useful in identifying the mechanism of ribbon formation.

Structure of the heliospheric current sheet from plasma convection in time-dependent heliospheric models

Context. The heliospheric current sheet is a plasma layer dividing the heliosphere into the regions of different magnetic field polarity. Since it is very thin compared to the size of the system, it is difficult to incorporate into the numerical models of the heliosphere. Because of the solar magnetic field reversals and the diverging and slowing down plasma flow in the outer heliosphere, the heliospheric current sheet is expected to have a complicated structure, with important consequences for transport processes in the heliosheath. Aims. We determine the shape and time evolution of the current sheet in selected time-dependent 3-D models of the heliosphere, assuming that the heliospheric current sheet is a tangential discontinuity convected by the plasma flow. Methods. We have derived the shape of the heliospheric current sheet at a given time by following the plasma flow lines originating at the neutral line on the source surface surrounding the Sun. The plasma flow was obtained from numerical MHD or gas-dynamical solutions. Results. The large-scale structure of the magnetic field polarity regions and the heliospheric current sheet in time-dependent asymmetric models of the heliosphere differs from the results obtained in simpler models. In particular, in the forward heliosheath it is characterized by secondary folds in the heliospheric current sheet that are caused by the solar wind latitudinal variation over the solar cycle. We present examples illustrating some cases of interest: a “bent” current sheet, and the HCS structure during the magnetic field reversal at the solar maximum. We also discuss the evolution of the magnetic polarity structure in the region close to the heliopause.

Comparison of Heliospheric Models with Observations of the Voyager and IBEX Spacecraft

Results of modeling the heliosphere are compared with observations of the Voyager spacecraft and the IBEX mission simultaneously. The MHD solutions are tested against observational data for different strengths and orientations of the local interstellar magnetic field (LIMF) used in the simulations for asymmetric magnetized solar wind flow. We show that the model reproduces approximately the position of the IBEX ribbon and the termination shock crossing distance for Voyager 2, when the LIMF vector lies in the proximity of the hydrogen deflection plane with the inclination angle to the local interstellar flow equal to 39° ± 9° and its magnitude is 2.4 ± 0.3 μG. In ecliptic coordinates this solution corresponds to the LIMF vector pointing from (longitude, latitude) = (227° ± 7°, 35° ± 7°).

ADVECTIVE TRANSPORT OF INTERSTELLAR PLASMA INTO THE HELIOSPHERE ACROSS THE RECONNECTING HELIOPAUSE

We discuss results of magnetohydrodynamical model simulations of plasma dynamics in the proximity of the heliopause (HP). The model is shown to fit details of the magnetic field variations observed by the Voyager 1 spacecraft during the transition from the heliosphere to the local interstellar medium (LISM). We propose an interpretation of magnetic field structures observed by Voyager 1 in terms of fine-scale physical processes. Our simulations reveal an effective transport mechanism of relatively dense LISM plasma across the reconnecting HP into the heliosphere. The mechanism is associated with annihilation of magnetic sectors in the heliospheric plasma near the HP.

Acceleration of solar wind ions to 1 MeV by electromagnetic structures upstream of the Earth's bow shock

We present measurements from the ESA/NASA Cluster mission that show in situ acceleration of ions to energies of 1 MeV outside the bow shock. The observed heating can be associated with the presence of electromagnetic structures with strong spatial gradients of the electric field that lead to ion gyro-phase breaking and to the onset of chaos in ion trajectories. It results in rapid, stochastic acceleration of ions in the direction perpendicular to the ambient magnetic field. The electric potential of the structures can be compared to a field of moguls on a ski slope, capable of accelerating and ejecting the fast running skiers out of piste. This mechanism may represent the universal mechanism for perpendicular acceleration and heating of ions in the magnetosphere, the solar corona and in astrophysical plasmas. This is also a basic mechanism that can limit steepening of nonlinear electromagnetic structures at shocks and foreshocks in collisionless plasmas.

Hyperchaotic intermittent convection in a magnetized viscous fluid.

We consider a low-dimensional model of convection in a horizontally magnetized layer of a viscous fluid heated from below. We analyze in detail the stability of hydrodynamic convection for a wide range of two control parameters. Namely, when changing the initially applied temperature difference or magnetic field strength, one can see transitions from regular to irregular long-term behavior of the system, switching between chaotic, periodic, and equilibrium asymptotic solutions. It is worth noting that owing to the induced magnetic field a transition to hyperchaotic dynamics is possible for some parameters of the model. We also reveal new features of the generalized Lorenz model, including both type I and III intermittency.

Analytical Model of Eddy Currents in a Reaction Sphere Actuator

A recently proposed technique to control the satellite attitude using a magnetically levitated sphere requires the development of suitable models of its dynamics. One of the phenomena that can affect motion of the system are eddy currents induced in the stator of the actuator due to time variable magnetic field generated by rotational motion of a permanent magnet rotor. We present an analytical model of the eddy currents for the actuator with eight-pole rotor. The model is derived using a second-order vector potential-based approach, and the solution is obtained in terms of spherical harmonic functions. This model allows us to study rotor rotations with constant angular frequency around an axis arbitrarily oriented with respect to both rotor and stator of the reaction sphere actuator.

Testing for Markovian character and modeling of intermittency in solar wind turbulence.

We present results of statistical analysis of solar wind turbulence using an approach based on the theory of Markov processes. It is shown that the Chapman-Kolmogorov equation is approximately satisfied for the turbulent cascade. We evaluate the first two Kramers-Moyal coefficients from experimental data and show that the solution of the resulting Fokker-Planck equation agrees well with experimental probability distributions. Our analysis provides evidence that the transfer of fluctuations from large to smaller eddies must be independent of the dynamics on large scales and in particular it must be independent of the driving mechanisms for solar wind turbulence. Our results also suggest the presence of a local transfer mechanism for magnetic field fluctuations in solar wind turbulence.

Identification of the dominant ULF wave mode and generation mechanism for obliquely propagating waves in the Earth's foreshock

We discuss mechanisms of the generation of ultralow frequency (ULF) upstream waves in the terrestrial foreshock that are essential for the acceleration of ions in space plasmas. The analysis is based on global hybrid kinetic simulations of the magnetosphere that provide realistic environment for the growth of the ULF waves in a quasi-radial configuration of the interplanetary magnetic field. We focus on a long-debated problem of the generation mechanism of oblique and parallel ULF waves and provide quantitative arguments in favor of the ion/ion cyclotron resonant instability. We also show that parallel propagating waves are predominantly generated in this configuration, but geometrical effects related to the phase space density in wave vector space lead to apparent predominance of obliquely propagating waves. Correspondence between the results outlined above and previously published experimental claims is thoroughly discussed and our results are shown to be consistent with spacecraft measurements.