Krzysztof Stasiewicz

Identification of widespread turbulence of dispersive Alfvén waves

A revised analysis of Freja observations shows that events interpreted previously as broadband turbulence in the frequency domain, f ∼1–500 Hz represent in fact turbulence of dispersive Alfven waves (DAW) covering a wide range of wave vectors k⟂. A satellites motion through broad Δk⟂, low frequency, ω < ωci, Alfvenic structures makes them appear as broadband waves at (Doppler) frequencies Δω = vs · Δk⟂ in the satellite reference frame. The identification is made with the measured ratio of δE/δB fluctuations which is consistent with the theoretical prediction for inertial Alfven wave spectrum at spatial scales λ⟂ ∼30–7000 m. In addition, we demonstrate that the multipoint measurements with double density probes provide independent confirmation of the spatial nature of the low frequency fluctuations.

Reinterpretation of mirror modes as trains of slow magnetosonic solitons

[1] It is shown that large amplitude periodic oscillations of the magnetic field measured frequently by spacecraft in the magnetosheath, and interpreted as mirror-mode waves/ structures, represent trains of slow-mode magnetosonic solitons. Nonlinear magnetosonic wave-trains with properties attributed previously to mirror-mode structures are obtained as exact solutions of Hall-MHD equations with anisotropic ion pressure. The theoretically derived properties of magnetosonic solitons are compared with multipoint measurements on Cluster satellites in the magnetosheath with plasma beta (plasma/magnetic pressure) ∼10. The computed properties of nonlinear waves: amplitudes, spatial scales, periodicity, and propagation velocities are consistent with in situ measurements in space.

On the origin of the auroral inverted-V electron spectra

On the basis of quasi-linear theory of ion-acoustic turbulence it is shown that the angular and energy distribution of the electron spectra observed in quasi-static inverted-V structures are natural products of electron heating and runaway processes occuring in a region of current-driven turbulence located at h ≈ 1 Re. The power law population J ∞ E−γ, with γ ≈ 1 observed in the energy range ~ 25–1000 eV, is interpreted as a quasi-stationary distribution of suprathermal electrons interacting resonantly with the ion sound waves. This spectrum is generated in the turbulent region and convectively transported earthward along the magnetic field lines. Field-aligned intense electron fluxes with collimation angle < 10° are explained as due to particles escaping from the turbulent region through the runaway cone—a characteristic feature of velocity-space in ion-acoustic turbulence. A complete, new interpretation of the observed electron spectra is given on the basis of the proposed physical acceleration mechanism along with many other implications of this theory.

The influence of a turbulent region on the flux of auroral electrons

The influence of low-frequency electrostatic turbulence on the flux of precipitating magnetospheric electrons is analyzed in the framework of the quasilinear kinetic equation. It is shown that an electron population in a turbulent region, with an electric field parallel to the ambient magnetic field, can be separated into two parts by introducing a pitch angle dependent runaway velocity vr(θ). Lower energy electrons with parallel velocity v∥ < vr are effectively scattered by plasma waves, so that they remain in the main population and are subjected to an anomalous transport equation. A distribution function f ∞ v−4 (or the particle flux vs energy J ∞ E−1) is established in this velocity range. Faster electrons with v∥ ≳ vr are freely accelerated by a parallel electric field, so that they contribute directly to hot electron fluxes which are observed at ionospheric altitudes. New expressions are derived for the magnetic-field aligned current and the electron energy flux implied by this model. These expressions agree well with empirical relations observed in auroral inverted-V structures.

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.

Multidimensional Hall magnetohydrodynamics with isotropic or anisotropic thermal pressure

We present a numerical solver for plasma dynamics simulations in Hall magnetohydrodynamic (HMHD) approximation in one, two and three dimensions. We consider both isotropic and anisotropic thermal pressure cases, where a general gyrotropic approximation is used. Both explicit energy conservation equation and general polytropic state equations are considered. The numerical scheme incorporates second-order Runge-Kutta advancing in time and Kurganov-Tadmor scheme with van Leer flux limiter for the approximation of fluxes. A flux-interpolated constrained-transport approach is used to preserve solenoidal magnetic field in the simulations. The implemented code is validated using several test problems previously described in the literature. Additionally, we propose a new validation method for HMHD codes based on solitary waves that provides a possibility of quantitative rigorous testing in nonlinear (large amplitude) regime as an extension to standard tests using small-amplitude whistler waves. Quantitative tests of accuracy and performance of the implemented code show the fidelity of the proposed approach.