Mohammed Shaaban

The interplay of the solar wind proton core and halo populations: EMIC instability

The kinetic properties of the solar wind protons (ions), like their temperature anisotropy and the resulting instabilities, are, in general, investigated considering only the proton core (or thermal) populations. The implication of the suprathermal halo components is minimized or just ignored, despite the fact that their presence in the solar wind is continuously reported by the observations, and their kinetic energy density may be significant. Whether they are originating in the corona or solar wind, the energetic particles may result from acceleration by the plasma turbulence or from the pitch angle scattering of the streaming protons by the self-generated fluctuations. The presence of suprathermal protons in the heliosphere suggests, therefore, a direct implication in resonant interactions, e.g., Landau and cyclotron, with plasma particles. This paper presents the results of a first investigation on the interplay of the proton core and suprathermal halo, when both these two populations may exhibit temperature anisotropies, which destabilize the electromagnetic ion (proton) cyclotron (EMIC) modes. These results clearly show that for conditions typically encountered in the solar wind, the effects of the suprathermals can be more important than those driven by the core. Remarkable are also the cumulative effects of the core and halo components, which change dramatically the instability conditions.

EFFECTS OF ELECTRONS ON THE ELECTROMAGNETIC ION CYCLOTRON INSTABILITY: SOLAR WIND IMPLICATIONS

In diffuse plasmas in space, particle–particle collisions are rare and inefficient, such that a plausible mechanism for constraining the temperature anisotropy of plasma particles may be provided by the resulting instabilities. The implication of the electromagnetic ion-cyclotron (EMIC) instability in the solar wind is still unclear because this instability is fast enough to relax the proton temperature anisotropy, but the 1 AU measurements do not conform to the instability thresholds predicted by the existing theories, which ignore the kinetic effects of electrons, assuming them to be isotropic. This paper presents a refined analysis of the EMIC instability in the presence of a temperature (T) anisotropy of electron (subscript e) population, i.e., enabling the identification of two distinct regimes of this instability that correspond to an excess of perpendicular temperature () or an excess of parallel temperature (). The growth rates, real frequencies, and threshold conditions are found to be highly sensitive to the electron temperature anisotropy, and electrons with inhibit the instability, while for the instability growth rates increase with the electron anisotropy. Moreover, the electron–proton temperature ratio becomes an important factor that stimulates the effect of the anisotropic electrons. The potential relevance of the new results in the solar wind is analyzed by contrasting the instability thresholds with the observed limits of the proton temperature anisotropy.

Effects of suprathermal electrons on the proton temperature anisotropy in space plasmas: Electromagnetic ion-cyclotron instability

In collision-poor plasmas from space, e.g., the solar wind and planetary magnetospheres, the kinetic anisotropy of the plasma particles is expected to be regulated by the kinetic instabilities. Driven by an excess of ion (proton) temperature perpendicular to the magnetic field (T⊥>T∥

Shaping the solar wind temperature anisotropy by the interplay of electron and proton instabilities

T_{perp}>T_{parallel}

Temperature anisotropy instabilities stimulated by the interplay of the core and halo electrons in space plasmas

), the electromagnetic ion-cyclotron (EMIC) instability is fast enough to constrain the proton anisotropy, but the observations do not conform to the instability thresholds predicted by the standard theory for bi-Maxwellian models of the plasma particles. This paper presents an extended investigation of the EMIC instability in the presence of suprathermal electrons which are ubiquitous in these environments. The analysis is based on the kinetic (Vlasov-Maxwell) theory assuming that both species, protons and electrons, may be anisotropic, and the EMIC unstable solutions are derived numerically providing an accurate description for conditions typically encountered in space plasmas. The effects of suprathermal populations are triggered by the electron anisotropy and the temperature contrast between electrons and protons. For certain conditions the anisotropy thresholds exceed the limits of the proton anisotropy measured in the solar wind considerably restraining the unstable regimes of the EMIC modes.

Clarifying the solar wind heat flux instabilities

A variety of nonthermal characteristics like kinetic, e.g., temperature, anisotropies and suprathermal populations (enhancing the high energy tails of the velocity distributions) are revealed by the in-situ observations in the solar wind indicating quasistationary states of plasma particles out of thermal equilibrium. Large deviations from isotropy generate kinetic instabilities and growing fluctuating fields which should be more efficient than collisions in limiting the anisotropy (below the instability threshold) and explain the anisotropy limits reported by the observations. The present paper aims to decode the principal instabilities driven by the temperature anisotropy of electrons and protons in the solar wind, and contrast the instability thresholds with the bounds observed at 1 AU for the temperature anisotropy. The instabilities are characterized using linear kinetic theory to identify the appropriate (fastest) instability in the relaxation of temperature anisotropies Ae,p = Te,p,⊥/Te,p,‖ 6= 1. The analysis focuses on the electromagnetic instabilities driven by the anisotropic protons (Ap ≶ 1) and invokes for the first time a dynamical model to capture the interplay with the anisotropic electrons by correlating the effects of these two species of plasma particles, dominant in the solar wind.

Beaming electromagnetic (or heat-flux) instabilities from the interplay with the electron temperature anisotropies

Two central components are revealed by electron velocity distributions measured in space plasmas, a thermal bi-Maxwellian core and a bi-Kappa suprathermal halo. A new kinetic approach is proposed to characterize the temperature anisotropy instabilities driven by the interplay of core and halo electrons. Suggested by the observations in the solar wind, direct correlations of these two populations are introduced as co-variations of the key parameters, e.g., densities, temperature anisotropies, and (parallel) plasma betas. The approach involving correlations enables the instability characterization in terms of either the core or halo parameters and a comparative analysis to depict mutual effects. In the present paper, the instability conditions are described for an extended range of plasma beta parameters, making the new dual approach relevant for a wide variety of space plasmas, including the solar wind and planetary magnetospheres.

Radiative transfer in finite volcanic eruption clouds

In the solar wind electron velocity distributions reveal two counter-moving populations which may induce electromagnetic (EM) beaming instabilities known as heat flux instabilities. Depending on plasma parameters two distinct branches of whistler and firehose instabilities can be excited. These instabilities are invoked in many scenarios, but their interplay is still poorly understood. An exact numerical analysis is performed to resolve the linear Vlasov-Maxwell dispersion and characterize these two instabilities, e.g., growth rates, wave frequencies and thresholds, enabling to identify their dominance for conditions typically experienced in space plasmas. Of particular interest are the effects of suprathermal Kappa-distributed electrons which are ubiquitous in these environments. The dominance of whistler or firehose instability is highly conditioned by the beam-core relative velocity, core plasma beta and the abundance of suprathermal electrons. Derived in terms of relative drift velocity the instability thresholds show an inverse correlation with the core plasma beta for the whistler modes, and a direct correlation with the core plasma beta for the firehose instability. Suprathermal electrons reduce the effective (beaming) anisotropy inhibiting the firehose modes while the whistler instability is stimulated.

Radiative transfer in finite participating atmospheric aerosol media

In space plasmas, kinetic instabilities are driven by the beaming (drifting) components and/or the temperature anisotropy of charged particles. The heat-flux instabilities are known in the literatu...

Firehose constraints of the bi-Kappa-distributed electrons: a zero-order approach for the suprathermal electrons in the solar wind

Radiation transfer through a volcanic aerosol medium has been studied. The radiation transfer properties of the medium as scattering, absorption and extinction coefficients are calculated using the Mie scattering theory. Average coefficients over the size parameter and the radiation wavelength are calculated. The radiation heat fluxes for volcanic eruption ash medium are calculated using the Variational Pomraning–Eddington approximation and compared with those obtained from the Galerkin method. The comparison showed very good agreement.