Ordinary Mode Instability Research Articles

Electron Temperature Anisotropy and Electron Beam Constraints from Electron Kinetic Instabilities in the Solar Wind

Abstract Electron temperature anisotropies and electron beams are nonthermal features of the observed nonequilibrium electron velocity distributions in the solar wind. In collision-poor plasmas these nonequilibrium distributions are expected to be regulated by kinetic instabilities through wave–particle interactions. This study considers electron instabilities driven by the interplay of core electron temperature anisotropies and the electron beam, and first gives a comprehensive analysis of instabilities in arbitrary directions to the background magnetic field. It clarifies the dominant parameter regime (e.g., parallel core electron plasma beta , core electron temperature anisotropy , and electron beam velocity V eb) for each kind of electron instability (e.g., the electron beam-driven electron acoustic/magnetoacoustic instability, the electron beam-driven whistler instability, the electromagnetic electron cyclotron instability, the electron mirror instability, the electron firehose instability, and the ordinary-mode instability). It finds that the electron beam can destabilize electron acoustic/magnetoacoustic waves in the low- regime, and whistler waves in the medium- and large- regime. It also finds that a new oblique fast-magnetosonic/whistler instability is driven by the electron beam with in a regime where and A ec < 1. Moreover, this study presents electromagnetic responses of each kind of electron instability. These results provide a comprehensive overview for electron instability constraints on core electron temperature anisotropies and electron beams in the solar wind.

Ordinary Mode Instability in a Cairns Distributed Electron Plasma

Employing the linearized Vlasov-Maxwell equations, a generalized dispersion relation for the ordinary mode is derived by employing the Cairns distribution function. The instability of the mode and its threshold condition is investigated. It is found that the temperature anisotropy χ = T‖/T⊥ > 1 required to excite the instability varies with density values whereas the growth rate is dependent on various parameters like non-thermality Λ, equilibrium number density n0 and temperature anisotropy. It is found that with the increase in the values of any of the parameters Λ, n0 and χ, the growth rate is enhanced and the k-domain is enlarged. The results are applicable for space plasma environments like solar wind.

Simulation and quasilinear theory of aperiodic ordinary mode instability

The purely growing ordinary (O) mode instability driven by excessive parallel temperature anisotropy for high-beta plasmas was first discovered in the 1970s. This instability receives renewed attention because it may be applicable to the solar wind plasma. The electrons in the solar wind feature temperature anisotropies whose upper values are apparently limited by plasma instabilities. The O-mode instability may be important in this regard. Previous studies of O mode instability have been based on linear theory, but the actual solar wind electrons may be in saturated state. The present paper investigates the nonlinear saturation behavior of the O mode instability by means of one-dimensional particle-in-cell simulation and quasilinear theory. It is shown that the quasilinear method accurately reproduces the simulation results.

Ordinary mode instability associated with thermal ring distribution

The purely growing ordinary (O) mode instability driven by excessive parallel temperature anisotropy has recently received renewed attention owing to its potential applicability to the solar wind plasma. Previous studies of O mode instability have assumed either bi-Maxwellian or counter-streaming velocity distributions. For solar wind plasma trapped in magnetic mirror-like geometry such as magnetic clouds or in the vicinity of the Earth's collisionless bow shock environment, however, the velocity distribution function may possess a loss-cone feature. The O-mode instability in such a case may be excited for cyclotron harmonics as well as the purely growing branch. The present paper investigates the O-mode instability for plasmas characterized by the parallel Maxwellian distribution and perpendicular thermal ring velocity distribution in order to understand the general stability characteristics.

The Electron Firehose and Ordinary-Mode Instabilities in Space Plasmas

The selfgenerated wave fluctuations are particularly interesting in the solar wind and magnetospheric plasmas, where Coulomb collisions are rare and cannot explain the observed states of quasi-equilibrium. Linear theory predicts that the firehose and the ordinary-mode instabilities can develop under the same conditions, confusing the role of these instabilities in conditioning the space-plasma properties. The hierarchy of these two instabilities is reconsidered here for nonstreaming plasmas with an electron temperature anisotropy $T_\parallel > T_\perp$, where $\parallel$ and $\perp$ denote directions with respect to the local mean magnetic field. In addition to the previous comparative analysis, here the entire 3D wave-vector spectrum of the competing instabilities is investigated, paying particular attention to the oblique firehose instability and the relatively poorly known ordinary-mode instability. Results show a dominance of the oblique firehose instability with a threshold lower than the parallel firehose instability and lower than the ordinary-mode instability. For larger anisotropies, the ordinary mode can grow faster, with maximum growth rates exceeding the ones of the oblique firehose instability. In contrast to previous studies that claimed a possible activity of the ordinary-mode in the small $\beta [< 1]$ regimes, here it is rigorously shown that only the large $\beta [> 1]$ regimes are susceptible to these instabilities.

Towards a complete parametrization of the ordinary-mode electromagnetic instability in counterstreaming plasmas. I. Minimizing ion dynamics

The ordinary mode instability can be driven by drifting bi-Maxwellian plasma particle distributions with and without temperature anisotropy. Here, the linear instability analysis is generalized for realistic settings, when the plasma streams are magnetized and hot enough. The new parametrization proposed in this study enables a better understanding of the interplay of counterstreaming and temperature anisotropy, providing the derivation of new regimes of the ordinary mode instability. Accurate analytical forms are derived for the instability conditions for general values of the temperature anisotropy, the streaming velocity, and the parallel plasma beta. To keep the analysis straightforward, the role of ions is minimized.

Inverse cascade feature in current disruption

We investigate the origin of waves leading to current disruption and dipolarization observed by a THEMIS satellite at XGSM ∼ −8 RE in the magnetotail near a substorm expansion onset on 29 January 2008. Continuous wavelet transform of the magnetic activity associated with current disruption shows a clear inverse cascade feature of waves starting at frequencies slightly below the ion cyclotron frequency to waves at low frequencies that correspond to the time scale of dipolarization. There was no other low‐frequency wave associated with the time scale of dipolarization. On the basis of this inverse cascade feature, we provide a preliminary theoretical analysis to propose that the initial excited waves causing eventual dipolarization in this event may originate from the drift‐driven electromagnetic ion cyclotron instability, which is an electromagnetic version of the Weibel instability, or an ordinary mode instability similarly driven by the cross‐field ion drift.

Regulation of solar wind heat flux by ordinary mode instability

The ordinary mode can frequently become unstable in the solar wind at 1 AU provided the ratio of halo to core electrons density does not exceed the value 0.05. The growth rates corresponding to the average conditions are typically ∼10ΩP (ΩP being the proton cyclotron frequency). Because of low threshold for onset of instability forβ⊥C ≳ 1 (whereβ⊥C is the transverse beta for the core electrons), the mode is expected to play an important role in regulating the solar wind heat flux at 1 AU.

Effect of the temperature gradient on the ordinary mode electromagnetic instability in a high-beta plasma

The effect of nonuniformities of temperature, magnetic field, and number density on the ordinary mode with temperature anisotropy is investigated. It is shown that in a high-β plasma the new and much stronger instability due to electron wave resonance with temperature gradient and temperature anisotropy occurs. The ordinary mode instability in an inhomogeneous plasma is possible under the condition T⊥,e (electron perpendicular temperature) &amp;gt; T‖,e (electron parallel temperature), in contrast to the previous results T‖,e &amp;gt; T⊥,e. The pitch angle diffusion of particles is also investigated.

Effects of solar wind composition, anisotropy, and streaming on ordinary mode electromagnetic instability

The stability of the low-frequency waves (ω < Ωi) propagating transverse to the magnetic field of the plasma, which is composed of electrons, protons, and α particles with anisotropic electron and ion temperatures, is explored. The threshold for the ordinary mode instability and the growth rates have a very strong dependence on the electron temperature anisotropy but have a comparatively weaker dependence on the ion temperature anisotropy, on the relative abundance of helium to hydrogen, and on the relative streaming of two ion species. The threshold for the instability of these low-frequency waves is (me/mp)½ times smaller than the one corresponding to high-frequency waves; however, for the relative abundance of helium to hydrogen up to 20% for the relevant known magnetic fields, particle densities, temperatures, and drifts, the solar wind remains below the threshold for this instability.

Negative pressure effect on the ordinary mode

Based on the linearized Vlasov-Maxwell equations, it is found that, without macroscopic current, the ordinary mode instability can occur with the negative pressure of the electrons. These electrons reduce the critical number density of hot electrons for the onset of instability.