Magnetic curvature and ion distribution function effects on lower hybrid drift instabilities

Recent DE‐2 data from the Langmuir probe, vector electric field, and ion drift meter instruments are employed to study equatorial spread F in the frequency regime of the low frequency drift and the lower hybrid drift instabilities. Strong electron density gradients observed along the orbital path through the topside equatorial F region correspond to regions of high electric field waves and large ion drift velocities. The electric field waves are seen in two distinct wavelength ranges which correspond to the parameter regimes of the low frequency drift and the lower hybrid drift instabilities. In the smaller of the two wavelength ranges the lower hybrid drift instability is found to be unstable, based on the ion drift velocity and the other plasma parameters measured on DE‐2, and using published theory. Thus there is experimental evidence that the lower hybrid drift instability may produce the observed short wavelength waves without invoking a cascading mechanism.

The behavior of the lower hybrid drift instability is investigated in the low drift velocity regime by including the effects of the magnetic field on the ions. It is found that the mode is transformed from the nonresonant lower hybrid drift instability to the resonant ion cyclotron drift instability as the drift velocity Vd decreases and is finally stabilized when Vd/Vti∼ (me/mi)1/2.

Carrying out a large scale 3D full kinetic simulation, we have shown that the effects of lower-hybrid waves at the edges of the current sheet provide a quick triggering of magnetic reconnection even in ion-scale current sheets. A thin embedded electron current layer is formed as a result of the non-linear evolution of the lower hybrid drift instability, which is sustained by accelerated meandering electrons around the neutral sheet, and the emergence of a thin electron current layer is subject to the quick reconnection triggering. However. there may be an upper-limit to the current sheet thickness for this type of quick triggering mediated by the lower-hybrid-drift instability alone because the lower-hybrid-drift wave activity becomes weaker in a thicker current sheet. To evaluate whether the quick triggering mechanism mediated by the lower-hybrid-drift instability is truly realistic, we performed a parametric study of magnetic reconnection with the lower hybrid drift instability. Consequently, we confirmed that the proposed scenario of the quick reconnection triggering is available with a real ion-electron mass ratio m/sub i//m/sub e/=1836. We also found that the upper-limit of the current sheet thickness depends sensitively on the strength of the guide magnetic field.

<p>The mass and energy transfer across Earth’s magnetopause is caused by a variety of different plasma processes. One of these processes is the Kelvin-Helmholtz instability (KHI), excited by the velocity shear between the fast-flowing magnetosheath plasma and the relatively stagnant magnetosphere. It has been frequently observed during periods of northward interplanetary magnetic field (IMF), however much less is known about its behaviour during southward IMF conditions.</p><p>We present the first Magnetospheric Multiscale (MMS) observations of KH waves and vortices at the dusk-flank magnetopause during southward IMF conditions on September 23, 2017. The instability criterion for the KHI was fulfilled during this event. The boundary normal vectors, obtained by using multi-point methods, are consistent with the predicted structures of the KH waves. We further performed a series of realistic 2D and 3D fully kinetic PIC simulations based on the plasma parameters observed during this MMS event. A comparison to results from these simulations demonstrated quantitative consistencies with the MMS data in many aspects such as the flow and total pressure variations in the KH waves, and the signatures of the non-linearly rolled up KH vortices including the Low Density Faster Than Sheath (LDFTS) plasma.</p><p>The simulations further showed that secondary instabilities are excited at the edges of the primary KHI. The Rayleigh-Taylor instability (RTI) can lead to the penetration of high-density arms into the magnetospheric side and disturb the structures of the vortex layer, leading to irregular variations of the surface waves. This can be an important factor in explaining the lower observational probability of KH waves during southward IMF than northward IMF. In the non-linear growth stage of the primary KHI, the lower-hybrid drift instability (LHDI) is excited at the vortex edges leading to efficient plasma mixing across the magnetopause.</p><p>The high-time resolution of MMS measurements demonstrated the occurrence of kinetic-scale plasma waves mainly on the low-density side of the edges of the KH waves. Given quantitative consistencies with the simulations, these waves can be interpreted as being generated by the LHDI. These observed waves form due to the strong density gradient between the two sides of the boundary layer and can lead to a flattening of the edge layers.</p><p><br>In this presentation, we will show the consistencies between MMS observations and 2D and 3D simulation runs focusing on the large-scale surface waves (KHI, RTI) and the small-scale fluctuations (LHDI) and outline the multi-scale properties of the observed KH waves during southward IMF.</p>

The current sheet structure modification by lower‐hybrid drift instability (LHDI) has been shown to lead to quick magnetic reconnection triggering (QMRT) even in a thick current sheet (Tanaka et al., 2005). The three‐dimensional simulations, however, have been limited to certain sets of parameters. Having learned which effects of LHDI are crucial for QMRT in these three‐dimensional results, we perform a series of two‐dimensional simulations of LHDI (reconnection itself not included) with various current sheet thickness D, guide field intensity B 0y , and the frequency ratio τ ( τ = ω pe /Ω e , where ω pe is the electron plasma frequency and Ω e is the electron gyrofrequency) and inspect how these parameters affect the QMRT process. While QMRT looked promising in the three‐dimensional studies with B 0y = 0 and τ = 1, the two realistic factors, nonzero guide field and τ larger than 1, requires rather small current sheet thickness for QMRT to be operative in the magnetospheric situations.

In this paper we examine the drift instability around the lower hybrid resonance (LHR) frequency in the presence of temperature and density gradients. The unstable mode which propagates with the electron drift velocity is strongly affected by the electron temperature and density gradients present in a direction perpendicular to the magnetic field. A generalised expression for the growth/damping rate for the lower hybrid drift mode in the presence of temperature and density gradients have been derived and discussed for suitable ionospheric plasma parameters. Expressions for marginal stability and maximum growth rate have also been calculated. Applications of these results to explain some of the observed features of satellite observations of ionospheric plasma have been indicated.

We use the Magnetospheric Multiscale mission to investigate electron‐scale structures at a dipolarization front. The four spacecraft are separated by electron scales and observe large differences in plasma and field parameters within the dipolarization front, indicating strong deviation from typically assumed plane or slightly curved front surface. We attribute this to ripples generated by the lower hybrid drift instability (LHDI) with wave number of kρe≃0.4 and maximum wave potential of ∼1 kV ∼kBTe. Power law‐like spectra of E⊥ with slope of −3 indicates the turbulent cascade of LHDI. LHDI is observed together with bursty high‐frequency parallel electric fields, suggesting coupling of LHDI to higher‐frequency electrostatic waves.

The lower-hybrid drift instability (LHDI) in a thin current sheet in the intermediate-wavelength (kyρiρe∼1, where ky, ρe, and ρi are the wave vector and the electron and ion gyroradii, respectively) regime for particles with κ velocity distribution is studied. The latter is more suitable for describing nonthermal distributions with an enhanced high-energy tail and includes the Maxwellian as a limiting case. It is shown that linear electromagnetic LHDI can be excited near the center of the current sheet. The growth rate decreases, but the electromagnetic component of the LHD mode increases with increase in hot particles.

The lower hybrid drift instability is investigated by means of a simple instructive method. It is found that there is a nonlinear resonant effect when the group velocity is close to the electron EXB drift velocity. This effect could form envelope solitons propagating in the azimuthal direction.

The effect of finite parallel wavenumber and electron temperature gradients on the lower hybrid drift instability is studied in the parameter regime corresponding to the TRX-2 device [Fusion Technol. 9, 48 (1986)]. Perturbations in the electrostatic potential and all three components of the vector potential are considered and finite beta electron orbit modifications are included. The electron temperature gradient decreases the growth rate of the instability but, for kz=0, unstable modes exist for ηe(=T′en0/Ten0)>6. Since finite kz effects completely stabilize the mode at small values of kz/ky(≂5×10−3), magnetic shear could be responsible for stabilizing the lower hybrid drift instability in field-reversed configurations.

A kinetic electrostatic eigenvalue equation for the lower-hybrid drift instability (LHDI) in a thin Harris current sheet with a guide field is derived based on the gyrokinetic electron and fully kinetic ion(GeFi) description. Three-dimensional nonlocal analyses are carried out to investigate the influence of a guide field on the stabilization of the LHDI by finite parallel wavenumber, k∥. Detailed stability properties are first analyzed locally, and then as a nonlocal eigenvalue problem. Our results indicate that at large equilibrium drift velocities, the LHDI is further destabilized by finite k∥ in the short-wavelength domain. This is demonstrated in a local stability analysis and confirmed by the peak in the eigenfunction amplitude. We find the most unstable modes localized at the current sheet edges, and our results agree well with simulations employing the GeFi code developed by Lin et al. [Plasma Phys. Controlled Fusion 47, 657 (2005); Plasma Phys. Controlled Fusion 53, 054013 (2011)].

The effect of the magnetic field curvature on magnetic islands in a tokamak is analyzed. It is demonstrated that the original investigation of this effect by Kotschenreuther et al. (1985) is inconsistent: on the one hand, the authors made the correct assumption that this is an ideal effect and, on the other hand, they described it in terms of the parameters characteristic of the "resistive ordering" approach, which is incompatible with the ideal approximation. More recent studies of the magnetic curvature effect have produced further ambiguities; as a result, a branch of the theory of magnetic islands has arisen that is based on the supposition that the effect under discussion can be described in terms of the Glasser-Greene-Johnson parameter D R. This branch is shown to be erroneous, because the parameter D R describes the plasma response to magnetic field perturbations on spatial scales of about the dimension of the linear resistive layer, while the characteristic spatial scale of the magnetic islands is much longer. It is concluded that the correct theory developed here for the magnetic curvature effect makes more optimistic predictions about its stabilizing role.

A transcollisional, electromagnetic fluid model, incorporating the parallel heat flux as a dependent variable, is constructed to treat electron drift turbulence in the regime of tokamak edge plasmas at the L - H transition. The resulting turbulence is very sensitive to the plasma beta throughout this regime, with the scaling with rising beta produced by the effect of magnetic induction to slow the Alfvénic parallel electron dynamics and thereby leave the turbulence in a more robust, non-adiabatic state. Magnetic flutter and curvature have a minor qualitative effect on the turbulence mode structure and on the beta scaling, even when their quantitative effect is strong. Transport by magnetic flutter is small compared to that by the flow eddies. Fluctuation statistics show that while the turbulence shows no coherent structure, it is coupled strongly enough so that neither density nor temperature fluctuations behave as passive scalars. Both profile gradients drive the turbulence, with the total thermal energy transport varying only weakly with the gradient ratio, . Scaling with magnetic shear is pronounced, with stronger shear leading to lower drive levels. Scaling with either collision frequency or magnetic curvature is weak, consistent with their weak qualitative effect. The result is that electron drift turbulence at L - H transition edge parameters is drift Alfvén turbulence, with both ballooning and resistivity in a clear secondary role. The contents of the drift Alfvén model will form a significant part of any useful first-principles computation of tokamak edge turbulence.

This paper discusses theoretical aspects of cross-field wave-particle transport at the magnetopause. The basic framework for calculating the transport based on second order-theory is reviewed. Instabilities that give rise to the waves which then lead to particle scattering and diffusion are discussed, and in particular, the lower hybrid drift instability is emphasized. The linear and nonlinear properties of this instability are presented, along with results from revelant computer simulations. Other potential diffusion processes at the magnetopause are also described. The overall conclusion, supported by wave and particle observations, is that wave-particle diffusion is a relatively weak process at the magnetopause. Further, it is argued that this line of research, while illuminating potential new mechanisms, cannot address the more important question of diffusion and transport on a spatial scale of many ion gyroradii over a temporal scale of many ion gyroperiods. A new paradigm to examine the global aspects of magnetopause transport on ion scales using hybrid code methods is presented. Results of two-dimensional hybrid simulations show that the structure of the magnetopause for northward interplanetary magnetic field is reproduced, and the resulting transport in this case is due to waves below the ion gyrofrequency. Diffusion in the presence of viscousmore » transport due to the kinetic Kelvin-Helmholtz instability is then discussed. The potential for the study of particle diffusion, viscous interactions and magnetic reconnection simultaneously through this paradigm is also addressed.« less

Plasma waves generated in a large vacuum chamber (20 × 30 m) by an electron beam of energy 0.5 to 2 keV, are measured. Both pre-beam plasma discharge (pre-BPD) and beam plasma discharge (BPD) states are investigated. The measured waves fall roughly into three categories: 1) a low-frequency spectrum with f << fee, fp; 2) a whistler mode spectrum; and 3) a high-frequency spectrum. The extremely low frequency waves are shown to be surface waves on a (nonneutral) plasma column but a higher frequency part of the low-frequency spectrum was not thoroughly investigated. It is speculated that it also represents surface waves on a nonneutral column. Its measured properties are consistent with the lower hybrid drift instability. The whistler mode spectrum seems to be a Cerenkov resonance with the lower Trivelpiece-Gould mode. It does not seem to play an important role in modifying the particle distributions. The high frequency spectrum is, as suggested by Rowland et al. [21], a Cerenkov resonance with the upper Trivelpiece-Gould mode. It is responsible for the major perturbation of the beam energy in BPD.