Lower Hybrid Drift Instability in the Ionosphere

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.

The linear Vlasov dispersion relation for electrostatic instabilities driven by a cross-field current due to density and temperature gradients in a unidirectional magnetic field is examined in the local approximation. Both electrons and ions are taken as magnetized, and propagation is in the plane defined by the drift velocities and the magnetic field. The effects of temperature gradients on three different instabilities are studied and compared. Electron (ion) temperature gradients parallel (antiparallel) to the density gradient enhance the maximum growth rate of the universal drift instability; but, an antiparallel ∇T for either species is necessary to enhance the growth of the lower hybrid drift or to lower the threshold of the ion cyclotron ion drift instability. Oblique propagation of the ion cyclotron ion drift instability corresponds to lower thresholds than at perpendicular propagation.

ABSTRACTIt is shown that ions can be accelerated to about 100 keV in the direction perpendicular to the magnetic field by the ExB mechanism of electrostatic waves. The acceleration occurs in discrete steps of duration being a small fraction of the gyroperiod and can explain observations of ion energization to 10 keV at quasi-perpendicular shocks and to hundreds keV at quasi-parallel shocks. A general expression is provided for the maximum energy of ions accelerated in shocks of arbitrary configuration. The waves involved in the acceleration are related to three cross-field current-driven instabilities: the lower hybrid drift (LHD) instability induced by the density gradients in shocks and shocklets, followed by the modified two-stream (MTS) and electron cyclotron drift (ECD) instabilities, induced by the ExB drift of electrons in the strong LHD wave electric field. The ExB wave mechanism accelerates heavy ions to energies proportional to the atomic mass number, which is consistent with satellite observations upstream of the bow shock and also with observations of post-shocks in supernovae remnants. The results are compared with other acceleration mechanisms traditionally discussed in the literature.

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.

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 plasma density and magnetic field are highly spatially and temporally variable in the Venus nightside ionosphere. This variability may affect the propagation and the generation of plasma waves. We determine how these irregularities are related to plasma wave activity. Previously, we have found two types of signals in the lower ionosphere, 100‐Hz narrow‐band signals and high‐frequency wideband bursts. We investigate how these two different wave phenomena depend on the structure of the ionosphere. We calculate the ratios of the instantaneous values of electron density, magnetic field, and electron temperature relative to their average (1 min) values, and also their gradient scale lengths. The 5.4‐kHz burst activity is related to density depressions and obvious electron temperature enhancements, but varies little with magnetic field strength. The 100‐Hz narrow‐band signals are often associated with significantly enhanced magnetic field and depressions of electron density. Both higher field strength and lower density result in a lower β (ratio of thermal pressure to magnetic pressure) plasma, which favors the propagation of whistler waves. We find that neither type of signal is associated with gradients of the magnetic field, but the 5.4‐kHz wideband signals have higher occurrence rates at higher electron density and temperature gradients, while the 100‐Hz narrow‐band burst rate increases only slightly for steeper gradients. A local gradient‐driven instability cannot explain most of the 100‐Hz narrow‐band signals, but some 5.4‐kHz signals may be associated with pressure gradients.

From a basic discussion of the behaviour of VLF waves propagated almost perpendicularly to the earth 's magnetic field, the lower hybrid resonance (LHR) frequency is seen as a boundary between compressional Alfv én waves and electrostatic sound waves. The disparity of the wavelengths involved leads to a fundamental difference between the characteristics of the AC electric field far from and close to the source.Some detailed observations of this field are presented, from an experiment that used both a mother-daughter technique for large distance measurements and a quadrupole probe technique for the local field, and that was performed in the equatorial ionosphere. The frequency dispersion of quasi-transverse (QT) waves received at a large distance and the amplitude-frequency response of the VLF quadrupole probe are compared with a theoretical analysis for the warm ionospheric plasma. From this analysis, the possibility is established of using the properties of fast and slow waves in active experiments, for measuring simultaneously the LHR, the electron density, a hybrid ion-electron thermal velocity and possibly the plasma drift velocity.

Growth of electrostatic waves around the lower hybrid resonance (LHR) frequency has been discussed. Expressions for growth rate of the excited electrostatic wave around the LHR frequency have been derived and evaluated for ionospheric plasma parameters. The results are in good agreement with the experimental observations reported by earlier workers.

In saturated Ohmic confinement regime of the experimental advanced superconducting tokamak, significant changes in the relative electron temperature fluctuations T̃e/T¯e, measured by a correlation electron cyclotron emission system, have been observed. It was found that T̃e/T¯e is strongly dependent on the normalized electron temperature and density gradient, R/LTe=R∇Te/Te and R/Lne=R∇ne/ne. T̃e/T¯e stay at a high level when R/LTe is within the range of 17−25 and R/Lne is within the range of 4−6. It has been also observed that the energy confinement degradation is correlated with the increase in electron temperature fluctuations. The linear stability analysis results show that the electron temperature fluctuations are mainly attributed to the ion temperature gradient modes.

Magnetic curvature is shown to have a relatively weak effect on lower hybrid drift instabilities, but ion temperature gradients can significantly alter stability in the broad sheath limit, ai≪Ln.

A rocket borne spectrometer was flown to measure absolute intensities of extreme ultraviolet spectral lines from the three ions O vi, Ne viii, and Mg x present in the Sun. From these measurements, intensity ratios of lines from O vi, ratios of lines from Ne viii, and ratios of lines from Mg x were formed. These experimental ratios were compared with ratios calculated by using specific theoretical values of the ionization equilibrium in which dielectronic recombination was included in the processes establishing ionization balance. The effects of the electron density and temperature gradient on the temperature distribution of the flux of the spectral lines in the solar atmosphere have been taken into account in the calculations of the ratios. The agreement between the experimental and calculated ratios is good for the ions Ne viii and Mg x and satisfactory for the ion O vi for which the calculated ratio is subject to large uncertainties. A reliable measurement of the electron temperature in the lower corona was obtained from the experimental ratios for Mg x. This experimental temperature is in good agreement with the emission temperature of the spectral lines of Mg x predicted from the theoretical values of the ionization equilibrium. The design and photometric calibration of a new rocket spectrometer developed to measure the intensity ratios over the broad spectral region 50 to 1250 A are also described.

<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>

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.