Cross-field Instability Research Articles

Fine structure and motion of the bow shock and particle energisation mechanisms inferred from Magnetospheric Multiscale (MMS) observations

Abstract. This study presents new observations of fine structure and motion of the bow shock formed in the solar wind, upstream of the Earth's magnetosphere. NASA's Magnetospheric Multiscale (MMS) mission has recorded data during 11 encounters with a shock oscillating with frequency of 1 mHz. Shocks move with a speed of 4–17 km s−1; have thickness of 100 km, i.e. an ion gyroradius; and represent cascades of compressional magnetic field and plasma density structures of increasing frequencies or smaller spatial scales. Induced density gradients initiate chains of cross-field current-driven instabilities that heat solar wind ions by the stochastic Ẽ×B wave energisation mechanism. The theoretical ion energisation limits are confirmed by observations. We have identified the ion acceleration mechanism operating at shocks and explained double-beam structures in the velocity space. The nature of this mechanism has been revealed as a stochastic resonant acceleration (SRA). The results provide for the first time a consistent picture of a chain of plasma processes that generate collisionless shocks and are responsible for particle energisation.

ELF magnetic fluctuations detected by Kaguya in deepest lunar wake associated with type-II protons

Magnetic fluctuations in the extremely low-frequency (ELF) range from 0.1 to 10 Hz were found by the Lunar Magnetometer (LMAG) of the magnetic field and plasma experiment (MAP) on board the spacecraft Kaguya in the deepest wake behind the moon, where the magnetic field is usually quiet. The fluctuations were compressional and non-monochromatic, showing no preferred polarization. They were often accompanied by “type-II entry” solar wind protons that were reflected by the dayside lunar surface or crustal magnetic field, gyrated around the solar wind magnetic field, then entered the deepest wake. The ELF waves persisted for 30 s to several minutes. The duration was often shorter than that of the type-II protons. Most of the waves were detected on the magnetic field lines disconnected from the lunar surface, along which the solar wind electrons were injected into the wake. Since a large cross-field velocity difference is expected between the type-II protons and the solar wind electrons injected along the magnetic field, some cross-field current-driven instability such as the lower hybrid two-stream instability is expected to be responsible for the generation of the waves.

Ion and electron accelerations by large‐scale shear Alfvén waves via cross‐field instabilities

Using two‐dimensional particle‐in‐cell simulations we study the linear and nonlinear effects of a large‐scale shear Alfvén wave (LS‐SAW) propagating parallel to the ambient magnetic field Bo. We demonstrate that ion polarization drift in the propagating SAW drives cross‐field instability (CFI) by ion‐electron coupling and produces electrostatic (ES) waves, which accelerate electrons parallel and ions perpendicular to Bo. Even though the large‐scale SAW has no parallel electric field E∥, the E∥ of the ES wave fields embedded in LS‐SAW provide a novel acceleration mechanism for the electrons moving with the group velocity of the LS‐SAW. Examining data from satellite observations, we suggest that the mechanism of CFI for plasma acceleration/heating powered by LS‐SAWs is distinctly possible on auroral field lines.

Low-frequency electrostatic waves in the ionospheric E-region: a comparison of rocket observations and numerical simulations

Abstract. Low frequency electrostatic waves in the lower parts of the ionosphere are studied by a comparison of observations by instrumented rockets and of results from numerical simulations. Particular attention is given to the spectral properties of the waves. On the basis of a good agreement between the observations and the simulations, it can be argued that the most important nonlinear dynamics can be accounted for in a 2-D numerical model, referring to a plane perpendicular to a locally homogeneous magnetic field. It does not seem necessary to take into account turbulent fluctuations or motions in the neutral gas component. The numerical simulations explain the observed strongly intermittent nature of the fluctuations: secondary instabilities develop on the large scale gradients of the largest amplitude waves, and the small scale dynamics is strongly influenced by these secondary instabilities. We compare potential variations obtained at a single position in the numerical simulations with two point potential-difference signals, where the latter is the adequate representation for the data obtained by instrumented rockets. We can demonstrate a significant reduction in the amount of information concerning the plasma turbulence when the latter signal is used for analysis. In particular we show that the bicoherence estimate is strongly affected. The conclusions have implications for studies of low frequency ionospheric fluctuations in the E and F regions by instrumented rockets, and also for other methods relying on difference measurements, using two probes with large separation. The analysis also resolves a long standing controversy concerning the supersonic phase velocities of these cross-field instabilities being observed in laboratory experiments.

A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002

Abstract. We present a coordinated ground- and space-based multi-instrument study of two magnetospheric substorm events that occurred on 1 September 2002, during the interval from 18:00 UT to 24:00 UT. Data from the Cluster and Polar spacecraft are considered in combination with ground-based magnetometer and HF radar data. During the first substorm event the Cluster spacecraft, which were in the Northern Hemisphere lobe, are to the west of the main region affected by the expansion phase. Nevertheless, substorm signatures are seen by Cluster at 18:25 UT (just after the expansion phase onset as seen on the ground at 18:23 UT), despite the ~5 RE} distance of the spacecraft from the plasma sheet. The Cluster spacecraft then encounter an earthward-moving diamagnetic cavity at 19:10 UT, having just entered the plasma sheet boundary layer. The second substorm expansion phase is preceded by pseudobreakups at 22:40 and 22:56 UT, at which time thinning of the near-Earth, L=6.6, plasma sheet occurs. The expansion phase onset at 23:05 UT is seen simultaneously in the ground magnetic field, in the magnetotail and at Polar's near-Earth position. The response in the ionospheric flows occurs one minute later. The second substorm better fits the near-Earth neutral line model for substorm onset than the cross-field current instability model. Key words. Magnetospheric physics (Magnetosphereionosphere interactions; Magnetic reconnection; Auroral phenomenon)

Dusty plasma cross-field instability excited by gravity

A cross-field instability is considered in a magnetized, dusty plasma acted upon by a gravity field, g , perpendicular to the magnetic field, B , permeating the plasma. Waves are produced traveling essentially with the dust grains zero-order motion. The condition for excitation of the instability is that the dust zero-order velocity perpendicular to both g and B exceed the dust-acoustic speed, C da . Numerical results are presented for an ionospheric type plasma and for a laboratory type plasma.

On the deviations of measured electron density profiles from IRI predictions for low latitudes

In situ electron density profiles obtained from the equatorial stations in Brazil using conventional Langmuir probes and High Frequency Capacitance probes are compared with the IRI predictions in the light of the spectral distribution of the plasma density irregularities observed. Plasma instability mechanisms, especially the Rayleigh-Taylor and the Cross-Field instability mechanisms are considered responsible for the generation of the plasma irregularities observed at different height regions along the electron density profile, in the post sunset ionosphere. A comparative study of the observed characteristics of the plasma irregularities with those expected from the theories can give us information on the reliability of the observed profiles. This reliability estimate is all the more important because of the fact that the techniques used for the measurement of electron density are known to be associated with some inherent problems. The presence or absence of plasma irregularities in a given height region can, therefore, give us information as to whether the deviations of the observed electron density profiles from the IRI predictions are genuine or not. From this comparative study one can also obtain information as to what are the physical parameters responsible for the observed deviations in the profiles and thereby suggest some possible improvements in the methods used for IRI predictions for low latitudes.

Some cross-field instabilities in magnetized dusty plasmas

It is shown that the resonant streaming energy associated with cross-field motions (due to the cross product of dc electric and magnetic fields) of the plasma particles can be coupled to low-frequency electrostatic waves. Conditions under which unstable waves appear are obtained for both collisionless and collisional dusty magnetoplasmas. The relevance of the present investigation to understanding the origin of nonthermal fluctuations in laboratory and space plasmas is pointed out.

Electron magnetohydrodynamic turbulence in a high-beta plasma. I. Plasma parameters and instability conditions

The interaction of a dense discharge plasma with a weak external magnetic field has been studied experimentally. The electron pressure exceeds the field pressure and forms a magnetic hole in the plasma interior. The ions are unmagnetized, while the electrons are in a transition regime from none to full magnetization. The electron confinement changes from Boltzmann equilibrium to magnetic confinement. The pressure balance equation does not describe the diamagnetism because ambipolar E×B drifts oppose the diamagnetic drift. The net drift exceeds the sound speed by an order of magnitude and produces a strong two-stream cross-field instability. Although its spectrum is close to the lower hybrid instability, there are significant differences from the classical lower hybrid instability, e.g., the presence of strong magnetic fluctuations. These fall into the regime of electron magnetohydrodynamics (EMHD) with unmagnetized but mobile ions. While the EMHD turbulence is the main focus of the two following companion papers, this first paper describes the plasma diamagnetism and basic parameters that lead to the instability.

Electron magnetohydrodynamic turbulence in a high-beta plasma. III. Conditionally averaged multipoint fluctuation measurements

A large discharge plasma is generated whose electron pressure exceeds that of an external magnetic field. A magnetic cavity exists in the plasma interior. The ions are unmagnetized while the electron magnetization varies from complete to none. In the region of pressure and field gradients a strong instability is observed. It is a cross-field instability driven by the electron diamagnetic drift through the unmagnetized ions, creating large density and magnetic field fluctuations near the lower hybrid frequency that propagate at the sound speed in the diamagnetic drift direction. The basic plasma parameters leading to the instability have been presented in Part I of three companion papers. Spectra, correlations, amplitude distributions, and magnetic hodograms derived from single-point fluctuation measurements have been presented in Part II. These led to the discovery of density cavities and current sheet formation by nonlinear wave steepening. The present Part III deals with multipoint fluctuation measurements using on-line conditional averaging, which resolves the structure of typical fluctuations in space and time. Propagation velocity and coherence of the flutelike density perturbations is measured. The topology of the magnetic fluctuations and associated current density is investigated and shown to consist of flux ropes of negative self-helicities. Superposition of the nonuniform mean field and fluctuating fields yields the net field and current density. Instantaneous field lines and magnitude distributions in three-dimensional space are presented for a turbulent high-beta plasma. The results suggest that the demagnetization of electrons in large positive density fluctuations provides a new saturation mechanism for the instability.

<i>Letter to the Editor</i>: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

Abstract. At dip equatorial stations in the Indian zone, spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N; 77.5°E; dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum. The implications of this finding are discussed.Key words. Ionosphere (equatorial ionosphere; ionospheric irregularities)

Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

At dip equatorial stations in the Indian zone. spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N: 77.5°E: dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum The implications of this finding are discussed.

Features of E region irregularities at the magnetic equator and in its vicinity

Rocketborne measurements of E region irregularities in scale size 1 km down to 1 meter were carried out from Thumba (dip = 0) and from SHAR (Sriharikota, dip = 13°). It appears that the irregularities are generated due to two stream instability and cross field instability over Thumba. Over SHAR the irregularities are generated due to cross field instability and wind shear mechanism.

Kinetic theory of low-frequency cross-field instability in a weakly ionized plasma. II

The consistent kinetic approach developed in Paper I [Ya. S. Dimant and R. N. Sudan, Phys. Plasmas 2, 1157 (1995)] is applied to obtain the general dispersion relation of the two-stream E×B instability in collisionally dominated weakly ionized plasmas for wave frequencies small compared to the ion–neutral collision frequency. This dispersion relation covers the whole low-frequency band from the asymptotic short-wave limit studied in Paper I to the long-wave limit. Previous theories employing simplified kinetic theory or fluid equations for electron behavior are only correct in the long-wave limit. The principal new results are that the threshold conditions for this instability and the growth rates are altered from those predicted by earlier simplified theories.

Kinetic theory of low-frequency cross-field instability in a weakly ionized plasma. I

A consistent kinetic theory is developed for the description of electrons under conditions of a low-frequency two-stream E×B instability in collisionally dominated, weakly ionized plasmas. Starting from the Boltzmann collision integral, a simplified kinetic equation for the electron distribution function has been derived, which takes into account strong pitch-angle scattering of electrons by neutrals, velocity dependence of the electron–neutral collision frequency, etc. Linearized equations describing small oscillations of the electron distribution function and ion density are presented. For the asymptotic case of short waves, the dispersion relation of the E×B instability has been obtained and analyzed under conditions typical for the lower ionosphere. Under certain conditions, the rigorous kinetic consideration yields substantial changes in results compared to previous theories. The general approach may be applied to other linear and nonlinear low-frequency processes in a weakly ionized plasma.

Spectral characteristics of low-frequency turbulence in toroidal collisional magnetoplasma

The dispersion and spectral characteristics of low-frequency fluctuations excited in collisional toroidal magnetoplasma is reported. The plasma produced under the operating conditions have magnetized electrons, unmagnetized ions, E∥∇n⊥B and ∇n antiparallel to effective gravity g. The low frequency (<ion-gyrofrequency) electrostatic fluctuations observed in density and potential are turbulent, travel mainly in azimuthal direction and are excited by the cross-field instability. The spectral indices of the k spectra for the density and potential fluctuations, as computed using pair probe technique, are −3.8±0.5 and −6.0±0.6, respectively. These results are in good agreement with the in situ measurements of fluctuations in equatorial electrojet and the earlier laboratory experiments on the cross-field instability. Numerical simulations and nonlinear theories, however, predict flatter spectra.

Experimental study of low-frequency turbulence in a toroidal plasma

Low-frequency turbulence has been studied in a plasma embedded in a pure toroidal magnetic field. A study of the variation of the spectral characteristics of the low-frequency turbulence with the variation in νin/Ωi has been carried out with two different sets of density and potential profiles. The spectral relation between density and potential fluctuations indicates that the Rayleigh–Taylor instability excites fluctuations in the long wavelength region, while the Rayleigh–Taylor driven drift instability is responsible for the short wavelength regime for νin/Ωi≪1. Spectral characteristics observed in regions where density gradients are parallel to gravity are similar to the case where density gradients are antiparallel to the gravity and support the view that the fluctuations generated in the bad curvature region are transported to the good curvature region. The results are in agreement with theoretical predictions, numerical simulations, in situ measurements, and other laboratory experiments on drift and Rayleigh–Taylor instabilities. For νin/Ωi≥1, the spectral characteristics change and may correspond to the collisional gradient drift instability or the cross-field instability.

Seasonal variations of equatorial spread-<i>F</i>

Abstract. The occurrence of spread-F at Trivandrum (8.5°N, 77°E, dip 0.5°N) has been investigated on a seasonal basis in sunspot maximum and minimum years in terms of the growth rate of irregularities by the generalized collisional Rayleigh-Taylor (GRT) instability mechanism which includes the gravitational and cross-field instability terms. The occurrence statistics of spread-F at Trivandrum have been obtained using quarter hourly ionograms. The nocturnal variations of the growth rate of irregularities by the GRT mechanism have been estimated for different seasons in sunspot maximum and minimum years at Trivandrum using h'F values and vertical drift velocities obtained from ionograms. It is found that the seasonal variation of spread-F occurrence at Trivandrum can, in general, be accounted for on the basis of the GRT mechanism.

A review of experiments on cross-field plasma instabilities.

If electrons drift relative to ions across a magnetic field, three types of electrostatic cross-field instability can occur at frequencies much greater than the ion cyclotron frequency. These have been predicted to cause particle heating and perhaps anomalous diffusion. The experiments on each type are described. The main observations include linear and nonlinear wave properties, together with the ion heating associated with the two low frequency modes.

A study of rocket measurements of ionospheric currents-III. Ionospheric currents at the magnetic dip equator

SUMMARY The 31 daytime rocket flight results within 0° or 2° dip latitude provide a good study of several issues leading to the following findings. In this zone, 60 per cent or more of ionospheric current systems have two layers. The lower layer has a peak density of 7 ± 4 A km-2 at an altitude of 106 ± 2 km, a half thickness at half peak of 5.7 ± 0.7 km and an altitude extent of 34 ± 9 km. The upper layer has a peak density of about 1 A km-2 at an altitude of 136 ± 8 km and an altitude extent of 19 ± 5 km. In some altitude structures, the single layer system is like a hybrid of the lower and the upper layers. The thickness of the equatorial electrojet decreases as its peak current density increases. From this and earlier findings, the electrojet contracts vertically and latitudinally as its peak current density increases. From the sample examined, there is not much difference between the intensities of the lower layers over India and Peru. However, the intensity of the upper layer over India is only about half of its intensity over Peru. The decrease of the electrojet current density in the afternoon is related more to the decrease in the east-west electrostatic field than to electron density or conductivity. AC electric field fluctuations caused by the cross-field instability have been observed in situ in the east-west direction only, in the frequency range of 5 to 120 Hz at altitudes of 85 to 105 km. It is shown that the geometrical and current density structures of the electrojet substantially influence the ratios of its internal to external magnetic field components, all of which initially increase in magnitude with dip distance. Within 0° to 2° dip latitude the ratios are 0.28 ± 0.08 for the northward component X, -0.17 ± 0.2 for the vertical component Z, and 0.23 ± 0.02 for the total field F. A proposed simple measure of day-to-day variability shows promise.