Finite Ion Gyroradius Research Articles

Kinetic Ballooning Instability of the Near‐Earth Magnetotail in Voigt Equilibrium

AbstractFor a long time, ballooning instabilities have been believed to be a possible triggering mechanism for the onset of substorm and current disruption initiation in the near‐Earth magnetotail. Yet the stability of the kinetic ballooning mode (KBM) in a global and realistic magnetotail configuration has not been well examined. In this paper, stability of the KBM is evaluated for the two‐dimensional Voigt equilibrium of the near‐Earth magnetotail based on an analytical kinetic theory of ballooning instability in the framework of kinetic magnetohydrodynamic (MHD) model, where the kinetic effects such as the finite gyroradius effect, wave‐particle resonances, and particle drifts motions are included usually through kinetic closures. The growth rate of the KBM strongly depends on the magnetic field line stiffening factor S, which is in turn determined by the effects of the trapped electrons, the finite ion gyroradius, and the magnetic drift motion of charged particles. The KBM is unstable in a finite intermediate range of equatorial βeq and only marginally unstable at higher βeq regime for higher Te/Ti values. The finite ion gyroradius and the trapped electron fraction enhance the stiffening factor that tends to stabilize the KBM in the magnetotail far away from Earth. On the other hand, the current sheet thinning destabilizes KBM in the lower βeq regime and stabilizes KBM in the higher βeq regime.

Solitary dispersive Alfven wave in a plasma with hot electrons, cold positrons and ions

The dispersion relation and the nonlinear behaviour of dispersive Alfven waves (DAWs) in a plasma, which is composed by hot electrons, cold positrons and ions, are studied from the multi-component fluid theory. It can be indicated from the linear dispersion relation that the coupling of positron acoustic waves with shear Alfven waves results from the effect of the finite ion gyroradius. In our mode, the pressure and the inertia of the positron acoustic wave are, respectively, provided by the hot electrons and the cold positrons, which are different from the ones in a plasma consisting of two-group positrons with different temperatures. Furthermore, the existence and properties of DAW solitons are analysed from the Sagdeev pseudopotential equation, which indicates the existence of super-Alfvenic or sub-Alfvenic solitary DAWs, as well as the amplitude and width of DAW solitons depend on the positron concentration and the electron temperature. When the effects of cold positrons can’t be neglected, there may coexist the compressive and rarefactive DAW solitons. When the concentration of positron is negligibly small, the properties of DAW solitons in our model are similar to the ones of kinetic Alfven wave solitons in a plasma composed by hot electrons and cold ions.

Dispersive Alfvén waves in a plasma with anisotropic superthermal particles

The dispersion of dispersive Alfvén wave in a low β plasma with anisotropic superthermal particles modeled by a bi-nonextensive distribution is derived from a kinetic way. The effect of anisotropic temperature on inertial Alfvén wave is so small that it is negligible. However, it will play an important role on the property of kinetic Alfvén wave (KAW). The numerical results reveal that the presence of superthermal electrons in the small wavenumber limit will lead the damping rate of the KAW bigger than the one with Maxwellian distribution. Whereas, the damping rate of KAW in the large wavenumber limit will decrease with the presence of superthermal electrons. When the effect of electron anisotropic temperature overwhelms the effect of finite ion gyroradius in the small wavenumber regime, the damping rate of KAW grows with the presence of electron temperature anisotropy. On the other hand, when the effects of finite ion gyroradius play a dominant role in the large wavenumber regime, the damping rate of KAW increases with the effective perpendicular and parallel electron temperatures.

The Kelvin–Helmholtz instability at Mercury: An assessment

The Kelvin–Helmholtz instability is believed to be an important means for the transfer of energy, plasma, and momentum from the solar wind into planetary magnetospheres, with in situ measurements reported from Earth, Saturn, and Venus. During the first MESSENGER flyby of Mercury, three periodic rotations were observed in the magnetic field data possibly related to a Kelvin–Helmholtz wave on the dusk side magnetopause. We present an analysis of the event, along with comparisons to previous Kelvin–Helmholtz observations and an investigation of what influence finite ion gyro radius effects, believed to be of importance in the Hermean magnetosphere, may have on the instability. The wave signature does not correspond to that of typical Kelvin–Helmholtz events, and the magnetopause direction does not show any signs of major deviation from the unperturbed case. There is thus no indication of any high amplitude surface waves. On the other hand, the wave period corresponds to that expected for a Kelvin–Helmholtz wave, and as the dusk side is shown to be more stable than the dawn side, we judge the observed waves not to be fully developed Kelvin–Helmholtz waves, but they may be an initial perturbation that could cause Kelvin–Helmholtz waves further down the tail.

Alfvén resonance heating in Uragan-2M torsatron

Uragan-2M is a medium-size torsatron with reduced helical ripples. This machine has the major plasma radius R = 1.7 m, the average minor plasma radius rp ≤ 0.24 m and the toroidal magnetic field B0 ≤ 2.4 T. The Alfven resonance heating in a high k‖ regime is advantageous for small size machines since it can be realized at smaller plasma densities than the minority and second harmonic heating. The Alfven resonance heating is examined numerically in the approximation of radially non-uniform plasma cylinder with identical ends. The numerical model for wave excitation and propagation accounts for the longitudinal electron thermal motion and the finite ion gyroradius which allow the model to treat correctly the propagation and damping of the kinetic Alfven wave in hot plasma. A compact antenna consisting of four loop elements is chosen to provide operation in a high k‖ regime. The major drawback of such an approach is the presence of plasma peripher y heating owing to unavoidable excitation of low k‖ Alfven resonances. Calculations show that, with the proper choice of heating regime, the periphery heating has an acceptable level and the major part of the power is deposited inside plasma column.

Parametric analysis of the two-fluid tearing instability

A two-fluid analysis of the current driven tearing instability is presented. It concentrates on the systematic investigation of the physics related to the plasma compressibility and to the contribution of the Hall term to the inductive electric field, for arbitrary values of the ion skin depth and of the magnitude of the magnetic guide field. The plasma compressibility is treated consistently for a wide range of the plasma beta that excludes only the extremely cold limit where the mode growth rate would become sonic or supersonic. Conversely, the effects associated with the finite ion gyroradius and the equilibrium density and temperature gradients are neglected. Seven parametric regions are identified, characterized by the relative strengths of the Hall and beta parameters. Five of them are amenable to asymptotic analyses yielding analytic dispersion relations and one allows a semi-analytic treatment. The singular, multi-layer structure of the tearing mode and the conditions under which the different components of the magnetic field diffuse are shown in detail for each of those parametric regions.

On the dispersion relation for the kinetic Alfvén wave in an inhomogeneous plasma

The kinetic Alfvén wave has been recognized as an important wave mode in magnetospheric plasmas and laboratory plasmas, and has potential application in many areas of cosmic plasma physics. The kinetic dispersion relation of this mode has been described including finite frequency and finite ion gyroradius corrections. Laboratory plasmas as well as plasmas in space often contain strong gradients perpendicular to the background magnetic field. In this case, the dispersion relation must be generalized to include changes in the plasma parameters on each side of the gradient. In the presence of such gradients, localized modes can be found in the plasma. Depending on the relative values of the Alfvén speed and the plasma beta across these gradients, these modes can be trapped within the cavity or enhancement or propagate across the gradient.

Gyrotropic guiding-center fluid theory for the turbulent heating of magnetospheric ions in downward Birkeland current regions. II

A new fluid theory in the guiding-center and gyrotropic approximation derivable from the ensemble-averaged Vlasov-Maxwell equations that included the effect of wave-particle interactions for weakly turbulent, weakly inhomogeneous, nonuniformly magnetized plasma was recently given by Jasperse, Basu, Lund, and Bouhram [Phys. Plasmas 13, 072903 (2006)]. In that theory, the particles are transported in one spatial dimension (the distance s along the magnetic field) but the turbulence is two-dimensional. In this paper, which is intended as a sequel, the above theory is used for quasisteady conditions to find: (1) a new formula for the perpendicular ion-heating rate per unit volume Ẇi⊥(s), where Ẇi⊥(s) decreases for large s by what we call the “finite ion gyroradius effect”; and (2) a new formula for the perpendicular ion temperature at low altitudes, Ti⊥(s). Parametrized calculations for Ti⊥(s) are also given.

Io‐Jupiter interaction: Alfvén wave propagation and ionospheric Alfvén resonator

A linear, one‐dimensional gyrofluid code has been used to determine the characteristics of propagating Alfvén waves and the ionospheric Alfvén resonator on a Jupiter‐Io flux tube. This model includes electron inertia, electron pressure gradient, and finite ion gyroradius effects, as well as the displacement current correction to prevent the Alfvén velocity from exceeding the speed of light. A quasi‐steady Vlasov code provides realistic density profiles along the flux tube as input parameters for the gyrofluid model. In this paper, we demonstrate that the majority of the wave energy from an initial pulse with a long wavelength (∼0.1 RJ) is unable to reach Jupiter's ionosphere without wave breaking, phase mixing, and/or other nonlinear processes; however, a significant energy flux may be transferred via high‐frequency, small‐wavelength waves to the ionosphere. The waves that reach the ionosphere stimulate an ionospheric Alfvén resonator which is generated between the ionospheric boundary and the first velocity peak of the Alfvén phase speed. The ionospheric density and scale height play important roles to determine the resonant frequency. The eigenfrequency decreases with increasing scale height and with increasing ionospheric density. The fundamental frequency and higher harmonics of the Alfvén resonator are comparable to the observed reoccurring frequency of S bursts between a few and hundreds of Hz. On the basis of this information, we suggest the Alfvén resonator as the likely driver explaining multiple occurrences of S bursts.

Nonlinear effects associated with dispersive Alfvén waves in plasmas

Large amplitude Alfvén waves are frequently found in magnetized space and laboratory plasmas. Our objective here is to discuss the linear and nonlinear properties of dispersive Alfvén waves (DAWs) in a uniform magnetoplasma. We first consider finite frequency (ω/ωci) and ion gyroradius effects on inertial and kinetic Alfvén waves, where ωci is the ion gyrofrequency. Next, we focus on nonlinear effects caused by DAWs. Such effects include plasma density enhancement and depression by the Alfvén wave ponderomotive force, electron Joule heating by the thermal Alfvén wave force, the generation of zonal flows due to the shear Alfvén wave mode couplings as well as the formation of localized Alfvénic structures and Alfvénic vortices. The relevance of our investigation to the appearance of nonlinear Alfvén waves in the Earth's auroral acceleration region, in the solar corona and in the large plasma device at UCLA is discussed.

Modeling of field‐aligned electron bursts by dispersive Alfvén waves in the dayside auroral region

A linear, one‐dimensional gyrofluid code including electron inertia, electron pressure gradient, and finite ion gyroradius effects is applied to an inhomogeneous dayside auroral field line to determine the characteristics of propagating Alfvén waves with small perpendicular wavelengths. Test particles are then used to study the behavior of both magnetosheath (100 eV) and background ionospheric electrons (2 eV) under the influence of dispersive Alfvén waves. Although the test particle approach is not self‐consistent, the gyrofluid/test particle simulation is able to reproduce many of the features observed by low‐altitude satellites. The test particle simulations verify results from previous studies, such as reproducing electron energy and pitch‐angle dispersions, and in doing so, validate the approach. The test particle simulations also show how resonant particles can lead to low‐energy field‐aligned electron bursts that are commonly observed in the dayside auroral region. The new results in this study reveal the plasma conditions necessary for electron resonance. We show that an increased mass density (significant O+ density) in the acceleration region is an essential prerequisite to generate an electron burst. The primary effect of the O+ is to decrease the phase speed of the Alfvén wave. Furthermore, the full gyrokinetic effects of the O+ act to produce a region in which the Alfvén speed profile is gradually slowing, which allows electrons remaining within the wave to lower altitudes. In these electron bursts, the energy gain experienced by the majority of electrons ranges from tens to hundreds of eV. The trapping occurs if the parallel electric field is substantial enough (∼0.2 mV/m) in the acceleration region to accelerate the background electrons. An integrated energy flux of accelerated electrons is estimated to be 3 erg s−1 cm−2, about 20% of the Alfvén wave Poynting flux.

Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling

Abstract. We show two examples from the CANOPUS array of the optical signatures of auroral arcs produced by field line resonances on the night of 31 January 1997. The first example occurs during local evening at about 18:00 MLT (Magnetic Local Time), where CANOPUS meridian scanning photometer data show all the classic features of field line resonances. There are two, near-monochromatic resonances (at approximately 2.0 and 2.5 mHz) and both show latitudinal peaks in amplitude with an approximately 180 degree latitudinal phase shift across the maximum. The second field line resonance event occurs closer to local midnight, between approximately 22:00 and 22:40 MLT. Magnetometer and optical data show that the field line resonance has a very low frequency, near 1.3 mHz. All-sky imager data from CANOPUS show that in this event the field line resonances produce auroral arcs with westward propagation, with arc widths of about 10 km. Electron energies are on the order of 1 keV. This event was also seen in data from the FAST satellite (Lotko et al., 1998), and we compare our observations with those of Lotko et al. (1998). A remarkable feature of this field line resonance is that the latitudinal phase shift was substantially greater than 180 degrees. In our discussion, we present a model of field line resonances which accounts for the dominant physical effects and which is in good agreement with the observations. We emphasize three points. First, the low frequency of the field line resonance in the second event is likely due to the stretched topology of the magnetotail field lines, with the field line resonance on field lines threading the earthward edge of the plasma sheet. Second, the latitudinal phase structure may indicate dispersive effects due to electron trapping or finite ion gyroradius. Third, we show that a nonlocal conductivity model can easily explain the parallel electric fields and the precipitating electron energies seen in the field line resonance.Key words. Magnetospheric physics (electric fields; energetic particles precipitating; current systems)

Ion escape from Mars in a quasi‐neutral hybrid model

Ion measurements made by the Phobos 2 spacecraft revealed an extensive, ever‐persisting escape of planetary ions in the Martian tail. Observations indicated that in large regions of the Martian tail, most ions were of planetary origin. The measurements suggested that the solar wind "pickup" of planetary ions is an important factor in the global Mars‐solar wind interaction. However, how in detail the newly formed planetary ions affect the formation of the observed global plasma and field regions remained unanswered. In this paper the escape of Martian ions is studied with a quasi‐neutral hybrid simulation model. The used hybrid model includes two ion species: the solar wind protons and the planetary ions. This approach makes it possible to study the asymmetries associated with the finite ion gyroradius as well as the difference between the motion of the solar wind protons and planetary ions. The source of the escaping ions is studied indirectly by producing planetary ions from two ion sources: the neutral corona (photoionization) and the ionosphere. The simulation is found to reproduce many observed features of the Mars‐solar wind interaction in general and those of the planetary ions in particular. Ion escape within the optical shadow forms an antisunward flowing beam of ∼1 keV planetary ions, much as observed. A total mass loss rate of ∼100–400 g s−1 is found to be large enough to reproduce several plasma and magnetic field features observed by Phobos2 on its circular orbits. Simulations produce a wide region behind the planet where the density and the particle flux of H+ ions are much smaller than their solar wind values, resembling the observed "proton cavity." The work also depicts how the "ionotail," formed mainly by the escaping planetary ions, is embedded in the Martian magnetotail. The runs also result in a notable asymmetry with respect to the direction of the convectice electric field in the solar wind which is caused by the finite ion gyroradius. The magnetopause also demonstrates the asymmetry: the magnetopause is a sharp boundary or a smooth transition layer, depending on the direction of the upstream convective electric field. Overall, the work illustrates the importance of the plasma‐neutral interactions in the global Mars‐solar wind interaction.

Dispersive shear Alfvén waves on model Tsyganenko magnetic field lines

We present a theory of low- m dispersive shear Alfvén waves (SAWs) that are excited on magnetic field lines specified by the Tsyganenko 1996 magnetic field model. Our dispersive nonlinear model is applied to field lines near the midnight meridional plane and is parameterized by averaged quantities along a field line. For inner plasma sheet ambient parameters, dispersion is dominated by finite electron temperature and ion gyroradius effects that lead to a poleward propagating arc that is in general agreement with simultaneous CANOPUS and FAST satellite observations. The computed fundamental mode frequency is in the 1–4 mHz range, and is in agreement with the measured frequency of nightside magnetospheric field line resonances (FLRs). We conclude that dispersive Alfvén waves excited in a stretched magnetic topology can explain many features of FLRs observed within diffuse auroral precipitation (in H β) near the inner plasma sheet.

Dipole Alfven Vortex with Finite Ion Larmor Radius in a Low-Beta Plasma

A set of nonlinear fluid equations which include the effects of ion gyroradius is derived to describe Alfven vortex. The correction of finite ion gyroradius to the Alfven vortex in the inertial region is much more significant than that in the kinetic region. The amplitude of the vortex is enhanced in both regions. The scale of the vortex in the kinetic region becomes larger whereas it becomes smaller in the inertial region.

Discrete Auroral Arcs and Nonlinear Dispersive Field Line Resonances

Dispersive effects in field line resonances (FLRs) are discussed in the context of potential structures, parallel currents, and auroral density cavities observed by the FAST satellite. Our model includes the Earth's dipole magnetic field, and accounts for electron inertia, electron thermal pressure, finite ion gyroradius effects, and field aligned variations of the plasma density and ambient electron and ion temperatures. For realistic backgound parameters, we show that finite plasma temperature effects determine the dynamics of FLRs and that solitary wave structures evolve out of the resonance region, producing deep density cavities above the polar ionospheres. Results are shown to be in reasonable agreement with ground and satellite observations, with the exception of the magnitude of low altitude electric fields.

Charge separation at a plasma–wall transition due to the finite ion gyro-radius

An Eulerian Vlasov gyrokinetic code is developed for the investigation of a plasma in E → × B → flow. The charge separation at a plasma–wall transition due to finite ion gyro-radius is studied. The model is two-dimensional in space and consists of a two-component collisionless plasma slab.

Kinetic effects on ideal ballooning stability of the TJ-II heliac device

Kinetic corrections to a previous ideal MHD ballooning stability study of the TJ-II heliac standard configuration are numerically calculated using a criterion derived from the gyrokinetic Maxwell equations for modes with perpendicular wavelengths on the scale of the ion gyroradius in a general magnetic geometry. In this criterion, finite ion gyroradius and trapped particle effects are retained at first order. The existence of temperature gradients and different ion and electron temperatures are accounted for. The relevant kinetic regime for TJ-II is found to be the intermediate frequency regime, with ωbi, ωti < γ < ωbe, ωte across all the relevant region whenever electron cyclotron resonance heating (ECRH) plasmas are considered. Therefore, only trapped electrons are included in the calculations. The threshold poloidal wavenumber mu for diamagnetic stabilization of unstable ballooning modes is obtained, and a reduction of about 30% of the instability region radial width has been found. The trapped electron contribution to the growth rate is shown to be destabilizing for all modes with poloidal wavenumber below this threshold and is evaluated perturbatively, showing that an important part of it is due to those electrons trapped in the ripple of the magnetic field.

Solitary kinetic Alfvén waves with finite ion gyroradius

A solitary kinetic Alfvén wave (SKAW) with a finite ion gyroradius is obtained by using kinetic theory. It is found that the existence of SKAW depends not only on the parallel wave number but also on the perpendicular wave number. In contrast to the results with cold ions, both the super-Alfvénic and the sub-Alfvénic solitons exist in the density compressive region. The amplitude and the parallel propagation velocity of the solitons are found to be enhanced, while the width is narrowed due to the finite ion gyrodius effect.

Kinetic Alfvén waves driven by velocity shear

Considering the effect of the finite ion gyroradius, the kinetic Alfvén wave in an anisotropic medium driven by a parallel velocity flow was investigated and the full dispersion relation was given. In deducing the dispersion relation, the coupling between electrostatic and electromagnetic oscillations, effects of the resonant electrons and the longitudinal motion of ions are considered. Based on the analytical and numerical calculations, two instability boundaries are found. They are 1&amp;lt;(k⊥dV0/dx)/kzωci&amp;lt;νA2/(Ti⊥/mi) and (k⊥dV0/dx)/kzωci&amp;gt;νA2/(Ti/mi). Resonant electrons serve as the main energy source in these two regimes, while the ion longitudinal motion only suppresses the mode in the second regime. In the boundary layer of the magnetopause, the temperature anisotropy also modifies the instability by lowering the frequency and reducing the growth rate. The above results show that the low-frequency Alfvén waves with the scale of perpendicular wavelengths greater than the ion gyroradius in the magnetopause can be efficiently driven by a velocity-sheared flow.