OBLIQUE EMISSION OF WHISTLER-MODE WAVES AROUND INTERPLANETARY SHOCKS

We present a study of whistler-mode wave generation and wave particle interaction in the vicinity of interplanetary shocks as observed by the Ulysses spacecraft. Generally the whistler-mode waves (measured in the frequency range 0.22–448 Hz) are observed downstream of the shocks where they persist for some hours. From the electron distribution functions (EDF) in the energy range 1.6 to 862 eV measured by the spacecraft, we compute the wave growth rate of the electromagnetic electron cyclotron and Landau instabilities for the case of oblique propagation of the waves with respect to the interplanetary magnetic field (IMF) B. In general, in agreement with the wave measurements, the instability grows mostly downstream of the shock fronts. Following the wave activity, velocity space diffusion of the electrons results in a marginally stable state with some sporadic fluctuations.     

Roles of Fast-Cyclotron and Alfvén-Cyclotron Waves for the Multi-Ion Solar Wind

Using linear Vlasov theory of plasma waves and quasi-linear theory of resonant wave–particle interaction, the dispersion relations and the electromagnetic field fluctuations of fast and Alfvén waves are studied for a low-beta multi-ion plasma in the inner corona. Their probable roles in heating and accelerating the solar wind via Landau and cyclotron resonances are quantified. In this paper, we assume that i) low-frequency Alfvén and fast waves, emanating from the solar surface, have the same spectral shape and the same amplitude of power spectral density (PSD); ii) these waves eventually reach ion cyclotron frequencies due to a turbulence cascade; iii) kinetic wave–particle interaction powers the solar wind. The existence of alpha particles in a dominant proton/electron plasma can trigger linear mode conversion between oblique fast-whistler and hybrid alpha–proton cyclotron waves. The fast-cyclotron waves undergo both alpha and proton cyclotron resonances. The alpha cyclotron resonance in fast-cyclotron waves is much stronger than that in Alfvén-cyclotron waves. For alpha cyclotron resonance, an oblique fast-cyclotron wave has a larger left-handed electric field fluctuation, a smaller wave number, a larger local wave amplitude, and a greater energization capability than a corresponding Alfvén-cyclotron wave at the same wave propagation angle θ, particularly at 80^°<θ<90^°. When Alfvén-cyclotron or fast-cyclotron waves are present, alpha particles are the chief energy recipient. The transition of preferential energization from alpha particles to protons may be self-modulated by a differential speed and a temperature anisotropy of alpha particles via the self-consistently evolving wave–particle interaction. Therefore, fast-cyclotron waves, as a result of linear mode coupling, constitute a potentially important mechanism for preferential energization of minor ions in the main acceleration region of the solar wind.     

Comparative study of ion cyclotron waves at Mars, Venus and Earth

Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind-Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus.     

Venus Express observations of ULF and ELF waves in the Venus ionosphere: Wave properties and sources

Electrical activity in a planetary atmosphere enables chemical reactions that are not possible under conditions of local thermodynamic equilibrium. In both the Venus and terrestrial atmospheres, lightning forms nitric oxide. Despite the existence of an inventory of NO at Venus like the Earth’s, and despite observations of the signals expected from lightning at optical, VLF, and ELF frequencies, the existence of Venus lightning still is met with some skepticism. The Venus Express mission was equipped with a fluxgate magnetometer gradiometer system sampling at rates as high as 128 Hz, and making measurements as low as 200 km altitude above the north polar regions of Venus. However, significant noise levels are present on the Venus Express spacecraft. Cleaning techniques have been developed to remove spacecraft interference at DC, ULF, and ELF frequencies, revealing two types of electromagnetic waves, a transverse right-handed guided mode, and a linearly polarized compressional mode. The propagation of both types of signals is sensitive to the magnetic field in ways consistent with propagation from a distant source to the spacecraft. The linearly polarized compressional waves generally are at lower frequencies than the right-handed transverse waves. They appear to be crossing the usually horizontal magnetic field. At higher frequencies above the lower hybrid frequency, waves cannot enter the ionosphere from below when the field is horizontal. The arrival of signals at the spacecraft is controlled by the orientation of the magnetic field. When the field dips into the atmosphere, the higher frequency guided mode above the lower hybrid frequency can enter the ionosphere by propagating along the magnetic field in the whistler mode. These properties are illustrated with examples from five orbits during Venus Express’ first year in orbit. These properties observed are consistent with the linearly polarized compressional waves being produced at the solar wind interface and the transverse guided waves being produced in the atmosphere.     

Kinetic equilibrium of nonlinear electromagnetic waves in electron-positron-ion magnetoplasmas

The problem of a one-dimensional, nonlinear, circularly polarized wave in a collisionless plasma is solved for the case in which the group velocity of the localized wave is zero relative to the plasma frame.For an electron-positron plasma the ponderomotive potential is repulsive, and both small and large amplitude waves in the presence of reflected and transmitted particles is discussed.The inclusion of an external magnetic field such that the cyclotron frequency is smaller than the wave frequency does not alter qualitatively the results of the fieldless case, except for the fact that an electrostatic potential is generated.For the case in which the cyclotron frequency is larger than the wave frequency, the resulting potential is attractive, and it is shown that no localized waves can coexist with confined and transmitted particles. If, however, an ionic component is presnt in the electron-positron plasma, such as may be the case in pulsar magnetospheres, localized waves may again be possible.     

Nonlinear Coupling of Langmuir Waves with Whistler Waves in the Solar Wind

Close temporal correlation between high-frequency Langmuir waves and low-frequency electromagnetic whistler waves has been observed recently within magnetic holes of the solar wind. In order to account for these observations, a theory is formulated to describe the nonlinear coupling of Langmuir waves and whistler waves. It is shown that a Langmuir wave can interact nonlinearly with a whistler wave to produce either right-hand or left-hand circularly polarized electromagnetic waves. Nonlinear coupling of Langmuir waves and whistler waves may lead to the formation of modulated Langmuir wave packets as well as the generation of circularly polarized radio waves at the plasma frequency in the solar wind. Numerical examples of whistler frequency, nonlinear growth rate and modulation frequency for solar wind parameters are calculated.     

Proton cyclotron instability in the drifting magnetoplasma

A general expression for the tensor of the dielectrical susceptibility in an anisotropic plasma with particle drifts is derived, and the dispersion equation is found for waves propagating in arbitrary direction with respect to the mean magnetic field. The dispersion equation is solved for the case of electromagnetic ion‐cyclotron waves. It is found that in the plasma of the auroral magnetosphere strong plasma instability may occur so that the value of the growth rate of the waves is of the order of the wave frequency. Besides, the plasma instability is excited at less values of the wave number if the magnetospheric altitude becomes larger.     

Polarization properties of low frequency electromagnetic cyclotron waves associated with magnetic clouds

Recent studies have revealed that there are a large number of low frequency electromagnetic cyclotron waves (ECWs) occurring in and around magnetic clouds (MCs) that are common magnetic structures in interplanetary space. Using magnetic field data from the STEREO spacecraft, this paper investigates polarization properties of ECWs associated with 120 MCs. Results show that the ECWs are highly transverse, strongly polarized waves with large ellipticities. Specifically, almost all of the waves take place with the ratios of transverse power to total power higher than 0.94, polarization degrees greater than 0.85, and ellipticities larger than 0.5. The mean values of these quantities can be up to 0.99, 0.96, 0.85, respectively. In particular, there is a tendency of ellipticities decreasing with respect to the wave normal angles for ECWs with left handed polarization. The decreasing tendency is consistent with the recent theory and simulation results.     

Cluster observations of Pc 1–2 waves and associated ion distributions during the October and November 2003 magnetic storms

Unusual wave activity in the Pc 1–2 frequency band (0.1–5 Hz) was observed by the Cluster spacecraft in association with the two large geomagnetic storms of late 2003. During the onset of the Hallowe’en storm on October 29, 2003, intense broadband activity between ∼0.1 and 0.6 Hz appeared at all 4 spacecraft on both sides of the magnetic equator at perigee (near 1400 UT and 08:45 MLT). Power was especially strong and more structured in frequency in the compressional component: a minimum in wave power was observed at 0.38 Hz, corresponding to the oxygen ion cyclotron frequency. Poynting vector calculations indicated that wave power was primarily directed radially inward rather than along the magnetic field. Narrowband purely compressional waves near 0.15 Hz appeared at higher dayside latitudes in the southern hemisphere. CIS ion spectrometer data during this pass revealed that O⁺ was the dominant energetic ion. During the recovery phase of the November storm, on November 22, 2003, predominantly transverse 1.8 Hz waves with peak-to-peak amplitude of 10 nT were observed by all four spacecraft near perigee at L=4.4. During this more typical Pc 1 event, wave power was directed along B, toward the northern ionosphere. An unusually polarized 2.3 Hz emission (with power in the radial and compressional, but not azimuthal directions) was observed at L=5.4–5.9, 10–15° south of the magnetic equator. We infer that this wave event may have been generated on lower L shells and propagated obliquely to Cluster's location. Consistent with other recent observations, anisotropic plasma sheet/ring current proton distributions appeared to be a necessary condition for occurrence of waves during both passes, but was not always a sufficient condition. The transverse waves of November 22 occurred in regions which also contained greatly increased fluxes of cool ions (E<1 keV). On both days, Cluster observed features not previously reported, and we note that the purely compressional nature of the October 29 events was not anticipated in previous theoretical studies. The fact that these unusually polarized waves occurred in association with very intense geomagnetic storms suggests that they are likely to be extremely rare.     

Electromagnetic Ion Cyclotron Waves in Multi-Ions Hot Anisotropic Plasma in Auroral Acceleration Region-Particle Aspect Approach

Using particle aspect approach, the effect of multi-ions densities on the dispersion relation, growth rate, perpendicular resonant energy and growth length of electromagnetic ion cyclotron wave with general loss-cone distribution function in hot anisotropic multi-ion plasma is presented for auroral acceleration region. It is observed that higher He⁺ and O⁺ ions densities enhance the wave frequency closer to the H⁺ ion cyclotron frequency and growth rate of the wave. The differential heating of He⁺ ions perpendicular to the magnetic field is enhanced at higher densities of He⁺ ions. The waves require longer distances to achieve observable amplitude by wave-particle interactions mechanism as predicted by growth length. It is also found that electron thermal anisotropy of the background plasma enhances the growth rate and reduces the growth length of multi-ions plasma. These results are determined for auroral acceleration region.     

Use of multi-point analysis and modelling to address cross-scale coupling in space plasmas: Lessons from Cluster

The properties of plasmas (in space) are fundamentally governed by both ‘cross-scale’ coupling and comparative temporal behaviour operating over the micro-, meso-, and (MHD-) fluid regimes: for example, under conditions of turbulence, during magnetic reconnection and in shocks and other plasma boundaries. These themes map to a number of related and overlapping, phenomena, where known phenomena play different roles in each theme. Detailed understanding of fundamental plasma processes therefore requires analysis of both theoretical models (to distinguish the collisionless from the collisional regimes) and multi-scale measurements (suitable to address issues of stationarity). In particular, the investigation of phenomena requires analysis techniques which can distinguish and quantify temporal behaviour and the multi-scale spatial behaviour. The analysis of existing, multi-point data sets has led to a number of data co-ordination methods, such as the four spacecraft analysis tools developed for cluster, and we consider examples here. Advanced analysis concepts may be investigated with suitable considerations of measurement quality:adequate sampling of phenomena (for example, to extract the necessary information on the mechanisms operating) requires suitable spacecraft configurations and directly relates to the measurement quality achievable. A particular issue is how to resolve temporal behaviour across the spatial regimes, so that the data set is suitably coordinated. With the addition of theoretical modelling (in the context of particular phenomena) both the space and laboratory plasma regimes may be compared and we give an example of nonlinear wave coupling across spatial scales in this context.     

火星上游离子回旋波的观测研究

离子回旋波(Ion Cyclotron Wave, ICW)是指频率接近离子回旋频率的一种等离子体波,它在火星上游广泛存在,卫星观测到的频率多在质子回旋频率附近. ICW是拾起离子时的副产物,成为新生行星质子存在的间接标志.火星上游ICW自1990年首次报道以来,受到广泛的关注.总结了火星上游ICW的研究进展,包括ICW事件的观测、ICW的产生机制、统计性质以及将来的研究趋势.     

Mode identification of terrestrial ULF waves observed by Cluster: A case study

Using multipoint measurements from the Cluster mission wave identification techniques are applied to observations of ULF waves made in the terrestrial foreshock with the aim of identifying the modes and properties of the waves taking into account the effects of a high beta plasma. The wave properties in the spacecraft and plasma rest frames are experimentally derived using minimum variance analysis. Two waves with periods of 30 and 3 s dominate the dynamic frequency spectrum. The results indicate that these waves propagate in the fast magnetosonic and Alfvén/Ion Cyclotron modes, respectively. Both waves propagate in the upstream direction in the plasma rest frame but are convected downstream in the spacecraft frame. The measured wave properties in the plasma rest frame are in good agreement with those obtained from the theoretical kinetic dispersion relation taking into account the effects of different plasma beta. The dispersion results show a rather significant deviation from fluid model, especially when high beta plasma conditions occur. These experimentally derived foreshock ULF wave properties are in good agreement with previous results but when the effects of a high beta plasma are considered it is not as straight forward to choose the correct wave mode branch.     

Stability of Electrostatic Electron Cyclotron Waves in a Multi-Ion Plasma

We have studied the stability of the electrostatic electron cyclotron wave in a plasma composed of hydrogen, oxygen and electrons. To conform to satellite observations in the low latitude boundary layer we model both the ionic components as drifting perpendicular to the magnetic field. Expressions for the frequency and the growth rate of the wave have been derived. We find that the plasma can support electron cyclotron waves with a frequency slightly greater than the electron cyclotron frequency ω _ce ; these waves can be driven unstable when the drift velocities of both the ions are greater than the phase velocity of the wave. We thus introduce another source of instability for these waves namely multiple ion beams drifting perpendicular to the magnetic field.     

Linear wave conversion processes in the theory of the proton whistler

The linear coupling between the different kinds of waves propagating in a warm plasma inhomogeneous along thex direction is investigated in order to locate the regions (,k) space where two of the roots of the characteristic equation coalesce. Firstly, using the approximation of geometrical optics the differential equation is derived and wave propagation at fixed wave numberk _z is studied in these special cases for which the characteristic equation reduces to a biquadratic. When the density gradient is parallel to the magnetic field, a detailed analysis of the different properties of the waves shows that the mechanism proposed by Gurnett and others to explain the characteristics of the proton whistler is unlikely to operate, even if a wave coupling occurs at the so called cross over frequency for small incidence angles. The only relevant process occurs when the density gradient is perpendicular to the magnetic field for waves propagating at small incidence angles. It is shown that, close to a coalescence point, but within the limit of the geometrical optics approximation, one of the WKB solutions is a mixed (transverse-longitudinal) mode which becomes purely longitudinal in the limit of large wave numbers. Consequently, as this wave has E almost parallel tok, coalescence implies that the waves are nearly longitudinal at the singular point, in agreement with other results. Next, application of the theory is made to some relevant space observations. It is shown that the proton whistler could be the result of a linear coupling between the extraordinary and the slow ion cyclotron waves close to the Buchsbaum resonance in ionospheric regions above 300 to 400 km where the H⁺ density begins to grow. Transformation conditions are given which favour the coupling mechanism in regions of strong latitudinal gradients. Finally, a comparison is made with experiment which confirms the principal features of the present theory.     

A further search on waves generated by solar energetic protons

It has been suggested that in the interplanetary medium Alfvén waves may be significantly amplified or damped during large solar proton events. This implies the increase or decrease of the ambient magnetic fluctuations in concurrence with the presence of the streaming particles, that we have analysed in a first study at times of eight proton events observed by Helios spacecraft (Valdés-Galicia and Alexander, 1997). However, it is not possible with interplanetary magnetic field measurements only to distinguish between waves moving away or towards the Sun in the frame of reference of the spacecraft. Plasma data for these eight events have now been made available to us and hence the energetic content of inward and outward propagating waves may be found, which is an important aid in our search for signatures left by the energetic protons. In the present work we incorporate the new information into the analyses of those events that in our first study showed more favourable evidence and therefore try to give a more definite answer as to whether it might be observed. The new results do not reinforce the evidence of our previous work, as they seem to be mildly consistent with the presence of the proton self-generated waves in just one of the three cases studied.     

Nonlinear model to study filamentation instability of circularly polarized dispersive Alfvén waves in solar wind plasmas

This paper examines the small-scale solar wind turbulence driven in view of the Alfvén waves subjected to ponderomotive nonlinearity. Filamentation instability is known to take place for the case of dispersive Alfvén wave (DAW) propagating parallel to the ambient magnetic field. The ponderomotive force associated with DAW is responsible for wave localization and these webs of filaments become more intense and irregular as one proceeds along the spatial domain. The ponderomotive force associated with pump changes with pump parameters giving rise to different evolution patterns. This paper studies in detail the nonlinear evolution of filamentation instability introduced by dispersive Alfven waves (DAWs) which becomes dispersive on account of the finite frequency of DAW i.e., pump frequency is comparable to the ion cyclotron frequency. We have explicitly obtained the perturbation dynamics and then examined the impact of pump magnitude on the driven magnetic turbulence using numerical simulation. The results show steepening at small scales with increasing pump amplitude. The compressibility associated with acoustic fluctuations may explain the variation in spectral scaling of solar wind turbulence as observed by Alexandrova et al. (Astrophys. J. 674:1157, 2008).     

Generation of hiss beyond plasmapause by magnetosonic waves

Magnetosonic waves near the harmonics of proton cyclotron frequency can become unstable in the presence of oxygen ions in the ring current. For cos θ = 0 (θ being the angle between the wave vector and the geomagnetic field) the growth rates are peaked at some optimum value of the oxygen ion density, whereas for cos θ ≠ 0 they are reduced with the increase of oxygen ion density. The presence of hot oxygen ions can generate instability near the harmonics of oxygen cyclotron frequency. The growth rates are enhanced with the increase of cos θ. This mechanism can generate discrete spectrum of ELF hiss beyond the plasmapause.     

Excitation of plasma waves in the ionosphere caused by atmospheric acoustic waves

The transformation of atmospheric acoustic waves into plasma waves in the ionosphere is investigated. The transformation mechanism is based on plasma wave exitation by growing acoustic waves, when a frequency/wavelength matching situation is reached. The interaction of acoustic and plasma waves occurs through collisions of neutral particles with ions. For the case of ion-sound waves, oscillations on ion cyclotron frequency and Alfvén waves is considered. A peculiarity of Alfvén waves is the wide frequency band which may be stimulated through wave-wave interaction.     

Proton cyclotron frequency phenomena in the topside ionosphere

Proton cyclotron echoes and spurs are phenomena related to the proton cyclotron frequency discovered on topside sounder ionograms from Canadian Alouette satellites. The echoes and spurs appears on the ionograms at apparent ranges which lead to a frequency close to the proton cyclotron frequency; the frequency is obtained by taking the reciprocal of the time elapsed between the transmission of the sounder pulse and the reception of the signal at the satellite. Aloutte II and ISIS and II ionograms for about sixty satellite passes were scaled to study the charateristics of these phenomena. Generally, proton cyclotron echoes and spurs occured on the ionograms at frequencies below the electron plasma frequency f_N, the echoes predominantly slightly above the electron cyclotron frequency f_H and the spurs just below f_N. They appeared most often when a harmonic of the electron cyclotron frequency nf_H(n = 1, 2, 3, 4) was approximately equal to one of the other characteristic frequencies, that is: (1) nf_H ≈ f_N, (2) nf_H ≈ f_zS, the frequency of the Z wave at the heght of the satellite, and (3) nf_H ≈ f_T, the upper hybrid resonance frequency.Proton cyclotron echoes, spurs and protein cyclotron wave patterns have many features in common in addition to their fundamental relationship with the proton cyclotron frequency. The echoes and spurs are observed most often when when nf_H overlaps one of the other characteristic frequencies, that is: nf_H ≈ f_N, nf_H ≈ f_zS, and nf_H ≈ f_T. The proton cyclotron wave pattern is observed under the first of the three conditions. It appears that the occurence of the phenomena is related to the plama conditions, the geographic location not being important in itself except that reflects different plasma conditions. Although proton cyclotron echoes and spurs were observed more often near the geomagnetic equator, consistent with the results of Matuura and Nishizaki,(8) they still observed at high latitudes even near the north geomagnetic pole.The echoes and spurs occur at frequencies below f_N, the echoes predominantly slightly above f_H and the spurs just below f_N. Generally it is easy to distinguish between the two since usually they appear separately or, if together, often an echo would terminate and a spur begin at a slightly different apparent range. But it is not always easy since sometimes it appeared that a proton cyclotron echo and a spur formed a continuous trace, suggesting that perhaps they may be different manifestations of the same phenomenon. Work is continuing in an attemp to understand the origin of proton cyclotron echoes, spurs, and proton cyclotron wave patterns.