Waves in space plasmas: Highlights of a Conference held in Hawaii, 7–11 February 1983

The Conference was called to bring together investigators of magnetospheric plasma waves having frequencies from VLF whistlers and emissions down through ELF and ULF to Pc5 long period pulsations. The emphasis was on the physics and techniques underlying the entire frequency range. Topics included wave electron interactions and electron precipitation, ray tracing and other methods to track down sources of VLF and ULF waves, VLF-ULF relationships, heavy ion effects in ULF propagation, and long period ULF waves.     

Coupling at the Atmosphere-Ionosphere-Magnetosphere Interface and Resonant Phenomena in the ULF Range

We discuss the electromagnetic processes in the ULF range which are important for the coupling between the atmosphere, ionosphere, and magnetosphere (AIM). The main attention is given to the Pc1–2 frequency ranges (f≈0.1–10 Hz) where some natural resonances in the AIM system are located. In particular, we consider the resonant structures in the spectra of the magnetic background noise related to the Alfvén resonances in the ionosphere as a possible diagnostic tool for studies of the ionospheric parameters. We also discuss the self-excitation of Alfvén waves in the ionosphere due to the AIM coupling and the role of such waves in the acceleration of electrons in the upper ionosphere and magnetosphere. Precipitation of magnetospheric ions due to their interaction with the ion-cyclotron waves is analyzed in relation to the ionospheric current systems, formation of partial ring current, and the influence of the ionosphere-magnetosphere feedback on the generation of such waves.     

Diagnostics of the magnetosphere and interplanetary medium by means of pulsations

The possibility of diagnostics of the interplanetary magnetic field using the data of ground observations of Pc3, 4 pulsations is investigated. Both empirical and theoretical foundations of this method are discussed. In the shorter-period range (Pc1, Pi1, Ipdp) the methods of qualitative diagnostics of non-stationary processes in the quiet and disturbed magnetosphere are considered. In conclusion, we shall briefly consider the problems, arising when estimating the amplitude of the hydromagnetic waves in the regions of generation.     

Observations of ion cyclotron waves near synchronous orbit and on the ground

Ion cyclotron waves (hereafter ICW's) generated in the magnetosphere by the ion cyclotron instability of 10–100 keV protons are now known to be the origin of short-period (0.1–5 Hz) electromagnetic field oscillations observed by synchronous spacecraft and on the earth's surface. Observations of the various wave characteristics, including spectral and polarization properties that lead to the identification of generation and propagation mechanisms and regions in the magnetosphere are described with reference to ATS-6, GEOS and ground-based wave data and interpreted using cold plasma propagation theory. The presence of heavy ions (O⁺, He⁺) dramatically modifies ICW magnetospheric propagation characteristics giving rise to spectral slots and polarization reversals. These properties may be used in plasma diagnostics. Finally satellite-ground correlations and techniques for determining the magnetospheric source position of ICW's not seen at synchronous orbit but observed on the ground as structured Pc1 pulsations are considered.     

Stimulation of VLF amplification in the magnetosphere

Among the various plasma instabilities that exert influence on the dynamic equilibrium state of the magnetosphere, the cyclotron-resonance interaction appears to be the most accessible to artificial stimulation. The strength of the interaction is sensitive to both the background magnetoplasma parameters and the hot energetic particle distribution. Thus, proper modification of one or more conditions can induce significant wave amplification at the expense of hot plasma energy density. Several methods of hot and cold plasma injection have been investigated with the linear theory to assess their effectiveness as a means of stimulating amplification.Only the interaction of VLF waves (3–30 kHz) with hot electrons (0.1–100 keV) is treated here. The injection of a dense jet of barium that travels upward along the geomagnetic field causes appreciable amplification when the jet is within 30° of the geomagnetic equator. Injection of a geosynchronous lithium cloud stimulates amplification of both VLF and ULF waves, but the magnitude depends critically on the state of geomagnetic activity. Conventional hot electron beams may also amplify narrow frequency bands, but the net wave energy is severely limited by the beam energy.Although the cyclotron-resonance is recognized as a dominant interaction in magnetospheric dynamics, its properties have never been confirmed quantitatively by appropriate spacecraft experiments. Controlled injections would provide important insight into this fundamental process because the induced amplification has a well-defined signature.     

Wave particle interactions as an energy transfer mechanism between different particle species

Recent improvements in experimental techniques and cooperative data analysis efforts have brought a lot of information on the basic mechanisms by which energy can be exchanged between different particle species in the collisionless magnetospheric or solar wind plasmas. Some of these mechanisms are reviewed. A particular emphasis is put on interactions which occur in the equatorial magnetosphere between energetic protons and electromagnetic ultra low frequency (ULF) waves and which are linked with He⁺ ion trapping and heating as well as with field-aligned suprathermal electron beam generation. The process by which ion conic distributions are produced by electrostatic ion cyclotron waves generated at high altitude along auroral field lines by drifting electrons is also discussed.     

Recent Developments in Magnetospheric Diagnostics Using ULF Waves

One of the most ubiquitous indicators of the state and topology of the magnetosphere are ultra-low frequency (ULF) waves. These may be continuously and inexpensively monitored from the ground using networks of magnetometers. The most robust measurable quantity provided by magnetometer networks is signal phase and this paper emphasizes the usefulness of this parameter in a variety of ULF wave diagnostic processes ranging from equatorial to high latitudes. This revised version was published online in August 2006 with corrections to the Cover Date.     

The menagerie of geospace plasma waves

Sounding rockets and satellites have discovered a large variety of plasma waves within the Earth's magnetosphere—geospace. These waves are found over a frequency range of millihertz to megahertz. The frequency ranges are generally associated with characteristic frequencies such as the plasma frequency and gyrofrequency. Most waves are generated by hot or streaming magnetospheric plasma; some waves are due to lightning discharges, to intentional man-made transmitters or to incidental radiation from power transmission systems. Propagation of waves from the observation region back to a probable source region can be modelled using ray tracing techniques in a model magnetosphere where the electron number density, ion composition and magnetic field vector is specified. Information in addition to the common amplitude-frequency-time spectrograms can be obtained from the received waves using multiple antennas and receivers. Cross-correlation of the wave electric and magnetic components can provide information on the wave polarization and direction of propagation and on the wave distribution function.     

Consequences of hydromagnetic waves on magnetospheric particle dynamics

Since long ULF waves are known to play a dominant role in the dynamics of energetic protons. Recent observations have shown that they also contribute to the heating and/or acceleration of both heavy ions and electrons. This review will deal with all the aspects of wave particle interactions that involve electromagnetic ULF waves in the equatorial region of the magnetosphere. We will consider successively Pc-3-4-5 pulsations, magnetosonic waves and ion cyclotron waves. In the latter case, for which both the experimental data and the theoretical interpretation are more advanced, we will discuss the linear, quasi-linear and nonlinear aspects of the interaction. Results of recent numerical simulations of this type of interaction will also be reported. A final section will be devoted to ULF waves observed in the vicinity of the Io torus.     

Morphology and physics of short-period magnetic pulsations

This review is devoted to the main problems of experimental and theoretical investigations of geoelectromagnetic waves in the frequency range from 0.1 to 5 Hz. These waves constitute the short-period subclass of so-called geomagnetic pulsations. The short-period pulsations are represented by Pc1, Pc2, Pi1, Ipdp types and some subclassifications. The understanding of the pulsation mechanisms provides an insight into the structure and dynamics of the Earth's magnetosphere. We focus our attention on Pc1 pearl pulsations and on the classical (evening) Ipdp, for which basic physical concepts have been established. Other types and varieties are outlined also, but in less detail. In these cases, the physical mechanism is not always clear (as, for example, in the case of morning Ipdp), and/or the morphology is still to be determined carefully (Pc2 and discrete signals in polar cusps as typical examples).Short-period pulsations are a spontaneous, sporadic phenomenon which undergo a certain evolution in the course of a magnetic storm. We consider the storm-time variation as a natural background, and we use this background to collect the information about the pulsations in an orderly manner. At the same time, together with the transient storm-time variation of pulsation activity, quasi-periodic variations take place, which are connected with the Earth's and Sun's rotation, Earth's orbital motion and solar cycle activity. The study of these regular variations allows us to have a new approach to the mechanisms of excitation and propagation of short-period geomagnetic pulsations.     

Plasma Waves in the Hermean Magnetosphere

The Hermean magnetosphere is likely to contain a number of wave phenomena. We briefly review what little is known so far about fields and waves around Mercury. We further discuss a number of possible phenomena, including ULF pulsations, acceleration-related radiation, bow shock waves, bremsstrahlung (or braking radiation), and synchrotron radiation. Finally, some predictions are made as to the likelihood that some of these types of wave emission exist.     

Substorm aspects of magnetic pulsations

The different types of magnetic pulsations occurring during magnetospheric substorms are analysed into the concept of polar substorms recently described in detail by Akasofu (1968). Special attention is thus paid, to the simultaneous occurrence of different types of micropulsations at different places around the earth, during the development of a substorm. Time lags between the appearance of micropulsations and other geophysical effects of the substorm are of fundamental importance in this respect. Relationships between the occurrence or spectral shape of micropulsations and the state of the magnetosphere, as determined by satellite measurements are also of interest. Recent theoretical studies about the origin of these micropulsations are reviewed: natural h.m. emissions are directly linked to the thermal plasma density, the high energetic particle fluxes and pitch angle distributions in the far magnetosphere (L 5–8). We can thus expect to be able to deduce some information about the changes of these quantities during substorms. New semi-quantitative work is reported, which tries to interpret the repetitive structure of SIP events in terms of thickness of the magnetospheric tail, and the frequency drift of IPDP's in terms of magnetospheric electric fields. The present knowledge about absorption and dispersion of hydromagnetic waves through the ionosphere or inside the submagneto-ionospheric guide is also stated, because not taking these effects into account could lead to misinterpretation of the data.     

Review of Pi2 Models

More than half a century after the discovery of Pi2 pulsations, Pi2 research is still vigorous and evolving. Especially in the last decade, new results have provided supporting evidence for some Pi2 models, challenged earlier interpretations, and led to entirely new models. We have gone beyond the inner magnetosphere and have explored the outer magnetosphere, where Pi2 pulsations have been observed in unexpected places. The new Pi2 models cover virtually all magnetotail regions and their coupling, from the reconnection site via the lobes and plasma sheet to the ionosphere. In addition to understanding the Pi2 phenomenon in itself, it has also been important to study Pi2 pulsations in their role as transient manifestations of the coupling between the magnetosphere and the ionosphere. The transient Pi2 is an integral part of the substorm phenomenon, especially during substorm onset. Key questions about the workings of magnetospheric substorms are still awaiting answers, and research on Pi2 pulsations can help with those answers. Furthermore, the role of Pi2 pulsations in association with other dynamic magnetospheric modes has been explored in the last decade. Thus, the application of Pi2 research has expanded over the years, assuring that Pi2 research will remain active in this decade and beyond. Here we review recent advances, which have given us a new understanding of Pi2 pulsations generated at various places in the magnetosphere during different magnetospheric modes. We review seven Pi2 models found in the literature and show how they are supported by observations from spacecraft and ground observatories as well as numerical simulations. The models have different degrees of maturity; while some enjoy wide acceptance, others are still speculative.     

Interplanetary field effect on the magnetosphere

This paper reviews recent developments in the understanding of the solar-wind magnetosphere interaction process in which the interplanetary magnetic field has been found to play a key role. Extensive correlative studies between the interplanetary magnetic field and the magnetospheric parameters have in the past few years yielded detailed information on the nature of the interaction process and have made possible to follow the sequence of events that are produced inside the magnetosphere in consequence of the solar-wind energy transfer. We summarize the observed effects of the interplanetary magnetic field, its north-south and east-west components in particular, found in various domains of the magnetosphere — dayside magnetopause, polar cap, magnetotail, auroral zone —, and present an overall picture of the solar-wind magnetosphere interaction process. Dungey's reconnected magnetosphere model is used as a frame of reference and the basic compatibility of the observations with this model is emphasized. In order to avoid overlap with other review articles in the series discussion on the energy conversion process inside the magnetosphere leading to the substorm phenomenon is kept to the minimal.     

High-Latitude Particle Precipitation and its Relationship to Magnetospheric Source Regions

Two central issues in magnetospheric research are understanding the mapping of the low-altitude ionosphere to the distant regions of the magnetsphere, and understanding the relationship between the small-scale features detected in the various regions of the ionosphere and the global properties of the magnetosphere. The high-latitude ionosphere, through its magnetic connection to the outer magnetosphere, provides an important view of magnetospheric boundaries and the physical processes occurring there. All physical manifestations of this magnetic connectivity (waves, particle precipitation, etc.), however, have non-zero propagation times during which they are convected by the large-scale magnetospheric electric field, with phenomena undergoing different convection distances depending on their propagation times. Identification of the ionospheric signatures of magnetospheric regions and phenomena, therefore, can be difficult. Considerable progress has recently been made in identifying these convection signatures in data from low- and high-altitude satellites. This work has allowed us to learn much about issues such as: the rates of magnetic reconnection, both at the dayside magnetopause and in the magnetotail; particle transport across the open magnetopause; and particle acceleration at the magnetopause and the magnetotail current sheets.     

The use of multiple receivers to measure the wave characteristics of very-low-frequency noise in space

Until now most very-low-frequency (VLF) radio noise experiments in the ionosphere, magnetosphere and solar wind have been able to provide only the amplitude and spectral characteristics of VLF phenomena. Experiments using multiple receivers to measure the amplitudes and relative phases of the magnetic and electric wave components, however, can give the wave characteristics in addition. Knowledge of both the spectral and the wave characteristics are desirable in making deductions about the noise source location and mechanism and about the properties of the propagation path. Expressions are derived for obtaining the electromagnetic wave characteristics — wave normal vector, Poynting vector and wave polarization — and the electrostatic wave characteristics — wave normal direction and field magnitude — from the amplitudes and relative phases of the wave components. The antenna systems capable of measuring the necessary wave components on payloads which are not spinning, spinning, or spinning and precessing are described. Consideration is given to the experimental technique of reducing payload interference, of transferring the required data to the ground and of obtaining the desired spatial, frequency, amplitude and phase resolution.The data obtained with such an experiment may represent the superposition of signals from multiple sources and multiple paths and from interference signals. Interpretation of these results is discussed and the use of the results of obtaining information on the source location and mechanism and on the propagation path properties is described. Recently several sounding rocket and satellite experiments capable of measuring some of the wave characteristics have been flown. The results concerning the wave normal directions for several different types of VLF noise phenomena are summarized.     

Long-period irregular magnetic pulsation,Pi3

In the 1973 Scientific Assembly, the International Association of Geomagnetism and Aeronomy proposed with Resolution No. 11 to settle two new classes of magnetic pulsations; Pc6 having a sinusoidal waveform with periods longer than 600 s and Pi3 having an irregular waveform with periods longer than 150 s. The present paper reviews the studies on these pulsations putting a stress on Pi3. the Pi3-type pulsations are further classified into Psc5, Psc6, Pip, Ps6 and another type. The pulsations Psc5 and Psc6 mean the damped-type pulsations associating with storm sudden commencement in the approximate period range from 150 to 600 s and longer than 600 s, respectively, while Pip and Ps6 are the quasi-sinusoidal pulsations associating with magnetospheric substorm in the appropriate period range from 100 to 400 s and from 5 to 40 min, respectively. In the present review paper a stress is further layed on morphology and theory of the Ps6-type Pi3 pulsation. The following two-snake model is concluded to be plausible. A current system with in-flowing field-aligned current, westward ionospheric current, and out-flowing field-aligned current expands toward both the dawn- and the dusk-sectors with the progress of magnetospheric substorm. Thus Ps6 is regarded to be due to a magnetic effect of a meandering of the current system during the expansion, which is compared with the meandering of two snakes along the auroral oval from the midnight point toward both the east and the west, respectively.     

Modelling of the electromagnetic field in the interplanetary space and in the Earth's magnetosphere

A magnetohydrodynamic model of the solar wind flow is constructed using a kinematic approach. It is shown that a phenomenological conductivity of the solar wind plasma plays a key role in the forming of the interplanetary magnetic field (IMF) component normal to the ecliptic plane. This component is mostly important for the magnetospheric dynamics which is controlled by the solar wind electric field. A simple analytical solution for the problem of the solar wind flow past the magnetosphere is presented. In this approach the magnetopause and the Earth's bow shock are approximated by the paraboloids of revolution. Superposition of the effects of the bulk solar wind plasma motion and the magnetic field diffusion results in an incomplete screening of the IMF by the magnetopause. It is shown that the normal to the magnetopause component of the solar wind magnetic field and the tangential component of the electric field penetrated into the magnetosphere are determined by the quarter square of the magnetic Reynolds number. In final, a dynamic model of the magnetospheric magnetic field is constructed. This model can describe the magnetosphere in the course of the severe magnetic storm. The conditions under which the magnetospheric magnetic flux structure is unstable and can drive the magnetospheric substorm are discussed. The model calculations are compared with the observational data for September 24–26, 1998 magnetic storm (Dst _min=−205 nT) and substorm occurred at 02:30 UT on January 10, 1997. This revised version was published online in August 2006 with corrections to the Cover Date.