Copyright 1995 Kluwer Academic Publishers.
Reprinted by
permission of Kluwer Academic Publishers.
The Plasma Wave Instrument (PWI) on the Polar spacecraft is designed to provide comprehensive measurements of plasma waves in the polar regions of the Earth's magnetosphere. The study of plasma waves in the Earth's polar regions has a long history extending over more than half a century. The first reported observation of a high-latitude plasma wave phenomena was in 1933 by Burton and Boardman [1933]. Using a telegraph line and a telephone receiver as a simple receiving system, Burton and Boardman discovered that bursts of very-low-frequency (VLF) radio "static" were sometimes correlated with flashes of auroral light. These observations were confirmed in later studies by Ellis [1957], Dowden [1959], Martin et al. [1960], and others using ground-based VLF radio receivers. Typically, the aurora-associated emissions were the strongest in the frequency range from about 1 to 20 kHz. Because the emissions tended to produce a hiss-like sound in the audio output of the radio receiver, they soon became known as "auroral hiss."
The launch of the first Earth-orbiting satellites in the late 1950s opened a new era in the study of VLF radio wave phenomena. In addition to auroral hiss, a wide variety of new wave phenomena were quickly discovered in the region of magnetized plasma around the Earth known as the magnetosphere. These waves soon came to be known as "plasma waves," since they depended on the presence of a plasma for their generation and propagation. In general, plasma waves can be divided into two types: electromagnetic and electrostatic. Electromagnetic plasma waves have both electric and magnetic fields and propagate at high speeds, usually much greater than the thermal speeds of the plasma particles, and over long distances, often one Earth radius (RE) or more. Auroral hiss is an example of an electromagnetic plasma wave emission. Electromagnetic waves in the Earth's magnetosphere can be further subdivided into two types, those that are confined to the immediate vicinity of the Earth, and those that can escape to great distances. The latter are often referred to as planetary radio emissions. These emissions have access to free space and do not require a plasma to propagate. The best known example of a terrestrial radio emission is auroral kilometric radiation [Gurnett, 1974], which is an intense radio emission produced along the Earth's auroral field lines in the frequency range from about 50 to 400 kHz. Electrostatic plasma waves have no magnetic field and propagate at low speeds, on the order of the plasma thermal speeds, and are often strongly damped, thereby severely limiting their range of propagation. The best known examples of electrostatic waves are probably the Langmuir wave [Tonks and Langmuir, 1929], which consists of an electrostatic oscillation near a characteristic frequency known as the electron plasma frequency, and the ion acoustic wave [Stix, 1962], which is similar to a sound wave in an ordinary gas.
A list of some of the most important types of plasma wave emissions observed in the polar regions of the Earth's magnetosphere is given in Table 1. This table lists the name of the emission, the mode of propagation, the electromagnetic (EM)/electrostatic (ES) character of the wave, the frequency range, and the generation mechanism. The frequency of the emission is controlled by certain characteristic frequencies of the plasma, the most important of which are the cyclotron frequency and the plasma frequency where e and m are the particle charge and mass, B is the magnetic field strength, îo is the permittivity of free space, and N is the number density. Separate cyclotron frequencies and plasma frequencies are defined for each species in the plasma. The cyclotron frequencies and plasma frequencies are often combined to form resonance and cutoff frequencies for various wave modes, such as the upper hybrid resonance frequency fUHR, the lower hybrid resonance fLHR, the right-hand cutoff fR=0, and the left-hand cutoff fL=0. Formulas for these various frequencies are given at the bottom of Table 1. For a further discussion of the relation of these characteristic frequencies to the various plasma wave emissions observed in the Earth's polar regions, see the reviews by Shawhan [1979] and Gurnett [1991].
The importance of plasma waves in the magnetospheric plasma lies in their resonant interactions with charged particles. Two types of resonant interactions can occur between a plasma wave and a charged particle, the Landau resonance and the cyclotron resonance. The Landau resonance occurs when the component of the phase velocity of the wave along the static magnetic field, /k , matches the particle velocity along the magnetic field (the symbol , indicates the component parallel to the magnetic field). Cyclotron resonance occurs when the angular velocity of the wave field around the magnetic field matches the angular velocity of the particle, or its harmonic, in a frame of reference moving along the magnetic field with the particle. These resonance conditions can be summarized by the formula which give the parallel resonance velocity as a function of the frequency, , and the wave number, k . The Landau resonance condition is given by n = 0, the first order cyclotron resonance is given by n = 1, and the higher order (harmonic) cyclotron resonances are given by n > 1.
When a resonant interaction occurs between a plasma wave and a charged particle, energy and momentum is exchanged between the wave and the particle. Which way the energy and momentum flows depends in detail on the distribution of particle velocities. Certain types of distribution functions, such as a beam or a loss cone, lead to wave growth or amplification, whereas others, such as an isotropic distribution, lead to damping. A beam is a region of the velocity distribution function that has a positive slope (i.e., f/ v > 0 for v > 0), and a loss cone is a conical region in velocity space, aligned along the magnetic field, that is strongly depopulated due to collisions with the atmosphere. In the auroral zone, for example, electron beams accelerated by parallel electric fields are believed to be responsible for the generation of auroral hiss via a Landau resonance [Gurnett, 1966; Gurnett and Frank, 1972; Laaspere and Hoffman, 1976; Maggs, 1976]. Auroral kilometric radiation (AKR) is believed to be generated by a loss cone in the auroral electron distribution via a relativistic n = 1 cyclotron resonance mechanism [Wu and Lee, 1979; Winglee, 1985; Zarka et al., 1986; Louarn et al., 1990]. One consequence of wave growth, particularly for cyclotron interactions, is that particles are scattered into the loss cone. For charged particles trapped in planetary magnetospheres, scattering into the loss cone is the primary mechanism by which particles are lost from the radiation belts [Kennel and Petschek, 1966]. Once generated all internally confined waves must eventually be damped. If the damping involves a resonant interaction, some of the particles can be accelerated to high energies. Such resonant acceleration processes are believed to be the primary mechanism by which nonthermal particle distributions known as conics are generated in the Earth's auroral zones [Shelley and Collin, 1991].
One of the primary objectives of the International Solar Terrestrial Program, of which the Polar spacecraft is a part, is to "understand the physical processes controlling the origin, entry, transport, storage, acceleration, and loss of plasmas in the Earth's neighborhood" [Final Report of the Science Definition Working Group, April 1979]. Since plasma waves clearly play an important role in the loss and acceleration of plasmas in the Earth's polar regions, it is important that the spectrum of plasma waves be adequately characterized by the Polar spacecraft. This is the primary purpose of the Polar plasma wave investigation. The general objectives of the investigation, as outlined in the proposal submitted to NASA for the Polar plasma wave investigation [Gurnett et al., 1988] are
The advanced design of the Polar plasma wave instrument and the other instruments on the Polar spacecraft will allow us to investigate many specific questions that have been left unanswered. For example, to understand the exact mechanisms involved in the generation of auroral kilometric radiation closely coordinated wave and plasma measurements must be made as the spacecraft crosses the auroral field lines. It is not known, for example, whether a loss-cone [Wu and Lee, 1979] or an electrostatically trapped particle distribution [Louarn et al., 1990] is responsible for the growth of the auroral kilometric radiation. The high-resolution plasma measurements on Polar, in conjunction with the very detailed plasma wave measurements, should allow us to answer questions of this type. A list of the investigators and institutions participating in this investigation is given in Table 2.