The Plasma and Suprathermal Ion Composition (PLASTIC) investigation provides the in situ solar wind and low energy heliospheric ion measurements for the NASA Solar Terrestrial Relations Observatory Mission, which consists of two spacecraft (STEREO-A, STEREO-B). PLASTIC-A and PLASTIC-B are identical. Each PLASTIC is a timeof-flight/energy mass spectrometer designed to determine the elemental composition, ionic charge states, and bulk flow parameters of major solar wind ions in the mass range from hydrogen to iron. PLASTIC has nearly complete angular coverage in the ecliptic plane and an energy range from ∼0.3 to 80 keV/e, from which the distribution functions of suprathermal ions, including those ions created in pick-up and local shock acceleration processes, are also provided.

Recent observations have suggested that flickering aurora is produced by a modulation of the field-aligned component of the electron flux within an auroral arc. It is proposed that a portion of the field-aligned electrons are of ionospheric origin and that these electrons are accelerated and their flux modulated by electromagnetic ion cyclotron waves that occur below the main acceleration region on auroral •rc field lines. A model of the electromagnetic ion cyclotron wave shows that the parallel phase velocity of the wave increases as the wave propagates toward the ionosphere. A test particle calculation shows that ionospheric electrons trapped or reflected by the wave are accelerated to energies of several keV and that their flux is modulated at the wave frequency. The relative amplitudes of the model wave electric fields are consistent with the observations of sm•llscale low-frequency ionospheric and magnetospheric electric fields near auroral •rcs of approximately 10 mV/m and 100 mV/m, respectively. The large-amplitude ion cyclotron waves also produce a ponderomotive force and a self-consistent ambipolar electric field. Energy considerations show that the downward energy flux in the electromagnetic ion cyclotron wave can be several percent of the total downward auroral electron energy flux. include the inverted V itself [e.g., Frank and Ackerson, 1971; Gumerr, 1972; Lin and Hoffman, 1982], the detailed electron and ion distributions which include upfiowing ion beams and electrons trapped between the potential and the mirror point [Evans, 1974; Shelley et al., 1976; Mizera and Fennell, 1977; Croley et al., 1978; Ghielmetti et al., 1978; Cladis and Sharp, 1979; Sharp et al., 1979; Gorney et al., 1981; Mizera et al., 1981a, b], and electrostatic shocks [Mozer et al., 1977, 1980; Hudson and Mozer, 1978; Totbert and Mozer, 1978; Temerin et al., 1981a, 1981b Bennett et al., 1983, Temerin and Mozer, 1984; Redsun et al., 1985]. However, not all features of auroral particle distributions can be explained by such quasi-static electric fields. Outside of inverted Vs, features of auroral particle acceleration such •s field-aligned counterstreaming electrons [Sharp et al., 1980; Collin et al., 1982], conics [Sharp et al., 1977; Klumpar, 1979] and suprathermal electron bursts [Johnstone and Winningham, 1982] are difficult to .explain by quasi-static electric fields. Even the inverted V electron precipitation shows features which cannot easily be explained by quasi-static electric fields. These include the narrowly field-aligned electrons that are seen not only at the edges of inverted Vs [Hoffman and Evans, 1968; O'Brien and Reasoner, 1971; Arnoldy et al., 1974; Totbert and Carlson, 1980; Carlson and McFadden, 1982] but also well within the inverted V [Lin and Hoffman, 1979a, b; Burchet al., 1979]. Another feature of inverted Vs and auroral arcs that is not explained by quasi-static electric fields is the several hertz periodicities that are sometimes seen in association with bright auroral arcs. These have been seen both i...

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The Plasma and Suprathermal Ion Composition (PLASTIC) investigation provides the in situ solar wind and low energy heliospheric ion measurements for the NASA Solar Terrestrial Relations Observatory Mission, which consists of two spacecraft (STEREO-A, STEREO-B). PLASTIC-A and PLASTIC-B are identical. Each PLASTIC is a timeof-flight/energy mass spectrometer designed to determine the elemental composition, ionic

Shear Alfvén waves with amplitudes >100 mV/m were observed on two recent sounding rocket flights: one flown from Poker Flat, Alaska, to an altitude of 1040 km in the evening auroral zone, and another flown from Sondre Stromfjord, Greenland, to an altitude of 770 km in the morning cusp region. The largest waveforms are best described as a series of step functions, rather than as broadband noise or as single frequency waves. Complete two‐dimensional E and B measurements at 4‐ms time resolution were made, showing a downward propagation direction and implying insignificant reflection from the ionosphere at frequencies greater than 1 Hz. Intense, field‐aligned, low‐energy electron fluxes accompany the waves. Acceleration of these electrons by the Alfvén waves, either through the resonant acceleration mechanism described by Temerin et al. (1986), or through the nonresonant mechanism described by Goertz and Boswell (1979), is shown to be feasible. The waves in at least one case have a sufficiently large ponderomotive potential to generate the observed density fluctuations of order 1.