Description of Data

The Fast Auroral Snapshot Explorer (FAST) was launched August 21, 1996, into a 4200 X 350 km orbit with 83° inclination. FAST carries the Time-of-flight Energy Angle Mass Spectrograph (TEAMS), the first instrument which simultaneously measures 3-D distributions of H⁺, He²⁺, He⁺, and O⁺ at 0.001–12 keV and also provides a mass spectrum over the range 1&150;60 amu/q [Möbius et al., 1998a].

Figure 1 shows an overview of particle and ELF wave data from the inbound northern auroral pass from orbit 1697 on January 25, 1997. From top to bottom, the panels show an electron energy spectrogram; the ion pitch angle distribution (all species); H⁺, He⁺, and O⁺ energy spectrograms; average energy per ion for each species; the H⁺-He⁺ ion hybrid frequency f_IH, which is calculated from the fractional densities of H⁺, He⁺, and O⁺ according to the formula given by Lund and LaBelle [1997]; and ELF electric and magnetic field frequency spectrograms. The energy spectrograms cover all pitch angles. Lines on the wave spectrograms indicate the local proton gyrofrequency f_cH⁺ ~ 190 Hz.

During the period 08:39–08:41 UT, FAST encountered an electron inverted V in which peak energies exceeded 20 keV, even though this orbit was geomagnetically quiet (Kp = 1). An ion conic was observed from just before 08:39 until 08:40:21, when an abrupt transition to a beam occurs. Three times during this interval, TEAMS detected increased fluxes and energies of He⁺ and O⁺. Each time, electromagnetic waves at 60–120 Hz, broadening of the ion pitch angle distribution, and increases in the energy of the inverted-V electrons are seen; however, little or no change is seen in the proton energies, which range from 3 to 200 eV. The EMIC waves dominate the electric field spectra in the ion conic; lower hybrid emissions are visible at 300–400 Hz but are about 20 dB weaker. Note that some of the EMIC waves lie as much as 20 Hz below f_IH, which is 80-100 Hz during the period the EMIC waves are observed. As can be seen in the figure, He⁺ is more efficiently heated than O⁺ during periods of EMIC activity: He⁺ typically falls in the energy range 100–5000 eV, while O⁺ extends from 20 to 3000 eV. As FAST entered the ion beam, the frequency of the electromagnetic waves increased to just below f_cH⁺ and broad-band electrostatic turbulence appeared below about 100 Hz; even here, the energies of He⁺ and O⁺ are nearly equal, whereas O⁺ normally has a higher energy than He⁺ in ion beams [Möbius et al., 1998b].

Figure 2 shows the velocity space distribution of H⁺, He⁺, O⁺, and electrons over one spin period at 08:39:40–45. The axes for the three ion species plots are scaled such that each plot covers the same energy range. The H⁺ and O⁺ distributions show classic conic signatures, with perpendicular velocities extending to about 100 km/s for H⁺ and 200 km/s for O⁺. The parallel temperature of O⁺ is significantly higher than that of H⁺. The pitch angle of the conic at the altitude of FAST (4140 km) is about 130°; unfolding the conic adiabatically implies a 90° pitch angle, and therefore a transverse heating region, at about 2400 km. The He⁺ ions lie almost entirely in the range v_perp = 100–400 km/s; they appear to have a higher minimum parallel energy and a somewhat flatter cone angle of about 110°. The latter cone angle indicates a second heating region near or above 3600 km. Two features stand out in the electron distribution: the inverted-V population, which includes the downgoing population at v_|| ~ -5 X 10⁴ km/s (an energy of about 10 keV) and some mirroring electrons, and a field-aligned component up to about 1 keV with a low perpendicular temperature.

Several other preferential acceleration events have been identified in the data. One clear trend emerges: whenever EMIC waves are unambiguously present, preferential acceleration of He⁺ occurs, while in the absence of EMIC waves energization of all ion species is comparable. This point is illustrated in Figure 3, which shows how the maximum energies of the three major species are related in 86 ion conic events, of which 24 occur simultaneously with EMIC waves. (Because the maximum energies are estimated, several points overlie one another.) All of the conics with EMIC waves (solid symbols) show He⁺ preferentially heated over H⁺ and most show He⁺ preferentially heated over O⁺; almost all of the other events (open symbols) fall along the line y = x in the graphs. That ion conics fall into two or more distinct classes based on what waves are observed is consistent with a recent statistical survey of Freja data [André et al., 1998]. Some of these events are accompanied by substantial increases in the He⁺ density; for example, significant amounts of He⁺ were detected during a preferential acceleration event observed on orbit 534 on October 9, 1996 (not shown), but He⁺ was almost entirely absent elsewhere in the pass.

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