Data

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

We examine here the five passes through the auroral zone which meet the criteria below. All of these passes are between 2100 and 0100 magnetic local time. The selected passes contain examples of both BBELF and EMIC waves associated with ion conics; this condition was imposed in order to allow a comparison between the two mechanisms at nearby locations (separation of less than 5° in invariant latitude, one hour in magnetic local time, and 200 km in altitude) under as nearly the same geomagnetic and solar activity conditions as possible. We further required that the satellite collected fast survey data (time resolution of 2.5 s for H⁺ and O⁺ and 5 s for He⁺ and a Nyquist frequency for E of either 1024 or 256 Hz) during both ion conic events. A summary of the events selected is given in Table 1.

Data from one of these five passes are shown in Figure 1, which shows two minutes of data from a nightside (22.9 MLT) auroral pass on January 24, 1997. This orbit was geomagnetically quiet (Kp = 1-). An ion conic associated with BBELF waves is seen at 12:45:25–12:45:45, and an ion conic associated with EMIC waves is seen at 12:46:10–12:46:30. The ion conic with the EMIC waves is followed immediately by an upward ion beam; this morphology is not unusual at these altitudes.

In this example the BBELF waves are electrostatic, although examples of BBELF waves with a magnetic component exist (not shown). The BBELF waves coincide with bursts of upgoing field-aligned electrons, which are a candidate for the free energy source of the waves.

The EMIC waves are seen as a narrowband emission (delta f/f ~ 0.1) at frequencies of 100–150 Hz; the local proton gyrofrequency, denoted by the black line in the bottom panel of Figure 1, is ~ 200 Hz. A second band of waves, which appears at ~ 40 Hz for a few seconds near 12:46:30, is consistent with EMIC waves on the He⁺ branch of the dispersion relation; similar waves were reported in Viking data [Gustafsson et al., 1990]. These waves occur along with bursts of field-aligned electrons at energies below the "inverted-V" peak, but the correlation is not one-to-one. Although some VLF whistler mode waves are seen at the same time as the EMIC waves, the EMIC waves are better correlated with the ion heating; this pattern has also been seen in Freja data [Erlandson et al., 1994; André et al., 1998].

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