High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990)) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from • 9 to 19 R•r in the Earth's magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on 2 years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements which we call bursty bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 •9 ergs/R•r 2. 17 R•r and were argued to have an (inferred) scale size of 15 R•r in the Yas• direction and fast shock properties. 1DeparUnent Statistical studies have also been used to characterize plasma sheet flows. Prior to 1986 only a few of these studies addressed plasma sheet flows irrespective of substorm phase [Caan et al., 1979; Hayakawa et al., 1982; Slavin et al. 1985, 1987]. For example, Hayakawa et al. [1982] using IMP 6 data from distances of-15 R•r < XOSM < -33 R•r showed that high-speed flows (V,, > 300 km/s) can occur in the plasma sheet within 1 Re from the expected neutral sheet position defined by the Russell and Brody [1967] model, so one might infer that these flows occurred in the CPS. However, the above statistical studies either did not distinguish between the CPS and its boundary or they concentrated in the distant-tail regions [Slavin et al., 1985, 1987]. The first statistical assessment of the significance of the near-Earth CPS for magnetotail transport was made by Huang and Frank [1986]. They constructed a data base using 128-512 s resolution plasma data from the ISEE 1 satellite. They applied the criterion that high-speed flows (Vi > 150 km/s) occurring 1.5 R•r or more away from the GSM equatorial plane are in the PSBL. They found that the average speed in the CPS was low (around 50 km/s) regardless of geomagnetic activity (based on the AE index). They showed (see also Figure 2 of Huang and Frank [ 1987]) that even if high-speed flows existed in their data set, these were not representative of the average properties of the CPS. They therefore argued that even if high-speed flows of short time or spatial scales may occur in the CPS, they are statistically insignificant compared to the vast majority of the (low flow velocity) data. The above study did not attempt to assess the relative contributions of the CPS and the plasma sheet boundary to magnetotail transport. A new statistical selection criterion to distinguish between the central plasma sheet and its boundary was proposed by Baumjohann et al. [1988] (subsequently referred to as BJeta188): ...

Using a common methodology to analyze data from the AMPTE/IRM and ISEE 2 satellites we report on the statistical properties of bursty bulk flow events (BBFs) in the inner plasma sheet (IPS). A positive correlation between BBFs and the AE index suggests that BBFs are predominantly geomagnetically active time phenomena. Earthward BBFs are more frequent close to midnight and away from Earth, up to a distance of ~19 RE. Tailward BBFs are very infrequent in the IRM data set and somewhat less infrequent in the ISEE 2 data set in the region of the satellites' spatial overlap, possibly due to the more active conditions prevailing during the ISEE 2 mission in that region. However, in both data sets the ratio of tailward to earthward BBFs increases with distance from Earth; more than 20% of all BBFs are anti-sunward tailward of X = -19 RE in the ISEE 2 data set. BBFs are responsible for 60-100% of the measured earthward transport of mass, energy and magnetic flux past the satellite in the regions of maximum occurrence rate, even though they last approximately 10-15% of the IPS observation time there. Thus BBFs represent the primary transport mechanism at those regions. The one-to-one correspondence between BBFs and substorm phase, as well as the relative contribution of BBFs to the total transport observed during substorms are questions that await further investigation based on multi instrument studies of individual events. vicinity of the neutral sheet was found to be quite dynamic in the AMPTE/IRM data set, contrary to conclusions based primarily on data from the ISEE satellites [Eastman et al., 1985]. In addition, the direction of the bursts of plasma flow in the ICPS is predominantly across the instantaneous magnetic field, rendering them possible significant contributors to the total measured magnetic flux transport. The high speed flows in the ICPS were studied on a case-bycase basis by Angelopoulos et al. [1992a]. The authors concluded that the rise-and-fall timescale of the flow bursts is of the order of a minute and that the bursts occur within 10-rain timescale flow enhancements termed bursty bulk flow events (BBFs). The bursts of flow are associated with ion heating and plasma sheet dipolarization. In a statistical study of BBFs in the AMPTE/IRM data set, Angelopoulos et al. [ 1992b] showed that such events are relatively infrequent (< 7% of the time in the plasma sheet and < 20% of the time in the ICPS) but can produce roughly half of the earthward mass and energy transport measured past the satellite during its 1985 magnetotail passes and most of the earthward magnetic flux transport. Thus such events represent important building blocks of magnetotail transport. High speed flows in the near-Earth central plasma sheet (CPS) have been reported in the past, in particular in data from the ISEE satellites [e.g., Hones, 1979; Nishida et al., 1981; Cattell and Mozer, 1984; Huang et al., 1987; Ohtani et al., 1992; Sergeev et al., 1992]. However, such flows have received little attention from the point of view of trans...

The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa.

On 3 January 2000, the Galileo spacecraft passed close to Europa when it was located far south of Jupiter's magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward Jupiter. This pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of Europa's internal field. The Galileo magnetometer measured changes in the magnetic field predicted if a current-carrying outer shell, such as a planet-scale liquid ocean, is present beneath the icy surface. The evidence that Europa's field varies temporally strengthens the argument that a liquid ocean exists beneath the present-day surface.