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Figure 3. Total dust production rate of Io assuming that the grains are ejected into a cone with 35° opening angle centered at Jupiter. Each vertical bar represents data from one Galileo orbit. The height of the bar shows the dust production rate derived from measurements between 10 to 30 Rj from Jupiter. The data have been corrected for a Jovian local time variation of the dust emission from the Io torus and for a long-term change of the dust instrument sensitivity (Kriiger et al. in prep.). The labels of individual Galileo orbits are indicated at the top. No dust stream measurements were collected during Galileo orbits 5 and 13.

total amount of dust emitted from Io per second can be calculated. With R — 0.1... 100 impacts per minute detected from 1996 to 2001 the average dust ejection rate is 10gs_1 to 10 kgs_1(Figure 3). If we take a typical value of lkg s-1 of dust and compare it with 1 ton sec-1 of plasma ejected from Io into the torus, the dust amounts to only 0.1% of the total mass released. These numbers indicate that Io's volcanic plumes are also a minor source for interplanetary dust compared with comets or main belt asteroids [33]. Io, however, turns out to be a major dust source for the Jovian system itself. The total mass of dust produced by Io as 10 nm-sized particles is comparable to the mass of dust ejected as micrometer-sized particles by the other Galilean satellites, which have no volcanic activity (Sect. 3).

The Jovian dust stream measurements can serve as a monitor of Io's volcanic plume activity. With Galileo imaging ten active plumes have been observed which is comparable with nine plumes seen by Voyager [34]. At least tow types of plumes can be distinguished: large, faint ones, with short-lived or intermittent activity (Pele-type) or small, bright, long-lived ones (Prometheus-type). The most powerful plume ever detected on Io, Pele, is the archetype of the first category and was observed to an altitude of more that 400 km [29]. Pele is also the location of the most stable high-temperature hot-spot on Io and is probably related to an active lava lake. Plumes are normally related to hot spots but not vice versa. The Pele plume is known to be rich in S2 gas as well as SO2 [35], Although it has been suggested that the Pele plume may be a pure gas plume, plume observations can also be interpreted as due to very fine (< 80 nm) particulates [29].

It is of special interest to see whether variations in the dust production rate deduced from the dust stream measurements can be related to the activity of the Pele or other plumes on Io, or to the total thermal output of the satellite. A correlation with the activity of the Pele plume seems most promising because only the most powerful plumes are expected to accelerate the grains to sufficient altitudes so that they can finally escape from the satellite [4,36].

The dust production of Io for individual orbits of Galileo is shown in Figure 3. Here, the vertical bars indicate the variation in the derived dust production rate if we vary the jovicentric distance at which the dust flux is taken between 10 and 30 Rj during one orbit. This reveals a strong variation in the dust production rate from orbit to orbit which is up to two orders of magnitude. If the plasma conditions in the Io torus and the Jovian magnetic field did not change too drastically from orbit to orbit, it reflects the variation of the activity of the Io plumes.

We have compared the dust production rate shown in Figure 3 with the total thermal output of Io deduced from Galileo near-infrared measurements (Spencer, personal comm.). Unfortunately, this did not give a clear picture. This negative result, however, is not too surprising because Io's overall thermal output is not very well correlated with plume activity. The Pele plume was observed in July 1995, July 1996, December 1996 and possibly July 1997 [34]. It was absent in June 1996, February 1997, June 1997 and July 1999. Although, the strong drop in the dust impact rate from December 1996 to February 1997 (E4 to E6 orbit) is consistent with these detections/non-detections, for other measurements it is not. Especially, the non-detection of the plume on 2 July 1999 is in contradiction with the large measured dust emission.

A correlation of the in-situ dust measurements with either Galileo or Earth-based imaging observations turns out to be very difficult because the imaging observations represent only sporadic glimpses. Many more observations would be needed to establish a firm link between the Galileo dust measurements and the activity of (an) individual plume(s) on Io. The picture is further complicated by the fact that the plume activity sometimes changes on timescales of days to weeks. Ideally, one would need imaging observations at exactly the same time as the dust measurements.

We have also estimated the Io dust production from the measurements of Galileo and Ulysses in interplanetary space out to 1 AU from Jupiter assuming again that the dust is uniformly distributed into a cone of 35°. This leads to unrealistically high dust production rates of more than 107 kgs-1. It indicates that this simple picture cannot be extrapolated to interplanetary space and that the dust is not distributed uniformly to such large distances. Rather, the dust particle trajectories must undergo some focussing effect due to electromagnetic interaction with the interplanetary magnetic field.

Additional evidence for such a focussing effect came from Galileo measurements in 2000 when the spacecraft has left the Jovian magnetosphere for the first time since 1995. Measurements outside the magnetosphere at a distance of ~ 280 Rj (0.13 AU) from Jupiter gave a surprisingly high impact rate of up to 10 impacts per minute (Figure 4). This value was comparable with the rates detected both in interplanetary space (Figure 1) and close to Jupiter during Galileo's early orbital mission (Figure 2).

In May and June 2000 (days 145 to 170), while Galileo was receding from Jupiter (from 10 to 170 Rj), the impact rate dropped by more than two orders of magnitude (from 0.05 to 0.0005 impacts per minute). This drop was close to the inverse square of the source

150 200 250 300 350 Day of year 2000

Figure 4. Impact rate of dust stream particles measured with Galileo between May and December 2000 (Galileo G28 orbit). Galileo perijove (at ~ 8 Rj distance from Jupiter) and apojove (~ 280Rj) are indicated. The impact rate has been averaged over a 40 h time interval. Due to different modes of dust instrument read-out the data have a time resolution varying from less than an hour to more than 20 days (28). The dust streams were outside the field of view of the dust detector before day 141 and again between day 350 and 363.

150 200 250 300 350 Day of year 2000

Figure 4. Impact rate of dust stream particles measured with Galileo between May and December 2000 (Galileo G28 orbit). Galileo perijove (at ~ 8 Rj distance from Jupiter) and apojove (~ 280Rj) are indicated. The impact rate has been averaged over a 40 h time interval. Due to different modes of dust instrument read-out the data have a time resolution varying from less than an hour to more than 20 days (28). The dust streams were outside the field of view of the dust detector before day 141 and again between day 350 and 363.

distance. When Galileo was outside the magnetosphere, beyond ~ 200 Rj from Jupiter (after day 180), the impact rate increased by about four orders of magnitude. Between August and October 2000 (days 230 to 280), Galileo remained more or less stationary with respect to Jupiter and Io, and the impact rate remained remarkably constant for about two months with roughly 1 impact per minute. Assuming - as before - that dust particles get ejected into a 35° cone, this leads to a dust production of Io of ~ 100 kg s-1. It seems unlikely that such a high dust production is maintained over such a long time period. More likely is a focussing effect of the grains due to the boundary between the Jovian magnetosphere and the interaction with the interplanetary magnetic field. Interestingly, the impact directions measured with Galileo indicate that the grains approached the sensor from a direction very close to the ecliptic plane. Similarly high impact rates were was also detected with the Cassini dust instrument [12] in September 2000 at ~ 0.3AU from Jupiter when the spacecraft was approaching the planet (Kempf et al., in prep.).

Frequency analysis of the Galileo dust impact rates measured beyond ~ 250 Rj did not reveal 5 and 10 h periodicities as was seen within the magnetosphere. Instead, a strong peak at Io's orbital period showed up in the frequency spectrum (A. Graps, personal comm.), much stronger than seen close to Jupiter.

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