Jupiter Dust Streams

2.1. Electromagnetically interacting dust

The impact directions of the dust stream particles measured with Galileo and Ulysses in interplanetary space were close to the line-of-sight direction to Jupiter. The approach direction of most streams, however, deviated too much from the direction to Jupiter to be explained by gravitational forces alone. This deviation was correlated with the magnitude and the direction of the interplanetary magnetic field [10] which implied that strong non-gravitational forces must have been acting on the grains. The observed 28 day period in the impact rate (Figure 1) was most likely caused by changes in the tangential component of the solar wind magnetic field which periodically accelerated the particles towards and away from the ecliptic plane [21,22]. Numerical simulations showed that only particles with velocities in excess of 200 kms-1 and radii in the range 5nm < s < 15 nm were compatible with the observations [23]. Larger (smaller) grains did not interact enough (interacted too strongly) with the interplanetary magnetic field to explain the observed impact directions. This demonstrated that the solar wind magnetic field acts as a giant mass-velocity spectrometer for charged dust grains.

Strong electromagnetic interaction of dust grains was also found with the Galileo detector within the Jovian magnetosphere. Figure 2 shows an example of the impact rate measured with Galileo in the inner part of the magnetosphere. During this and most other times when Galileo collected data in this spatial region, the impact rate fluctuated with 5 and 10 h periodicities and the fluctuations typically exceeded 2 orders of magnitude. Furthermore, the impact directions of the grains and the measured charge rise times and charge amplitudes which were used to derive particle speeds and masses showed similar fluctuations [24], These fluctuations were correlated with the position of Galileo in the Jovian magnetic field (cf. bottom panel of Figure 2). Due to a 9.6° tilt of Jupiter's magnetic axis with respect to the planet's rotation axis the magnetic equator sweeps over the spacecraft in either up- or downward direction every 5 h.

In addition to the 5 and 10h periods which are compatible with Jupiter's rotation period, a modulation of the impact rate with Io's orbital period (42 h) could also be recognized during some time intervals (e.g. Galileo orbits E4 [24], G7 [25] and C9 [26]) while at other times an Io modulation was missing (e.g. Galileo orbit G2, Figure 2). A detailed frequency analysis of a two year dataset showed Io's orbital frequency as a "carrier frequency" and primary source of the Jovian dust streams [27]. Jupiter's magnetic field frequency modulates Io's frequency signal, giving rise to modulation sidelobe products seen around first order (10 h) and harmonic (5 h) Jupiter magnetic field frequencies. These modulation products confirm Io's role as a primary source of the Jovian dust streams. Io as a source can best explain the time series analysis results showing Io's orbit periodicity.

An Io source is also compatible with the deduced particle sizes of ~ 10 nm: photometric observations of the Io plumes obtained with Voyager imply a size range of 5 to 15 nm [28], in agreement with numerical simulations [23]. Recent Hubble Space Telescope (HST) observations constrained the grains to be smaller than 80 nm [29]. Hence, given the ejection speeds of more than 200 kms-1, Io turned out to be a source for interplanetary and interstellar dust!

Figure 2. Top: Impact rate of Jovian dust stream particles measured in September 1996 (Galileo's G2 orbit). In the time period shown the spacecraft approached the planet from 60 to 10 Rj jovicen-tric distance (Jupiter radius, Rj = 71,492 km). Note the strong fluctuations with 5 and 10 h periods. Bottom: Height of Galileo spacecraft above Jovian magnetic equator. A dipole tilted by 9.6° with respect to Jupiter's rotation axis has been assumed.

3 4 September 1996

3 4 September 1996

Figure 2. Top: Impact rate of Jovian dust stream particles measured in September 1996 (Galileo's G2 orbit). In the time period shown the spacecraft approached the planet from 60 to 10 Rj jovicen-tric distance (Jupiter radius, Rj = 71,492 km). Note the strong fluctuations with 5 and 10 h periods. Bottom: Height of Galileo spacecraft above Jovian magnetic equator. A dipole tilted by 9.6° with respect to Jupiter's rotation axis has been assumed.

The suggested mechanism to eject dust grains from within the Jovian magnetosphere matched the size and velocity range of the observed stream particles by recognizing that these grains become positively charged in the Io plasma torus and can get accelerated by Jupiter's corotational electric field [22,30,31]. As grains traverse the various plasma regions in the torus, however, their charge will not remain constant. Dust grains escaping Io's plumes first enter the cold plasma torus where they become negatively charged (~ —3 V). Grains that reach the outer hot regions of the torus change their sign of charge to positive +3 V) because of secondary electron emission. Once positively charged, grains will be accelerated by the outward pointing corotational electric field. They will leave the Jovian system if their radii are between about 9 and 180 nm [24], Smaller grains remain tied to the magnetic field lines and gyrate around them like ions do, whereas bigger grains move on gravitationally bound orbits which are - depending on the particle size - more or less affected by the Lorentz force. Recent investigations showed that a higher secondary electron yield which leads to potentials of —5 V in the cold torus and +5 V elsewhere gives better agreement with the observations [32],

Since Io is located very close to Jupiter's equatorial plane, the particles are to a first order approximation accelerated outward along this plane. Because of the 9.6° tilt of Jupiter's magnetic field with respect to the planet's rotation axis, however, the particles also experience a significant out-of-plane component of the Lorentz acceleration: particles continuously released from Io move away from Jupiter in a warped dust sheet which has been nick-named 'Jupiter's dusty ballerina skirt' [30]. A detector attached to a spacecraft moving in Jupiter's equatorial plane detects an increased number of particles when this dust sheet sweeps over its position. The 5 and 10 h fluctuations in the dust impact rate as well as the impact directions of grains observed by Galileo [24] can be explained with this scenario of electromagnetically coupled dust grains. However, only grains within a narrow size range around 10 nm are in agreement with the observed features. Smaller and larger stream particles were not detected with the Galileo dust instrument.

The charge of a particle escaping from the Io torus strongly depends on variations in the plasma density and temperature in space and time and thus is a function of Io's position at the time of particle release. In fact, the position where a particle is released from the torus is correlated with Io's position (Graps, personal comm.). In addition, the torus shows a strong dawn-to-dusk asymmetry in the plasma conditions that influence the escape of the dust particles. Grain charges are more negative on the dawn side of the torus where a lower electron temperature leads to a reduced secondary electron production. Particles on the dawn side remain captured in the torus for longer times because of their lower positive charge. Six years of Galileo dust stream measurements clearly show a variation of the flux with Jovian local time: significantly higher dust fluxes were measured on the dawn and on the dusk sides than on the noon side of Jupiter (Kriiger et al., in prep.) as predicted by numerical modelling [31]. Thus, the Jovian dust streams serve as tracers of the plasma conditions in the Io torus.

The fly-by of the Cassini spacecraft at Jupiter in December 2000 provided a unique opportunity for a two-spacecraft time-of-flight measurement (Cassini-Galileo) of particles from one collimated stream from the Jovian dust streams. Particles in a stream were detected with Galileo as the spacecraft was inside the Jovian magnetosphere close to the orbit of Europa (about 12 Rj), and then particles in the same stream were detected by Cassini outside the magnetosphere (at 140 Rj). The Cassini data imply that particles of different sizes have different phases with respect to Jupiter's rotation (Kempf et al., in prep.), a result which was also seen in earlier Galileo data [24]. The comparison of the measurements from both dust instruments, however, is hampered by the higher detection sensitivity of the Cassini detector with respect to the Galileo detector. Both instruments have detected stream particles with different sizes and, hence, different phases. The analysis is ongoing and more detailed modelling to describe the phase relation of different-sized particles is in progress. The present analysis indicates particle speeds of about 400 km s-1. This value is in agreement with speeds for 5 nm particles as derived from dynamical modelling and earlier studies of the Jovian dust stream dynamics [23].

The Cassini dust instrument is equipped with a time-of-flight mass spectrometer which measures the elemental composition of dust grains with a mass resolution M/AM ss 100. During Cassini's approach to Jupiter impact spectra of a few hundred dust stream particles have been measured and their chemical composition reflects the chemistry found on Io. With the Cassini instrument the surface composition of a satellite other than our Moon has been measured directly.

2.2. Io as a source of dust in the Jovian system

How significant is Io as a source of cosmic dust? How does the amount of dust ejected compare with other dust sources in the Solar System? With a simple calculation we can derive the total dust production rate of Io. Given the spread of Io dust along and away from Jupiter's equatorial plane, we assume a cone-shaped emission pattern of dust originating at Jupiter. We assume a cone opening angle of 35° and isotropic dust emission towards all jovigraphic longitudes. Although Galileo measurements were obtained only along the Jovian equatorial plane, this opening angle is justified by the Ulysses measurements. Ulysses measured the dust streams at 35° jovigraphic latitude after Jupiter fly-by (cf. Figure 1). For a given impact rate R, particle density p = 2gem-3, particle radius a = 10 nm, a detector sensitive area of A — 0.02 m2 and a cone radius r = 30 Rj the

Was this article helpful?

0 0

Post a comment