JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. AI2, PAGES 21,245-21,251, DECEMBER 1, 1993 The Dusty Ballerina Skirt of Jupiter M. HORANYI Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder G. MORFILL Max-Planck-lnstitut fiir ExtraterrestrishePhysik, Garching, Germany E. GRON Max-Planck-lnstitut fiir Kernphysik, Heidelberg, Germany We suggest a model to explain the unexpected recurrent dust events that were observedduring the Jupiter encounter by the dust detector on board the Ulysses spacecraft. This model is based on dust-magnetosphere interactions.Dust particlesinside the Jovian magnetosphere collect electrostatic charges and their interaction with the magnetic and electric fields can lead to energization and subsequent ejection. We discuss the dustyregions (ring/halo, "gossamer" ting) and also Io as potential sources for the Ulysses events. This model favors Io as a source. The massand velocity range of the escaping particles are compatiblewith the observations, and we also suggest internal periodicitiesto explain the recurrent nature of the Ulysses dust events. 1. INTRODUCTION There are many exciting phenomena associated with the interaction of magnetosphericfields and plasmas with the embedded dust grains. Lorentz resonances [$chaffer and Burns, 1987], gyrophase drifts due to compositionaland/or plasma density and/or plasma temperature gradients [Northrop et al., 1989], transport due to charge [Morrill et al., 1980a, b] or magnetic field fluctuations[Consolrnagno, 1983], shadow resonance [Horanyi and Burns, 1991] and the coupling between radiation pressure and electrodynamic forces [Horanyi et al., 1992], for example, might all contrib- ute to shaping the fine dust distribution in planetary rings. The ting-halo transition at Jupiter or the structure of Sat- urn's E ring are examples where the observed radial and vertical structureclearly demonstrates the effect of magneto- sphericperturbations.The dust becomes an integral compo- nent of the magnetosphere since it acts as a source/sinkof the plasma. The produced low-energy photo electrons and secondary electrons or the sputtered off ions might signifi- cantly alter the magnetospheric plasma distribution, for example. Though many of these processesare now recog- nized, dusty planetary magnetospheres still hold surprises. During its recent encounter with Jupiter the Ulysses spacecraft encountered recurrent streams of very small dust particles [Griin et al., 1993]. The impact geometry suggests that the grains originatefrom within the Jovian system. The mass of thedetected stream particles is 1.6 x 10 -16_< m --< 1.1 X 10 -14 g, and their estimated velocity is 20 < v < 56 km/s; the uncertainty in estimating the massis a factor of 10, and the uncertainty in estimating the velocity is a factor of 2. Assuming an average density of 1 g/cm 3, the size range of these particles is 0.03 < a < 0.14 /am. Perhaps, the most puzzling part of the observation is the quasi-periodic nature Copyright 1993 by the American GeophysicalUnion. Paper number 93JA02588. 0148-0227/93/93 JA-02588505.00 of the events. The first analysis showed that the streams were spaced approximately 500 Rj (Rj = 7.1 x 10 4 km, the radius of Jupiter) apart; they occurred with a period of 28 -+ 3 days (close to the rotation period of the Sun). Subsequent analysis of the data identified additional events, perhaps suggesting a new period of approximately 13 ___ 1 days before the encounter [Baguhl et al., 1993]. As a possible explanation, we have suggested [Horanyi et al., 1993] a new phenomenon: the acceleration and subse- quent ejection of small grains from planetary magneto- spheres. In this paper we explore this original model in detail. Io, as previously suggested [Johnson et al., 1980], is a likely source of submicron sized dust grains. These small grainsbecome chargedin the Jovian magnetospheric plasma environment, and their motion is strongly affected by elec- tromagnetic forces [Morrill et al., 1980a, b]. In a certain size range, grains gain energy from the corotating electric field and escape. We will demonstrate that the escape direction depends on Io's position and the orientation of the corotating magnetic field. The observedperiodicity can be explained as a combination of the periods related to Io's motion in geographic and magneticcoordinates.The period, mass, and velocity range of the escaping dust grains in our model is consistentwith the Ulysses findings. 2. CHARGING OF DUST PARTICLES To investigate the dynamics of small grains in the Jovian magnetosphere, we have to calculate their charge. There are various charging currents, all functions of the plasma com- position, temperature, density, radiation field and also the material properties, velocity, and previous charginghistory of the dust grains [Whipple, 1981]. In the Jovian environment we expect the electron and ion thermal fluxes and the secondary electron fluxes to be the most important contrib- utors; the sum of these determines the evolution of a grain's charge Q via the current balance equation 21,245