SM33A-1244 1340h
Effect of IMF By and Dipole Tilt on the Interaction between the Solar Wind and the Jovian Magnetosphere
The interplanetary magnetic field (IMF) at Jupiter_fs orbit is mainly in the Y-JSE direction. The Parker spiral angle is 11 degrees at the Jovian orbit (5.2 AU). In addition Jupiter has a dipole tilt of 9.8 degrees. In this paper will use a three dimensional global magnetohydrodynamic (MHD) simulation code to model the influence of IMF BY and dipole tilt on the configuration of Jupiter_fs magnetosphere and thereby extend our previous results in which we only considered the effects of IMF BZ. To simulate Jupiter_fs rapidly rotating magnetosphere with dipole tilt, IMF Bz and By we have extended our MHD model to handle the full three dimensional Jovian magnetosphere by removing the symmetry boundary condition at the equator. In this presentation we will present results with IMF BZ =XX and By=0.42nT. We used a simulation box with nearly 108 cells (600,400,400) and grid spacing of 1.5RJ. We will use the results of this simulation we to demonstrate the basic structure of Jovian magnetosphere as a function of IMF direction and dipole tilt.
SM33A-1245 1340h
ULF Wave Activity in the Terrestrial Polar Cap and Polar Cap Boundary Regions as Observed by Cluster
Ultra-low frequency (ULF) wave activity over the northern and southern terrestrial polar caps and at polar cap boundary regions is analyzed using Cluster Electron Drift Instrument (EDI), Flux Gate Magnetometer (FGM) and Cluster Ion Spectrometry (CIS) data. The February and March Cluster orbits lie mainly in the noon-midnight meridional plane with perigee at midnight. The open, polar cap for these orbits is bounded on one side by the nightside plasmasheet boundary layer and on the other by the dayside, high-latitude, poleward cusp boundary. Preliminary statistics of oscillation characteristics for the February and March orbits of 2001-2004 are presented. These statistics, as well as selected, energetic events, suggest an organization of the data in terms of three main magnetic field line topologies: those field lines adjacent to the poleward dayside cusp, those extending into the tail-lobe, and those adjacent to the nightside plasmasheet boundary layer. Possible source mechanisms suggested by this organization are discussed.
SM33A-1246 1340h
S-Bursts and the Jupiter Ionospheric Resonator
We explore the possibility of electron acceleration by inertial Alfven waves in Jupiter's upper ionosphere. We discuss our results in the light of the results from Crary [1997] who investigated electron acceleration at the Io torus boundary. We show that electron acceleration by Alfven waves is possible in both regions and improve upon Crary's results by incorporating a more accurate plasma model along the Jupiter-Io flux tube. Furthermore, we suggest that resonant eigenmodes in Jupiter's ionosphere (~30 Hz) may of the source of modulation of S-bursts. Since the Alfven wave eigenmode phenomena and electron acceleration are seen on Earth, we rely heavily on analogy with earth-based observations to advance our models.
SM33A-1247 1340h
Has the configuration of Jupiter's polar aurora changed significantly since the Voyager epoch?
At Earth, over 70 percent of auroral power during active periods resides within the diffuse auroral components, despite the disproportionate interest in discrete auroral processes. At Jupiter, present day interest also resides overwhelmingly on discrete auroral processes associated with field-aligned currents driven by planetary rotation. However, during the Voyager epoch of 1979 through the 1980's, interest was focused elsewhere. It was believed that bright aurora of Jupiter extend in latitude down to the vicinity of the magnetic footprints of the plasma torus of Io, based on Voyager ultraviolet spectrometer (UVS) observations. And, it was believed that diffuse auroral processes generate this Io-associated aurora, specifically with the precipitation of energetic ions, most importantly S and O. Ion energy intensities were measured sufficient to deliver about 60 ergs per square cm over the 7 to 9 RJ region with strong wave scattering, sufficient to deliver about 30 trillion W of energy. Since that time (beginning in the early 1990's), near-Earth imaging of the aurora has revealed that the brightest aurora of Jupiter typically occurs on field lines that map along field lines to between 20 and 30 RJ, and certainly no closer to Jupiter than 15 RJ. Discussions of the differences between Voyager and more recent observations have focused on the limitations of the Voyager measurements. However, the particles that were identified during the Voyager epoch as being responsible for the bright aurora are just the same particles that in 1995 were observed by Galileo to have been dramatically depleted, by a factor of 5 in total energy density and perhaps even more for heavy ions. Could the change in the perceived configuration of the aurora since the Voyager epoch represent a true change in the configuration of Jupiter's magnetosphere? Specifically, has a key component of Jupiter's aurora, the diffuse component, dimmed dramatically since that epoch? Here we update the evidence that the configuration of Jupiter's magnetosphere has changed by showing that the depletion of the near-torus energetic ion populations have persisted and probably substantially deepened in more recent times. We review concurrent evidence that the depletion of the energetic ions is caused by enhancements in the neutral gases emanating from Io since the Voyager epoch. We argue that following the Voyager epoch, Jupiter's aurora may have become even less Earth-like than it is now in terms of the balance between diffuse and discrete components, and that the volcanoes of Io have a here-to-fore unappreciated role in establishing that balance.
SM33A-1248 1340h
Thermal Plasmas in Jupiter's Magnetosphere: Considerations for Auroral Processes
Measurements of thermal plasmas from Jupiter's plasma sheet and plasma torus provide context for consideration of auroral processes. It is now thought by many that the main ring of auroral emission at Jupiter is a response to the breakdown of corotation in the middle magnetosphere and that the emissions are associated with the system of electric currents that communicate stresses between the ionosphere and the magnetosphere. Several mechanisms have been proposed to account for the transfer of energy that drives the IR, visible, and UV emissions. In this report we review observational evidence of the breakdown of corotation, and we report on the occurrence of field-aligned streams of hot electrons that may be a signature of processes in the ionosphere. We consider implications for models of magnetosphere-ionosphere coupling at Jupiter.
SM33A-1249 1340h
Nonlinear Interaction of Io's Disturbance System With the Inhomogenous Jovian Magnetosphere
We present nonlinear, time-dependent, three-dimensional simulations of the magnetohydrodynamic interaction of Io's disturbance system with the inhomogeneous Jovian magnetosphere. We have expanded the well-known ZEUS-3D code to generate our model in idealized MHD. Since field and plasma in the vicinity of Io and its Alfven Wing are strongly disturbed according to observations, we focus on the nonlinear nature of the interaction and thereby extend previous work. Moreover, we discuss the transition from the linear to the strongly nonlinear case and observe profound differences. Of particular importance are multiple wave reflections at the torus boundaries and the Jovian ionosphere which have been treated only linearly in the past. Our magnetospheric model is idealized by assuming straight field lines in the initial condition. Io is implemented as a sphere of neutral gas. The Simulation is carried forward in time until a steady state is reached in Io's vicinity and its rest frame. The results are applicable to the Jovian and Saturnian system etc.
SM33A-1250 1340h
Multi-fluid Simulations of Ganymede's Magnetosphere: The Role of Heavy Ions in the Magnetotail
The magnetosphere of Ganymede, embedded in the Jovian magnetosphere, presents us with a unique case study of a sub-alfvenic flow interaction in the presence of continuous reconnection. We study the interaction using multi-fluid simulations that have been benchmarked against existing magnetometer data and UV observation. These simulations are a useful tool in determining the role of heavy ions, present from both the ionosphere of Ganymede and the corotational magnetosphere of Jupiter, in the tail region of Ganymede. Heavy ions are important not only for energy and momentum transport in the magnetosphere, but are also required to precipitate through the tenuous atmosphere and sputter the icy surface of Ganymede. It is shown that heavy ions from Ganymede's magnetosphere are accelerated via reconnection in the tail and enabled to precipitate down to the surface on the leading hemisphere (tail side). Incident plasma from Jupiter's magnetosphere is also heated and precipitates on both the flow facing (trailing) and tail (leading) hemispheres. Polar outflow of Ganymede's ionosphere was also studied and found to contribute significantly to the local population of Jupiter's magnetosphere.
SM33A-1251 1340h
Plasma sources and lifetimes within the Jovian magnetosphere
The Jovian magnetosphere is fed by plasma from Io, the icy Galilean moons and the Jovian ionosphere. The plasmas from these different sources not only differ in mass composition and outflow rate, but also have access to different regions. The mixing of these plasmas plays a critical role in determining the overall size and structure of the Jovian magnetosphere. 3-D multi-fluid simulations that incorporate the heavy and light ion interactions are used to model the mixing of heavy ion torus plasma with light ion outflow from the Jovian magnetosphere. It is shown that at radii smaller than the Io torus the plasma is completely co-rotational and that the main loss mechanism is to the Jovian ionosphere. Light ions continue to have essentially no departure from being fully co-rotational out to about 13 R$_J$, while the heavy torus ions show some slippage with lifetimes of the order of about 60 hrs. Between 13 $R_J$ and about 20 $R_J$ lifetimes of both light and heavy ions is of the order of about 100 hrs. These characteristics are most strongly dependent of the torus plasma density rather than the ionospheric outflow rate. Beyond this distance, deviations from co-rotation becoming an increasing larger effect and lifetimes decrease. The properties of the ionospheric outflows play an increasingly important role in this region. The co-rotation boundary and position of the magnetosphere are shown to move significantly under the convection of torus and ionospheric plasma from the inner magnetosphere into the outer magnetosphere.
SM33A-1252 1340h
Magnetic field disturbances in the Jovian magnetosphere due to large dynamic pressure enhancements in the solar wind
In order to understand the Jovian magnetospheric response to large enhancements of solar wind dynamic pressure, we investigate magnetic field disturbances observed by the Galileo spacecraft located inside the Jovian magnetosphere. A lack of solar wind monitoring just outside the Jovian magnetosphere is complemented by performing one-dimensional magnetohydrodynamic (MHD) simulation. Solar wind data obtained from the Wind and ACE spacecraft in the vicinity of the Earth are used as input parameters of the solar wind simulation. A verification test of simulated solar wind parameters is performed by comparing them with actual solar wind parameters obtained by the Ulysses spacecraft in the vicinity of the Jovian orbit. The estimated error of shock arrival time is <2 days if the acute angle of Jupiter-Sun-Earth is <50°. We select 9 events that meet a criterion of event selection: an increase of solar wind dynamic pressure >0.3 nPa in a few hours at the Jovian orbit. Characteristic changes of magnetic field disturbances are found for all of the 9 events. For 8 of the 9 events, the rectangular waveform with the rotation period of about 10 hours becomes irregular in response to the large dynamic pressure change. Furthermore, it is found that the amplitude of the turbulent magnetic field in the ultra-low frequency (ULF) range of 0.3 - 10 mHz increases simultaneously with and/or after irregular waveform changes, in proportion to the maximum amplitude of solar wind dynamic pressure enhancements.
SM33A-1253 1340h
Coupling the Io plasma torus to Jupiter's ionosphere: Cassini observations of longitudinal variations
The ionization of $\sim$1 ton/second of neutral material from Io's atmosphere produces a dense ($\sim$2000 cm$^{-3}$) plasma torus. Plasma coupling works to enforce corotation between the torus and Jupiter's magnetic field; however, significant corotational breakdown is found in the middle magnetosphere between 20-30 R$_J$ in the centrifugal equator plane and, interestingly, the breakdown point maps to Jupiter's main auroral oval. In addition, the torus is known to deviate from corotation by up to 3% near Io's orbit (6 R$_J$). Recent observations of the torus by the Cassini Ultraviolet Imaging Spectrograph (UVIS) have revealed significant azimuthal variations in ion composition along with roughly 1.4% deviation from corotation ($\sim$1 km/s at the orbit of Io). We find that the amplitude of the azimuthal compositional asymmetry varies with time. For the minor ion species of S II and S IV, the amplitude varies between 5-20%, while the major ion species of S III and O II remain relatively constant with amplitudes ranging between 2-5%. This amplitude of the azimuthal compositional asymmetry appears to be modulated by the magnetic longitude of the plasma, such that when the peak in the S II mixing ratio (and correspondingly, the minimum in S IV) is aligned with a System III longitude of $\sim$200$^\circ$, the amplitude is greatest. We are able to model this behavior with a longitudinal modulation of corotational lag and/or a hot electron source. We will discuss the implication of the Cassini observations with regard to magnetosphere-ionosphere coupling at Jupiter.
SM33A-1254 1340h
Spatially and Chemically Segregated Energization of Jupiter's Magnetosphere
The small-scale magnetic field fluctuations in Jupiter's middle magnetosphere undergo a turbulent cascade which transports the energy in the fluctuations to smaller length scales. This kind of turbulence is in essence a bath of weakly, but non-linearly interacting Alfven waves. The turbulent cascade is non-dissipative, but when the length scales of the fluctuations approach kinetic length scales their energy is converted into particle acceleration and heating. Key kinetic length scales for Alfvenic turbulence are the electron inertial length scale and the ion-acoustic gyroradius. We show in our presentation that the electron inertial length scale is the crucial dissipation length scale inside of 27 RJ (Jovian Radii) while the ion-acoustic gyroradius dominates outside of 27 RJ. The electron inertial length scale maximizes at the high latitudes, while the ion-acoustic gyroradius maximizes in the equatorial current sheet for the heaviest ions. This ordering implies that the Jovian magnetosphere is energized inside of 27 RJ predominantly at high latitudes where the lightest ions (i.e., protons) prevail. Outside of ~27 RJ, the heavier magnetospheric ions (i.e., sulfur) are energized in the equatorial current sheet. Observational support for this segregated energization picture comes from recent observations of energetic particles ($>$50 keV; Mauk et al. [2004]) which show that protons dominate the total energy flux inside of about 25 RJ and sulfur ions dominate the energy flux outside of ~25 RJ.