SM53A-1077
New Horizons Exploration of the Jovian Magnetotail
A key issue of the jovian magnetosphere is how plasma is lost from the system down the magnetotail. Are large plasmoids ejected sporadically via explosive reconnection events (e.g. by analogy with Earth) or is there a steady "drizzle" of plasma from small scale disconnections of highly stretched out magnetic flux tubes? Furthermore, is there a significant solar-wind-driven Dungey cycle with a return, Jupiter-ward flow from a distant X-line? Again, by analogy with Earth, how much solar wind plasma enters the high latitude magnetopause mixes with magnetospheric plasma (from Io)? Voyager observations indicated burst of material flowing away from Jupiter between ~100-200 Rj on the dawn flank and showed that the magnetotail of Jupiter extends past the orbit of Saturn. Galileo measurements indicated periodic bursts of material being ejected tailward on the nightside as well as occasional supercorotational bursts on the in the morning plasma sheet. But very little is known about structure and processes on the dusk flank and at the critical distances of a few hundred Rj. Thus, the fortunate traversal down the magnetotail of the New Horizons spacecraft, on its way to Pluto, has provided a fantastic opportunity to address the above scientific questions.
SM53A-1078
Ion abundance ratios in the Jovian magnetotail
The New Horizons mission offered an exceptional opportunity to explore the distant Jovian magnetotail to over 2600 Jovian Radii. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) is a multi- directional time-of flight spectrometer measuring ions from ~15 keV to over 1 MeV, and electrons from 25 keV through ~0.5 MeV. Included in the measurements made by PEPSSI are proton, helium, oxygen, and sulfur observations. The ion abundance ratios offer distinct information on the sources and sinks in the Jovian system. Previous observations of ion abundances are available only out to ~ 150 Jovian Radii. During the 100+ day exploration of the magnetotail, PEPSSI encountered numerous spatial structures containing a high energetic particle number density. We examine the composition of these dynamic structures and compare the ion abundances with those found in more quiescent regions. We report on the composition ratios as a function of distance down the Jovian magnetotail.
SM53A-1079
Plasma Sheet Magnetic Field Fluctuations Characterized by Galileo at Jupiter
Spectral indices from power spectra of the magnetic field fluctuations observed within the Earth's plasma sheet appear to confirm the presence of turbulence in the magnetotail. Using Galileo magnetometer data mainly from Jupiter's magnetotail, we have determined spectral indices from power spectra within the frequency range of tenths of mHz to 10 mHz for approximately 100 plasma sheet intervals over a range of local times principally in the noon to midnight sector and at radial distances between 20 and 60 RJ. We find spectral indices centered near -2.3 for the radial and azimuthal components of the field, but typically closer to -2.0 for the north-south component. The most probable values do not appear to depend strongly on local time but local time coverage is spotty. When examined as a function of the radial distance, the spectral index decreases with radial distance from values near -1.4 to 2.0 in the range 20-30 RJ to values near -2.3 to -2.6 in the range 50-60 RJ.. The observed spectral indices are not those expected for homogenous fully developed turbulence (-5/3) or MHD turbulence (- 3/2). For some of our observations, the large negative indices may relate to fluctuations in the dissipative regime. We will discuss our observations in the context of the spectral indices and other derived values.
SM53A-1080
Numerical Simulations of the Jovian Magnetosphere: Influence of the Solar Wind
The Earth and the jovian magnetospheres are shaped by the solar wind, displaying gigantic magnetic tails on the night side. Due to the intrinsic variability of the solar wind, the global shape of the Earth magnetosphere fluctuates with time; the influence of the solar wind velocity, density, pressure and other characteristics were extensively studied and are now more or less understood. On the other hand, the interactions between the jovian magnetosphere and the solar wind are still poorly comprehended. The extremely strong internal magnetic field, the fast rotation and the presence of the Io torus (included in our model by the mean of a source term) are specific to Jupiter and should lead to a magnetosphere hardly comparable with the magnetosphere of the Earth. In order to study this phenomena, we performed three dimensional numerical simulations of the jovian magnetosphere in the framework of magnetohydrodynamics. We present here an extensive parameters study showing the response of the jovian magnetosphere to different solar wind parameters.
SM53A-1081
Dynamic Events in the Jovian Magnetotail: A Statistical Study From Magnetic Field Measurements
The Galileo mission to Jupiter provided magnetic field and energetic particle measurements of the Jovian plasma sheet in the near and distant tail. Dynamic events associated with reconnection have been observed in the Jovian magnetotail [ Russell et al., 1998]. These events are thought to imply the presence of a nearby neutral point. In the magnetic field data, they are characterized by an intensification of Bθ, the component of the magnetic field perpendicular to the equator. We have analyzed magnetometer data from Galileo using all intervals with time resolution of 24 s per vector or better in regions of the magnetotail between 1800 and 0600 local time and at radial distances beyond 30 RJ to identify dynamic events occurring in the Jovian magnetotail. Our criteria require that Bθ increase by at least a factor of 2 over background levels and concurrently a bend of field lines with respect to the radial direction change. Change of the bend-back angle indicates that rotational speed of the plasma is either increasing or decreasing to conserve angular momentum as plasma flows toward Jupiter or down the tail, respectively. Our analysis has identified nearly 1,000 dynamic events in 28 orbits including 7,500 hours of data. These events occur as far down the tail as 142 RJ. There is no evidence of a significant asymmetry in the frequency of events around midnight, though we do find that the frequency of the events decreases near midnight. A previous statistical study of particle bursts in the Jovian magnetotail [ Woch, Krupp, and Lagg, 2002], found that the events were concentrated in the dawn sector and noted the location of a statistical separatrix dividing predominantly inward and outward flow. Our results also suggest a similar contour separating the flow patterns.
SM53A-1082
Alfvenic Acceleration of Io Torus Electrons
The observed auroral spot at the Io footprint indicates that electron acceleration is occurring along the Io fluxtube. Su et al.[2006] has demonstrated using a gyro-fluid Alfven wave simulation that Alfven waves originating from the torus can partially propagate into the Jovian Magnetosphere. We will present a study showing where resonant electron acceleration due to the Alfvenic parallel electric field occurs. We will also show the dependence of this acceleration on local plasma parameters such as density and temperature. We will compare these results to acceleration studies in the Terrestrial magnetosphere. Su, Y.-J., S.T. Jones, R.E. Ergun, F. Bagenal, S. E. Parker, P. A. Delamere, and R. L Lysak, Io-Jupiter interaction: Alfven wave propagation and ionospheric Alfven resonator, J. Geophys. Res., 111, A06211, doi:10.1029/2005JA011252, 2006.
SM53A-1083
The Jovian Radiation-Belt Emission: Features and Time Variability.
We present new results that identify the role of various physical mechanisms in explaining the distribution of high energy electrons trapped in Jupiter's inner radiation belts. Our results indicate the inner radiation belts are linked to processes very close to the planet (such as the ring system and inner satellites) and more distant phenomena (such as that ongoing in the outer magnetosphere and magnetotail). Short term variations in the radiation belts may be a proxy to processes linked to variability in Jupiter's ring system (as observed by New Horizons), whereas long term variations may be linked to distant solar wind interactions with Jupiter. Using a newly modified model and comparing to the observed features in Jupiter's synchrotron emission, we investigate the long- and short-term variations, the brightness distribution, beaming curve and radio spectrum. Specific implications on the importance of phenomena taking place in the inner magnetosphere (interactions with moons and dust, synchrotron mechanism and particle transport) are reported. Mechanisms linked to solar activity and potentially responsible for temporal changes in the Jovian decimetric emission will also be discussed.
SM53A-1084
On the observation of ‘kronian' energetic events: combined observations of radio and magnetic signatures
In a recent paper [Louarn et al, 2007], we have reported on radio signatures observed at Saturn by the Cassini RPWS experiment which are strikingly similar to the jovian ‘energetic events' observed by Galileo [Louarn et al, 1998]. They consist of sudden intensifications of the auroral radio emission (SKR) followed by the detection of a periodic narrowband radiation which most likely originates from Saturn's plasma disk. At Jupiter, these radio signatures are associated with magnetic perturbations suggesting that dynamical processes develop in the magnetodisc, most certainly due to radial plasma transport. We examine whether comparable magnetic perturbations are observed in Saturn's system, in conjunction with radio ‘events'. From an initial analysis, it appears that such associated magnetic perturbations are systematically observed. Examples will be given and discussed for: (1) strong perturbations in the plasma disc/plasma sheet and (2) compression effects seen when CASSINI is in the flank of the magnetosphere.
SM53A-1085
Ion Flows in Saturn's Nightside Magnetosphere
Within Saturn's magnetosphere, plasma produced by the system of rings and moons continually loads the field lines. To avoid unlimited mass build-up, the release of plasma is predicted downtail either as a planetary wind or during substorm-like processes. The newly emptied flux tubes will then return to the dayside, depleted of plasma, to complete the circulation. The goal of the present study is to determine the equatorial flows in the night-side region to explore this scenario. Does the plasma appear to corotate? Are radial flows observed both outwards suggestive of tailward plasma release and inwards indicating the return of newly closed field-lines? If so, at what radial distances and local times are these observed? What plasma characteristics do these flows exhibit? Approximately 30 entries into the plasma sheet have been identified between October 2005 and April 2007 when Cassini was between 5 RS and 45 RS from Saturn, at local times between 5 to 20 LT, and close to the equatorial plane. This data set includes the preliminary events previously presented by Sittler et al., [2007a, b, c], where both outward and inward flows super-imposed on rotational motion were identified. For each plasmasheet encounter we employ data from the Cassini Plasma Spectrometer (CAPS) to determine preliminary ion flow directions and magnitude, as well as the ion and electron energy spectra and the ion composition. Our preliminary results find that the ion flows are generally in the corotation direction, but significantly lagging the corotation speed. There is also generally an outward (tailward) component to the derived velocities. Of the events surveyed, the IMS instrument was looking tailward (and therefore able to see inward flows) approximately 60 % of the time. However, inward flows (i.e. VR < 0) were only observed approximately 10 % of the time, demonstrating the rarity of such signatures in this region. Moreover, the ion composition persistently has a strong water-group component, rather than being relatively depleted of heavy ions as expected for recently emptied flux tubes returning to the inner magnetosphere. E. C. Sittler et al., Cassini Observations of Saturn's Dawn-Magnetotail Region: Preliminary Results, Magnetospheres of the Outer Planets (MOP), San Antonio, TX, June 25-29, 2007a. E. C. Sittler et al., Cassini Observations of Saturn's Dawn-Magnetotail Region: Preliminary Results, Europlanet, EPSC2007-A-00428, Potsdam, Germany, August 19-24, 2007b. E. C. Sittler et al., Cassini Observations of Saturn's Magnetotail Region: Preliminary results, Fall AGU, San Francisco, CA, December 10-14, 2007c.
SM53A-1086
On the Boundary Between a Proto-star Magnetosphere and its Protoplanetary Disk
We have investigated how the Magneto Rotational Instability (hereafter MRI) affects the structure of the boundary between a proto-star magnetosphere and its proto-planetary disk. 3D resistive MHD simulations of MRI in a proto-planetary disk, especially near its inner-edge that separates the magnetic field dominated proto-starfs magnetosphere and the gas dominated disk, have been conducted. Cylindrical co-ordinates have been used while the vertical structure has been neglected (cylindrical disk approximation). In our 2D simulations on the radial-vertical plane, we modeled the inner-edge by giving a strong magnetic field on the magnetospheric side where the MRI cannot grow, and the disk part is threaded by a weak magnetic field which makes the region to be MRI unstable. In the disk, with the radial variation of the angular velocity that decreases with distance from the central star, the MRI grows faster in the inner region than in the outer part. Then the MRI becomes most active in the inner part of the disk, less in the outer part, and none in the magnetosphere. This situation is also achieved by considering a magnetic resistivity that increases with distance. Putting a magnetic resistivity in the outer region representing the gdead zoneh gives the same spatial profile of MRI activity. In these situations where MRI is confined to the inner-part of the disk, we have found that the profile of the angular velocity is strongly changed to have a discontinuous jump at the inner-edge, which is both large and sharp enough to destabilize the Kelvin- Helmholtz Instability in r-phi plane. These results suggest that the disk inner-edge is heavily perturbed by the MRI and possibly by the secondarily induced KHI.
SM53A-1087
Modelling of Io-Jupiter decameter arcs, emission beaming and energy source
The electrodynamic interaction between Io and Jupiter is known to lead to accelerated electrons in/near the Io flux tube. These electrons produce intense radio emissions in the hecto-decameter range, with specific arc shapes in the time-frequency plane depending on the hemisphere of origin of the emission and on the Io-Jupiter- observer geometry. Assuming radio wave generation by the cyclotron-maser instability and a Jovian magnetic field model, we simulate tf arc shapes as a function of the radio emission beaming and of the lead angle between the radio emitting field line and the instantaneous Io field line. An excellent fit is obtained for loss-cone driven emission, obliquely beamed in a hollow cone at ~70 degrees from the source magnetic field within a ~1 degree thick beam. The radio beaming angle obtained from our simulations is found to be consistent with empirically fitted beaming angle function, assuming a supplementary refraction effect in the dense regions of the Io flux tube. The lead angle giving the best fit is ~30 degrees in both hemispheres, not fully consistent with the propagation of the perturbation generated by Io via Alfven waves. Simulation of future Juno radio observations are briefly discussed.
SM53A-1088
Cassini Observations of Saturn's Magnetotail Region: Preliminary Results
Using Cassini thermal plasma, hot plasma and magnetic field observations for several intervals between the dawn meridian of Saturn's outer magnetosphere and Saturn's magnetotail region, we investigate the structure of the magnetotail, plasma and magnetic field properties within tail-like current sheet regions and ion flows within the magnetotail regions. We use Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS), Electron Plasma Spectrometer (ELS) observations, MIMI LEMMS ion and electron observations and Cassini magnetometer data (MAG) to characterize the plasma environment. IMS observations are used to measure plasma flow velocities from which one can infer rotation versus convective flows. IMS composition measurements are used to trace the source of plasma from the inner magnetosphere (protons, H2+ and water group ions) versus an external solar wind source (protons and He++ ions). A critical parameter for both models is the strength of the convection electric field with respect to the rotational electric field for the large scale magnetosphere. For example, are there significant return flows (i.e., negative radial velocities, VR < 0) and/or plasmoids (VR > 0) within the magnetotail region? Initial preliminary evidence of such out flows and return flows was presented by Sittler et al. [2007a,b]. This talk complements the more global analysis by McAndrews et al. [2007].
SM53A-1089
On the Physics of the Interaction of a Rotating Magnetic Field with a Magnetized Plasma
The interaction of Rotating Magnetic Fields (RMF) with plasmas is a fundamental plasma physics problem with implications to fusion related Field-Reversed Configurations (FRC), space propulsion, astronaut protection from cosmic rays in long interstellar travel, control of energetic population in the radiation belts and near zone processes in pulsar magnetospheres. An important but not yet explored application of RMF is as an efficient radiation source of MHD and whistler waves in space plasmas. Despite its importance the basic plasma physics understanding of the interaction of rotating magnetic fields with magneto-plasmas, the scaling laws that control it and the range of potential applications to space plasmas remains unexplored. To zero order a magnetic field rotating at a rate w in a plasma drives plasma currents due to the difference in mass between electrons and ions. The electrons quickly come to a co-rotation with RMF, generating a differential azimuthal current whose maximum is given by Jtheta = nwr . The RMF can be generated either by a pair of polyphase coils, superconducting or else, or a rotating permanent magnet. Key questions include the depth of penetration of the field in the plasma, the spatiotemporal structure of the induced magnetic field as a function of the RMF and plasma parameters and the spatial decay rate magnetic field. Flux conservation arguments indicate that the induced field will decay slower than 1/r**2 and 2D simulation studies indicate that depending on the plasma beta it falls as 1/r**n with n smaller than 1.5. Our preliminary simulations indicate that penetration lengths exceeding 20-30 collisionless skin depths can be reached. The paper will present a combination of analytic/computational results along with preliminary experiments conducted using the Large Plasma Device (LAPD) located at UCLA that emphasize the RMF properties for generating MHD and whistler waves. This work was sponsored by ONR MURI Grant 5-28828
SM53A-1090
Effect of Field-Aligned Potentials on Angular Momentum Transfer at Jupiter
We present a time-independent model of Jupiter's rotationally driven aurora based on angular momentum conservation including the effects of a field-aligned potential and a variable Pedersen conductivity. We modify our existing model to include the momentum transfer within Jupiter's neutral atmosphere. The field-aligned potential arises from field-aligned current limitation at high-latitude and changes the mapping of the electric fields between the ionosphere and the magnetosphere. The primary effect of the field-aligned potential is to actually enhance coupling by increasing current flow from the ionosphere to the magnetosphere. The net result is an equatorial mapping location for the main auroral oval at ~25 RJ. Our model reproduces many of the observed characteristics of Jupiter's main auroral oval including the energy flux into the ionosphere (~2 to ~30 mW/m2), the width of the aurora at the ionosphere (~1000 km), and field-aligned potentials consistent with the observed electron energies (~30 to ~200 keV). We also investigate the effect of the magnetic field geometry on the total radial current produced and present preliminary results for Saturn.
SM53A-1091
Terrestrial AKR Beaming: Implications for Jovian DAM Beaming Models
Recent studies of terrestrial auroral kilometric radiation (AKR) angular beaming patterns using the WBD instrument on the Cluster spacecraft array demonstrate that emission from individual bursts is confined to a narrow solid angle within a plane containing the magnetic field vector at the source and tangent to the auroral oval. Assuming that the electron cyclotron maser instability (CMI) mechanism is driven by a shell distribution, the propagation direction at the source must be nearly perpendicular (k||~0). Since the angular beaming is observed at low tangent plane longitudes, the rays are must be refracted upward within the auroral cavity, typically θr >45o. We apply these results to models of Jovian decametric emission (DAM) which is also assumed to be generated by the CMI mechanism above the auroral zone. If the terrestrial and Jovian mechanisms and source environments are indeed similar, then Jovian DAM models invoking loss-cone electron distributions to explain the DAM angular power pattern are likely not correct. We suggest that a shell distribution is driving the DAM emission and that refraction causes the observing beaming angles.