SM44A-01 INVITED
New Horizons Plasma Observations of Jupiter's Magnetotail to >2500 RJ
(*On behalf of the New Horizons Plasma Science Team.) Observations from the New Horizons spacecraft as it traversed down the jovian magnetotail to >2500 jovian radii (RJ) reveal a remarkable diversity of plasma populations and structures. At ~~100 RJ the ions evolved from a hot plasma disk distribution to slower flows down the tail that persisted and became increasingly variable in flux and mean energy. The plasma was highly structured with numerous sharp discontinuities, gradual boundaries, and apparent plasmoids, and contained a variable mixture of plasma from Io and Jupiter's ionosphere with intense bursts of ionospheric H+ and H3+. We observed 10 hour quasi-periodicities in flux far from the plasma disk at ~~450 RJ, and again at ~~1500 RJ, just inside the magnetopause and magnetopause boundary layer. We also found a 3-4 day quasi-periodicity in flow speed, most prominently from ~~600-1000 RJ, which may be indicative of large, expanding plasmoids moving down the tail. This talk summarizes the findings of McComas et al. [2007] and provides an update on the team's subsequent analysis efforts. McComas, D.J., F. Allegrini, F. Bagenal, F. Crary, R.W. Ebert, H. Elliott, A. Stern, and P. Valek, Diverse Plasma Populations and Structures in Jupiter's Magnetotail, Science, 2007.
SM44A-02 INVITED
Energetic Particles in the Jovian Magnetotail
New Horizons observed the unexplored, distant Jovian magnetotail to > 2500 RJ. A high-temperature, multi-species population of energetic particles is characteristic of regions closer to the planet. Velocity dispersions, anisotropies, and compositional variation seen in the deep-tail (> ~500 RJ) with a ~3- day periodicity suggest similar events that were observed at ~100 RJ in Galileo data. The signatures suggest plasma streaming away from the planet and injection sites in the near-tail region (~200 to 400 RJ) that could be related to magnetic reconnection events. The observed tail structure remains coherent at least as far as the first outbound crossing into the magnetosheath. Preliminary estimates suggest that this hot plasma population is sufficient to balance the pressure of the interplanetary magnetic field to this distance, maintaining the magnetotail structure.
SM44A-03
Characteristics of low energy ions observed by New Horizon/SWAP during Jupiter's mangetotail flyby
On February 28, 2007, the New Horizon (NH) spacecraft began traversing a path nearly straight down Jupiter's enormous magnetotail. Beginning on DOY 82 and throughout the period of time in which NH remained in the tail region, the spacecraft was spinning at a rate of 5 rotations per minute (RPM) with its high-gain antenna pointing toward Earth. A "herringbone" pattern of low energy ions was observed by the Solar Wind Around Pluto (SWAP) instrument once NH was spinning [McComas et al., 2007]. SWAP was designed to make coincidence measurements of ions from 35 eV/q to 7.5 keV/q with a 276° × 10° field-of-view. With calibration parameters provided by the SWAP instrument team, we are able to estimate the ion characteristics by constructing observed signatures from a 3D phase-space density model. Densities and speeds in the inner magnetosphere (< 100 RJ) were obtained to be consistent with previous missions. The dominant velocity component of plasmas is in the tailward direction. The observed ion thermal speeds are relatively low when compared to the bulk velocities indicating that Jupiter's magnetotail is filled with high Mach number plasmas. Low-energy ion moments (density, three components of velocity, and temperature) of selected periods from NH/SWAP will be presented for the first time during the flyby of Jupiter's magnetotail. McComas, D. J., F. Allegrini, F. Bagenal, F. Crary, R. W. Ebert, H. Elliott, A. Stern, and P. Valek, Jupiter's Magnetotail: Diverse Plasma Populations and Structures, Science, submitted, 2007.
SM44A-04
Velocity Dispersion and Spatial Structures in the Jovian Magnetotail
The unique trajectory of the New Horizons spacecraft almost directly down the center of Jupiter's magnetotail allowed continuous energetic particle and plasma measurements to 2600 Jovian radii (RJ)---over 15 times further than any other spacecraft. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument during this period detected clear evidence of at least eight instances of velocity dispersion of 30 keV--1 MeV ions from 880 to 1950 RJ down the tail. This is consistent with what would occur if all particle energies were simultaneously injected and allowed to propagate. One such event, beginning on day 118 of 2007 at 1350 RJ and lasting just over a day, showed superposed spatial and temporal structure, with dispersive signatures abruptly cutoff and new signatures appearing, apparently as the spacecraft moved back and forth between three separate regions. These regions appear to be well connected to a common injection site but are dominated by distinct dispersive events with onsets separated by hours. We will report on the dispersion events in the PEPSSI data, and, in the case of the day 118 event, estimate the size and motion of the spatial structures.
SM44A-05
Properties of Ions in Jupiter's Middle Magnetosphere
A survey of plasmas in Jupiter's middle magnetosphere and plasma torus reveals many properties of the thermal ions near the orbits of the Galilean moons. A surprising increase in the temperature of the ions is noted at a distance from the planet just slightly larger than the radius of Ganymede's orbit. This happens also to be at a location that may be conjugate to the main ring of auroral emissions. Thus, there are several plausible mechanisms for heating ions in that region, including various auroral phenomena, but also pickup from Ganymede. The heating is predicted to have important consequences for the electrodynamics in the auroral region. Observationally, this is known to be a region threaded by beams of keV electrons, and a causal connection is possible, though the beams are not an expected feature of most models of the aurora. The observations are considered, in part, in the context of their effects on the moons, and also of possible effects of the moons on plasma populations. These plasma parameters are derived from the low-rate survey data during Galileo's prime and extended missions, and they are now being readied for inclusion in NASA's Planetary Data System.
SM44A-06
Periodic Variations of Low Energy Ion Fluxes In Jupiter's Magnetotail
The New Horizons spacecraft traveled ~2000 Rj down Jupiter's magnetotail. The objective of this study is to quantify the periodicities present at several distances along the New Horizons trajectory through Jupiter's magnetotail. Initial examination of Solar Wind Around Pluto (SWAP) low energy ion measurements indicate distinct periodicities with periods close to 10 hours as well as longer periods of 3-4 days. Similar periodicities were observed with previous missions that explored within 200 Rj of the planet, and more recently by the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument, also on New Horizons in the 30keV – 1 MeV ion intensities. In this study we focus on shorter periods closer to 10 hours since those are more distinct and occur both close and far from the planet. Approximately 10 hour periodicities in the count rates were observed near 430 Rj and again near 1500Rj just prior to New Horizons entering the magnetosheath. In Jupiter's magnetosphere each set of SWAP measurements consist of two consecutive 32 second sweeps followed by a 256 sec gap. Since the SWAP measurements have uneven time spacing, using Lomb-Scargle periodograms is a more appropriate method for determining the spectrum than using a power spectrum derived from a Fast Fourier Transform (FFT). We will verify the Lomb-Scargle periodograms by determining the power spectrum with the FFT using only the first sweep of each set of observations since this provides the regular time spacing required for FFTs. We hope to gain insight into the physical implications by comparing intervals at different distances from Jupiter, and by comparing SWAP and PEPSSI measurements of the same time intervals.
SM44A-07
Magnetic Mapping of the Outer Jovian Magnetosphere Onto the Auroral Pattern.
We use a newly developped magnetic field model to map the outer Jovian magnetosphere down to the ionosphere. In addition to the usual terms representing the internal planetary field and the current disc contribution (both taken from the Connerney et al. (1998)'s VIP4 model), the model, derived from a terrestrial one, includes terms related to the magnetosphere/solar-wind interaction, and generated by magnetopause currents shielding the internal and current disc components, and by cross-tail currents and their closure currents on the magnetopause, as well as some partial penetration of the IMF. The distant magnetosphere region, from the magnetopause to the magnetodisc is mapped down along the magnetic field lines to the polar ionosphere, and compared to auroral features derived from UV images taken with the Hubble Space Telescope. We investigate the limit between corotating features and features fixed in a Sun-Jupiter magnetic frame of reference. We also check the mapping of polar cap boundary field lines against in-situ Ulysses measurements. The effect of the dynamical magnetosphere/solar-wind coupling on the polar auroral structure, and in particular on the main auroral oval and on the polar cap boundary is studied by varying the model input parameters.
SM44A-08
Multi-Fluid Simulations of the Jovian Magnetotail under Varying IMF Conditions
The New Horizons spacecraft observed large cohesive density structures at great lengths down the Jovian magnetotail. Multi-fluid simulations that separately track the solar wind protons and heavy ions from the Galilean moons are used to investigate potential processes that can lead to such enhanced density structures. There are different dynamical regimes to consider. First, the inner magnetosphere is strongly rotationally driven. In the outer magnetosphere, the system can be subjected to torques from changes in the interplanetary magnetic field (IMF). For example, the IMF at 5 AU is largely tangential and the dominant y-component of the IMF can lead to local time asymmetries of the plasma sheet. Rotation of this IMF component can produce additional stresses on the plasma sheet that can lead to modified plasma flows down the tail. These processes as seen through the densities and energization of different ion species from the inner magnetosphere into the distant tail are discussed.
SM44A-09
Rotational dynamics of the Jovian magnetosphere
We present results from a 3D global magnetohydrodynamic (MHD) simulation of the magnetosphere of Jupiter with the goal of understanding how features of the magnetosphere relate to either the presence of Iogenic mass, to the rotation of the magnetosphere, to solar wind variations or a combination of these drivers. Using the model we will perform idealized simulations that allow us to isolate the features of the magnetosphere and the drivers that affect them. In addition, we have recently conducted a series of simulations to look at the dawn-dusk asymmetries of the current sheet thickness as observed by Galileo. These studies indicate that Jupiter's magnetic field responds dynamically to the stretching in the tail and the quasi-periodic release of plasmoids down the tail. We will explore the simulation results as they relate to both magnetospheric configuration and dynamics.
SM44A-10
Topology of Plasma Flow: Intertwining of the Dungey and the Vasyliunas Cycles
The configuration of plasma flow in the magnetospheres and magnetotails of Jupiter and Saturn can be described in terms of two basic patterns: magnetospheric circulation imposed by solar wind flow past an open magnetosphere (the "Dungey cycle") and rotationally driven escape of plasma down the magnetotail (the "Vasyli\=unas cycle"). It is often assumed that return (sunward) flow of plasma in the magnetotail is an essential requirement of the Dungey cycle, and that therefore any observational or theoretical obstacles to return flow imply limitations to the cycle. In fact, the only requirement is return of the magnetic flux, not necessarily of the plasma. Both cycles can be described from a unified viewpoint as topological configurations that combine removal of the plasma with circulation of the magnetic flux. Under certain geometrical conditions, part of the return plasma flow from the nightside X-line of the Dungey cycle can proceed tailward and escape down the magnetotail as part of the plasma outflow in the Vasyli\=unas cycle.