SM41B-1653
The Dependence of Saturn's Magnetosphere and Polar Ionosphere on Solar Wind Dynamic Pressure and IMF Direction
The configuration and dynamics of the Earth's magnetosphere are largely controlled by the solar wind while Jupiter's magnetosphere is dominated by its massive rotating equatorial current sheet and plasma source at Io. Saturn has a rapid rotation equivalent to Jupiter but its magnetic field strength is comparable to that of the Earth. Thus Saturn is often called the planet whose characteristics are a mixture of those at Jupiter and the Earth. In a recent simulation study of the configuration of Saturn's magnetosphere for no interplanetary magnetic field (IMF), southward IMF and northward IMF we found that the Kronian magnetosphere always has vortices and turbulent convection which result from the interaction between the solar wind, corotation and magnetospheric convection. However the detailed configuration of the energy flux and field aligned currents (FACs) in the polar region are a function of IMF direction. In particular, the simulation results indicate that the vorticity causes intense FACs which are strongest near dawn. These results suggest that the Kronian magnetosphere can be quite different from both Jovian and terrestrial magnetospheres. In this study we have examined the effects of solar wind dynamic pressure and IMF BY component on Saturn's magnetosphere. We found that high dynamic pressures did little to change the Kronian convection for northward IMF while for no/southward IMF the convection patterns in the magnetosphere were changed as compared to the recent our study. The magnitudes of the energy flux and FACs are enhanced but the distributions are not changed. For the cases including IMF By component, the configurations of the magnetosphere and polar ionosphere are a mixture of those with no IMF and northward IMF. In this talk we will present these simulation results in detail and discuss the relation of the magnetospheric dynamics (in particular the convection) to the polar dynamics.
SM41B-1654
Simulations and Observations of Vortices Near Saturn's Dawn Magnetopause
We have used a global magnetohydrodynamic simulation of the interaction of the solar wind with Saturn's magnetosphere and Cassini observations to investigate vorticity in the morning magnetosphere. We find that large vortices form for northward IMF in a region near the morning magnetopause with large velocity shear. The vortices are associated with strong field aligned currents. The vortices do not form in simulations with southward IMF or low velocity shear. For our initial simulations we assumed a source of plasma from the moon Enceladus of 2X1029 AMU/s. This is near the upper limit on the Enceladus plasma source. In this study we will present results from simulations which span the entire range of estimated Enceladus sources. In the simulations the boundary vortices cause magnetic field oscillations in the Kronian magnetosphere. The oscillations have periods of one to three hours. We have examined Cassini magnetic field observations1 in the morning and find similar oscillations. In this talk we will present the results from a statistical study of the observed oscillations and a comparison of their properties with the simulation results. 1 Daugherty, M. K., S. Kellock, A. P. Slootweg, N. Achilles, S. P. Joy and J. N. Mafi, CASSINI MAG CALIBRATED SUMMARY AVERAGED, C-E/SWS/J/S-MAG-4-SUMM-AVERAGED-V1.0, NASA Planetary Data System, 2007.
SM41B-1655
The Saturnian Ring Current as Revealed Through Combined Plasma, Energetic Particle and Magnetic Field Measurements. Inertial or Pressure Gradient Driven?
Combined data from the MAG, MIMI and CAPS instruments onboard Cassini are used to study the driving mechanism and physical characteristics of the ring current at Saturn. The Saturnian plasma environment is adequately sampled in the eV to MeV energy range with MIMI/CHEMS and LEMMS, and CAPS/IMS and ELS sensors. All available equatorial plane passes of Cassini are considered separately between radial distances of 6 and 17 RS, covering September 2005 to July 2006 (Rev 14 to 26) and June to September 2007 (Rev 46 to 49). Anti-symmetric local time sectors are compared where possible and an analysis of the observed orbit-to-orbit variations is also undertaken. Magnetic field modelling and a statistical approach are also employed to address each of the radial force balance equation terms and determine how their contributions to the total current vary with radial distance. Initial results suggest that the azimuthal current density attributed to the plasma pressure gradient reaches its maximum contribution to the total current density in the region 9-10 RS and appears comparable to the inertial current density term being of order of 10-11 A/m2.
SM41B-1656
Planetary period oscillations in Saturn's magnetosphere: Phase relation of equatorial magnetic field oscillations and Saturn kilometric radiation modulation
We examine the planetary-period oscillations in Saturn's magnetic field observed by the Cassini spacecraft on 23 near-equatorial periapsis passes in the inner magnetosphere spanning October 2004 to July 2006. Overall, we find that the phase of the magnetic oscillations is well organized by the long-timescale modulation phase of Saturn kilometric radiation (SKR) determined over the same interval by Kurth et al.~(2007), suggesting that the slow period variation of the latter relates to inner magnetosphere processes. The relative phases of the oscillations in the spherical polar r and φ magnetic field components imply the presence of a quasi-uniform equatorial field rotating near the SKR period, while the sense of the θ component indicates that the perturbation field lines form loops with apices in the Northern Hemisphere. No consistent evidence is found for a sign reversal in any field component across the equatorial plane, within ±20° in latitude. The relative SKR phasing is such that the peak radio power occurs when the r and θ component maxima lie at ~0200 LT~±~2 hours. However, a slow drift of the magnetic phase relative to the SKR phase is also discerned, amounting to~~75° over the study interval. This drift lies within the envelope of scatter in the SKR phase determinations, suggesting that it represents the refinement of a common periodicity. A revised magnetic phase or longitude model is derived that should form an improved organizational system for oscillatory phenomena observed during this interval of the Cassini mission. The magnetic oscillations are also found to exhibit pass-to-pass phase "jitter" about the long-term variation, of RMS amplitude~~20°, with r and φ strongly correlated, but not θ. The relation with the solar wind-modulated short-timescale phase variations reported in SKR data by Zarka et al.~(2007) remains to be investigated, though the latter are 5 times larger in magnitude.
SM41B-1657
Relationship Between Saturn Kilometric Radiation Emissions and Kronian Magnetotail Activity, as Compared with the Terrestrial Case
Several examples of plasmoid passage associated with substorm-like reconnection in the magnetotail of Saturn have been discovered using data from the Cassini magnetometer [Jackman et al., 2008]. Some of these events (along with several others) have subsequently been found to be roughly associated with bursts of Saturn Kilometric Radiation (SKR) emission. We explore this link in detail, with particular emphasis on expansions of the radio emission to lower frequencies, signifying motion of the source along the field lines. We compare with observations of Auroral Kilometric Radiation (AKR) at Earth, which has been shown in some cases to have a separate low-frequency component associated with substorm onset [Morioka et al., 2007] We also wish to estimate the recurrence rate of kronian substorms. Previously for the case of the terrestrial magnetosphere, it has been found that the substorm phenomenon is identified with a component of the probability distribution of durations for which the AU or AL indices are above or below a fixed threshold, respectively [Freeman, Watkins and Riley, 2000]. After exploring the correlation between terrestrial substorms and AKR (which can be a reasonable proxy for the AE indices), we then apply the threshold crossing technique to Saturn by using data from the Cassini Radio and Plasma Wave Science (RPWS) instrument. We look at the probability distributions of Saturn's radio emissions in different frequency bands above and below fixed thresholds, with a view to revealing kronian substorms as a distinct separate population with a characteristic scale. References: Freeman, M.P., N.W. Watkins and D.J. Riley, (2000), Evidence for a solar wind origin of the power law burst lifetime distribution of the AE indices, Geophys. Res. Lett., 27, 8, 1087-1090. Jackman, C. M., C. S. Arridge, N. Krupp, E. J. Bunce, D. G. Mitchell, W. S. Kurth, H. J. McAndrews, M. K. Dougherty, C. T. Russell, N. Achilleos, A. J. Coates, and G. H. Jones (2008), A multi-instrument view of tail reconnection at Saturn, J. Geophys. Res., doi:10.1029/2008JA013592, in press. Morioka, A., Y. Miyoshi, F. Tsuchiya, H. Misawa, T. Sakanoi, K. Yumoto, R.R. Anderson, J.D. Menietti, and E.F. Donovan, (2007), Dual structure of auroral acceleration regions at substorm onsets as derived from auroral kilometric radiation spectra, J. Geophys. Res., 112, doi:10.1029/2006JA012186.
SM41B-1658
Modelling of the Saturnian Kilometric Radiation (SKR)
The Saturnian Kilometric Radiation (SKR), discovered by the Voyager spacecraft in the 1980's, is observed quasi-continuously by Cassini since 2003. Study of 3 years of SKR observations by RPWS (Radio and Plasma Wave Science) revealed three recurrent features of SKR dynamic spectra : (i) discrete arcs, presumably caused by the anisotropy of the radio emission pattern combined to the observer's motion, (ii) an equatorial shadow zone around the planet (observed near perikrones) and (iii) signal extinctions at high northern latitudes. We model these features using the code PRES (Planetary Radio Emission Simulator) that assumes radio emissions to be generated via the Cyclotron Maser Instability for simulating observed dynamic spectra. We show that observed arc-like structures imply radio sources in partial (~90%) corotation, located on magnetic field lines of invariant latitude 70° to 75°, and emitting at oblique angle from the local magnetic field with a cone angle that varies with frequency. Then, based on the previously demonstrated conjugacy between UV and SKR sources, we successfully model the equatorial shadow zone as well as northern latitude SKR extinctions assuming time variable radio sources distributed along field lines with footprints along the daily UV oval measured from HST images.
SM41B-1659
Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra
Ultraviolet spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been compared to synthetic spectra of electron-excited H2 in order to derive various auroral characteristics, such as the energy of the primary precipitating electrons and the H2 temperature at the altitude of the aurora. Two physical processes have been exploited: the absorption by hydrocarbons in the FUV and H2 self-absorption in the EUV. We find energies in the range 10-18 keV, which locates Saturns's aurora between 0.1 and 0.3 μ bar. We also determined that the auroral H2 emission is characterized by a temperature of ~400K, consistent with temperatures measured in the infrared, but much higher than what is expected from equatorial atmospheric models. These new results bring valuable constraints on both polar atmospheric models and theoretical studies of the ionosphere-magnetosphere coupling.
SM41B-1660
The Degree of Correlation of Jovian and Saturnian Auroral Emissions With Solar Wind Conditions
While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere
with the solar wind, auroral emissions on Jupiter and Saturn are thought to be driven primarily by internal
processes, with the main energy source being the planets' rapid rotation. Limited evidence has suggested
there might be some influence of the solar wind on Jupiter's aurorae, and indicated that auroral storms on
Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of
auroral processes on solar wind conditions, a large campaign of observations of these planets has been
undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft
and solar wind conditions both propagated from one AU and measured near each planet. The data indicate a
consistent brightening of both the auroral emissions and Saturn Kilometric Radiation (SKR) at Saturn close
in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical
relationship between Saturnian auroral processes and solar wind conditions. This correlation has been
strengthened by the final campaign observations in Feb. 2008. At Jupiter the situation is less clear, with
increases in total auroral power seen near the arrival of solar wind forward shocks, while little increase has
been observed near reverse shocks. In addition, auroral dawn storms have been observed when there was
little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian
auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data
set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of
Jupiter and Saturn.
http://www.bu.edu/csp/PASS/main.html
SM41B-1661
The variation of different components of Jupiter's auroral emission
In this paper we use the extensive HST data set obtained over two month-long campaigns in 2007 to determine the long term variability of the different components of Jupiter's auroras. We define three regions on the planet's disc, i.e. the main oval, the low latitude, and high latitude auroras, and extract the UV auroral power emitted therefrom. The high latitude region was also further divided into the polar inner and polar outer regions. We discuss the temporal variation of these parameters with reference to the auroral morphology and estimated solar wind conditions projected to Jupiter's orbit from data obtained at Earth orbit. We show that the auroral morphology was very different between the first and second campaigns. In the first campaign the emitted power originated mainly from the main oval and the high latitude regions, which roughly correlated, and exhibited enhancements that are suggested to be associated with compression regions. In the second campaign the high latitude and main oval auroras were generally dimmer overall and less variable, while the low latitude region was populated with bright, patchy emission. We show that a particular auroral morphology is probably associated specifically with compression regions, i.e. over longitudes greater than approx. 180 degrees the main oval is bright and located approx. 1 degree poleward of its previous location, while over smaller longitudes the main oval is not bright or well defined. Instead there is bright emission originating from the contiguous poleward region in the afternoon/dusk sector where bright, sometimes multiple arcs form. It remains unclear, however, whether this state is a response to the initial shock or some other event within the rapidly-varying compression regions. We also show that the dawn storm events, typically associated with intense dawn side main oval auroras also result in the brightening of the high latitude auroras, even to the very highest latitude components, which presumably map to a very different region of the magnetosphere. However, apart from the dawn storms and bright poleward arcs in the afternoon/dusk sector, the power emitted from the poleward auroras is generally uncorrelated with that of the main oval.
SM41B-1662
Long-term Brightness Variations of the Io UV Footprint
Since the finding of the UV Io footprint in 1996, the successive UV instruments on board the Hubble Space Telescope (HST) allowed us to considerably improve the understanding of the Io-Jupiter electro-magnetic interaction and its auroral counterpart. It has been shown that the Io footprint is generally formed by one bright spot preceded or followed by secondary spots whose relative positions are linked to the location of Io in the plasma torus. We also know that these spots experience brightness variations from minutes to hours. The Io footprint brightness varies over hours with the longitude of Io in the Jovian magnetic field (System III longitude) but until recently, huge gaps existed in the longitude coverage. Part of these gaps has now been filled during the latest HST imaging campaign and a more complete spot brightness versus Io System III longitude diagram emerges. Additionally, we compare spot brightness between images obtained a few minutes apart but from opposite hemispheres. Based on images gathered from 1997 to 2007 with the STIS and the ACS cameras, we also show that the footprint morphology and the spots brightness, including their relative brightness, can vary significantly from one year to another. Finally, we discuss the brightness variations from hours to years in terms of plasma torus density and position of Io in the plasma torus as well as in Jovian magnetic field.
SM41B-1663
AUTOMATIC SEGMENTATION OF JUPITER'S AURORA ON HST IMAGES
The aurora on Jupiter (as well as Saturn) can be studied with a high sensitivity and resolution by HST UV imaging and spectroscopy. We present preliminary results of automatic detection and segmentation of Jupiter's auroral emissions as observed by HST. We use a dynamic algorithm for partitioning the underlying pixel grid of an image into regions according to a prescribed homogeneity criterion. The algorithm consists of an iterative procedure that dynamically constructs a Voronoi Diagram or Voronoi tessellation, until the intensity of the underlying image within each region is classified as homogeneous. The computed tessellations allow the extraction of quantitative information about the auroral features such as mean intensity, latitudinal and longitudinal extents. We comment on the possibility of performing a statistical survey of the macroscopic spatial behaviour of the aurora and looking for scale-invariant characteristics, a property of dynamic systems known as self organised criticality.
SM41B-1664
FUSE Observations of Jovian Aurora at the Time of the New Horizons Flyby
At the time of the New Horizons flyby of Jupiter on 28 February 2007, there was a five-day window of opportunity during which the Far Ultraviolet Spectroscopic Explorer (FUSE), despite the loss of three of its four reaction wheels, could be stably pointed at Jupiter's position on the sky. FUSE was an orbiting spectroscopic observatory capable of spectral resolution better than 0.4~Å for extended sources in the wavelength range 905--1187~Å, together with very high sensitivity to weak emissions. Three orbits of observations were obtained in a point-and-stare mode beginning at 16:50 UT on 02 March 2007 of which for the first two the FUSE 30" × 30" aperture was centered on the north polar aurora. During each orbit the count rate was constant with time indicating that the target remained fully in the aperture during the entire exposure. These spectra will be compared with those obtained by FUSE in October 2000, December 2000, and January 2001, in terms of FUV luminosity and derived H2 vibrational population. We will also place these data in the context of the ultraviolet images obtained by the Hubble Space Telescope one Jovian rotation before and one after the FUSE observations.
SM41B-1665
The Aurora and Magnetic Field of Uranus
Resolution of the details of a planetary magnetic field from magnetometer measurements made during a single flyby can be limited by the incomplete geometrical sampling of the trajectory. This problem was especially severe for the only spacecraft visit to Uranus, that of Voyager 2 in 1986. Fortunately, auroral footprints serve as additional constraints that may be used to determine the higher multipole moments of the planetary field. This approach has been used by Connerney (JGR 103:11,929, 1998) to improve the magnetic field model of Jupiter. In the present work, this approach is applied to improving the resolution of the magnetic field of Uranus. The earlier determination of Uranus' auroral emission distribution (Herbert JGR 99:4143, 1994) from scans by the Voyager 2 Ultraviolet Spectrometer (UVS) has been improved by incorporating more observations and using more powerful analysis techniques. However, the resulting new estimate of the locus of the auroral oval does not match the expectations derived from the Voyager magnetometer (Connerney et al., JGR 92:15,329, 1987, Q3 model). Accordingly, a search has been initiated for planetary magnetic field model coefficients that agree both with the new auroral locus and also with the magnetic field observations. This search is more ambiguous than that at Jupiter, because the source of the aurora is not clearly defined, but a reasonable starting assumption is that it lies at constant L shell (maximum field-line distance from Uranus). Based on this assumption, preliminary results confirm the Q3 model quadrupole moment's large magnitude but disagree slightly with its orientation. Further analysis will be presented at the meeting. Support from the NASA Outer Planets Program made this work possible, and is gratefully acknowledged. Part of this work was done while a guest investigator at l'Institut d'Astrophysique de Paris (IAP/CNRS), whose hospitality is appreciated.
SM41B-1666
Auroral X-ray Emission at Jupiter
Auroral emissions from Jupiter have been observed across the electromagnetic spectrum including ultraviolet and X-ray emissions. X-ray emissions with a total power of about 1GW were observed by the Einstein Observatory, the Roentgen satellite (ROSAT), Chandra X-ray Observatory (CXO), and XMM-Newton. Previous studies (Cravens et al. 1995, Kharchenko et al. 1998, 2006, and Liu and Schultz 1999) have shown that precipitating energetic sulfur and oxygen ions can produce the observed X-rays. Sulfur and oxygen ions in the outer magnetosphere are presumably accelerated by field-aligned potentials before they precipitate into the atmosphere (Cravens et al. 2003). Collisions with atmospheric neutrals remove most of the incident ions' orbital electrons and subsequent charge transfer collisions produce X-rays. This study theoretically models the ion precipitation using cross sections for these charge exchange processes and an empirical stopping power. X-ray luminosities are determined for several monoenergetic beams. Our model also shows the altitude dependence of the X-ray photons produced and accounts for the change in observed X-ray power due to opacity effects.
SM41B-1667
Observations of the Jovian X-ray Emission by Chandra and XMM Newton
Three different kinds of aurorae are generally seen in UV and near-IR observations of Jupiter: (1) Aurorae at the satellite footprints (2) the main oval and (3) transient signatures at higher latitudes called polar emissions. Jovian x-rays are primarily associated with the polar emissions, and are particularly intense in a "hot spot" region near 60-70 degrees north latitude and 160-180 degrees system III longitude. The auroral x- rays are primarily due to line emission from precipitating energetic O and S ions (300-800 eV photons), with a small fraction due to electron bremsstrahlung (> 2000 eV photons). Here we present new x-ray observations obtained simultaneously by Chandra and XMM Newton at the end of February 2007, during the New Horizons Jupiter flyby. These observations confirm the presence of O and S emissions in separate bands around 500-700 eV and 200-400 eV respectively, with only minor spatial and temporal variations in the brightness ratio between the two bands. These observations thus indicate a rather steady O/S ratio in the magnetospheric source region of the x-ray aurora (thought to be near the dayside magnetopause).
SM41B-1668
The Relationship of Jupiter's Non-Io-DAM Emissions and Aurora Phenomena
It is widely believed that non-Io-DAM is auroral radio emission from the Jovian aurora. We have analyzed non-Io-DAM occurrence dependence from 4 MHz to 16 MHz based on the System III central meridian longitude (CML) and calculated the occurrence probability for each frequency by using the Cassini RPWS data during Cassini's Jupiter flyby. As a result of this analysis, the two peaks of non-Io-B and non-Io-A occurrence probability showed a dramatic change in longitude between 9 MHz and 16 MHz. At 16 MHz two peaks of probability occurred at 160° and 240° CML. As the frequency decreases to 9 MHz, the two peaks converged to become one peak near 205° CML at 9 MHz. This peak gradually disappeared below 9 MHz. Based on Jupiter's magnetic VIP4 model, an angular beaming model was made to explain these observational results by taking into account the decreasing cone half-angle of the emitting cone from 16 MHz down to 9 MHz. By using our model we found the active magnetic flux tubes of non-Io-B and non-Io-A sources are localized at about 180° ± 10° of System III longitude projected on Jupiter's surface. Our results are close to the longitudinal values of previous observations of UV aurora phenomena. The close relationship between non-Io-DAM emission and the UV aurora can be considered.
SM41B-1669
Statistical studies of non-Io Jupiter DAM emissions observed at Nançay during solar cycle 23 (1995-2001)
Non-Io (or Io-independent) Jupiter decameter emissions observed by the Nançay decametric array during the rising phase of solar cycle 23 (1995-2001) are studied. About 700 emissions have been recorded during that period. From the statistical studies, several conclusions were obtained. It was observed that most of emissions (~91%) are Right-Hand elliptically polarized and from the so-called "A" source (i.e. northern hemisphere and for Jovian longitude of observer in the range ~200° to 320°). The relative contributions to emissions is (in decreasing order) from sources "A", "B" (northern, 100°-200°), C (southern, 300°-400°), and D (southern, 0°-80°). Considering all emissions, the average duration is ~22 min, the average maximum frequency is ~24 MHz and the average power is ~2.4 dB above the sky background. RH emissions reach higher frequencies than LH (Left-Hand) ones. Their duration and average power are similar, but peak values are higher for RH emissions. The number of emissions roughly follows the sunspot cycle. In relation to the solar cycle variation, it was observed that Jupiter radio emissions have on the average a longer duration during the rising phase. The maximum frequency of emissions is similar on the average for all phases, but extreme values (maximum) are seen during rising and solar maximum epochs. The emissions power is higher during the rising and maximum phases (similar in both) than during solar minimum.
SM41B-1670
Modeling the particle acceleration near the Io FootPrint and the associated radio emissions
The Jovian S-bursts are bursty decameter emissions related to the Io-Jupiter interaction. A typical S-bursts is an emission with a duration of some tens of millisecond which presents a characteristic drift in frequency. It can be related to the presence of electron beams in adiabatic motion above the Io FootPrint. However an S-burst storm presents many discreet structures, which repeat with a quasi-period of few Hertz and whose shape can sensibly differ from the most typical one. We show that these characteristics reveal the dynamical processes in the vicinity of the Io FootPrint, such as acceleration by Alfvén waves, electric potential drops, ion and electron holes... Simulations of the emissions related to the presence of all these acceleration processes will be presented and compared to observations.
SM41B-1671
State of the art in the Jovian radio arc modelization
The Jovian radio arcs are decameter emissions related to the Io-Jupiter interaction. Their arc shape in the time frequency plane is known to be due to geometrical visibility effect on sources spread along an "active" magnetic field line. The state of the art in their modelization will be presented, including some recent results which permit to deduce from their shape the position of the active field line relative to the Io field line and the emitting particle energy. This modeling is permitted by a numerical code, which was also used to model the Cassini observation of Saturn. The interest on such a modelization of the future observations of JUNO will be discussed.
SM41B-1672
The energy of electron beams near the Io footprint derived from radio-astronomy observations.
The energy of Jupiter-Io auroral electrons can be inferred through the analysis of decametric radio emissions, especially that of millisecond bursts. Jovian millisecond (or S-)bursts are intense impulsive decametric radio bursts drifting in frequency in tens of milliseconds. Previous analyses suggest that S-bursts are cyclotron-maser emission in the flux tubes connecting Io or Io's wake to Jupiter. Electrons are thought to be accelerated from Io to Jupiter. Near Jupiter, a loss cone appears in the magnetically mirrored electron population, which is able to amplify extraordinary (X) mode radio waves. Most of the theories about their origin include an interpretation of their frequency drift. We have analysed two sets of data collected at the DAM Nançay and Karkhov radio-telescopes; they are consistent with the above scenario. In addition, we confirm that the frequency drift df/dt(f) is negative on average and decreases in absolute value at high frequencies, consistent with the adiabatic theory of particles motion. We find a typical energy of 4 keV for the emitting electrons. In many cases, we find evidence of localized ~ 1 kV electric potential jumps at high latitudes along magnetic field lines connecting Io or Io's wake to Jupiter. These potential jumps are stable over tens of minutes and propagate upward with a velocity of the order of the sound velocity. We will discuss the consistency of such ~ 4 keV electron populations with other energy estimates derived from observations of the UV Jupiter-Io auroras.
SM41B-1673
A Searchlight Beam Model of Jupiter's Decametric Radio Emissions
It has long been recognized that there is a marked long-term periodic variation in Jupiter's integrated radio
occurrence probability. The period of the variation is on the order of a decade. Carr et al. [1970] showed that
such variations are much more closely correlated with Jovicentric declination of the Earth (De). The range of
the smoothed variation of De is from approximately +3.3 to -3.3 degrees. This De effect was extensively
studied and confirmed by Garcia [1996]. It shows that the occurrence probability of the non-Io-A source is
clearly controlled by De at 18, 20, and 22 MHz during the 1957-1994 apparitions. We propose a new model
to explain the De effect. This new model shows that the beam structure of Jupiter radio emissions, which has
been thought of like a hollow-cone, has a narrow beam like a searchlight, which can be explained by
assuming that the three dimensional shape of the radio source expands along the line of the magnetic field.
Various computer graphics illustrate the concept of this searchlight beam model.
http://jupiter.kochi-ct.jp/cg/
SM41B-1674
Polarization measurements of Jovian radio emissions at high-magnetic latitudes observed by Ulysses spacecraft
In early 2004, the Unified Radio And Plasma wave (URAP) experiment onboard Ulysses measured the Stokes parameters of Jovian kilometric radiations at northern high-latitudes during the gdistant encounterh flyby. Ulysses observed from more than +80 deg to less than +10 deg of Jovicentric latitude for several months. The observation indicated that quasi-periodic (QP) bursts and narrowband kilometric emissions (nKOM) have LH polarization (V~+1). Therefore, we conclude that these emissions are LO mode waves. This is consistent with several previous observations at low-latitudes (~0 deg) and mid-latitudes (~-40 deg) (e.g., Daigne and Leblanc, 1986; MacDowall et al., 1993). On the other hand, it was shown that broadband kilometric radiations (bKOM) have RH polarization (V~-1) at high-latitudes. This result does not agree with our previous study based on the combination of observed location of bKOM and ray tracing, which showed these emissions as LO mode waves (Kimura et al., 2008). We confirmed by additional ray tracing analyses that a solution can allow LO mode waves to be observed with RH polarization at high-latitudes. This result would constrain generation and propagation processes for Jovian radio components observed at high-latitudes.
SM41B-1675
Effect of Field-Aligned Potentials on Magnetospheric Dynamics at Jupiter and Saturn
We present a time-independent model of Jupiter's rotationally driven aurora based on torque balance between the ionosphere and magnetosphere, including the effects of a field-aligned potential and a variable Pedersen conductivity. The field-aligned potential arises from field-aligned current limitation at high-latitudes and changes the mapping of the electric fields between the ionosphere and the magnetosphere. We apply a Vlasov study to determine the location and extent of the field-aligned potential drop along the flux tube and to derive the relation between voltage and parallel current. The primary effect of the field-aligned potential is to enhance coupling by increasing angular momentum transfer 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 from 2 mW/m2 to 30 mW/m2, the width of the aurora at the ionosphere (1000 km) and field-aligned potentials consistent with observed electron energies from 30 keV to 200 keV. We apply our model to Saturn and find that the magnetospheric plasma at Saturn may make a significant contribution to Saturn's main auroral oval. Our model at Saturn reproduces the angular velocity profiles determined from Cassini and Voyager data.
SM41B-1676
Resolving Jupiter's Aurora with the Juno Suite of Magnetospheric Instruments
The Juno mission, a Jupiter polar orbiting program that begins its 1-year mission at Jupiter in 2016, resolves critical questions regarding the processes that drive Jupiter's dramatic aurora and tests the universality of the processes that generate aurora and couple magnetized planets to their space environments. Jupiter's magnetosphere is known to be powered very differently than is Earth's, specifically by strong planetary rotation rather than by the interplanetary environment controlled by the solar wind. And yet, we see hints that some of the fundamental processes that drive Jupiter's aurora are the same as those that drive Earth's aurora. Is the global aurora regulated by a global pattern of electric currents that connects Jupiter's magnetosphere to its polar ionosphere along magnetic field lines? Does particle acceleration occur as a result of the scarcity of field-aligned current carriers, and where does the acceleration occur? Are the dominant acceleration processes the same as those that dominate at Earth or do other processes prevail? How do the auroral processes that generate emissions modify the energy and momentum coupling between the magnetosphere and ionosphere? Juno addresses and answers these and other critical questions by flying a capable suite of in situ particles and fields magnetospheric instruments through Jupiter's low-altitude polar regions, and combining those measurements with UV and IR imaging of the polar auroral displays. Here we review the critical issues and questions regarding Jupiter's aurora that Juno addresses, and show how they are resolve by the Juno mission and instruments.
SM41B-1677
The Juno Jovian Auroral Distributions Experiment (JADE)
The Jovian Auroral Distributions Experiment (JADE) is a part of the suite of instruments carried on Juno to directly explore, for the first time, Jupiter's polar magnetosphere in general and auroral processes, in particular. JADE comprises a single-head ion mass spectrometer (JADE-I) and three identical electron energy per charge (E/q) analyzers (JADE-E) to measure the full auroral electron and ion particle distributions. Together these three-dimensional distributions 1) determine the particle populations that precipitate into the auroral regions and produce the observed auroral emissions; 2) measure the leakage of Jovian ions out along the field; and 3) address the acceleration processes at work in the regions both around and below the spacecraft. This paper briefly summarizes the JADE scientific objectives, instrumentation, and capabilities.
SM41B-1678
The Ultraviolet Spectrograph (UVS) on Juno
Juno, a NASA New Frontiers mission, plans for launch in August 2011, a 5-year cruise (including a flyby of Earth in October 2013 for a gravity boost), and 14 months around Jupiter after arriving in August 2016. The spinning (2 RPM), solar-powered Juno will study Jupiter from a highly elliptical orbit, in which the spacecraft (for about 6 hours once every 11 days) dives down over the north pole, skims the outermost atmosphere, and rises back up over the south pole. This orbit allows Juno avoid most of the intense particle radiation surrounding the planet and provides an excellent platform for investigating Jupiter's polar magnetosphere. Part of the exploration of Jupiter's polar magnetosphere will involve remote sensing of the far-ultraviolet H and H2 auroral emissions, plus gases such as methane and acetylene which add their absorption signature to the H2 emissions. This hydrocarbon absorption can be used to estimate the energy of the precipitating electrons; since more energetic electrons penetrate deeper into the atmosphere and the UV emissions they produce will show more absorption. Juno will carry an Ultraviolet Spectrograph (UVS) to make spectral images of Jupiter's aurora. UVS is a UV imaging spectrograph sensitive to both extreme and far ultraviolet emissions in the 70-205~nm range that will characterize the morphology and spectral nature of Jupiter's auroral emissions. Juno UVS consists of two separate sections: a dedicated telescope/spectrograph assembly and a vault electronics box. The telescope/spectrograph assembly contains a telescope which feeds a 0.15-m Rowland circle spectrograph. The telescope has an input aperture 40×40~mm2 and uses an off-axis parabolic primary mirror. A flat scan mirror situated at the front end of the telescope (used to target specific auroral features at up to ±30° perpendicular to the Juno spin plane) directs incoming light to the primary. The light is then focused onto the spectrograph entrance slit, which has a 'dog- bone' shape 6° long, in three 2° sections of 0.2°, 0.05°, and 0.2° width (projected onto the sky). Light entering the slit is dispersed by a toroidal grating which focuses the UV bandpass onto a curved microchannel plate (MCP) cross delay line (XDL) detector with a solar blind UV- sensitive CsI photocathode, which makes up the instrument's focal plane. Tantalum shielding surrounds the detector assembly to protect the detector and the adjacent detector electronics from high-energy electrons. The main electronics box is located in the Juno vault. Inside are two redundant high-voltage power supplies (HVPS), two redundant low-voltage power supplies, the command and data handling (C&DH) electronics, heater/actuator activation electronics, scan mirror electronics, and event processing electronics. An overview of the UVS design and scientific performance will be presented.
SM41B-1679
The Juno Magnetic Field Investigation (MAG): Exploration of the Polar Magnetosphere
The Juno MAG investigation is part of a fields and particles instrumentation package that will explore Jupiter's polar magnetosphere and aurorae. The Juno mission design provides 32 high inclination orbits equally spaced in longitude; the Juno spacecraft will repeatedly pass through Jupiter's auroral curtains over both poles. The MAG investigation is implemented with a pair of highly accurate vector fluxgate magnetometers mounted on a dedicated magnetometer boom at 10 and 12 m from the spacecraft body. The MAG boom is itself a 4 m rigid appendage supported at the outer extremity of one of three solar array assemblies that radiate from the body of the spacecraft. Each vector fluxgate sensor is mounted on an optical bench in close proximity with two precision non-magnetic star cameras provided by the Danish Technical University (John Lief Jorgensen) which allow accurate attitude determination at each of the MAG sensors. Both MAG sensors are sampled at a native rate of 64 vector samples/s throughout periapsis, and averaged and decimated elsewhere throughout the orbit to reduce demand on s/c telemetry. The MAG investigation will map out the distribution of field aligned currents in the polar regions, and contribute to understanding of Jovian aurorae, magnetosphere-ionosphere coupling, and angular momentum transfer in the Jovian system.
SM41B-1680
A Wave Investigation for the Juno Mission to Jupiter
A primary objective of the Juno mission is to explore the polar magnetosphere of Jupiter. While Ulysses briefly attained latitudes of ~48 degrees, this was at relatively large distances from Jupiter (~8.6 RJ). Hence, the polar magnetosphere of Jupiter is largely uncharted territory and, in particular, the auroral acceleration region has never been visited. Juno's mission includes ~30 orbits of Jupiter with inclination of 90 degrees and a perijove between the atmosphere and the rings at low latitudes. The auroral suite of instruments on Juno includes a Waves investigation designed to explore the plasma wave and radio wave emissions associated with Jupiter's polar magnetosphere, and, especially, the auroral acceleration region. The Juno Waves instrument utilizes a short electric dipole antenna and a body-mounted search coil magnetometer that will enable an initial reconnaissance of the plasma waves important in accelerating auroral charged particles and identifying in situ the sources of hectometric and decametric radio emissions. The instrument will survey the magnetic component of waves from 50 Hz to 20 kHz and will cover the range from 50 Hz to above 40 MHz for the electric component. During the 6 hours centered on perijove, the instrument will collect complete electric and magnetic spectra with at a cadence of one spectrum per second. The instrument also has a burst capability that can be triggered to collect waveforms from 50 Hz to 150 kHz and a 1 MHz band including the electron cyclotron frequency. Also, the new viewing geometry of the various radio emissions coupled with modeling [e.g. Hess et al., J. Geophys. Res., in press, 2008] should allow the identification of radio source locations and extent far better than achieved up to now.
SM41B-1681
Modeling Magnetic Field Topology at Jupiter with the Khurana Magnetic Field Model
To explore the degree of coupling between the interplanetary magnetic field (IMF) and Jupiter's magnetosphere, we traced magnetic field lines from the polar region of the planet using the Khurana [1997, 2005] magnetic field model. We used a parameterized definition of the Jovian magnetopause created by Joy et al. [2002] that varies with the value of the solar wind dynamic pressure. We searched for field lines that cross the magnetopause and that potentially connect to the interplanetary magnetic field. We further explored the variation on magnetic field structure with local time orientation of Jupiter's dipole (i.e. Central Meridian Longitude) as well as upstream solar wind and IMF conditions.
SM41B-1682
The Juno Synchrotron Emission Investigation
Juno's close polar orbit is ideal for studying Jupiter's polar magnetosphere. Although Juno's orbit is designed
to avoid Jupiter's intense radiation belts, Juno carries a suite of microwave radiometers that are capable of
remotely sensing the synchrotron emission originating from the relativistic electrons trapped close to Jupiter.
The microwave radiometry investigation (MWR) is primarily focused on investigating the dynamics and
composition of Jupiter's deep atmosphere, however, the experiment will in fact provide a detailed
investigation of Jupiter's synchrotron emission. The MWR suite is comprised of six radiometers operating at
a range of wavelengths between one and fifty centimeters. As with the radio telescope observations from the
Earth, the radiometers are sensitive to the relativistic electrons emitting synchrotron emission as they gyrate
in Jupiter's strong magnetic field. The proximity of the spacecraft to Jupiter allows for a clear separation of
atmospheric and synchrotron emission, something not possible using Earth based radio telescopes. During
the 6 hours centered on perijove, the MWR instrumnet will collect complete observations of Jupiter's
synchrotron emission from a variety of vantage points including above the poles looking down, and from
within the radiation belts looking out. Because the synchrotron emission is a directional phenomena
dependent on both the orientation of the magnetic field and the pitch angle of the emitting electron, Juno will
provide an unprecedented investigation into the energy, L-shell, and pitch angle distribution of the high
energy electrons making up Jupiter's radiation belts.
http://www.juno.wisc.edu
SM41B-1683
JEDI -- The Jupiter Energetic Particle Detector for the Juno mission
The Juno mission provides the first opportunity to conduct an in-depth exploration of Jupiter's polar magnetosphere. The high-inclination, low-periapsis orbit provides in-situ access to three critical regions: the auroral magnetic field lines, the equatorial magnetosphere, and the polar ionosphere. The Jupiter Energetic Particle Detector Instrument (JEDI) is one of several Juno magnetospheric instruments that will work together to resolve critical scientific questions about these novel environments, most importantly how Jupiter's dramatic aurora is generated. JEDI measures the energy, spectra, mass species (H, He, O, S), and angular distributions of the higher energy charged particles that: 1) are accelerated at low altitude by Jovian auroral processes, 2) precipitate into Jupiter's upper atmosphere, 3) heat and ionize the Jovian upper atmosphere, and 4) populate Jupiter's inner magnetosphere. JEDI is a compact, light-weight, time-of-flight (TOF) spectrometer that makes 3-parameter TOF and energy ion measurements, 2-parameter TOF-only ion measurements, and single parameter electron measurements in the 10-keV to 10-MeV ion and the 25-keV to 1-MeV electron energy range. The rapid spacecraft motion and slow spacecraft rotation requires that JEDI simultaneously and continuously resolve both magnetic loss cones at every position inside of ~3RJ. To achieve these measurements JEDI uses multiple sensors, each with six angular sectors evenly distributed in a 160° x 12° fan. Through these multiple views JEDI continuously samples within a 360° plane roughly normal to the spacecraft spin axis with full-sky coplanar coverage achieved each spacecraft spin. JEDI with its low resource requirements and rad-hard, high-speed electronics will make the demanding scientific observations required by the Juno mission.
SM41B-1684
Optimization of neutral atom imagers
Abstract: The interactions between plasma structures and neutral atom populations in interplanetary space can be effectively studied with energetic neutral atom imagers. For neutral atoms with energies less than 1 keV, the most efficient detection method that preserves direction and energy information is conversion to negative ions on surfaces. We have examined a variety of surface materials and conversion geometries in order to identify the factors that determine conversion efficiency. For chemically and physically stable surfaces smoothness is of primary importance while properties such as work function have no obvious correlation to conversion efficiency. For the noble metals, tungsten, silicon, and graphite with comparable smoothness, conversion efficiency varies by a factor of two to three. We have also examined the way in which surface conversion efficiency varies with the angle of incidence of the neutral atom and have found that the highest efficiencies are obtained at angles of incidence greater then 80°. The conversion efficiency of silicon, tungsten and graphite were examined most closely and the energy dependent variation of conversion efficiency measured over a range of incident angles. We have also developed methods for micromachining silicon in order to reduce the volume to surface area over that of a single flat surface and have been able to reduce volume to surface area ratios by up to a factor of 60. With smooth micro-machined surfaces of the optimum geometry, conversion efficiencies can be increased by an order of magnitude over instruments like LENA on the IMAGE spacecraft without increase the instruments mass or volume. Work was supported by grant ACT-05-40 from the ESTO office of NASA