Longitude System Research Articles

Energetic Electron Patterns in the New SLS5 Longitude System

AbstractEnergetic electron (110–300 keV) fluxes collected during the entire 13‐year Cassini mission are organized in the new Saturn longitude system (SLS5). This system is based on Saturn radio emissions, varies slowly in time and has a north and a south branch. When the fluxes are organized in SLS5 longitudes and magnetodisk coordinates, they display an m=1 wave pattern that is quasi‐linear in longitude‐range maps. These spiral patterns are easily recognized in energetic electrons and have positive slopes a few degrees of longitude per RS (1 RS =60,268 km), similar to that observed in both particles and magnetic fields earlier in the mission. The patterns are most prominent at elevations +2 RS (SLS5 north) or −2 RS (SLS5 south) relative to the magnetodisk center. The results indicate the persistence of this pattern for essentially half a Saturnian year, suggesting that this pattern is a quasi‐permanent feature of the magnetosphere, much the same as its plasma sheet or magnetopause.

An SLS5 Longitude System Based on the Rotational Modulation of Saturn Radio Emissions

AbstractDespite the axisymmetry of Saturn's internal field, Saturn radio emissions like Saturn kilometric radiation (SKR) are modulated due to planetary rotation. With the completion of Cassini mission in September 2017, we now have around 14 years of observation of Saturn radio emissions, roughly from southern solstice to northern solstice. In this study, we extend the SLS4 longitude system to the end of the Cassini mission using a phase tracing method. The new Saturn longitude system (SLS5) organizes the observed SKR maxima around 0° subsolar longitude in both northern and southern hemispheres and can be used to organize other phenomena observed in Saturn's magnetosphere, for example, hot plasma injection events. SKR is modulated like a clock when the main source on the morning side is visible. To convert the observed phase to the clock phase, the phase of the morning side source, we also define a second longitude system SLS5*, which takes the spacecraft position into account based on a simple visibility model.

The aurorae of Uranus past equinox

AbstractThe aurorae of Uranus were recently detected in the far ultraviolet with the Hubble Space Telescope (HST) providing a new, so far unique, means to remotely study the asymmetric Uranian magnetosphere from Earth. We analyze here two new HST Uranus campaigns executed in September 2012 and November 2014 with different temporal coverage and under variable solar wind conditions numerically predicted by three different MHD codes. Overall, the HST images taken with the Space Telescope Imaging Spectrograph reveal auroral emissions in three pairs of successive images (one pair acquired in 2012 and two in 2014), hence 6 additional auroral detections in total, including the most intense Uranian aurorae ever seen with HST. The detected emissions occur close the expected arrival of interplanetary shocks. They appear as extended spots at southern latitudes, rotating with the planet. They radiate 5–24 kR and 1.3‐8.8 GW of ultraviolet emission from H2, last for tens of minutes and vary on timescales down to a few seconds. Fitting the 2014 observations with model auroral ovals constrains the longitude of the southern (northern) magnetic pole to 104 ± 26° (284 ± 26°) in the Uranian Longitude System. We suggest that the Uranian near‐equinoctial aurorae are pulsed cusp emissions possibly triggered by large‐scale magnetospheric compressions.

Evidence of m = 1 density mode (plasma cam) in Saturn's rotating magnetosphere

AbstractCassini field and plasma data measured in the rotating Saturn Longitude System 3 (SLS3) coordinate system show positive evidence of structure whose dominant azimuthal wave number is m = 1: a long‐lived, nonaxisymmetric, cam‐shaped, global plasma distribution in Saturn's magnetosphere. Previous studies have identified evidence of this plasma cam in wave‐derived electron density data and in Cassini Plasma Spectrometer (CAPS) W+ ion counts data. In this paper we report the first comprehensive analysis of CAPS ion moments data to identify the m = 1 density cam. We employ a multiyear, multispecies database of 685,678 CAPS density values, binned into a 1 RS by 4.8° discretized grid, spanning 4–19 RS. Fourier (harmonic) analysis shows that at most radial distances the dominant azimuthal mode is m = 1, for both W+ and H+ ion distributions. The majority (63%) of m = 1 ion peaks are clustered in an SLS3 quadrant centered at 330°. The plasma cam's existence has important implications for the global interchange‐driven convection cycle and is a clue to solving the mystery of the rotational periodicities in Saturn's magnetosphere.

Seasonal evolution of Saturn’s polar temperatures and composition

The seasonal evolution of Saturn’s polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7–16μm thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60° from northern winter/southern summer (2004, Ls=293°) through the equinox (2009, Ls=0°) to northern spring/southern autumn (2014, Ls=56°). The hot tropospheric polar cyclones that are entrained by prograde jets within 2–3° of each pole, and the hexagonal shape of the north polar belt, are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by ≈30° longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. Tropospheric temperature contrasts between the cool polar zones (near 80–85°) and warm polar belts (near 75–80°) have varied in both hemispheres, resulting in changes to the vertical windshear on the zonal jets in the upper troposphere and lower stratosphere. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75°S (by approximately −5K/yr), coinciding with a depletion in the abundances of acetylene (0.030±0.005ppm/yr) and ethane (0.35±0.1ppm/yr), and suggestive of stratospheric upwelling with vertical wind speeds of w≈+0.1mm/s. The upwelling appears most intense within 5° latitude of the south pole. This is mirrored by a general warming of the northern polar stratosphere (+5K/yr) and an enhancement in acetylene (0.030±0.003ppm/yr) and ethane (0.45±0.1ppm/yr) abundances that appears to be most intense poleward of 75°N, suggesting subsidence atw≈-0.15mm/s. However, the sharp gradient in stratospheric emission expected to form near 75°N by northern summer solstice (2017, Ls=90°) has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. The peak stratospheric warming in the north occurs at lower pressure levels (p<1mbar) than the peak stratospheric cooling in the south (p>1mbar). Vertical motions are derived from both the temperature field (using the measured rates of temperature change and the deviations from the expectations of radiative equilibrium models) and hydrocarbon distributions (solving the continuity equation). Vertical velocities tend towards zero in the upper troposphere where seasonal temperature contrasts are smaller, except within the tropospheric polar cyclones where w≈±0.02mm/s. North polar minima in tropospheric and stratospheric temperatures were detected in 2008–2010 (lagging one season, or 6–8years, behind winter solstice); south polar maxima appear to have occurred before the start of the Cassini observations (1–2years after summer solstice), consistent with the expectations of radiative climate models. The influence of dynamics implies that the coldest winter temperatures occur in the 75–80° region in the stratosphere, and in the cool polar zones in the troposphere, rather than at the poles themselves. In addition to vertical motions, we propose that the UV-absorbent polar stratospheric aerosols entrained within Saturn’s vortices contribute significantly to the radiative budget at the poles, adding to the localised enhancement in the south polar cooling and north polar warming poleward of ±75°.

Good outcomes of percutaneous fixation of spinal fractures in ankylosing spinal disorders

IntroductionThe ankylosed spine is prone to trauma even with after application of force at low energy levels. Multi-level vertebral bony fusions produce long lever arms, susceptible to fracture, with an increased risk of neurological injury. Additional problems result from delayed presentation and osteoporosis. These patients are also often at high risk of complications, making conventional open spinal surgery less appealing. We present the outcomes of percutaneous fixation and its advantages in this high risk group of patients. MethodsA retrospective review of a series of 10 patients with a diagnosis of either ankylosing spondylitis or diffuse idiopathic skeletal hyperostosis (DISH). All patients had sustained a spinal fracture between January 2009 and January 2013 and underwent percutaneous fixation using Medtronic longitude system (Minneapolis, USA) with Polyaxial screws. All were followed up with outcomes, complications and functional scores (Oswestry Disability Index (ODI) and Pain Visual Analogue scores (VAS). ResultsThe mean patient age was 68. There was a delayed presentation in seven patients, of which two presented with neurological compromise. The neurological deficit did not change with surgery and there were no neurological complications as a result of surgery. The mean length of stay was 24 days, with no direct surgical complications. The mean drop in haemoglobin level was 2.1, with three patients requiring a blood transfusion. The patients were followed up to a mean of 22 months, with a mean ODI of 16 and pain VAS of 1.1. At the time of follow up, two patients had died with no loss to follow up. DiscussionEven minor trauma can result in fracture in the ankylosed spine, requiring a high index of suspicion from the physician. The risks of missing such a fracture are significant neurological injury. The biomechanics of the spine are significantly altered, and treatment is demanding. We propose that minimally invasive spinal surgery can achieve good outcomes, low complication rates and high rates of satisfaction.

Longitude dependences of Saturn's ultraviolet aurora

AbstractBased on periodicities in the kilometric radio emissions, the Saturn Longitude System 4 (SLS4) was used to organize the far ultraviolet (120–150 nm) aurora observed by the Ultraviolet Imaging Spectrograph on the Cassini spacecraft. Individual Ultraviolet Imaging Spectrograph pixels were projected onto the ionosphere of Saturn, transformed into the SLS4 north and SLS4 south longitude systems, accumulated over all over auroral observations from 2007 to early 2009, and binned into 1°×1° bins of colatitude. The intensity of the northern aurora showed little variation in its SLS4 north system, but the intensity of the southern aurora exhibited an enhancement of over ~10 kR between ~140–280° SLS4 south longitude. This enhancement may represent the auroral signature of a southern ionospheric vortex proposed in MHD models of Saturn's magnetosphere to explain its periodicities. The loci of the northern intensity peaks and the 3 kR boundaries varied little over 360° of longitude, while the equatorward boundary of the southern aurora varied by ~5° in SLS4 south longitude, reaching its most equatorward location of ~23° colatitude between 100° and 180° longitude. The polygonal centroids of the aurora in both north and south were consistent with offsets of no more than ~1° in both hemispheres.

Percutaneous transvertebral fixation of thoracolumbar fractures with monoaxial and polyaxial pedicle screws

Objective To evaluate effect of percutaneous transvertebral fixation of thoracolumbar fractures using both monoaxial and polyaxial pedicle screws.Methods The study involved single-level thoracolumbar fracture patients without neurological deficits undergone minimally invasive percutaneous fixation with CD-Horizon Longitude system from January 2010 to December 2011.Three level vertebras were immobilized by six pedicle screws.They were randomized into two groups:control group fixed with polyaxial screws for the fractured vertebra as well as the upper and lower adjacent vertebras and study group fixed with monoaxial screws for the superior adjacent vertebra and polyaxial screws for the injured vertebra and inferior adjacent vertebra.General information,height of fracture vertebral body,Gardner segmental kyphotic deformity,GSKD) and local kyphosis angle were assessed and compared between groups after operation so as to estimate correction ability of kyphosis deformity.Visual analogue scale (VAS) and Oswestry disability index (ODI) were used to evaluate alteration of back pain and lower limb function.Incidence of screw breakage was also compared between groups.Results Totally,30 patients were enrolled in the trial and received an average follow-up of 10.5 months (range,6-18 months).Two groups were similar in general information (sex,age,fracture levels,and fracture types) and perioperative information (operation time,intraoperative blood loss,proportion of antalgic supplement and hospital stay).GSKD and local kyphosis angle were significantly higher in study group than in control group after operation [(15.0 ± 5.4) °vs (8.8 ± 5.4) °,(16.0-6.5) ° vs (9.0 ± 5.5) ° respectively,P ＜ 0.01].While,correction loss of GSKD and local kyphosis was similar between study and control groups [(2.6 ±1.3)°vs (1.9 ± 1.5)°,(3.7 ±2.1)° vs (2.7 ±2.2)° respectively,P ＞0.05].Correction of anterior vertebral body height and correction loss of posterior vertebral body height were superior in study group to control group [(33.0 ± 11.4) ％ vs (22.7 ± 11.4) ％,(1.2 ± 1.2) ％ vs (2.5 ± 2.1) ％ respectively,P ＜ 0.05].Correction loss of anterior vertebral body height and correction of posterior vertebral body height were similar between study and control groups [(2.8 ± 2.7) ％ vs (5.4 ± 4.7) ％,(5.0 ±4.8) ％ vs (3.1 ± 2.9) ％ respectively,P ＞ 0.05].Besides,there were no significant differences of VAS and ODI between the two groups in the same period after operation.Conclusion Combined use of monoaxial and polyaxial pedicle screws in minimally invasive percutaneous fixation improves kyphosis deformity and correction ability of anterior height and decreases correction loss of posterior height of the fractured vertebral body. Key words: Spinal fractures; Thoracic vertebrae; Lumbar vertebrae; Fracture fixation,internal; Surgical procedures,minimally invasive

Ordering of injection events within Saturnian SLS longitude and local time

AbstractPeriodicities similar to the Saturn kilometric radio (SKR) emissions have been observed throughout the magnetosphere in both the magnetic field and the plasma. An outstanding question is what mechanism links these periodicities between the inner and outer magnetospheres. It had been postulated that the interchange instability, where narrow injections return the magnetic flux carried by the bulk plasma outflow, could play a role in determining the periodicities, but early analysis found no ordering of the injection events in the SKR‐derived Saturnian longitude system (SLS). In this study, we reexamine this possibility by limiting our data set to the young injection events observed by the Cassini radio and plasma wave science instrument. We find that the young injection events observed near midnight local time are strongly ordered by SLS. Further, this ordering varies with the Saturnian season. Pre‐equinox, the northern hemisphere's longitude system controls the event occurrence. Post‐equinox, the events are ordered by the southern hemisphere‐derived longitude system. We suggest that this may be an effect in the variations in ionospheric conductivity or due to change in the magnetosphere's orientation relative to the solar wind.

Magnetic field topology of the RS CVn star II Pegasi

The dynamo processes in cool active stars generate complex magnetic fields responsible for prominent surface stellar activity and variability at different time scales. For a small number of cool stars magnetic field topologies were reconstructed from the time series of spectropolarimetric observations using the Zeeman Doppler imaging (ZDI) method. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg. We collected high-resolution circular polarisation observations of II Peg using the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover 12 epochs spread over 7 years. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Magnetic inversions using these data reveals evolving magnetic fields with typical local strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent magnetic and temperature mapping technique, we do not find a clear correlation between magnetic and temperature features in the ZDI maps. Neither do we confirm the presence of persistent azimuthal field rings found in other RS CVn stars. Reconstruction of the magnetic field topology of II Peg reveals significant evolution of both the surface magnetic field structure and the extended magnetospheric field geometry. From 2004 to 2010 the total field energy drastically declined and the field became less axisymmetric. This also coincided with the transition from predominantly poloidal to mainly toroidal field topology. A qualitative comparison of the ZDI maps of II Peg with the prediction of dynamo theory suggests that the magnetic field in this star is produced mainly by the turbulent alpha^2 dynamo rather than the solar alphaOmega dynamo. Our results do not show a clear active longitude system, nor is there an evidence of the presence of an azimuthal dynamo wave.

Os objetivos da circunavegação da U.S. Exploring Expedition (1838-1842): longitude, mapeamento náutico e instituição das coordenadas geográficas modernas

This article analyzes the main objectives of the scientific voyage to circumnavigate the earth, undertaken by the United States from 1838 to 1842. Charting was one of the most important of the scientific and strategic goals of the exploratory voyage. The initiative for the undertaking was the search for exact positioning on the high seas after the establishment of the longitude system, when nautical charts and maps from various countries were compared, and other, new ones were drawn. The United States participated in this international effort, leading to the creation of its own cartographic system.

Stardust-NExT, Deep Impact, and the accelerating spin of 9P/Tempel 1

The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13 year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 ± 0.025°/dy (41.335 ± 0.005 h) prior to the 2000 perihelion passage, 210.448 ± 0.016°/dy (41.055 ± 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 ± 0.030°/dy (40.783 ± 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 ± 0.012°/dy (40.827 ± 0.002 h) in the interval 2006–2010 following the 2005 perihelion passage. The period decreased by 16.8 ± 0.3 min during the 2000 passage and by 13.7 ± 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 ± 0.003°/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations. The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4–15) pole and longitude system. The possibility of a 180° error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42. We find that a net torque in the range of 0.3–2.5 × 10 7 kg m 2 s −2 acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than ∼1° per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H 2O out-gassing from the surface is largely responsible for the observed spin-up.

Longitude dependences of energetic H+ and O+ at Saturn

The charge‐energy‐mass spectrometer instrument in the Cassini spacecraft measured differential fluxes of protons (2.8–236 keV) and oxygen ions (8.8–236 keV) from July 2004 to August 2007. The fluxes were bin‐averaged in Saturn longitude system (SLS) longitude within ±5 RS of the equator and between 8 and 12 RS in radial distance (1 RS = 60,238 km) to determine their global morphology. The 3‐year time period is the range of validity of the SLS, which is based on a variable period of Saturn kilometric radiation. Fluxes at all energies of H+ and O+ display an essentially sinusoidal variation in longitude, often with peak‐to‐trough ratios of 2:1. For E &lt; 77 keV, the maxima are consistently at ∼70° longitude (the minima are at ∼230°). For E &gt; 77 keV; the maxima shift to ∼250°. The ion distributions closely resemble those of energetic neutral hydrogen and neutral oxygen atoms observed by the ion neutral camera onboard Cassini.

Physical properties of (2) Pallas

Ground-based high angular-resolution images of asteroid (2) Pallas at near-infrared wavelengths have been used to determine its physical properties (shape, dimensions, spatial orientation and albedo distribution). We acquired and analyzed adaptive optics (AO) J/H/K-band observations from Keck II and the Very Large Telescope taken during four Pallas oppositions between 2003 and 2007, with spatial resolution spanning 32–88 km (image scales 13–20 km/pixel). We improve our determination of the size, shape, and pole by a novel method that combines our AO data with 51 visual light-curves spanning 34 years of observations as well as archived occultation data. The shape model of Pallas derived here reproduces well both the projected shape of Pallas on the sky (average deviation of edge profile of 0.4 pixel) and light-curve behavior (average deviation of 0.019 mag) at all the epochs considered. We resolved the pole ambiguity and found the spin-vector coordinates to be within 5° of [longitude, latitude] = [30°, −16°] in the Ecliptic J2000.0 reference frame, indicating a high obliquity of about 84°, leading to high seasonal contrast. The best triaxial-ellipsoid fit returns ellipsoidal radii of a = 275 km, b = 258 km , and c = 238 km . From the mass of Pallas determined by gravitational perturbation on other minor bodies ( 1.2 ± 0.3 ) × 10 - 10 M ⊙ , [Michalak, G., 2000. Astron. Astrophys. 360, 363–374], we derive a density of 3.4 ± 0.9 g cm - 3 significantly different from the density of C-type (1) Ceres of 2.2 ± 0.1 g cm - 3 [Carry, B., Dumas, C., Fulchignoni, M., Merline, W.J., Berthier, J., Hestroffer, D., Fusco, T., Tamblyn, P., 2008. Astron. Astrophys. 478 (4), 235–244]. Considering the spectral similarities of Pallas and Ceres at visible and near-infrared wavelengths, this may point to fundamental differences in the interior composition or structure of these two bodies. We define a planetocentric longitude system for Pallas, following IAU guidelines. We also present the first albedo maps of Pallas covering ∼80% of the surface in K-band. These maps reveal features with diameters in the 70–180 km range and an albedo contrast of about 6% with respect to the mean surface albedo.

Periodicity in Saturn's magnetosphere: Plasma cam

Plasma ion data from the Cassini Plasma Spectrometer (CAPS) are examined for all orbits from October 25, 2004 through Dec. 26, 2007. To eliminate effects of incomplete angular coverage, data are only used from the CAPS anode that is closest to viewing into the corotational flow and within 20° of that flow. The data are plotted in the SKR‐based SLS3 longitude system. The result is a cam‐shaped distribution in radial distance and SLS3 that has an outer lobe extending beyond 20 RS at SLS3 longitudes in the range ∼270° − 50°. The western edge of this outer lobe maps to the inner extent of a previously observed spiral pattern of periodic ion enhancements, which had the magnetic signature of plasmoids at distances &gt;35 Rs. The plasma cam and the plasmoid spiral emanating from it are responsible for plasma periodicities observed at radial distances beyond ∼15 Rs in Saturn's magnetosphere.

Minimally Invasive Multilevel Percutaneous Correction and Fusion for Adult Lumbar Degenerative Scoliosis

Prospective evaluation of 12 patients undergoing surgery for lumbar degenerative scoliosis. To assess the feasibility of minimally invasive spine surgery (MIS) techniques in the correction of lumbar degenerative deformity. Patient age, comorbidities, and blood loss may be limiting factors when considering surgical correction of lumbar degenerative scoliosis. MIS may allow for significantly less blood loss and tissue disruption than open surgery. Twelve patients underwent circumferential fusion. The age range of these patients was 50 to 85 years (mean of 72.8 y). Of the 12 patients, 7 were men and 5 were women. All patients underwent direct lateral transpsoas approach for discectomy and fusion with polyetheretherketone cage and rh-BMP2. All fusions to the sacrum included L5-S1 fusion with the Trans1 Axial Lumbar Interbody Fusion technique. Posteriorly, multilevel percutaneous screws were inserted using the CD Horizon Longitude system. Radiographs, visual analog scores (VAS), and treatment intensity scores (TIS) were assessed preoperatively and at last postoperative visit. Operative times and estimated blood loss were recorded. Mean number of segments operated on was 3.64 (range: 2 to 8 segments). Mean blood loss for anterior procedures (transpsoas discectomy/fusion and in some cases L5-S1 interbody fusion) was 163.89 mL (SD 105.41) and for posterior percutaneous pedicle screw fixation (and in some cases L5-S1 interbody fusion) was 93.33 mL (SD 101.43). Mean surgical time for anterior procedures was 4.01 hours (SD 1.88) and for posterior procedures was 3.99 hours (SD 1.19). Mean Cobb angle preoperatively was 18.93 degrees (SD 10.48) and postoperatively was 6.19 degrees (SD 7.20). Mean preoperative VAS score was 7.1; mean preoperative TIS score was 56.0. At mean follow-up of 75.5 days, mean VAS was 4.8; TIS was 28.0. A combination of 3 MIS techniques allows for correction of lumbar degenerative scoliosis. Multisegment correction can be performed with less blood loss and morbidity than for open correction.

Statistical analysis of injection/dispersion events in Saturn's inner magnetosphere

In the inner magnetosphere of a rapidly rotating planet such as Jupiter and Saturn, radial transport of plasma mainly comprises hot, tenuous plasma moving inward and cold, denser plasma moving outward. A distinctive phenomenon resulting from the drift dispersion of injecting hot plasma provides direct evidence for this convective motion. Particle instruments aboard the Cassini spacecraft, including the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS), have made numerous observations of such signatures. The statistics of the properties of such events are studied in this paper by analyzing CAPS data from 26 Cassini orbits. A statistical picture of their major characteristics is developed, including the distributions of ages, longitudinal widths, radial distances, and longitudes and local times of injection. An unexpected longitude modulation of these events appears in the old SLS longitude system, which is based on the Saturn kilometric radiation (SKR) observations by Voyager around 1980, while no such modulation seems to exist in the new SKR longitude system of the Cassini era. A Lomb periodogram analysis, however, reveals no significant periodic modulation of these events. The injection structures are found to occupy a small fraction (∼5–10) of the available longitude space.

On the cause of Saturn's plasma periodicity

Periodic plasma enhancements are examined for all Cassini orbits from December 29, 2005 through September 7, 2006. The events, which have UT durations of 3–4 hours, are centered near SLS3 longitude 10° at radial distances near 15 RS and at larger W longitudes at larger distances, reaching 180°W by 49 RS. Magnetic‐field data within the events and outside 30 to 35 RS show signatures of neutral‐sheet crossings and magnetic reconnection (i.e., plasmoids). We conclude that plasmoids move outward from 30–35 RS along a spiral path that rotates with the planet. The duration of these events is similar to that of SKR events, and they are ordered in the SKR‐based SLS3 longitude system. A conceptual model, in which the plasmoids are triggered in the pre‐midnight quadrant following (with a predictable delay) the appearance of SKR at the magnetopause and then propagate outward in a rotating spiral pattern, can explain the connection among periodicities observed in Saturn's charged particles, magnetic fields, and kilometric radiation.

An update to a Saturnian longitude system based on kilometric radio emissions

The period of Saturn kilometric radiation modulation as determined by Voyager forms the basis for a longitude system (SLS) recognized by the International Astronomical Union. However, Ulysses and Cassini observations have shown that this modulation period varies by the order of one percent on timescales of a few years and, hence, does not represent the internal rotation period of the planet. A new longitude system was proposed based on ∼2 years of Cassini observations of the kilometric radio emissions and accounts for the variable radio period (SLS2) valid over the time interval from day 001, 2004 through day 240, 2006. Early uses of this longitude system have revealed a number of magnetospheric phenomena which appear to be locked to the radio period, such as variations in the external magnetic field, the plasma density in the inner magnetosphere, and enhanced intensities of energetic ions. Analysis of the radio emissions since the new system was proposed revealed that the radio period continued to evolve, even showing a second, shorter period at times. The subsolar longitude of the peak of Saturn kilometric radio emissions begins to deviate from that given by the SLS2 system almost immediately after the previous analysis interval. Here, we provide a definition for SLS3, an extension to the longitude system valid over the interval from day 001, 2004 through day 222, 2007 based on variable period radio emissions.

Influence of Saturnian moons on Saturn kilometric radiation

Similar to past studies at Jupiter, we conduct an investigation of possible associations of radio emission occurrence probability with the orbital phases of Saturnian moons. We use a new definition of the Saturn longitude system (SLS) based on the results of Kurth et al. (2007). This paper presents results of our findings to date, sampling a large portion of the Radio and Plasma Wave Science (RPWS) instrument data over the frequency range 12 kHz &lt; f &lt; 16 MHz. We also investigate the intensity of Saturn kilometric radiation (SKR) as a function of the local time and subsolar longitude. When Titan is near a local time of midnight, there is a significant increase in the occurrence probability of SKR and a diminution of SKR when Titan is near local noon and afternoon. This indicates Titan may play a role in the process of substorm generation at Saturn perhaps due to its large plasma wake. Rhea displays a marginal orbital phase “control” of a subset of the SKR. In the past there have been conflicting reports of the absence of SKR emission at particular local times of Dione. We find no long‐term, statistical influence of Dione on SKR occurrence probability. In addition, we find no significant statistical influence of Enceladus or Tethys on SKR occurrence probability.