Search

Saturn's magnetosphere is an efficient emitter of energetic neutral atoms (ENAs), created through charge exchange of energetic ions with the extended neutral cloud originating from the icy moon Enceladus. We present an analysis using the complete image set captured by Cassini’s Ion Neutral Camera (INCA) to characterise Saturn’s average ENA morphology. Concentric tori are formed around the planet by oxygen and hydrogen ENAs, with intensity peaks between 7-10 Rs radial distance, with a ~1-2 Rs dayside offset. Nightside intensity is brighter than the dayside, likely the result of enhancements following large-scale plasma injections from the magnetotail, and influence of the noon-midnight electric field. Global intensity is clearly modulated with the near-planetary rotation period. This Cassini-era profile of Saturn's ENA emission advances our understanding of how volcanic moons can influence plasma dynamics in giant magnetospheres and is timely ahead of the planned JUICE mission, which carries the first dedicated ENA detector to Jupiter.

Motivated by potential effects of the Earth's rotation on the Deepwater Horizon oil plume, we conducted laboratory experiments on salt-water point plumes in a homogeneous rotating environment across a wide range of Rossby numbers 0.02 < Ro < 1.3. We report a striking physical instability in the plume dynamics near the source: after approximately one rotation period, the plume tilts laterally and starts to precess anticyclonically. The mean precession frequency ω scales linearly with the rotation rate Ω as ω≈0.4Ω. We find no evidence of a critical Rossby number above which precession ceases. We infer that a conventionally defined Rossby number is not an appropriate parameter when the plume is maintained over a long time: provided Ω ≠ 0, rotation is always important to the dynamics. This indicates that precession may occur in persistent oceanic or atmospheric plumes even at low latitudes.

The location and motion of Saturn’s equatorial current sheet is the result of an interplay between a quasi-static deformation that varies in radial distance and local time, impulsive perturbations that produce large-scale displacements, quasi-periodic perturbations near the planetary rotation period, and wave-like structures on shorter timescales. This study focuses on the latter, aperiodic wave pulses with periods from 1-30 minutes, that are unrelated to the quasi-periodic ‘flapping’ with a period near that of Saturn’s rotation. Cassini magnetometer data were surveyed for these aperiodic structures and then fitted to a simple model in order to estimate the properties of the waves.The model consists of a modified Harris current sheet model deformed by a Gaussian pulse wave function. This then allows for the extraction of wave parameters and current sheet properties. In particular we show an increase in current sheet scale height with radial distance from Saturn, an increase in the wave amplitude with radial distance, and the resolution of propagation directions using the wave vector fitted by the model. The dominant propagation direction is found to be radially outwards from Saturn.

This paper presents lightning mapping array (LMA) observations of corona discharges from a windmill and its lightning protection tower in winter thunderstorms in Japan. Corona discharges from the windmill, called windmill coronas, and those from the tower, called tower coronas, are distinctly different. Windmill coronas occur with periodic bursts, generally radiate larger power, and possibly develop to higher altitudes than tower coronas do. A strong negative electric field is necessary for the frequent production of tower coronas but is not apparently related with windmill coronas. These differences are due to the periodic rotation of the windmill and the moving blades which can escape space charges produced by corona discharges and sustain a large local electric field. The production period of windmill coronas is related with the rotation period of the windmill. Surprisingly, for one rotation of the windmill, only two out of the three blades produce detectable discharges and source powers of discharges from these two blades are different. The reason for this phenomenon is still unclear. For tower coronas, the source rate can get very high only when there is a strong negative electric field, and the source power can get very high only when the source rate is very low. The relationship between corona discharges and lightning flashes is investigated. There is no direct evidence that corona discharges can increase the chance of upward leader initiation, but nearby lightning flashes can increase the source rate of corona discharges right after the flashes. The peak of the source height distribution of corona discharges is about 100 m higher than the top of the windmill and the top of the tower. Possible reasons for this result are discussed.

In Saturn's magnetosphere, ubiquitous fluctuations with a period of ~10.7 hours have been observed in Saturn Kilometric Radiation (SKR), auroral emissions, the magnetic field, the electron density, and energetic particle fluxes. Here we characterize previously unstudied periodicities in plasma properties inside of 15 RS near the equatorial plane. Although periodically varying magnetic perturbations rotate relatively smoothly (spinning), plasma properties do not. The phase of the peak value of plasma density or pressure perturbations can change substantially across a few hours of local time or RS. As a means of interpreting observations, we use a magnetohydrodynamic simulation that generates field-aligned currents centered at 70° invariant latitude in Saturn's southern ionosphere and rotating at the SKR period. The simulation reproduces many periodic features of the data including not only spinning perturbations but also global scale compression and expansion (breathing). Simulated plasma properties are also modulated by periodic large scale north-south motion (flapping) in regions beyond ~15 Saturn radii (RS), which we do not analyze here. Inside of 15 RS, plasma responds to a superposition of spinning and breathing at the spin period, developing perturbations that peak at different phases depending on what is measured and where. Strong compressional effects act impulsively over a limited range of rotation phase. Superposition of local and global scale variations produces phase jumps across short distances and can introduce multiple peaks in the variation of plasma properties within one rotation period, accounting for anomalies in the phase-dependence of periodic fluctuations identified in the sparse data available.

During certain portions of the Cassini mission to Saturn, Cassini made repeated and periodic crossings of the magnetospheric current sheet that lies near the magnetic equator and extends well down the magnetospheric tail. These repeated crossings are part of the puzzling set of planetary period variations in numerous magnetospheric properties that have been discovered at Saturn. During 2010 these periodic crossings often display asymmetries such that the northbound crossing occurs faster than the southbound crossing or vice versa, while at other times the crossings are more symmetric. The character of the crossings is well organized by the relative phase of the northern versus southern perturbation currents inferred in earlier analyses of the magnetic field observations. Further, the dependence of the character of the crossings on the relative phase is consistent with similar asymmetries predicted both by the dual rotating current systems inferred from magnetic field observations and by global MHD models that incorporate the effects of hypothesized atmospheric vortices. The two models are themselves in generally good agreement on those predictions. In both models the asymmetries are attributable to a periodic thickening and thinning of the magnetospheric current sheet, combined with a periodic vertical flapping of the sheet. The Cassini observations thus provide additional observational support to such current systems as a likely explanation for many of the known magnetospheric planetary period variations.

International audience; We investigate planetary period oscillations (PPOs) in Saturn's magnetosphere using Cassini magnetic field and Saturn kilometric radiation (SKR) data over the interval from late 2012 to the end of 2015, beginning 3 years after vernal equinox and ending 1.5 years before northern solstice. Previous studies have shown that the northern and southern PPO periods converged across equinox from southern summer values 10.8 h for the southern system and 10.6 h for the northern system and near coalesced 1 year after equinox, before separating again with the southern period 10.69 h remaining longer than the northern 10.64 h. We show that these conditions ended in mid-2013 when the two periods coalesced at 10.66 h and remained so until mid-2014, increasing together to longer periods 10.70 h. During coalescence the two systems were locked near magnetic antiphase with SKR modulations in phase, a condition in which the effects of the generating rotating twin vortex flows in the two ionospheres reinforce each other via hemisphere-to-hemisphere coupling. The magnetic-SKR relative phasing indicates the dominance of postdawn SKR sources in both hemispheres, as was generally the case during the study interval. In mid-2014 the two periods separated again, the northern increasing to 10.78 h by the end of 2015, similar to the southern period during southern summer, while the southern period remained fixed near 10.70 h, well above the northern period during southern summer. Despite this difference, this behavior resulted in the first enduring reversal of the two periods, northern longer than southern, during the Cassini era.

Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere over 90 times as thick as present day Earth's. Billions of years ago the picture may have been very different. We have created a suite of 3D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present day Venus orbital parameters, an ocean volume consistent with current theory and measurements, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than about 16 Earth days, despite an incident solar flux 46-70% higher than modern Earth receives. At its current rotation period of 243 days, Venus's climate could have remained habitable until at least 715 million years ago if it hosted a shallow primordial ocean. These results demonstrate the vital role that rotation and topography play in understanding the climatic history of exoplanetary Venus-like worlds being discovered in the present epoch., Comment: 13 pages, 4 Figures, submitted to Geophysical Research Letters

We examine planetary period oscillations (PPOs) observed in Saturn's magnetospheric magnetic field data from the time of Saturn's equinox in 2009. In particular we focus on the time period commencing February 2011, when the oscillations started to display sudden and unexpected changes in behaviour at ~100-200 day intervals. These were characterised by large simultaneous changes in the amplitude of the northern and southern PPO systems, together with small changes in period and jumps in phase. Nine significant abrupt changes have been observed in the post-equinox interval to date, commencing as the Sun started to emerge from a long extended solar minimum. We perform a statistical study to determine whether these modulations in PPO behaviour were associated with changes in the solar and/or upstream solar wind conditions. We report that the upstream solar wind conditions show elevated values of solar wind dynamic pressure and density around the time of PPO behavioural transitions, as opposed to before and after these times. We suggest that abrupt changes in PPO behaviour may be related to significant changes in the size of the Saturnian magnetosphere in response to varying solar wind conditions.

Many of the periodic variations observed in Saturn's magnetosphere can be linked directly to the presence of a rotating pattern of field-aligned currents that link the northern and southern ionospheres with each other and with the magnetosphere. Such a current system is incorporated in a magnetohydrodynamic simulation that has previously been shown to reproduce many of the observed periodic properties of the system. Here the simulation is used to investigate a range of phenomena that can be attributed to the effects of compressional waves launched from the rotating current sources. The compressional waves are found to drive the flapping of the plasma sheet and the expansion and contraction of the magnetopause in each rotation period. Because the compressional perturbations weaken as they rotate from morning to evening around the dayside of the magnetosphere, the boundary develops a strong morning-evening asymmetry. A fit to the shape is provided that may be useful in further investigation of magnetopause properties, but there is already evidence of the proposed asymmetry in the observations of Clarke et al. (2010a).

We present a detailed investigation of the solar rotation effects on the Martian high-latitude (~63°N–81°N) ionosphere using the electron density (Ne) data measured by Mars Global Surveyor and solar XUV and EUV fluxes measured by SOHO under high (2000–2001), medium (2003), and low (2005) solar activity conditions. A fast Fourier transform spectral analysis method is used to estimate the amplitude of the rotation period in these parameters. This method clearly reveals the presence of solar rotation effects in the Martian ionospheric Ne at all altitudes (90–220 km), peak electron density (NmM2), and total electron content under the three solar activity conditions. These effects are in phase with the solar UV fluxes (corrected for the Martian orbit). The period of rotation effect (~26 days) is the same at all altitudes, though its amplitude is strongest at the ionospheric M2 peak (~135–140 km, ~3.5–6% of the mean values) and has a secondary enhancement at the M1 peak (~110–115 km). The effect of solar rotation on the M2 peak is larger during medium solar activity (2003) than during high solar activity (2000–2001). The effect, however, is absent in the ionospheric peak height (hmM2). The rotation effects on Mars are also compared with those on the Earth. Unlike at Mars, the Earth's high-latitude ionosphere shows no clear solar rotation effect, though the effect is observed clearly at lower latitudes.

We investigate the long-term motion of Saturn's north pole hexagon and the structure of its associated eastward jet, using Cassini imaging science system and ground-based images from 2008 to 2014. We show that both are persistent features that have survived the long polar night, the jet profile remaining essentially unchanged. During those years, the hexagon vertices showed a steady rotation period of 10 h 39 min 23.01 ± 0.01 s. The analysis of Voyager 1 and 2 (1980–1981) and Hubble Space Telescope and ground-based (1990–1991) images shows a period shorter by 3.5 s due to the presence at the time of a large anticyclone. We interpret the hexagon as a manifestation of a vertically trapped Rossby wave on the polar jet and, because of their survival and unchanged properties under the strong seasonal variations in insolation, we propose that both hexagon and jet are deep-rooted atmospheric features that could reveal the true rotation of the planet Saturn.

The recent survey by Andrews et al. (2012) of the separate northern and southern ~10.7 h periodic magnetic signals in Saturn's magnetosphere limits very much their governing current systems. The existence of signals with pure or close to pure northern or southern periods in respective polar caps taken with the relatively narrow bandwidth of the signals indicates that the actual periodicities are imposed independently from northern and southern polar regions, i.e., the open field line regions. Field-aligned currents must flow on the boundaries of these regions to exclude signals from the other hemisphere. Equatorward of the polar cap, on closed magnetic shells, there are distinct north and south “cam” source currents, the distinction being made clear by a difference in polarization. We outline the consequences for the governing current systems and the implications for sustaining the energy and power dissipation in the system.

[1] We report ground-based measurements of the polar middle atmosphere made using a 230–250 GHz passive microwave radiometer deployed at Troll station (72°01′S 02°32′E, L shell of L = 4.8), Antarctica. Our observations show enhanced mesospheric nitric oxide (NO) volume mixing ratio (VMR) during a series of small recurrent geomagnetic storms in the 2008 austral winter, reaching 1.2 ppmv on day 200 (18 July). The Lomb normalized periodogram of the NO VMR time series averaged over 65-80 km for days 130 to 220 of 2008 (9 May to 7 August) shows a peak exceeding the 95% confidence limit at 25.8 days, close to the synodic rotation period for low-latitude solar coronal holes. The highest correlations between the radiometer NO VMR data and trapped and quasi-trapped electron count rates for L = 3.5-5.5 from the Polar Orbiting Environment Satellites 90° telescope are for the >30 keV (90e1) channel (rmax = 0.56, lag time of 5.1 days) and >100 keV (90e2) channel (rmax = 0.57, lag time of 4.4 days). Maximum correlation between NO VMR and the >700 keV (90P6) channel data is lower but lag times are close to zero. Superposed epoch analyses for the eight most significant geomagnetic storm periods and three Carrington rotations (2070-2072) within the 90 day observation period indicate that significant NO abundance observed at 65-80 km in the Antarctic mesosphere may be produced directly by >200 keV electron precipitation or originate from a source at higher altitudes, e.g., production by >30 keV electrons followed by downward transport.

[1] Suggestions that the planetary period oscillations (PPOs) observed in Saturn's magnetosphere may be driven or influenced by neutral atmospheric perturbations motivate an exploratory comparison of PPO rotation periods with available tropospheric and stratospheric determinations. Nonpolar atmospheric rotation periods occupy the range ~10.2–10.7 h associated with the latitudinal jet structure, are similar north and south, and are independent of season, while PPO periods lie in a narrower partly overlapping range ~10.6–10.8 h, are persistently shorter north than south, and undergo a seasonal cycle. In this cycle, widely separated north-south PPO periods during southern summer converge across equinox to values lying within the atmospheric west jet band, remaining well-separated from east jet periods. Closest convergence occurred 1 year post equinox, contemporaneously with the switch in seasonal thunderstorm activity from Southern to Northern Hemispheres. Since most large-scale atmospheric phenomena are related to the west jets, rotating with closely similar periods, they also rotate with periods close to the PPOs under post equinoctial conditions but not otherwise. Specifically, post equinox northern PPOs rotate with a period close to the southern thunderstorms, as well as the north polar spot and hexagon features, while the post equinox southern PPOs rotate with a period close to the pre-equinox northern “string of pearls” and the first colocated post equinox northern thunderstorm, the Great White Spot event. However, even under these conditions, no consistent correspondences in period are found at a detailed level, which taken together with the lack of correspondence at other times does not suggest a direct physical link exists between these phenomena.

[1] A simple wavy magnetodisk model can explain periodicities in energetic charged particles observed in Saturn's outer magnetosphere (>20 RS). The model's free parameters are the tilt of magnetodisk in inner magnetosphere (~1.8°), speed of outgoing spiral wave (~8 RS/h), critical radius (~10 RS), and period of rotation (10.64 h). The fidelity of the model is not judged by a least squares fit to the actual data, but rather by the model's accuracy in reproducing the Lomb periodogram of the periodicities. The model accurately simulates the main spectral feature near ~10.7 h plus a secondary (“dual”) period near ~10.95 h. The ability of the wavy magnetodisk with one period to produce the observed dual periodicity in the observations suggests that models having “dual” periods need not be invoked to explain some of the periodicities in Saturn's outer magnetosphere.

[1] The origin of the periodicities in the radio, plasma, and magnetic fields of Saturn has long been debated. Given the high degree of alignment of Saturn's dipole with its rotation axis, no strong rotational periodicities are expected. However, Cassini data demonstrated the existence of such periodicities not only in Saturn's kilometric radio emissions (SKR) but in the plasma and magnetic field signatures. It is shown that the development of the centrifugal interchange instability that originates from mass loading from the Enceladus torus contains information about the planetary rotation period. However, the planetary period is masked by high-frequency components of the instability. The presence of Titan is shown to damp the high-frequency components and enables the fundamental frequency near the planetary rotation frequency to grow at the expense of the high-frequency components. As a result, the interchange instability is seen to change from one where five to seven large interchange fingers dominate to one where there are about three which cause the modulation of magnetospheric parameters near the planetary period. This modulation includes the movement of the magnetopause, the injection of energetic particles into the inner magnetosphere and the plasma density at high latitudes, both of which control SKR. Controlling factors on the frequency include Titan's drag on the plasma sheet which produces asymmetries between the Northern and Southern Hemispheres, solar wind conditions, and the density of the Enceladus plasma torus. The resultant magnetic perturbations are shown to have similar size and frequency as that seen in Cassini data.

[1] In this report, we modify the model of Arridge et al. (2011) which was proposed to describe the shape of the magnetodisk of Saturn and its north-south oscillations near the planetary rotation period. The modification is based on the recent results of Provan et al. (2011, 2012) and Andrews et al. (2012), who have shown that the magnetic modulations near the current sheet exhibit dual periodicities, and derived magnetic phase functions for the northern and southern period modulations from the magnetic field data. Using a modified model in which the effects of both modulations are included, specifically related to the modulations of the radial component of the magnetic field, we show that the oscillations can be modeled without the use of an arbitrarily assigned oscillation phase value as employed in the previous work of Szego et al. (2012). In addition, the best fit amplitudes of the two oscillations are found to be nearly constant for a majority of the magnetodisk data analyzed, such that a single model with one fixed parameter fits well to the data for a majority of the passes.

[1] We examine the “planetary period” magnetic field oscillations observed in the “core” region of Saturn's magnetosphere (dipole L ≤ 12), on 56 near-equatorial Cassini periapsis passes that took place between vernal equinox in August 2009 and November 2012. Previous studies have shown that these consist of the sum of two oscillations related to the northern and southern polar regions having differing amplitudes and periods that had reached near-equal amplitudes and near-converged periods ~10.68 h in the interval to ~1 year after equinox. The present analysis shows that an interval of strongly differing behavior then began ~1.5 years after equinox, in which abrupt changes in properties took place at ~6- to 8-month intervals, with three clear transitions occurring in February 2011, August 2011, and April 2012, respectively. These are characterized by large simultaneous changes in the amplitudes of the two systems, together with small changes in period about otherwise near-constant values of ~10.63 h for the northern system and ~10.69 h for the southern (thus, not reversed postequinox) and on occasion jumps in phase. The first transition produced a resumption of strong southern system dominance unexpected under northern spring conditions, while the second introduced comparably strong northern system dominance for the first time in these data. The third resulted in suppression of all core oscillations followed by re-emergence of both systems on a time scale of ~85 days, with the northern system remaining dominant but not as strongly as before. This behavior poses interesting questions for presently proposed theoretical scenarios.

[1] Today, it is widely accepted that during its early evolution, the Earth experienced a magma ocean that covered most of its surface. The separation of metal from silicate was much facilitated in the environment of such a magma ocean. The differentiation mechanism is known as the “metal‒rain scenario”. Our study will focus on the settling dynamics of these metal droplets. Because of the low viscosity of molten silicate and a higher rotation period of the Earth at that time the rotation has a potentially strong influence on the dynamics of the magma ocean. We use numerical 3D fluid simulations to analyze the combined effects of strong rotation and convection on the settling of the iron droplets. We show that the influence of rotation on the settling depends on the latitude. At the poles, the influence of rotation is only marginal. At the equator, three different scenarios can be distinguished. First, at low rotation rates, the particles form a dense layer at the bottom. Second, for strong rotation, the particles stay mostly suspended and layers form in the temperature field. Third, at higher rotation rates, the particles form a ribbon‒like structure in the middle of the box. The influence of rotation on the iron droplets may lead to a scenario where part of the iron is kept in the mantle instead of transported to the core. This would have a strong influence on the later states of the differentiation process and the amount of siderophile elements in the mantle.

We present a time-dependent MHD study of the controlling effects of the Mars crustal field on atmospheric escape. We calculate globally integrated planetary ion loss rates under quiet solar conditions considering the continuous rotation of crustal anomalies with the planet. It is found that the rotating crustal field plays an important role in controlling atmospheric escape. Significant time variation of ∼20% and ∼50% is observed during the entire rotation period for O+ and for O2+ and CO2+, respectively. The control is exerted mainly through two processes. First, the crustal magnetic pressure over the subsolar regime controls solar wind penetration and mass loading and therefore the escaping planetary ion source. There is a strong negative correlation between the magnetic pressure and ion loss, with a time lag of

The unprecedented single shot precision of the new-born Matera Laser Ranging Observatory (MLRO), that can reach a scattering down to a few millimeters on LAGEOS orbit, discloses new chances in studying the high frequency dynamics. In this work we present the very first LAGEOS-2 observations in terms of range residuals and discuss the cause of the high frequencies noticed since the testing phase of the MLRO system. There are sufficient theoretical and experimental evidences to interpret those signals as rotational signatures of the spinning satellite and the first quantitative results we obtained indicates a rotation period of 23.5 s on May 31, 2000.

Analysis of all the existing magnetic field data from Saturn's magnetosphere is carried out. These data arise from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2). In order to carry out a direct comparison between the three magnetic field data sets, they are all expressed in the same planetocentric spherical polar coordinate system. In the cases of Pioneer 11 (whole encounter) and Voyager 2 (inbound) a surprising perturbation of a few nanoteslas is observed (with a period close to Saturn's rotation period) in the radial and/or azimuthal components of the magnetic field. It is shown that a dipole tilt and/or current sheet crossings are unlikely causes for such periodic perturbations. This is in agreement with the negligible magnetic dipole tilt resulting from the existing models of Saturn's magnetic field.

The Unified Radio and Plasma Wave (URAP) experiment on the interplanetary spacecraft Ulysses is able to detect the Saturnian kilometric radiation (SKR) thanks to the high sensitivity of the receiver and in spite of the remoteness of the planet (8–13 AU). Our knowledge about Saturn comes essentially from the observations by the two Voyager spacecraft. Ulysses allows us to reassess the main properties of the SKR as they had been observed by Voyager 1 and 2: average flux density, spectrum, periodicity, and polarization. A striking difference between the results obtained from Voyager and those obtained from Ulysses is the periodicity of the radio emission linked to the planetary rotation (10 hours 39 min 24 s): The period deduced from Ulysses' observations is not constant and may differ by 1% from that of Voyager. The northern and southern sources of SKR are distributed along magnetic field lines which are fixed in local time. We interpret the source location by a Kelvin-Helmholtz instability developing on the flanks of Saturn's magnetopause, in the morningside. This instability could be at the origin of the acceleration of the particles responsible for the radio emission. Because of the solar wind fluctuations, the position of the magnetopause and the magnetohydrodynamic flow around it are modified so that the zone of Kelvin-Helmholtz instability slowly moves, implying a drift of the radio sources in local time. We examine and discuss the possibility for this drift to be the cause of the variations observed for the SKR period.

The observed local time asymmetry of the Io plasma torus is generally attributed to the presence of a persistent dawn-to-dusk electric field in the Jovian magnetosphere. The local time asymmetry is modulated at the System 3 rotation period of Jupiter's magnetic field, suggesting that the dawn-to-dusk electric field may be similarly modulated. We argue that such a System 3 modulation would have a profound disruptive effect on the observed torus structure if the torus were to corotate at exactly the System 3 rate: the torus would be a resonantly forced harmonic oscillator, and would disintegrate in a few rotation periods, contrary to observations. This destabilizing effect is independent of, and in addition to, the more familiar effect of the centrifugal interchange instability, which is also capable of disrupting the torus in a few rotation periods in the absence of other effects. We conclude that the observed (few percent) corotation lag of the torus is essential to preserving the observed long-lived torus structure by detuning the resonant frequency (the torus drift frequency) relative to the forcing frequency (System 3). A possible outcome of this confinement mechanism is a residual radial oscillation of the torus at the beat period (∼10 days) between System 3 and the torus drift period.

The power spectral density (frequency dependence of the variance) of the annual Stuiver radiocarbon record (Δ14C) contains a major line with a 17.5 ± 0.5 (2σ) year period which is proposed to result from a hitherto unexpected forcing of the cosmic ray (CR) flux at the top of the atmosphere. The only known natural period with a corresponding value is the synodic Saros cycle, the retrograde period of rotation of the Moon's nodal plane; 18.5 years is the sidereal nodal regression period. It is proposed that the intersection of the Moon's downstream diamagnetic solar wind cavity with the Earth at the Saros period is the source of the apparent cosmic ray anomaly leading to this periodic change of Δ14C; arguably, eclipsing and/or a low energy acceleration of the GCR could take place, though the evidence from the Fourier spectrum appears to favor acceleration.

The minimum corona observed with the Mark-III K-coronameter during the Whole Sun Month is compared with observations taken in June 1991 close to the last solar maximum. For both periods the three-dimensional structure of the electron density is reconstructed from coronagraph images covering a full rotation period of the Sun. This is accomplished by using methods of computer tomography specially adapted for the inversion of coronagraph images. The global structure is compared with a potential-field source-surface expansion of the photospheric magnetic field. The result affirms the model of a streamer belt around the Sun. Contrary to conventional models, however, the streamer belt shows a remarkable longitudinal density variation. A deviation between the magnetic field extrapolation and the reconstructed density enhancements occurs in the vicinity of an intense bipolar magnetic region. It can probably be attributed to the presence of nonnegligible currents in this region. We derive scale-height temperatures of 1.3 to 1.9 × 106 K from selected radial density profiles for the Whole Sun Month.

Ultralow-frequency (ULF) pulsations are an important class of phenomena characterizing the dynamics of the terrestrial magnetosphere and are usually interpreted as standing magnetohydrodynamic waves in the magnetospheric plasma of planet Earth. Spacecraft observations indicate that the Hermean and Jovian magnetospheres also support such eigenoscillations. Whether ULF waves exist in the magnetosphere of the planet Saturn is the topic of this work. On the basis of a simple plasma mass distribution model, periods of Kronian eigenoscillations are estimated. Values comparable to the planetary rotation period are found, which makes the existence of standing waves improbable. Despite this significant difference in Earth's magnetosphere it is shown that this does not inhibit the resonant coupling between fast mode type magnetopause surface waves and local propagating Alfven modes. Against this background we examine a pattern in the magnetic field data recorded by Voyager 1 during its encounter with Saturn. The analysis of the pattern and a modeled satellite crossing give indications that resonant mode coupling was detected.

Using low-frequency spectra of the wind speed, density, and magnetic field strength, we show that the near-streamer belt solar wind at solar minimum exhibits many harmonics of fundamental frequencies corresponding to 26- and 34-day periods. Nearly all the low-frequency peaks in the spectra can be explained by these harmonics. The 26-day period is that of coronal hole rotation, and the 34-day period is naturally associated with the photospheric rotation period at about 70° S latitude. Thus we find evidence that the wind flow near 25° S comes from a region poleward of 60° S, consistent with magnetic field models.

We measured the rotation period of Jupiter's inner magnetosphere with precision previously unattainable, using 35 years of observations of the Jovian decametric radiation at the University of Florida Radio Observatory at frequencies between 18 and 22.2 MHz. The new rotation period is the weighted mean of 13 independent 24-year average determinations. Each of these was found by measuring the drift of the histogram of occurrence probability versus System III (1965) central meridian longitude over an interval of approximately 24 years. The measured drift was used to correct the System III (1965) period to obtain the new value. Our weighted mean is 9 hours 55 min 29.6854 s, with a standard deviation of the weighted mean (σ) of 0.0035 s. This new rotation period is 7.4σ shorter than that of the System III (1965), indicating that the latter is in need of revision. Our measurements indicate an upper limit of about 4 ms/year on any possible Jovian rotation period drift.

We report our recent measurements of horizontal electric fields from multiple stratospheric balloon platforms during the Extended Life Balloon-Borne Observatories (ELBBO) experiment. The ELBBO project involved launches of five superpressure balloons into the stratosphere from Dunedin, New Zealand, during November and December 1992. Most of the balloons floated at a constant altitude of 26 km for over 3 months, covered a wide range of latitudes from the south pole to 281°S, and circled around the southern hemisphere several times. The electric field measurements from the ELBBO experiment have confirmed the earlier discovery of a new source of stratospheric horizontal electric fields [Holzworth, 1989], with wider latitude coverages than the original finding. This newly found horizontal electric field has an average magnitude of 10 to 20 mV/m at night and 100 to 150 mV/m in the daytime. Its field vector has a unique characteristic of counterclockwise rotation in the southern hemisphere, with the rotation period near the atmospheric inertial wave period. The horizontal scale size of this new-source E field was found to be near 500 km, of the order of the horizontal wavelength of the inertial wave in the stratosphere. This new-source E field is a steady background feature of the stratosphere, with local disturbance from underlying thunderstorms or other sources. At high latitude, a dawn-to-dusk electric field mapped down from the magnetosphere was observed at the local midnight, with a larger amplitude than that of the new-source E field. One of the implications of the new-source E field is that the neutral dynamic processes are strongly coupled into the electrodynamic processes in the stratosphere, making this region electrically active and not passive as previously thought by many.

The modulation lanes in Jupiter's decametric radio spectra were discovered by Riihimaa [1968]. We have developed a model for the mechanism responsible for their production in which the free parameters have been adjusted to provide a very close fit with the observations. In our model, a grid-like interference screen composed of field-aligned columns of enhanced or depleted plasma density is located near Io's orbit close to the longitude of the sub-Earth point. The column spacing is typically about 140 km. As a band of frequency components emitted from near the foot of an excited tube of magnetic flux passes through the screen, interference patterns of slightly different orientations are produced by the different frequencies. The corotation of this set of interference patterns with Jupiter results in the sloping modulation lanes of the observed dynamic spectrum. Newly calculated results indicate that (1) the Io-B and Io-A radiations are emitted from the northern hemisphere, while that from Io-C comes mainly from the southern hemisphere, (2) the half-angle of the assumed hollow-cone emission beam for Io-B is typically 60°, with a variation of a few degrees, and (3) the equatorial lead angle of the radio-emitting previously excited flux tube ahead of the flux tube through Io at the same instant is more variable, 50° being a typical value for Io-B. Modulation lanes from Io-unrelated emission were also successfully modeled, the assumption in this case being that some source of energy other than a direct encounter with Io excited the flux tube containing the emitting radio source. The L shell value for non-Io-A was less clearly defined but was definitely between 4 and 7. Sensitive measurements of the lane modulation depths for Io-B and Io-A indicated that the decay time for the columns of the interference screen is of the same order of magnitude as Jupiter's rotation period. Although the predominant modulation lane periodicity of about 2 s indicates an upper limit on Io-B source width of 70 km along the direction of increasing longitude, finer structure that was present in at least one case suggests an upper limit of only 20 km.

The result of a measurement of the period of rotation of Jupiter's inner magnetosphere with unprecendented precision is presented. The measurement was made from the University of Florida database of 35 apparitions of Jovian decametric observations at frequencies of 18, 20, and 22 MHz between 1957 and 1994. The mean of our 24 independent measurements was 9h55m29s.685, and the standard deviation of the mean was 0s.0034. The System III (1965) Jovian rotation period value that is currently accepted by the International Astronomical Union is greater than our value by 7.4 times our standard deviation; it appears to be in need of revision. We set an upper limit of 27 milliseconds per year on a possible drift of the rotation period as measured by our method.

We report the discovery with the Ulysses unified radio and plasma wave (URAP) instrument of features in the Jovian hectometer (HOM) wavelength radio emission spectrum which recur with a period about 2–4% longer than the Jovian System III rotation period. We conclude that the auroral HOM emissions are periodically blocked from “view” by regions in the torus of higher than average density and that these regions rotate more slowly than System III and persist for considerable intervals of time. We have reexamined the Voyager planetary radio astronomy (PRA) data taken during the flybys in 1979 and have found similar features in the HOM spectrum. Contemporaneous observations by Brown (1994) show an [SII] emission line enhancement in the Io plasma torus that rotates more slowly than System III by the same amount as the HOM feature.

[1] Jovian quasiperiodic (QP) radio bursts are suspected to be associated with relativistic particle accelerations occurring with a quasiperiodicity between a few minutes and a few tens of minutes in Jupiter's polar magnetosphere. Understanding the excitation and propagation of QP bursts could help us to better understand this periodic energization process. A first necessary step is to measure the wave mode, source location, and directivity of QP bursts. For that purpose, we performed a statistical analysis of goniopolarimetric measurements of QP bursts made with the Radio and Plasma Wave Science investigation (RPWS) onboard Cassini spacecraft during the Jupiter flyby of 2000–2001. We studied two groups of QP bursts on 22 and 23 December 2000, and we found consistent source directions about 50 RJ north of Jupiter with an error bar ≤20 RJ. Statistics of the Stokes parameters indicate that QP bursts are partially left-handed polarized (V > 0, Q, U

[1] Despite the high degree of axial symmetry of Saturn's internal magnetic field, rotation-associated periodicities are evident in Saturn's electromagnetic radiation, its magnetic perturbations and its particle populations. Although close to the mean rotation period of the cloud tops, the electromagnetic period drifts slightly over a time scale of years and, at high latitudes, differs for sources in the north and south. The source of the periodicity remains a mystery. The model investigated here places the momentum source in the upper atmosphere/ionosphere, with the wind patterns in the two hemispheres rotating about the spin axis at different rates typical of the 2005–2006 southern summer for which Cassini data have been extensively analyzed. A source at low altitudes would account for the persistence of phase following solar wind disruption of magnetospheric flow patterns but might not produce appropriate magnetospheric responses. However, a magnetohydrodynamic simulation in which vortical winds in the ionosphere drive field-aligned currents into the magnetosphere shows that the dual sources account nearly quantitatively for many measured magnetospheric responses. This paper focuses on the magnetotail where the model is shown to reproduce many well-documented results of data analysis including the features that appear distinctly at each of the two periods and those that appear as a carrier signal with amplitude modulation and phase shifts. In particular, the model accounts for current sheet flapping and modulation of the plasma sheet thickness and for the periodic structure of density enhancements at high latitudes at different periods in the north and the south.

[1] Saturn's magnetospheric structure and the intensity of radio frequency emissions from its immediate surroundings are modulated at close to the planet's rotation period. Analogous rotation-modulated variations at Jupiter are readily interpreted as effects of the non-axisymmetric intrinsic magnetic field. At Saturn, to the contrary, the high level of axial symmetry in the intrinsic field suggests that the periodicity is not internally imposed. A number of mechanisms have been proposed to account for the observations. Each model explains a subset of the observations in a qualitative manner, but no quantitative models yet exist. Here, using a magnetohydrodynamic simulation, we investigate the magnetospheric perturbations that arise from a localized vortical flow structure in the ionosphere near 70° S-latitude that rotates at roughly the rate of planetary rotation. The model reproduces nearly quantitatively a host of observed magnetospheric periodicities associated with the period of the dominant (southern) radio frequency emissions during the Cassini epoch including rotating, quasi-uniform magnetic perturbations in the equatorial plane, rotating mass density perturbations, periodic plasmoid releases that we associate with observed bursts of energetic neutral atoms (ENAs), periodic oscillations of magnetospheric boundaries, current sheet flapping, and periodic modulation of the field-aligned currents linked to Saturn's kilometric radiation (SKR). The model is not unique but is representative of a class of models in which asymmetric flows in the (as yet unmeasured) upper atmosphere couple to the ionosphere and generate currents that flow into the magnetosphere. It can be extended to include the second periodicity that has been associated with SKR emissions in the northern hemisphere.

[1] Saturn's magnetosphere is replete with magnetospheric periodicities; magnetic fields, plasma parameters, energetic particle fluxes, and radio emissions have all been observed to vary at a period close to that of Saturn's assumed sidereal rotation rate. In particular, periodicities in Saturn's magnetotail can be interpreted in terms of periodic vertical motion of Saturn's outer magnetospheric plasma sheet. The phase relationships between periodicities in different measurable quantities are a key piece of information in validating the various published models that attempt to relate periodicities in different quantities at different locations. It is important to empirically extract these phase relationships from the data in order to distinguish between these models, and to provide further data on which to base new conceptual models. In this paper a simple structural model of the flapping of Saturn's plasma sheet is developed and fitted to plasma densities in the outer magnetosphere, measured by the Cassini electron spectrometer. This model is used to establish the phase relationships between magnetic field periodicities in the cam region of the magnetosphere and the flapping of the plasma sheet. We find that the plasma sheet flaps in phase with Br and Bθ and in quadrature with the Bϕ component in the core/cam region. The plasma sheet phase also has a strong local time asymmetry. These results support some conceptual periodicity models but are in apparent contradiction with others, suggesting that future work is required to either modify the models or study additional phase relationships that are important for these models.

[1] Initial Voyager observations of Saturn kilometric radiation (SKR) indicated that the modulations in emitted power near the ∼11 h planetary rotation period are “strobe like,” varying with a phase independent of observer position, while subsequent Cassini studies of related oscillations in the magnetospheric magnetic field and plasma parameters have shown that these rotate around the planet with a period close to the SKR period. However, analysis of magnetic oscillation data over the interval 2004–2010 reveals the presence of variable secular drifts between the phases of the dominant southern period magnetic oscillations and SKR modulations, which become very marked after Cassini apoapsis moved for the first time into the postdusk sector in mid-2009. Here we use a simple theoretical model to show that such phase drifts arise if the SKR modulation phase also rotates around the auroral oval, combined with a highly restricted view of the SKR sources by the spacecraft due to the conical beaming of the emissions. Strobe-like behavior then occurs in the predawn-to-noon sector where the spacecraft has a near-continuous view of the most intense midmorning SKR sources, in agreement with the Voyager findings, while elsewhere the SKR modulation phase depends strongly on spacecraft local time, being in approximate antiphase with the midmorning sources in the postdusk sector. Supporting evidence for this scenario is provided through an independent determination of the variable rotation period of the southern magnetic field perturbations throughout the 6 year interval.

[1] We investigate magnetic field oscillations near the planetary rotation period in Saturn's magnetosphere observed during the initial near-equatorial phase of the Cassini mission. Phase determinations on 28 periapsis passes during this ∼2 year interval display pronounced nonrandom “jitter” relative to the ∼10.8 h modulations in the dominant southern Saturn kilometric radiation (SKR) emissions. Phase deviations in the radial and azimuthal components are strongly positively correlated, while being anticorrelated with the phase deviations in the colatitudinal component. This suggests the presence in the equatorial magnetosphere of superposed weaker field oscillations at the ∼10.6 h period of the northern SKR modulations, the phase deviations being shown to be periodic near the corresponding ∼23 day “beat” period. Modeling the effect of the northern period oscillations shows that their amplitude is ∼30%–40% of the southern period oscillations, producing phase deviations of ∼±25°. The relative phasing of the northern period radial and azimuthal fields is such as to form a rotating quasi-uniform field, as for the southern period oscillations, while the phasing of the colatitudinal component indicates perturbation field lines arched with apices pointing to the south, opposite to the southern period field lines that are arched with apices pointing to the north. The northern period field points sunward at northern SKR maxima, consistent with previous observations of the northern polar oscillations and opposite to the southern period field that points tailward at southern SKR maxima. The results support the view that the field oscillations are due to two auroral current systems that rotate with differing periods in the two hemispheres.

We present a focused parameter study of solar wind - magnetosphere interaction for the young Sun and Earth, $~3.5$ Ga ago, that relies on magnetohydrodynamic (MHD) simulations for both the solar wind and the magnetosphere. By simulating the quiescent young Sun and its wind we are able to propagate the MHD simulations up to Earth's magnetosphere and obtain a physically realistic solar forcing of it. We assess how sensitive the young solar wind is to changes in the coronal base density, sunspot placement and magnetic field strength, dipole magnetic field strength and the Sun's rotation period. From this analysis we obtain a range of plausible solar wind conditions the paleomagnetosphere may have been subject to. Scaling relationships from the literature suggest that a young Sun would have had a mass flux different from the present Sun. We evaluate how the mass flux changes with the aforementioned factors and determine the importance of this and several other key solar and magnetospheric variables with respect to their impact on the paleomagnetosphere. We vary the solar wind speed, density, interplanetary magnetic field strength and orientation as well as Earth's dipole magnetic field strength and tilt in a number of steady-state scenarios that are representative of young Sun-Earth interaction. This study is done as a first step of a more comprehensive effort towards understanding the implications of Sun-Earth interaction for planetary atmospheric evolution.

The two main, low frequency radio components discovered at Neptune by the Planetary Radio Astronomy experiment carried aboard Voyager 2 have a well defined periodicity at about 16.1 hour. By analyzing all the available data, i.e. about 60 days around the closest approach for the 'Burst' component and 15 days for the 'Smooth' component, we determine, for each component, the best estimate of the radio period. We conclude that the two estimates are not statistically different. While the two kinds of radio emissions have very different characteristics (in particular their frequency ranges and beaming properties), and probably correspond to different emission processes, their modulation is very likely due to the rotation of the planetary magnetic field tied to the core of the planet, as it has already been assumed for the other giant planets. The deduced estimate of the sidereal rotation period of Neptune is 16.108 +/- 0.006 (or 16h06.5m +/- 0.04 m).

During the encounter of Ulysses with Jupiter, we have measured two components of the dc electric field and deduced from them the flow speed in the Io toms, as well as the presence of a polar cap region end what we interpret as a cleft region. Within the toms the flow speed is approximately equal to the speed of a plasma corotating with Jupiter but has significant deviations. The dominant deviations have an apparent period of the order of Jupiter's rotation period, but this might be a latitudinal effect. Other important periods are about 40 min and less than 25 min.

Although Pluto was discovered in 1930, our modern view of this distant world only began taking shape in 1976. Prior to that time, the technology for studying this faint, 14th magnitude object was simply too immature. Virtually all that was known before 1976 was that Pluto circles the Sun in an unusually eccentric 248-year orbit (ranging from 29.5 to 49.4 AU) that is tilted far (17°) from the ecliptic—the plane in which the other planets orbit, that Pluto's rotation period is 6.39 days, that its intrinsic surface color is reddish, and that its rotational lightcurve is the strangest and most variegated of the planets. Since 1976, however, the pace and diversity of discoveries have increased dramatically. First, methane ice was discovered on Pluto's surface, which gave a clear indication that Pluto was formed in the outer solar system rather than simply ejected to it from another region. Next came the discovery of Pluto's large satellite Charon (usually pronounced “Sharon”) in 1978. Lying just 19,400 km from Pluto (

It has recently been shown using Cassini radio data that Saturn kilometric radiation (SKR) emissions from the Northern and Southern hemispheres of Saturn are modulated at distinctly different periods, similar to 10.6 h in the north and similar to 10.8 h in the south, during the southern summer conditions that prevailed during the interval from 2004 to near-equinox in mid-2009. Here we examine Cassini magnetospheric magnetic field data over the same interval and show that two corresponding systems of magnetic field oscillations that have the same overall periods, as the corresponding SKR modulations, to within similar to 0.01% are also present. Specifically, we show that the rotating quasi-dipolar field perturbations on southern open field lines and the rotating quasi-uniform field in the inner region of closed field lines have the same period as the southern SKR modulations, although with some intervals of slow long-term phase drift of unknown origin, while the rotating quasi-dipolar field perturbations on northern open field lines have the same period as the northern SKR modulations. We also show that while the equatorial quasi-uniform field and effective southern transverse dipole are directed down tail and toward dawn at southern SKR maxima, as found in previous studies, the corresponding northern transverse dipole is directed approximately opposite, pointing sunward and also slightly toward dawn at northern SKR maxima. We discuss these findings in terms of the presence of two independent high-latitude field-aligned current systems that rotate with different periods in the two hemispheres.

It is well known that a wide range of kronian magnetospheric phenomena, including the Saturn kilometric radiation (SKR), exhibit oscillations near the planetary rotation period. However, although the SKR is believed to be generated by unstable auroral electrons, no connection has been established to date between diurnal SKR modulations and UV auroral power. We use an empirical SKR phase determined from Cassini observations to order the ‘quiet time’ total emitted UV auroral power as observed by the Hubble Space Telescope in programs during the interval 2005–2009. Our results indicate that both the northern and southern UV powers are dependent on SKR phase, varying diurnally by factors of ∼3. We also show that the UV variation originates principally from the morning half of the oval, consistent with previous observations of the SKR sources.

[1] Here we report the discovery of a well-defined plasma density boundary at high latitudes in Saturn's magnetosphere. The boundary separates a region of relatively high density at L less than about 8 to 15 from a region with densities nearly three orders of magnitude lower at higher L values. Magnetic field measurements show that strong field-aligned currents, probably associated with the aurora, are located just inside the boundary. Analyses of the anisotropy of energetic electrons show that the magnetic field lines are usually closed inside the boundary and open outside the boundary, although exceptions sometimes occur. The location of the boundary is also modulated at the ∼10.6 to 10.8 hr rotational period of the planet. Many of these characteristics are similar to those predicted by Brice and Ioannidis for the plasmapause at a strongly magnetized, rapidly rotating planet such as Saturn.

[1] The Radio and Plasma Wave Science instrument (RPWS) on board the Cassini spacecraft has detected numerous narrowband radio emissions in Saturn's magnetosphere from September 2005 to May 2007. Typically, the narrowband radio emissions occur after an intensification of Saturn kilometric radiation (SKR) and last for several days. These emissions often occur near 5 kHz, with a modulation period of about 10.8 h, similar to the planet's rotation period. Other apparently associated bands also occur at higher frequencies, typically from 20 to 30 kHz. Both 5 kHz and 20 to 30 kHz emissions tend to decrease in intensity with increasing time and slowly drift upward in frequency after the onset. Our study shows that the narrowband radio emissions tend to be observed on the nightside at a radial distance range from 5 to 50 Saturn radii and tend to be observed more frequently at higher latitudes. The rotational modulation of narrowband radio emissions in Saturn's magnetosphere acts like a flashing light rather than a rotating beacon, similar to the source of SKR. These emissions are believed to be generated by mode conversion from anisotropy-driven electrostatic instabilities near the upper hybrid frequency. Comparisons with Cassini neutral particle imaging data show that transient hot plasma clouds, corotating with the planet inside the orbit of Titan, might be the source for narrowband radio emissions. Comparisons with Cassini Magnetometer data reveal that the narrowband radio emissions tend to occur 1 or 2 days after the compression of the magnetotail.

[1] We investigate the determination of the rotation period of Saturn's interior using magnetic field measurements from Cassini. First, we vary the rotation period and search for the period that yields the smallest rms misfit of a magnetic field model to the data. Second, we search for the period that yields the most power in the non-axisymmetric components of a magnetic field model. Neither method enables us to determine the rotation period. However, we are able to place a bound on the magnitude of the non-axisymmetric component of Saturn's magnetic field finding it to be no greater than 4–5% of the axisymmetric component.

We present simulations of atmospheres of Earth-like aquaplanets that are tidally locked to their star, that is, planets whose orbital period is equal to the rotation period about their spin axis, so that one side always faces the star and the other side is always dark. As extreme cases illustrating the effects of slow and rapid rotation, we consider planets with rotation periods equal to one current Earth year and one current Earth day. The dynamics responsible for the surface climate (e.g., winds, temperature, precipitation) and the general circulation of the atmosphere are discussed in light of existing theories of atmospheric circulations. For example, as expected from the increasing importance of Coriolis accelerations relative to inertial accelerations as the rotation rate increases, the winds are approximately isotropic and divergent at leading order in the slowly rotating atmosphere but are predominantly zonal and rotational in the rapidly rotating atmosphere. Free-atmospheric horizontal temperature variations in the slowly rotating atmosphere are generally weaker than in the rapidly rotating atmosphere. Interestingly, the surface temperature on the night side of the planets does not fall below ~240 K in either the rapidly or slowly rotating atmosphere; that is, heat transport from the day side to the night side of the planets efficiently reduces temperature contrasts in either case. Rotational waves shape the distribution of winds, temperature, and precipitation in the rapidly rotating atmosphere; in the slowly rotating atmosphere, these distributions are controlled by simpler divergent circulations. The results are of interest in the study of tidally locked terrestrial exoplanets and as illustrations of how planetary rotation and the insolation distribution shape climate.