Magnetosphere Of Saturn Research Articles

Distribution and Properties of Magnetic Flux Ropes in Titan's Ionosphere

AbstractTitan's magnetic environment is a dynamic and unique place. We detect 85 flux ropes during all the Cassini flybys of Titan from 2005–2017. Analysis describing the location of flux ropes in Titan's ionosphere as well as where Titan is in Saturn's magnetosphere shows that the flux ropes are more often found when Titan is in the noon sector of Saturn's magnetosphere. A secondary peak of occurrence is found in the postmidnight area, where it is expected that Saturn's magnetosphere is highly dynamic. We also find that the flux rope occurrence is correlated with the average electron density profile of Titan's ionosphere. A force‐free model is utilized to estimate the radii and core magnetic field strength of the flux ropes. We find a large range of radii from 150–500 km with a small number of very large flux ropes (500–1,000 km) and a range of core field strengths of 1–15 nT, again with some much larger values(20–40 nT). The model also shows that more flux ropes are right‐handed than left‐handed in twist; however, we are unable to determine if there is a physical reason or if this is due to an observer bias. Additionally, we evaluate the goodness of fit for the model in each instance and conclude that, on average, the flux ropes may be better represented by a non‐force‐free model.

Modeling the Temporal Variability in Saturn's Magnetotail Current Sheet From the Cassini F‐ring Orbits

AbstractThe Cassini spacecraft completed 20 highly repeatable orbits during northern summer at Saturn, known as the F‐ring orbits, of which 18 are considered in this study. The spacecraft traversed Saturn's magnetotail current sheet during each apoapsis pass between 16 and 22 Saturn radii over 2‐day intervals and revealed a highly variable radial magnetic field from orbit to orbit. The solar wind and planetary period oscillations (PPOs) are significant sources of temporal variability in the Saturnian magnetosphere. PPOs refer to dual magnetic perturbation systems, one in each hemisphere, which have been observed to modulate the position and thickness of the magnetotail current sheet with a 10.7‐hr periodicity. Thus, we employ a model which considers dual‐modulation effects of the northern and southern PPO systems, together with a model of variable solar wind forcing on the magnetotail current sheet, to investigate their combined temporal effects on the radial magnetic field in the magnetotail. For all 18 F‐ring orbits considered, the modeled radial fields show excellent overall agreement with the temporal variability in the large‐scale structure of the observed radial fields (root mean square error 1.5 nT for 80% of the orbits). The amplitudes of the northern PPO modulations are well constrained between 0.3 and 0.5 Saturn radii, and they exceed the southern modulations by a factor of 1.3. The solar wind forcing is observed to be highly variable from orbit to orbit.

Convection in the Magnetosphere of Saturn During the Cassini Mission Derived From MIMI INCA and CHEMS Measurements

AbstractAn extensive analysis of Cassini Ion and Neutral Camera (INCA) and Charge Energy Mass Spectrometer (CHEMS) measurements of ~6–231 keV ion anisotropies acquired during selected spin and stare periods for nearly all mission orbits has been completed. Based on this analysis, we find that the computed azimuthal speed of Saturn's magnetodisk plasma increases within the inner and middle magnetosphere. Beyond the orbit of Titan, in the magnetotail, the calculated rotation speed remains roughly constant with increasing distance from Saturn. The component of convection parallel to Saturn's spin axis is smaller and on average nearly zero. The radial speed of plasma shows distinct local time dependence and increases outward with increasing distance down the magnetotail. The magnetodisk flow remains primarily azimuthal to large distances, indicating the plasma is still under the influence of Saturn as it transits across the nightside. Tailward flows have been observed in the region near the dawn and dusk magnetotail flanks. The plasma flow in the predawn quadrant is much more disorganized than that in the premidnight quadrant. The hydrogen and oxygen hot ion temperatures increase with decreasing distance to Saturn. Some plasma evidently escapes into a dusk boundary layer at the dusk magnetotail flank, while the remaining plasma primarily moves across the magnetotail and is entrained into the flow of a boundary layer at the dawn flank of the magnetotail. When scaled to the magnetopause standoff distance and corotation fraction, the convection speed of plasma in the magnetospheres of Jupiter and Saturn is similarly organized.

Saturn's Open‐Closed Field Line Boundary: A Cassini Electron Survey at Saturn's Magnetosphere

AbstractWe investigate the average configuration and structure of Saturn's magnetosphere in the nightside equatorial and high‐latitude regions. Electron data from the Cassini Plasma Spectrometer's Electron Spectrometer (CAPS‐ELS) is processed to produce a signal‐to‐noise ratio for the entire CAPS‐ELS time of operation at Saturn's magnetosphere. We investigate where the signal‐to‐noise ratio falls below 1 to identify regions in the magnetosphere where there is a significant depletion in the electron content. In the nightside equatorial region, we use this to find that the most planetward reconnection x‐line location is at 20–25 RS downtail from the planet in the midnight to dawn sector. We also find an equatorial dawn‐dusk asymmetry at a radial distance of >20 RS, which may indicate the presence of plasma‐depleted flux tubes returning to the dayside after reconnection in the tail. Furthermore, we find that the high‐latitude magnetosphere is predominantly in a state of constant plasma depletion and located on open field lines. We map the region of high‐latitude magnetosphere that is depleted of electrons to the polar cap to estimate the size and open flux content within the polar caps. The mean open flux content for the northern and southern polar caps are found to be 25 ± 5 and 32 ± 5 GWb, respectively. The average location of the open‐closed field boundary is found at invariant colatitudes of 12.7 ± 0.6° and 14.5 ± 0.6°. The northern boundary is modulated by planetary period oscillations more than the southern boundary.

An Investigation of Flow Shear and Diamagnetic Drift Effects on Magnetic Reconnection at Saturn's Dawnside Magnetopause

AbstractAt Saturn, solar wind driving of the magnetosphere is predicted to be reduced due to the extreme rotationally driven interior. As a result, the rotationally driven plasma corotates with the planet, possibly out to the magnetopause. Modeling suggests that this corotation should set up conditions that are unfavorable for the onset of magnetic reconnection on the dawnside magnetopause due to the generation of a significant flow shear. The flow shear model being tested makes use of idealized steady state models and a dominant east‐west interplanetary magnetic field orientation with a slight northward component. When the flow shear is larger than the Alfvèn speed, the model indicates that reconnection is suppressed. In this study, we employ observations from Cassini, when the interplanetary magnetic field has a northward component, to test this flow shear suppression model and whether reconnection is suppressed by diamagnetic drifts. Observations from the electron spectrometer on the Cassini Plasma Spectrometer suite of instruments show that reconnection does occur at the dayside magnetopause of Saturn, even in locations where modeling suggests that conditions are unfavorable for its onset. The inconsistency between modeling and observations may be caused by the conditions and assumptions within the model itself. Furthermore, the occurrence of reconnection on the dawnside magnetopause in general may hint at additional complexities in the global structure of Saturn's magnetosphere.

The evolution of rogue wave triplets and super rogue waves in superthermal polarized space dusty plasma

This present investigation has been instigated to examine the impact of polarization force on modulational instability of dust acoustic (DA) waves and transition of rogue wave triplets to super rogue waves in dusty plasma composed of negative dust as fluid, Boltzmannian electrons, and superthermal ions. The presence of superthermal ions has remarkably altered the impact of polarization force. An increment in ion superthermality index restricts the polarization parameter toward smaller values. By adopting the reductive perturbation technique, the nonlinear Schrödinger equation (NLSE) is procured that determines the modulational instability of the dust acoustic waves. It is observed that the effect of polarization force is constricted by the wavenumber domain in the advent of the instability region. The rational solution of NLSE describes the evolution of dust acoustic rogue wave triplets, which further transform into the super rogue waves by means of superposition of triplets. It is remarked that the amalgamation of polarization force and superthermal ions have an explicit impact on the characteristics of different kinds of dust acoustic rogue waves. It is intensified that our present theoretical pronouncements may shed light on the salient features of different kinds of DA rogue waves in laboratory experiments and space/astrophysical regions, especially in Saturn's magnetosphere, planetary rings, and comet tails, etc.

Evolution of dust ion acoustic soliton in the presence of superthermal electrons

Propagation of solitary wave in dusty plasmas started to draw the attention of the physicists since the early 90s. The presence of superthermal particles seems to have a great impact on such waves, as they indicate the existence of non-thermal systems. It has been observed that the superthermal population is capable of altering the nature of the plasmas waves. In the present paper, the effect of the superthermal electron population on the dust ion acoustic solitary wave has been explored. The plasma is considered un-magnetized and composed of two components of superthermal electrons (of two distinct temperature) along with positive ions, and negative dust particles. A major part of the work has been concentrated on the stability of the solitary structures considering the effect of the superthermal parameter. In addition, the dust charge has been considered as a variable and a detailed analysis has been provided on the same. The proposed plasma model is most suitable for analyzing Saturn magnetosphere and can be extended to any space plasmas with superthermal population.

Local Time Variation in the Large‐Scale Structure of Saturn's Magnetosphere

AbstractThe large‐scale structure of Saturn's magnetosphere is determined by internal and external factors, including the rapid planetary rotation rate, significant internal hot and cold plasma sources, and varying solar wind pressure. Under certain conditions the dayside magnetospheric magnetic field changes from a dipolar to more disk‐like structure, due to global force balance being approximately maintained during the reconfiguration. However, it is still not fully understood which factors dominantly influence this behavior, and in particular how it varies with local time. We explore this in detail using a 2‐D force‐balance model of Saturn's magnetodisk to describe the magnetosphere at different local time sectors. For model inputs, we use recent observational results that suggest a significant local time asymmetry in the pressure of the hot (>3 keV) plasma population, and magnetopause location. We make calculations under different solar wind conditions, in order to investigate how these local time asymmetries influence magnetospheric structure for different system sizes. We find significant day/night asymmetries in the model magnetic field, consistent with recent empirical studies based on Cassini magnetometer observations. We also find dawn‐dusk asymmetries in equatorial current sheet thickness, with the varying hot plasma content and magnetodisk radius having comparable influence on overall structure, depending on external conditions. We also find significant variations in magnetic mapping between the ionosphere and equatorial disk, and ring current intensity, with substantial enhancements in the night and dusk sectors. These results have consequences for interpreting many magnetospheric phenomena that vary with local time, such as reconnection events and auroral observations.

The Dynamics of Saturn's Main Aurorae

AbstractSaturn's main aurorae are thought to be generated by plasma flow shears associated with a gradient in angular plasma velocity in the outer magnetosphere. Dungey cycle convection across the polar cap, in combination with rotational flow, may maximize (minimize) this flow shear at dawn (dusk) under strong solar wind driving. Using imagery from Cassini's Ultraviolet Imaging Spectrograph, we surprisingly find no related asymmetry in auroral power but demonstrate that the previously observed “dawn arc” is a signature of quasiperiodic auroral plasma injections commencing near dawn, which seem to be transient signatures of magnetotail reconnection and not part of the static main aurorae. We conclude that direct Dungey cycle driving in Saturn's magnetosphere is small compared to internal driving under usual conditions. Saturn's large‐scale auroral dynamics hence seem predominantly controlled by internal plasma loading, with plasma release in the magnetotail being triggered both internally through planetary period oscillation effects and externally through solar wind compressions.

Spectral Signatures of Adiabatic Electron Acceleration at Saturn Through Corotation Drift Cancelation

AbstractThe energetic electron spectra of Saturn's radiation belts are studied using Cassini's 13 years of measurements by the Magnetosphere Imaging Instrument/Low Energy Magnetospheric Measurement System detector. We find that between L shells (L) of 10 and 4.5 the differential flux spectrum of 0.3‐ to 1.6‐MeV electrons evolves from a single power law to two power law functions separated at an energy cutoff (Ec). We show that inside L∼8, Ec has an L shell dependence that tracks consistently the energy of corotation drift cancelation (or resonance) ECDR, that is, the energy at which magnetic electron drifts and azimuthal corotation cancel, and is not associated with the absorbing effects of Saturn's moons. Ec also deviates from what conservation of the first adiabatic invariant would dictate. Our results verify that electrons around ECDR can be transported radially very efficiently by variable convective flows, such as those related to the noon‐midnight electric field in Saturn's magnetosphere.

Study of relativistic beam of electron on whistler mode waves for subtracted distribution in Saturnian magnetosphere

Very low frequency electromagnetic waves in the magnetosphere of Saturn are the focus of the present study. Hot injected beam effect on these waves like whistler mode waves for relativistic and non-relativistic subtracted bi-Maxwellian distribution function have been studied in the presence of perpendicular A.C electric field. By using the method of characteristics solutions and kinetic approach, expression for dispersion relation and growth rate has been derived via detailed calculations and derivations. By changing parameters of plasma like: ac frequency, thermal anisotropy, number density, parametric analysis has been done. The influence of AC frequency on Doppler shift is analyzed, and the comparative study of the effects of oblique and parallel propagating waves on the growth rate has been done. Some different results were found using the subtracted bi-Maxwellian distribution function discussed with the results of bi-Maxwellian distribution function. It can be seen that the effective parameters for generating whistler mode waves are not only temperature anisotropy, but also relativistic factor, alternating field frequency, subtracted distribution amplitude and the width of the loss cone distribution function, which have been discussed in the results and discussion section.

Driving Planetary Period Oscillations From the Hall Conducting Layer of Saturn's Upper Atmosphere

AbstractWe develop a simple physical model of the planetary period oscillations (PPOs) observed in Saturn's magnetosphere. The model couples together flows and currents in five two‐dimensional systems: an upper and lower atmospheric layer in each hemisphere, and the equatorial magnetosphere. We neglect the curved geometry of the planet and the magnetosphere but use a simple scaling argument to account for the very different sizes of the two systems. We show that it is possible to drive a PPO current system with many of the observed properties by invoking a twin vortex system in the Hall conducting layer of either hemisphere. The twin vortex system is able to generate divergent currents because our model includes a representation of the latitudinal variation of conductance. The large inertia and low Pedersen conductance of this layer of the atmosphere means that the twin vortex system and the PPO currents have a very stable rotation velocity and a very long dissipation timescale, explaining the long term persistence and stability of the PPO current systems. For a range of realistic parameters, part of the PPO current system closes through the equatorial magnetosphere, and part through the opposite hemisphere, as observed. We make predictions about the phase relationships between these closure currents that allow our model to be tested. Although the wind speeds required to produce the observed currents are implausibly large, interaction with enhanced electron density in the auroral regions reduces the necessary wind speeds to plausible values.

Survey of Saturn Whistler Mode Hiss Intensity

AbstractWe have conducted an extensive survey of whistler mode hiss emission at Saturn using nearly the complete data set from the Cassini Radio and Plasma Wave Science investigation. We distinguish the hiss emission by frequency and intensity threshold criteria. The survey includes all longitudes and latitudes >25° for r < 10 RS, limited to the availability of plasma density data. The results indicate that hiss intensity is highest at high latitude, and the level of intensity is larger than chorus by at least an order of magnitude. After a rapid increase in intensity starting near L = 13, the hiss intensity remains relatively constant extending to L > 100. The intensity of the hiss as a function of frequency has a shallow peak near 35 Hz but remains always in the range ~5 × 10−5 nT2 < I < ~10−3 nT2. These survey results will be important in modeling the scattering interactions of whistler mode emissions with electrons in the Saturn magnetosphere. Based on recent particle and field observations, sources of Saturn hiss occur on and near the footprints of field‐aligned currents, plasma injections, and reconnected field lines.

Generation of oblique electromagnetic wave by hot injection electron beam with parallel AC electric field in the magnetosphere of Saturn

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Plasma particles, waves and fields are the major factors on which present knowledge about Saturn’s magnetosphere is based from the three flyby missions. For the present work, our examination is focused to study those waves who have frequency lower than electron cyclotron frequency in Saturnian magnetosphere. In this paper, we investigate the growth rate of whistler mode waves in magneto-plasma of Saturn. A dispersion relation for parallel propagating whistler mode waves has been applied to the magnetosphere of Saturn. The effect of electron density, temperature anisotropy, number density and A.C frequency on the temporal growth rate of the whistler mode emission is studied. For the given magnetic field and external electric field configuration, the general dispersion relation has been derived.

Sources, Sinks, and Transport of Energetic Electrons Near Saturn's Main Rings

AbstractThe inner boundary of Saturn's electron radiation belts, near the planet's A ring (∼2.27 Rs), is studied using Cassini's proximal orbit measurements. We find that variable convective flows transport energetic electrons to the A ring, which absorbs them instantaneously, forming the inner belt boundary. These flows are also responsible for a variable and longitudinally asymmetric boundary configuration. Prenoon, the boundary oscillates toward and away from the A ring with a 2‐week period. Postnoon, it maps persistently near the F ring (∼2.32 Rs) and coexists with localized MeV electron intensity enhancements (microbelts). We propose that the microbelts contain electrons in drift resonance with corotation, trapped in local time‐confined trajectories, which result from the aforementioned convective flows. The microbelts' collocation with the F ring implies either a local, secondary electron production due to galactic cosmic ray collisions with F ring dust or an enhanced resonant electron trapping due to an electrodynamic interaction between the F ring and Saturn's magnetosphere.

Analytical study of Whistler mode waves for relativistic plasma with AC electric field in inner magnetosphere of Saturn

The population of plasma present in the rotationally dominated inner magnetosphere of Saturn is identified by the observed plasma waves in the magnetosphere. Whistler mode emissions along with electrostatic cyclotron emissions often with harmonics are a common feature of Saturnian inner magnetosphere. We present the outcomes of a study of very large amplitude Whistler mode waves characteristics inside the magnetosphere of radial distance of less than 15 $$\hbox {R}_{\mathrm{s}}$$ . Whistler mode waves with temperature anisotropy in the magnetosphere of Saturn have been studied in the present work. Observations of Whistler mode emissions from Cassini Radio and Plasma Wave Science instruments have been obtained. Whistler mode waves were investigated using the method of characteristic solution by kinetic approach, in the presence of AC field. The observations made by space probes Voyager 1 and 2 and Cassini, launched by NASA, showed that charged particles are trapped in planet’s magnetic field lines. In 2004, the Cassini encounter with Saturn revealed that magnetosphere of Saturn exhibits Maxwellian distribution. So, the dispersion relation, real frequency and growth rate were evaluated using ring distribution function. Effect of AC frequency, temperature anisotropy, energy and number density of particles was found. Temperature anisotropies greater than one ( $$\hbox {T}_\bot {/}\hbox {T}_{\Vert } >1$$ ) and pancake-like distribution can generate Whistler mode emissions of the warmer plasma population. The study also extended to oblique propagation of Whistler mode waves in presence of AC electric field. However, when relativistic factor $${{\upbeta } }=\sqrt{\hbox {1}-\frac{\hbox {v}^{\mathrm {2}}}{\hbox {c}^{\mathrm {2}}}}$$ increases, growth rate decreases. Through comprehensive mathematical analysis, it was found that when Whistler mode waves propagate parallel to the intrinsic magnetic field of Saturn, its growth is enhanced more than in the case of oblique propagation. Results are also discussed while computing the rate with which the wave grows for a particular wavenumber.

Flux Tube Entropy and Specific Entropy in Saturn's Magnetosphere

AbstractThe motivation of this paper is to discuss the dynamical processes in Saturn's magnetosphere from the plasma entropy perspective. Saturn's magnetosphere is stabilized by a radially increasing profile of flux tube entropy and destabilized by a radially decreasing profile of flux tube content. The traditional radial transport scenario suggested that the magnetic flux with heavy flux tube content moves from the inner magnetosphere to the outer magnetosphere, stretching the magnetic field into a magnetodisc configuration. Subsequently, magnetic flux with low flux tube entropy generated by magnetodisc reconnection circulates back to the inner magnetosphere. However, the low‐specific entropy plasma with a narrow distribution in Saturn's inner magnetosphere suggests a significant nonadiabatic cooling process during the inward motion. The flux tube entropy analysis suggests that energetic particles dominate the total flux tube entropy in the magnetodisc region, and newly closed field lines generated by magnetodisc reconnection are likely to be transported into the inner magnetosphere. Based on the flux tube entropy constraint, this study demonstrates that the radial transport process in Saturn's magnetosphere can also be achieved via middle‐latitude double reconnection, driven by a low‐latitude interchange instability. This process does not involve significant latitudinal convection of magnetic flux in the ionosphere nor does it significantly modify the radial flux tube entropy profile.

Quantifying Mass and Magnetic Flux Transport in Saturn's Magnetosphere

AbstractRadial transport is an important dynamical process in Saturn's internally driven magnetosphere. Radial transport is presumed to occur by a centrifugally driven interchange instability, determined by the gradient of flux tube content and flux tube entropy. Plasma produced in the inner magnetosphere must be transported radially outward. The outward motion of the plasma stretches the magnetic field lines, leading to magnetic reconnection. Reconnection allows the mass to be transported radially while allowing the flux to circulate back to the inner magnetosphere. Both radial transport of mass and magnetic flux in Saturn's magnetosphere have been estimated based on Cassani Plasma Spectrometer data provided by Wilson et al. (2017, https://doi.org/10.1002/2017JA024117) and suggesting the radial transport rate of plasma of around 55 kg/s. The net magnetic flux transport should be 0, but the data suggest a net outward magnetic flux transport indicating the existence of different possible transport mechanisms in Saturn's magnetodisc.

Planetary Period Oscillations in Saturn's Magnetosphere: Comparison of Magnetic and SKR Modulation Periods and Phases During Northern Summer to the End of the Cassini Mission

AbstractWe compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres.

Three‐Dimensional Currents in Saturn's Magnetosphere

AbstractObservations of Saturn's magnetic field from July 2004 to December 2016 have been combined to allow a numerical calculation of ∇ × B throughout the magnetosphere, thus permitting the first global mapping of Saturn's currents in three dimensions. The azimuthal ring current Jϕ dominates the magnetospheric currents, peaking near the orbit of the moon Rhea and having a total magnitude of over 10 mega‐Amperes integrated within the L‐shell of Titan. In meridional cross section, the ring current has a teardrop shape rather than a block shape. Jϕ also has a noticeable local time asymmetry, being the largest in the midnight and dawn sectors. The meridional currents Jρ and Jz are smaller than the ring current by an order of magnitude; they flow radially outward near the equator and return at higher latitudes in a looping pattern reminiscent of Hadley cell atmospheric convection. There are two such loops: one in the northern hemisphere and one in the south, flowing in opposing directions. In the noon sector, a strong Jz current may indicate the dayside cusp.