Extended Plasma Sheet Research Articles

Study of Electromagnetic Electron Cyclotron Waves for Kappa Distribution with AC Field in the Magnetosphere of Saturn

Parallel propagating electromagnetic electron cyclotron (EMEC) waves in the extended plasma sheet (~12RS) and in the outer magnetosphere (~18RS) of Saturn have been studied. A dispersion relation for parallel propagating relativistic EMEC waves has been applied to the magnetosphere of Saturn, and comparisons have been made with the data of Voyager 1 at these radial distances. The detailed investigations for EMEC waves have been done in the presence of the perpendicular AC electric field, using the kappa distribution function. The relativistic temporal growth rate is calculated by the method of characteristic solution with the data provided by Voyager 1. The effect of the suprathermal electron density, temperature anisotropy, frequency of AC electric field, thermal energy of ions, and relativistic factor on the temporal growth rate of EMEC wave emission has been studied. The simulation results show that the growth of parallel propagating EMEC waves is significantly affected by variations in the temperature anisotropy, electron density, ion thermal energy, and relativistic factor in both the extended plasma sheet and the outer magnetosphere of Saturn. The temperature anisotropy (T⊥/T║), ion thermal energy (KBT║i), and electron density (n0) have been found to be a major source of free energy for parallel propagating EMEC waves in both regions.

Study of whistler mode waves for ring distribution function in Saturn’s magnetosphere

In present paper whistler mode waves in magnetosphere of Saturn have been investigated. Linear properties of ring velocity distribution function are used to derive dispersion relation for whistler mode waves in ambient magnetic field of Saturn in extended plasma sheet of planet’s magnetosphere. The derivation for perturbed distribution function, dispersion relation and growth rate have been determined by using the method of characteristic and kinetic approach. Analytical expressions for growth rate and real frequency of whistlers propagating parallel to magnetic field direction are attained. The analysis shows that temperature anisotropy, increases in number density and energy density increases the growth rate of whistler waves along with significant shift in wave number. It has been shown that whistler mode waves have grown due to loss of perpendicular kinetic energy of ring electrons. Calculations have been performed at two radial distances in Saturn’s magnetosphere. The results are of importance in analyzing observed VLF emissions over wide spectrum of frequency range in Saturnian magnetosphere. The analytical model developed can also be used to study various types of instabilities in planetary magnetospheres.

A simulation approach of high-frequency electrostatic waves found in Saturn’s magnetosphere

Using a particle-in-cell simulation, the characteristics of electron plasma and electron acoustic waves are investigated in plasmas containing an ion and two electron components. The electron velocities are modeled by a combination of two κ distributions. The model applies to the extended plasma sheet region in Saturn’s magnetosphere where the cool and hot electron velocities are found to have low indices, κc≃2 and κh≃4. For such low values of κc and κh, the electron plasma and electron acoustic waves are coupled. The model predicts weakly damped electron plasma waves while electron acoustic waves should also be observable, although less prominent.

Identification of Saturn's magnetospheric regions and associated plasma processes: Synopsis of Cassini observations during orbit insertion

Saturn's magnetosphere is currently studied from the microphysical to the global scale by the Cassini‐Huygens mission. During the first half of 2004, in the approach phase, remote sensing observations of Saturn's magnetosphere gave access to its auroral, radio, UV, energetic neutral atom, and dust emissions. Then, on 1 July 2004, Cassini Saturn orbit insertion provided us with the first in situ exploration of Saturn's magnetosphere since Voyager. To date, Saturn orbit insertion is the only Cassini orbit to have been described in common by all field and particle instruments. We use the comprehensive suite of magnetospheric and plasma science instruments to give a unified description of the large‐scale structure of the magnetosphere during this particular orbit, identifying the different regions and their boundaries. These regions consist of the Saturnian ring system (region 1, within 3 Saturn radii (RS)) and the cold plasma torus (region 2, within 5–6 RS) in the inner magnetosphere, a dynamic and extended plasma sheet (region 3), and an outer high‐latitude magnetosphere (region 4, beyond 12–14 RS). We compare these observations to those made at the time of the Voyager encounters. Then, we identify some of the dominant chemical characteristics and dynamical phenomena in each of these regions. The inner magnetosphere is characterized by the presence of the dominant plasma and neutral sources of the Saturnian system, giving birth to a very special magnetosphere dominated by water products. The extended plasma sheet, where the ring current resides, is a variable region with stretched magnetic field lines and contains a mixture of cold and hot plasma populations resulting from plasma transport processes. The outer high‐latitude magnetosphere is characterized by a quiet magnetic field and an absence of plasma. Saturn orbit insertion observations enabled us to capture a snapshot of the large‐scale structure of the Saturnian magnetosphere and of some of the main plasma processes operating in this complex environment. The analysis of the broad diversity of these interaction processes will be one of the main themes of magnetospheric and plasma science during the Cassini mission.

Stochastic Acceleration of Energetic Particles in the Magnetosphere of Saturn

Voyager's plasma probe observations suggest that there are at least three fundamentally different plasma regimes in Saturn: the hot outer magnetosphere, the extended plasma sheet, and the inner plasma torus. At the outer regions of the inner torus some ions have been accelerated to reach energies of the order of 43 keV. We develop a model that calculates the acceleration of charged particles in the Saturn's magnetosphere. We propose that the stochastic electric field associated to the observed magnetic field fluctuations is responsible of such acceleration. A random electric field is derived from the fluctuating magnetic field – via a Monte Carlo simulation – which then is applied to the momentum equation of charged particles seeded in the magnetosphere. Taking different initial conditions, like the source of charged particles and the distribution function of their velocities, we find that particles injected with very low energies ranging from 0.129 eV to 5.659 keV can be strongly accelerated to reach much higher energies ranging from 22.220 eV to 9.711 keV as a result of 125,000 hitting events (the latter are used in the numerical code to produce the particle acceleration over a predetermined distance).

Filling and emptying of the plasma sheet: Remote observations with 1–70 keV energetic neutral atoms

This paper shows the first energetic neutral atom (ENA) observations of the extended plasma sheet, taken with the Medium Energy Neutral Atom (MENA) imager on the IMAGE spacecraft. We show that ENA emissions can be routinely observed back to several tens of RE deep in the magnetotail when IMAGE is in an appropriate orbital position. Enhanced emissions (high plasma sheet densities) are associated with high solar wind densities and with super dense plasma sheet observations at geosynchronous orbit. We examine two magnetospheric storm intervals where plasma sheet loading begins prior to the storms and continues under all IMF BZ orientations, reaching its maximum during the peaks of the storms. For several days following these storms ENA emissions are weak, indicating that the plasma sheet is depleted after the storms. This study indicates that routine ENA observations of the plasma sheet content could become an important part of space weather monitoring.

Nature and location of the source of plasma sheet boundary layer ion beams

Onsager et al. (1991) have put forward a model of the formation of the plasma sheet boundary layer which relies on a steady source of plasma from a spatially extended plasma sheet, together with steady equatorward and earthward E×B convection of field lines due to reconnection at a downtail neutral line. This model is a synthesis of earlier proposals and it explains such features as an electron layer exterior to the ion boundary layer, ion velocity dispersion, counter streaming beams, low‐speed cutoffs in the beams. It also explains the apparent evolution of the ion beams through “kidney bean” shaped velocity‐space distributions toward quasi‐isotropic shells without invoking pitch angle scattering or energy diffusion. In this paper we explore two ramifications of the model: (1) low‐speed cutoffs observed in the ion distribution functions contain information about the distance to the neutral line downtail and (2) values of phase space density at the low‐speed cutoffs are the same as those in the distribution just earthward of the reconnection site. In principle we can map, as a function of time, the downtail neutral line distance and establish whether or not it is retreating during substorm recovery. We can also reconstruct the plasma distribution function near the neutral line to see if it is most consistent with mantle or plasma sheet plasma. We perform this analysis using ISEE Fast Plasma Experiment data for two plasma sheet recovery events, one on March 1, 1978, and the other on April 18, 1978. On March 1, 1978, we find evidence for an initial retreat from around 110 to 160 RE in the first 15 min; little further retreat occurs thereafter. On April 18, 1978, the neutral line location ranges from as little as 40 RE tailward of the satellite to as much as 200 RE, but there is no evidence for a systematic retreat. The reconstructed ion distributions for these events are most consistent with a plasma sheet origin for the March 1 case and possibly plasma mantle or low‐latitude boundary layer for the April 18 case.

Survey of low‐energy plasma electrons in Saturn's magnetosphere: Voyagers 1 and 2

This paper is a survey of the low‐energy plasma electron environment within Saturn's magnetosphere made by the plasma science experiment (PLS) during the Voyager encounters with Saturn. Over the full energy range of the PLS instrument (10 eV to 6 keV) the electron distribution functions are clearly non‐Maxwellian in character; they are composed of a cold (thermal) component with Maxwellian shape and a hot (suprathermal) non‐Maxwellian component. A large‐scale positive radial gradient in electron temperature is observed, increasing from less than 1 eV in the inner magnetosphere to as high as 800 eV in the outer magnetosphere. This increase in electron temperature explains the observed order of magnitude increase in plasma sheet thickness with increasing radial distance from Saturn. Scale heights of the cold heavy ion plasma can be as small as 0.2 Rs in the inner magnetosphere and as much as 3 Rs in the outer magnetosphere. Many of the observed density variations can be attributed to changes in density scale height without a change in plasma flux tube content. Three fundamentally different plasma regimes have been identified from the measurements: (1) the hot outer magnetosphere, (2) the extended plasma sheet, and (3) the inner plasma torus. The hot outer magnetosphere is a region within which the suprathermal electrons are the dominant contributors to the electron pressure, and at times to the electron density. Near the noon meridian, the electrons display a highly time dependent behavior with order of magnitude changes in density and temperature, which can occur in less than 96 s. Sudden density enhancements of cold plasma occur, which are thought to be either “plumes” associated with Titan (Eviatar et al., 1982) or plasma “blobs”, (Goertz, 1983). The extended plasma sheet, with mean inner and outer boundaries of 7 and 15 Rs, respectively, has enhanced levels of cold plasma relative to that in the hot outer magnetosphere, although the suprathermals continue to dominate the electron pressure. Plasma with energy less than 6 keV from this region is an important contributor to the ring current, and a significant current system is probably present between 6 and 8 Rs. The inner plasma torus is a region of reduced electron temperature (as low as 1 eV), enhanced equatorial densities (as high as 100/cm³), and reduced scale height (as small as 0.2 Rs). Localized reductions in electron temperature are observed near the L shells of Tethys, Dione, and possibly Rhea. The suprathermal electrons are observed to be severely depleted within the inner plasma torus, relative to that in the extended plasma sheet and hot outer magnetosphere. The energy dependence of the depletions indicate an interaction with dust or plasma waves at times; interactions with neutral gas or plasma ions may also contribute to the depletions. The data also indicate an association between the appearance of suprathermal electrons and the observed emission of whistler mode waves reported by Gurnett et al. (1981).

Dawn-dusk asymmetry of the tail region of the magnetosphere of Saturn and the interplanetary magnetic field

It is suggested that as a result of the dawn-dusk asymmetry of the magnetospheric structure of Saturn, caused by the interplanetary magnetic field, the Voyager 1 spacecraft remained in the closed region (the extended plasma sheet) during its entire traversal through the tail region, so that it did not have an opportunity to penetrate into the high latitude lobe (the open region).

Plasma Observations Near Saturn: Initial Results from Voyager 2

Results of measurements of plasma electrons and poitive ions made during the Voyager 2 encounter with Saturn have been combined with measurements from Voyager 1 and Pioneer 11 to define more clearly the configuration of plasma in the Saturnian magnetosphere. The general morphology is well represented by four regions: (i) the shocked solar wind plasma in the magnetosheath, observed between about 30 and 22 Saturn radii (RS) near the noon meridian; (ii) a variable density region between approximately 17 RS and the magnetopause; (iii) an extended thick plasma sheet between approximately 17 and approximately 7 RS symmetrical with respect to Saturn's equatorial plane and rotation axis; and (iv) an inner plasma torus that probably originates from local sources and extends inward from L approximately 7 to less than L approximately 2.7 (L is the magnetic shell parameter). In general, the heavy ions, probably O(+), are more closely confined to the equatorial plane than H(+), so that the ratio of heavy to light ions varies along the trajectory according to the distance of the spacecraft from the equatorial plane. The general configuration of the plasma sheet at Saturn found by Voyager 1 is confirmed, with some notable differences and additions. The "extended plasma sheet," observed between L approximately 7 and L approximately 15 by Voyager 1 is considerably thicker as observed by Voyager 2. Inward of L approximately 4, the plasma sheet collapses to a thin region about the equatorial plane. At the ring plane crossing, L approximately 2.7, the observations are consistent with a density of O(+) of approximately 100 per cubic centimeter, with a temperature of approximately 10 electron volts. The location of the bow shock and magnetopause crossings were consistent with those previously observed. The entire magnetosphere was larger during the outbound passage of Voyager 2 than had been previously observed; however, a magnetosphere of this size or larger is expected approximately 3 percent of the time.