R-3 Repository :: Browsing by Author "Hill, Thomas W."

Browsing by Author "Hill, Thomas W."

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A model for plasma transport in a corotation-dominated magnetosphere
(1988) Pontius, Duane Henry, Jr; Hill, Thomas W.
The gross structures of the magnetospheres of the outer planets are decided by processes quite different from those predominant in that of the earth. The terrestrial plasmapause, the boundary beyond which plasma motion is principally determined by magnetospheric interaction with the solar wind, is typically inside geosynchronous orbit. Within the plasmasphere, rotational effects are present, but gravity exceeds the centrifugal force of corotation. In contrast, the Jovian plasmasphere extends to a distance at least twenty times farther than synchronous orbit, affording a large region where rotational effects are expected to he clearly manifest (Brice and Ioannidis, 1970). The goal of this thesis is to develop an appropriate theoretical model for treating the problem of plasma transport in a corotation dominated plasmasphere. The model presented here is intended to describe the radial transport of relatively cold plasma having an azimuthally uniform distribution in a dipolar magnetic field. The approach is conceptually similar to that of the radial diffusion model in that small scale motions are examined to infer global consequences, but the physical understanding of those small scale motions is quite different. In particular, discrete flux tubes of small cross section are assumed to move over distances large compared to their widths. The present model also differs from the corotating convection model by introducing a mechanism whereby the conservation of flux tube content along flowlines is violated. However, it is quite possible that a global convection pattern co-exists with the motions described here, leading to longitudinal asymmetries in the plasma distribution.
A study of Uranian magnetospheric convection
(1990) Ye, Gang; Hill, Thomas W.
In order to understand and explain the low-energy plasma structures observed by the PLS experiment on Voyager 2 in the Uranian inner magnetosphere, an analytic and self-consistent model of a time-dependent solar-wind driven convection system at Uranus has been developed in the corotating coordinate system. Many important results of this model agree with the observations very well. Because of the unusual orientation of the planetary rotation and magnetic dipole axes, magnetic merging on the dayside magnetopause varies as a function of planetary spin, in response to the changing orientation of the planetary magnetic field relative to the upstream interplanetary magnetic field, which is assumed to have a fixed direction for many planetary rotations. Therefore the magnitude of the solar-wind driven convection electric field varies sinusoidally in time with the 17.2 hr planetary spin period, even though the field direction is fixed in the corotating frame in a direction analogous to the dawn-to-dusk direction in the Earth's magnetosphere. By assuming conservation of the first adiabatic invariant we find that the "hot" (few keV) protons observed by the PLS experiment in the inner magnetosphere may be convected Sunward from a pick-up source provided by electron impact ionization of the neutral torus of the outermost satellite Oberon. Under the time-dependent convection field this hot plasma forms a ring-current shielding layer in the region L = 5 $\sim$ 7, similar to an Alfven layer because the hot plasma convection timescale ($\sim$20 days) is much larger than the 17.2 hr period of variation of the convection field. Inside of the shielding layer the time-averaged electric field is much smaller than the time average of the imposed field. The sinusoidal oscillation of the imposed electric field, however, is not significantly shielded by the shielding layer because the shielding timescale ($\sim$30 hr) is longer than the 17.2 hr oscillation period. A fraction of the hot plasma is therefore able to penetrate the shielding layer to form a trapped ring-current population. This trapped ring-current population is sufficiently long-lived to undergo charge-exchange and inelastic collisions with the widely distributed neutral hydrogen corona, resulting in the energy degradation of the "hot" component and the simultaneous appearance of the "intermediate" (few 100 eV) and "warm" (few 10 eV) components evident in the PLS results in the region between L = 5 and L = 7. The region 2 Birkeland current system, in our model, is concentrated near the region of the ring-current shielding layer. By analogy with the Earth's magnetosphere, the lower boundary of the Uranian aurora is predicted by mapping the location of the shielding layer in the magnetic equatorial plane along the magnetospheric magnetic field lines onto the Uranian ionosphere.
Analytical and numerical modeling of the electromagnetic structure of geospace
(1995) Ding, Cheng; Hill, Thomas W.
Geospace is a vast complex system. A portable quantitative model of the geospace electromagnetic structure is the foundation of many researches in the area of space plasma physics. Work described in this dissertation uses both the analytical and numerical approaches to bring the existing models several steps closer to the ultimate model. An analytical model is developed to simulate the magnetic effect of the magnetopause current by solving the Laplace equation in a complex geometry. The procedure involves the minimization of an integral quantity to determine the coefficients of a harmonic expansion for the magnetic scalar potential of the magnetopause field. This procedure can shield any kind of interior magnetic field, or open the closed magnetosphere up with an arbitrary normal component distribution over the magnetopause. The Toffoletto and Hill 1993 open magnetosphere model is improved by adding a new physical module of the shielded ring current field, and modifying its additional tail field to eliminate the shortcomings that are evident. The new improved version has a better representation of the magnetic field in the inner magnetosphere. A numerical model for the electric potential on closed field lines is developed by extending the polar-cap potential into the low-latitude ionosphere using a finite element method. This significantly enhances the model's capability. By mapping the electric potential into the magnetosphere, the associated magnetospheric plasma convection can be examined to assess the reasonability of the normal component distribution on the magnetopause.
Centrifugally Driven Radial Convection of Plasma in Saturn's Inner Magnetosphere
(2011) Chen, Yi; Hill, Thomas W.
Theoretical studies have pointed out that in the inner part of a rapidly rotating magnetosphere such as that of Jupiter and Saturn, the dominant driving factor of radial plasma transport is the centrifugal interchange instability. After the in-situ observations of the Cassini spacecraft became available, the major observable signature of radial plasma convection is reported as a series of longitudinally localized injections and simultaneous drift dispersions of hot tenuous plasma from the outer magnetosphere. The Cassini Plasma Spectrometer (CAPS) and the Cassini Magnetospheric Imaging Instrument (MIMI) have observed signatures of these processes frequently, providing direct evidence for Saturn's magnetospheric centrifugally driven convective motions, in which the radial transport of plasma comprises hot, tenuous plasma moving inward and cooler, denser plasma moving outward. With methods similar to those of Hill et al. [2005], we study the statistics of the properties of such events by analyzing CAPS data from 26 Cassini orbits. A statistical picture of their major characteristics is developed, including the distributions of ages, longitudinal widths, radial distances, and longitudes and local times, which are all consistent with previous results. An unexpected longitude modulation of these events appears in the old (SLS) longitude system, while no such modulation seems to exist in the new (SLS2/SLS3) longitude system. A Lomb periodogram analysis, however, reveals no significant periodic modulation of these events. We further extend the statistical sample of these injection/dispersion events and find that the inflow channels occupy only a small fraction (~ 7%) of the total available longitudinal space, indicating that the inflow sectors are much narrower than the outflow sectors. Furthermore, we assume that the plasma is largely confined to a thin equatorial sheet, and calculate its thickness by deriving the centrifugal scale height profile based on the CAPS observations. We also present the radial and longitudinal dependences of flux tube mass content, as well as the total ion mass between 5 and 10 Saturn radii. Combining these results, we estimate a global plasma mass outflow rate ~ 280 kg/so
Density structures in the Jovian magnetosphere
(1994) Ansher, Jay Alan; Hill, Thomas W.
This paper continues the work of Ansher et al. (1992) by identifying and examining density structures in Jupiter's magnetosphere. The 110 hours of data used are from a 4-second temporal resolution density data set derived from plasma wave instruments on board both Voyager 1 and Voyager 2. One hundred five structures are identified. They are believed to be the same type of structures as seen by Ansher et al. (1992) and are found to have sharp density gradients at the boundaries, average scale sizes of about one Jovian radius, and typical density variations between 50% and 200% of the background. Many structures show good correlation with the magnetic field data. In addition, the existence of the density structures has little if any dependence on radial distance from Jupiter, System III longitude, or magnetic latitude. Comparison with four plasma transport models indicates that the observed structures resemble flux tubes of varying plasma content compared to the background density. These findings are in agreement with those of Ansher et al. (1992).
Mapping magnetic flux tubes and field aligned currents using two representative magnetospheric models
(1993) Ding, Cheng; Hill, Thomas W.
Among available quantitative magnetospheric models, the Tsyganenko models represent empirical modeling, while the Toffoletto and Hill model is theoretical analysis. By mapping flux tubes between the ionosphere and the magnetosphere, those two models are analyzed and compared. Both the implied and actual field-aligned currents (FAC) are calculated in both models. The implied FAC, required to maintain the model field in magnetostatic equilibrium and given by the Vasyliunas equation, has the same order of magnitude as observational data, even in non-equilibrium models. The actual FAC, given by Ampere's law, is much smaller than the implied FAC, which means no large extraneous FAC is included in those models. Several new approaches imply that none of these models are in magnetostatic equilibrium, while the discrepancies are considerably larger for the Tsyganenko models. The magnetic effects of a preliminary FAC model have been evaluated in the Toffoletto and Hill theoretical model.
Modeling the Plasma Convection in Saturn's Inner Magnetosphere
(2013-09-16) Liu, Xin; Hill, Thomas W.; Toffoletto, Frank R.; Lenardic, Adrian
Saturn's magnetosphere is unique in the solar system. The rotation-driven convection consists of alternating channels of cool plasma from an interior source moving outward and hot plasma from outside moving inward, making Saturn’s inner magnetosphere a dynamical region. This thesis describes work on developing numerical models to simulate the plasma convection pattern in Saturn's inner magnetosphere. Chapter 2 introduces the numerical Rice Convection Model (RCM), a multi-fluid model that was originally developed for Earth’s magnetosphere. We adapt it for Saturn’s conditions in this thesis. In Chapter 3, we show results of initial RCM simulation runs, in which only cool plasma from the interior source is considered. We also include the Coriolis force and the pickup effect. Because the cool plasma is much denser than the hot plasma and always dominant in determining the convection pattern, it is important and necessary to investigate it first. Chapter 4 compares several cool plasma source models and determines the one that produces the best simulation results when compared to Cassini spacecraft observations. In Chapter 5, we add the finite temperature and associated plasma pressure of the cool plasma. The effect of ionospheric Pedersen conductance is also investigated. Finally in Chapter 6, we add hot plasma at the outer boundary, and simulate the V-shape signatures of the injection-dispersion events, which are considered the most definitive evidence of rotation-driven convection in Saturn's inner magnetosphere. Our simulations conform to the observed fact that wider, slower outflow channels of cooler, denser plasma alternate with narrower, faster inflow channels of hotter, more tenuous plasma. Comparisons between simulated and observed results show great consistency.
Overview of Jovian Aurora's observed features and its possible origin
(2003) Gong, Bin; Hill, Thomas W.
As on Earth, Jupiter's auroral emission comes from the upward Birkeland currents. The VIP4 magnetic field model maps the main oval to the middle magnetosphere (r ∼ 30 RJ). However, the interpretation of the observed features of the jovian aurora and its link to the middle magnetosphere remains an open question. This thesis reviews the main observational features and existing models of Jupiter's aurora. After comparing the merits and disadvantages of each model, we outline future possible improvements of the models.
Plasma sheet dynamics induced by plasma mantle
(1988) Liu, Weining William; Hill, Thomas W.
Is the magnetic field in the Earth's magnetotail static? if yes, why? if not, what causes the magnetic field to change and how does it evolve with time? These questions have haunted magnetospheric physicists for the past decade. Although significant progress has been made in this area of research, a consensus still does not exist. This thesis approaches the problem from the most fundamental basis--Faraday's law relating the curl of the electric field to the time variation of the magnetic field. If we can reach an independent theory that relates the electric field to the magnetic field, the whole problem can, at least in principle, be solved. This thesis pursues the problem both physically and mathematically. Our answers to the questions listed at the beginning are: (1) the magnetic field is generally not static; (2) the change is powered by the energy transfer from the solar wind to the magnetosphere, the agent that effects the change is plasma injection from the high-latitude plasma mantle; (3) the time-dependence is closely related to the velocity distribution of the mantle plasma; A decrease of B$\sb{\rm z}$ in the near tail and a flux buildup at the farther end of tail are two primary features of the time evolution; (4) a dense, drifting plasma mantle causes an intensive reconfiguration in the plasma sheet and is likely to lead to plasma sheet instability. The general results of the thesis are supportive of Hones' phenomenological model of the tail evolution (Hones, 1977).
Rice convection model simulations of the centrifugal interchange instability in the magnetospheres of Jupiter and Saturn
(2009) Wu, Han; Hill, Thomas W.
Radial plasma transport in Jupiter's and Saturn's magnetospheres is driven by the centrifugal interchange instability. Simulations with the Rice Convection Model (RCM) of the injection-dispersion phenomenon in Saturn's magnetosphere produce interchange convection cells stretching outward from the outer edge of the plasma torus centered near the orbit of Enceladus. We first use an idealized cold plasma torus near L = 4 (the orbit of Enceladus) as an initial distribution. Small perturbations in the torus region grow into interchange convection cells with alternating sectors of inflow and outflow. We show that the velocity-dependent Coriolis effects can be included by an effective Hall conductance, which is first implemented here using the grid based RCM. The simulation shows the following effects introduced by the Coriolis force: (1) bending of the convection cells in the retrograde direction, (2) slowing of their growth, and (3) broadening the out-moving fingers. Our simulation results support and distinguish the predictions made by Vasyliunas [GRL, 21, 401, 1994] and Pontius [GRL, 24, 2961, 1997]. Then we address two curious properties revealed by Cassini observations. (1) Several magnetospheric properties, including electron density, are modulated by the SKR longitude of the spacecraft [e.g., Gurnett et al., Science, 316 , 442, 2007]. A two-cell corotating convection model has been invoked by Gurnett et al. and by Goldreich and Farmer [JGR, 112, A05225, 2007] to explain this modulation. We have simulated this system by imposing an initially asymmetric plasma distribution in the Enceladus torus region, and find that the initial distribution must be at least ∼40% asymmetric in order to give a factor ∼2 asymmetry at 5 Saturn radii as reported. (2) The inflow sectors ("injection events") are much narrower in longitude than the interspersed outflow sectors. There is no obvious explanation for this in linear instability theory, nor in previous RCM simulations without an active plasma source. Our inclusion of an extended active continuous plasma source, combined with the Coriolis effects, provides a possible explanation. Finally, the initial results of inclusion of the pickup currents are reported.
Solar wind control of the open magnetosphere: Comparison of GGS/polar images and theory
(2001) Urquhart, Andrew Lee; Hill, Thomas W.
This investigation explores the connection between the open polar cap magnetic flux phiPCF and interplanetary conditions. phi PCF is determined from GGS/Polar VIS Earth Camera far ultraviolet observations of the aurora borealis. Observations from the GGS/Wind SWE and MFI instruments are used to characterize the interplanetary conditions. Additional observations from the IMP-8 PLA and MAG instruments are used to evaluate solar wind propagation time delay estimation methods so that the GGS/Wind observations can be better associated with the GGS/Polar observations. This allows the GGS/Wind observations to be used to estimate the polar cap potential &phis;PCP values associated with the GGS/Polar phiPCF values. Statistical methods are applied to determine a proxy relationship between &phis;PCP and phiPCF. The Rice Field Model (RFM) is modified to accept phi PCF as a configuration parameter, and RFM polar caps are produced using phi PCF determined both directly from the GGS/Polar images and by the proxy relationship from the GGS/Wind data. The RFM is able to produce polar caps with the same areas and open magnetic fluxes as the GGS/Polar observations, but the agreement in the polar cap shapes and locations leaves opportunities for further improvements.
The Water Vapor and Dust Plumes of Enceladus
(2014-02-11) Dong, Yaxue; Hill, Thomas W.; Reiff, Patricia H.; Semmes, Stephen
Enceladus is the most active moon of Saturn. Its south polar plume, composed mostly of water vapor and ice grains, is one of the groundbreaking discoveries made by the Cassini spacecraft. During Cassini’s E2, E3, E5 and E7 encounters with Enceladus, the Ion and Neutral Mass Spectrometer (INMS) measured high neutral water vapor densities up to ~10^9 cm-3 (Waite et al., 2006; Teolis et al., 2010; Dong et al., 2011). We have constructed a physical model for the expected water density in the plumes, based on supersonic radial outflow from one or more of the surface vents. We apply this model to possible surface sources of water vapor associated with the multiple jets observed in the visible dust plumes (Spitale and Porco, 2007). Our model predictions fit well with the INMS measurements of neutral water vapor density along the E3, E5, and E7 trajectories. The fit is optimized by values of outflow velocity in the range ~550–750 m/s and values of total source rate in the range ~1.5 − 3.5 10^28 H2O molecules/s ~ 450 – 1050 kg/s. The model can be extended to incorporate the jet features within the plume observed during the E7 encounter. The dust (ice grain) plumes of Enceladus have been observed by multiple Cassini instruments. We propose a composite ice grain size distribution covering a continuous size range from nanometer to micrometers, by combining the CAPS (Cassini Plasma Spectrometer) nanograin size distributions (Hill et al., 2012) and the CDA (Cosmic Dust Analyzer) and RPWS (Radio and Plasma Wave Science) dust power-law size distribution (Kempf et al., 2008; Ye et al., 2012, 2013). We also study the grain charging process using the RPWS-LP (Langmuir Probe) data (Morooka et al., 2011). Based on the size distribution and charge per grain, the densities, source rate, motion, and currents of the ice grains can be calculated. We found that the grains ~2-20 nm dominate in both charge density and number density. The total grain mass density is likely to be ~ 1- 10 times that of the water vapor, and the grain mass loading rate is ~100 kg/s. The motion of the charged grains in Saturn’s magnetosphere implies a transition from ion-like motion to neutral-like motion as the grain size increases from ~nm to ~μm. The grains carry a total current of ~10^5 A at Enceladus, which leads to a different current system from that of the ion pick-up and associated Birkeland currents. The grain current system may be dominant or at least comparable to the ion pick-up current system in accounting for the magnetic perturbations observed near Enceladus.
Variations of Jovian aurora induced by changes in solar wind dynamic pressure
(2005) Gong, Bin; Hill, Thomas W.
The jovian aurora contains a persistent main oval encircling each magnetic pole, which is associated with the upward field-aligned currents in the corotation enforcement current system. It has been suggested by two recent studies that the brightness of the main oval should become temporarily dimmer ∼ 1 hr after arrival of a shock wave in the solar wind, compressing the magnetosphere abruptly, because the difference between the angular velocity of the plasma in the magnetosphere and the rigid planetary rotational speed becomes smaller. But recent observations at Jupiter and Saturn have reported the opposite: the auroral oval brightens, and moves poleward, after the arrival of a solar wind shock. In this thesis, I will quantitatively include the flywheel effect of the neutral gas in the ionosphere in the coupling current system to explain this discrepancy and show that the corotation enforcement current should reverse and strengthen after a compression, and thereby temporarily cause the main oval to become brighter and move poleward. I will also show the differences between the night side and the day side in steady state and after a compression event by applying two different magnetic field models fitted from observations, and try to qualitatively explain the dawn-dusk asymmetry by introducing a region-1 current system analogous to that at Earth, which arises from the detailed interaction between solar wind and magnetosphere. Generally, I expect the day side sector of the main oval to brighten more than the night side sector, and the dawn sector to brighten more than the dusk sector.

Browsing by Author "Hill, Thomas W."

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