R-3 Repository :: Browsing by Author "Freeman, John W., Jr."

Browsing by Author "Freeman, John W., Jr."

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A comparison of the magnetospheric specification model, the Garrett model and satellite data for the geosynchronous electron fluxes
(1991) Nagai, Akira; Freeman, John W., Jr.
The Magnetospheric Specification Model (MSM) calculates electron and ion fluxes that may endanger spacecraft. This thesis is to evaluate the electron flux levels specified by the MSM by comparison with the Garrett model output and spacecraft observations for the large magnetic storm of April 1988. The MSM is a magnetospheric physics model which uses ground-based and satellite data as input. The Garrett model, on the other hand, is a statistical model based on average geosynchronous electron fluxes. The MSM flux enhancement predictions are in better temporal agreement than the other model. The largest error of the MSM is associated with flux dropouts which are observed by the spacecraft but not predicted by the MSM. The other possible error sources are (1) the MSM does not properly represent extreme thinning of the plasma sheet, (2) the MSM tends to overestimate the convection electric field.
A comparison of the magnetospheric specification model, the Hardy et al. model, and satellite observations for precipitating auroral electron energy fluxes
(1992) Lambour, Richard Lee; Freeman, John W., Jr.
A semi-quantitative comparison has been made of the observed and calculated precipitating electron energy fluxes for the April 1988 magnetic storm. Electron energy fluxes were calculated by the Rice Magnetospheric Specification Model (MSM), a comprehensive model of the inner magnetospheric environment, and by the Hardy et al. model, a statistical model of electron precipitation in the auroral zone. The MSM correlates better with the observed fluxes than does the Hardy et al. model in terms of auroral boundaries, latitudinal profile and extent, and the actual magnitude of the energy flux. The sources of error in the MSM are probably: (1) Artificial flux dropouts created near the ionospheric projection of the model outer boundary, (2) an overestimate of the convection electric field, and (3) errors in locating the polar cap boundary.
A computational investigation of solar energetic particle trajectories in model magnetospheres
(1999) Orloff, Seth Michael; Freeman, John W., Jr.
This work studies the dynamic behavior of solar energetic particles (SEPs, defined as protons and electrons with energies of 1 MeV to 1 GeV) by simulating their motion in model electromagnetic fields. Because of the hazards they pose to orbiting spacecraft and manned spaceflight operations, these species must be included in modern modeling efforts, including forthcoming space weather models. In this thesis, we describe an original computer program called the Solar Energetic Particle Tracer (SEPTR). As part of an operational computer model, SEPTR calculates the upper rigidity cutoffs for particles within an evolving magnetosphere. We use a feature of SEPTR to calculate late a global ionospheric grid of rigidity cutoffs using the DGRF 1980 magnetic field model. Rather than map rigidity space in order to find the cutoffs, we use an algorithm designed to locate the upper cutoff in a minimum of time, which is more appropriate for an operational model. Comparisons to similar calculations by Smart and Shea [1985] support the validity of our method. A second feature of SEPTR is the ability to use data of SEP or X-ray fluxes in order to generate differential energy spectra for magnetospheric protons. The rigidity cutoffs are then applied to the proton spectrum in order to provide a representation of the local flux. Because we calculate actual particle trajectories rather than making use of popular approximations, our code is ideal for studying the limitations of adiabatic theory. We examine the expansion of the first adiabatic invariant series in terms of ordering parameter, identify examples of non-adiabatic behavior, and specify a form of the first adiabatic invariant that is appropriate for full particle tracing. The dayside magnetopause frequently has a geometry with a minimum in magnetic field strength off the magnetic equatorial plane. This feature leads to particle drift paths that move into the Northern or Southern polar cusp. Our model suggests a possible concentration of energetic electrons in these regions as well as chaotic radial diffusion when the particle resumes equatorial mirroring.
A suprathermal ion accelerator
(1967) Young, David T; Freeman, John W., Jr.
To facilitate calibration of the Rice ALSEP/SIDE instrument a low energy (thermal to 4.0 kev) positive ion beam of variable current density (10-12 to 10-8 amp/cm2) has been developed. A high perveance electron beam of 0.15 to 0.40 kev is formed using electrostatic lenses alone, without recourse to the customary confining axial magnetic field. The electron beam is introduced into a region where ionization occurs by electron collisions with any of several species of gas introduced into the vacuum system for this purpose. Ions are then extracted by an asymmetric electric field designed to produce a minimal increase in the width of their initial Maxwellian energy distribution. An investigation of the spreading of the ion energy distribution due to dispersion of the electron beam and inherent lens defects is carried out using arguments taken primarily from statistical mechanics and electrostatic field theory. Once removed from the ionization region, ions are focused into a beam and accelerated or decelerated to the desired energy by conventional ion optics.
Application of an empirically-derived polytropic index for the solar wind to a solar wind shock propagation model
(1994) Totten, Tracy Lynn; Freeman, John W., Jr.
Data from the Helios 1 spacecraft have been used to determine an empirical value for the polytropic index for the free-streaming solar wind. Application of this non-adiabatic polytropic index to a two-dimensional solar wind computer model to simulate the effects of thermal heat conduction has been investigated. The current project involves the insertion of this empirically-derived polytropic index into a magnetohydrodynamic model of solar wind propagation. This computer model is used to predict the time for shocks originating at the Sun to travel to Earth. This information is important for the protection of Earth-orbiting satellites. The model is a two and one-half-dimensional numerical code that solves the magnetohydrodynamic equations using the two-step Lax-Wendroff scheme. The shock jump ratios of the plasma parameters are determined using the Rankine-Hugoniot relations. In addition, the shock model requires a representative background solar wind as an initial condition. The original background solar wind is similar to the results obtained by Parker (Astrophysical Journal, 1958) and Weber and Davis (Astrophysical Journal, 1967). Changes to this initial condition are made by applying the non-adiabatic polytropic index to a three-dimensional, steady-state, magnetohydrodynamic model of the solar wind. The adjustments to the steady-state model produce a background solar wind that compares well to Helios 1 data. This new background solar wind is used as the initial condition for the 2D shock model. The shock model is also adjusted to include the effects of heat conduction. Comparison of model results with observational data indicate that these changes produce average transit times that are only 45 minutes late. Before the changes to the 2D shock model and its initial solar wind condition were made, the average prediction time was two hours late. Adjusting the shock model to include the effects of heat conduction but using the original background solar wind produces an average transit time that is less than one hour early. A few specific events are discussed in greater detail.
Calibration of the Rice magnetospheric specification and forecast model for the inner magnetosphere
(1994) Lambour, Richard Lee; Freeman, John W., Jr.
A quantitative comparison has been made between observed low-energy ($\sim$1 keV $-$ 30 keV) particle fluxes in the inner magnetosphere (r $<$ 6.6 Earth radii) and those calculated by the Rice Magnetospheric Specification and Forecast Model (MSFM). The MSFM, which was delivered on February 28, 1994, is an operational computer model of the terrestrial magnetospheric environment which is designed to facilitate US Air Force spacecraft operations. The model will be used as a diagnostic tool for spacecraft charging and other anomalies in a real time operational setting as well in post-event analysis. The MSFM specifies electron and ion (H$\sp+$ and O$\sp+$) fluxes of energies between 100 eV and 100 keV in the distance range of 2-10 R$\sb{\rm E}$ on a two-dimensional grid. Comparison of CRRES observations from the August 26-27, 1990 magnetic storm with output from the progenitor of the MSFM, the Magnetospheric Specification Model (MSM), showed that the modeled electron fluxes were being depleted by the MSM loss algorithm before they could convect into the inner magnetosphere, thus creating an unacceptably large discrepancy between the output and the data. Based on the CRRES observations, adjustments were made to the loss algorithm which greatly enhanced the accuracy of the modeled fluxes in the inner magnetosphere, and these adjustments were incorporated into the MSFM. The MSFM was then subjected to an extensive test program in which the model was run for six unique magnetic storm intervals, and the accuracy of the modeled electron and ion fluxes at geosynchronous orbit and in the inner magnetosphere were quantified by calculation of standard deviations (RMS errors) between the extensive set of observational data assembled for each interval, and the model output. Overall, the accuracy of the MSFM electron and ion fluxes in the inner magnetosphere is quite good; the model moves roughly the right number of particles to roughly the right location. However, due to a fundamental lack of knowledge about the low-energy plasma environment in the inner magnetosphere, some inadequacies still exist in the MSFM loss algorithm which should be corrected when feasible to further improve its accuracy.
Design and calibration of the Suprathermal Ion Detector Experiment (SIDE)
(1969) Shane, Robert Lee; Freeman, John W., Jr.
The SIDE is one of the experiments for the Apollo Lunar Surface Experiments Package (ALSEP). It is designed to measure mass, energy per unit charge, and flux of positive ions at the surface of the moon. The SIDE contains two analyzers. One is for low energy ions; it consists of E X B velocity analyzer and a cylindrical curved plate energy analyzer aligned with a channeltron electron multiplier. The other is for high energy ions; it consists of a curved plate analyzer only. The combination of energy and velocity data provides both energy and mass analysis of the low energy ions. The high energy ions are energy analyzed only. Each element of the analyzers is discussed separately according to ion trajectories, passbands, and design characteristics. The calibration procedures are described, sample curves given, and results presented.
Determination of the polytropic index of the free-streaming solar wind
(1994) Totten, Tracy L.; Freeman, John W., Jr.
Observations of solar wind temperatures near the Earth indicate that heating of the solar wind plasma exists. An alternate approach to finding explicit heating terms for the energy equation is to use a polytropic approximation. Using data from the Helios 1 spacecraft, an empirical value for the polytropic index of the solar wind is found to be independent of speed state, within statistical error, and has an average value of 1.47. Application of this empirically derived index to a solar wind computer model is examined by comparing the MHD energy equation and the polytropic relation. The result is obtained that the polytropic index can replace the adiabatic index in the MHD energy equation to simulate the effects of heat conduction if the assumptions are made that the heat conduction flux has a specific form and the particle pressure has no explicit time dependence. Justifications and limitations of this approach are discussed.
Magnetic fields in protoplanetary disks
(1996) Reyes-Ruiz, Mauricio; Freeman, John W., Jr.
We study the origin, configuration and effects of magnetic fields in protoplanetary disks. Standard accretion disk models are adopted for protoplanetary disks to determine their electrical properties. A new computational approach to calculate the two-dimensional large scale magnetic field in thin disks, is used to study two scenarios previously proposed for the origin of magnetic fields in protoplanetary disks. We first consider the possibility that the accretion flow in a protoplanetary disk drags an external, uniform and vertical magnetic field to the required configuration to launch winds centrifugally. Results depend strongly on the magnetic Prandtl number of the prescribed turbulent motions in the disk. For fiducial values of such parameter, magnetic field dragging is unlikely to yield a configuration capable of driving cold winds centrifugally. For this to happen, the Prandtl number must be reduced by almost two orders of magnitude from the expected value, and still dragging in the disk's outer portions will be ineffective. In the second part of this thesis we calculate the magnetic configuration from an $\alpha\Omega$ dynamo operating inside a protoplanetary disk. A vacuum is assumed outside the disk. We incorporate a saturation mechanism for the dynamo instability to model the back reaction of the Lorentz force on the turbulent motions. This allows us to study the feasibility of achieving a wind-conducive magnetic configuration from the interaction of the dynamo field and a weak, externally generated, magnetic field. In general, our results indicate that some combinations of disk models and exterior magnetic field strengths result in portions of the disk being threaded by open field lines with the right configuration to drive winds. In summary, dynamo magnetic fields may be sustained in extended portions of protoplanetary disks for times comparable to the lifetime of the disk. However, the existence of an intermediate region, where the low ionization does not allow the field to be regenerated, is a general feature of viscous protoplanetary disk models. The contribution of the generated magnetic fields to the transport of angular momentum through the accretion disk, can be comparable to the effect of the turbulent viscosity.
Magnetosheath plasma at 60 Re
(1972) Fenner, Martha Ann; Freeman, John W., Jr.
The Apollo 12 Suprathermal Ion Detector Experiment on the surface of the moon observes magnetosheath plasma as the moon passes out of the earth's magnetosphere. The first six months of SIDE data in the sheath were analyzed for this thesis. A composite display of the energies and magnitudes of fluxes was presented for each magnetosheath crossing. Interesting features of the sheath were labeled on the composite display and later illustrated by individual energy spectra. For the magnetically disturbed month of July, 1970, a third type of display presented all of the energy spectra plotted vertically versus time. The observed features of the sheath include: 1. High energy peaks (~1kev to 3kev) associated with the boundaries and with disturbances in the sheath 2. A sharp transition between two intensity levels, suggesting a sudden exit from a high density region or a sudden change in flow direction 3. Two types of magnetopause crossings 4. Hydromagnetic waves or turbulence near the boundaries 5. A strong correlation between sheath data and geomagnetic activity.
Plasma flow at the magnetopause
(1968) Warren, Carlos Scott; Freeman, John W., Jr.
On January 13 and 14, 1967, an unusual compression of the magnetosphere during an intense magnetic storm caused the magnetopause to pass inside the orbit of the geostationary spin-stabilized satellite ATS-I. At the time of the boundary crossing, the satellite was located at 6.6 earth radii, approximately two hours past the noon meridian on the dusk side. The boundary crossing was marked by rapid magnetic field changes at the satellite position, and anomalously high anisotropic ion fluxes detected by the on-board Rice suprathermal ion detector. The Rice ion detector samples fluxes in the equatorial plane of the earth, in approximately 12° increments. There are 20 differential energy passbands covering 0-50 ev and two integral passbands that sample energies > 0 ev and >50 ev. A complete energy spectrum is covered in 112 seconds, while a 360° angular scan is completed in approximately 0.64 seconds. During the almost one hour the magnetopause was in the satellite's vicinity, several fluctuations in the local magnetic field and ion directional distributions indicated multiple boundary crossing by the ATS-I. In addition to the pure magnetosheath and pure magnetospheric ion flow patterns, a new component of ion flow was found close to, inside of, and along the magnetospheric boundary. The streaming energy of this flow along the boundary was less than that observed in the magnetosheath, but greater than that observed in the pure magnetosphere. This new component is thought to be the "return" flow (to the tail) for the magnetospheric thermal plasma found to be convecting sunward at points deeper within the magnetosphere.
Plasma flow in the magnetosphere under the influence of large scale electric field
(1968) Chen, Abel Jer-Jiunn; Freeman, John W., Jr.
The plasma flow is obtained by setting up a mathematical model, using the most appropriate geomagnetic field model and adopting the idea of equivalent potential in the magnetosphere The field model consists of four components: 1) Dipole main field; 2) Dayside magnetosphere boundary current; 3) Quiet day ring current; and 4) Infinite neutral current sheet suggested by Mead. The driving mechanisms are due to co-rotation with the Earth, the gradient drift, and gross electric field drift. The drift paths for thermal plasma and higher energy particles are presented. Some geophysical phenomena associated with the flow pattern, such as the development of asymmetric ring current, the formation of plasmapause and its characteristics, are discussed.
Recurring ion events at the lunar surface
(1972) Lindeman, Robert Allen; Freeman, John W., Jr.
Solar plasma disturbance observed by a suprathermal ion detector on the moon
(1972) Medrano-Balboa, Rene Adalid; Freeman, John W., Jr.
An interplanetary plasma disturbance was observed from the Moon by the Rice University Suprathermal Ion Detector Experiment (SIDE) on November 19, 1970 starting at 00h 16min 37sec U.T. The time of observation after the start of the disturbance was 130 minutes. The energy range covered by the detector is from 10 eV to 3500 eV in 20 steps. Major particle flux enhancements are found to be between energies 250 eV and 1000 eV. Fluxes at other energies are relatively unaffected. The detectors were pointing at 26° from the Sun's radial direction, the usual direction of the solar wind plasma flow. The gross structure of the event consists of two major enhancements in both the particle fluxes and velocities. The first enhancement included the initial 78 minutes, and was immediately followed by the second enhancement. Both enhancements may correspond to 2 solar flares from the same heliographic location. Significant flux enhancements in the fine structure are found to be density enhancements. These enhancements appeared many times in the first 78 minutes and less often in the rest of the observing time. Random variations in both flux and velocity in the last 30 minutes of the data was observed. Secondary peaks appear in the differential energy spectra, most of which are tentatively identified as a-particles. The locations of these peaks appear to correlate with the valleys between flux enhancements of the fine structure. Other secondary peaks at M/Z = 4 are suggested to indicate the presence of 4He+.
Statistical modelling of dynamic auroral fluxes
(1989) Shade, John William; Freeman, John W., Jr.
In order to obtain a better understanding of auroral processes, statistical models have been formulated that characterize the spatial dependences of the aurora. These efforts include the Hardy probability model, the Hardy average model, and the Evans average model. Each differs in its technical approach but all three attempt to characterize electron energy fluxes at any given location in the auroral zone. In an attempt to describe the limitations of each model and perhaps make suggestions on how one can improve them, we reduced a six month sampling of DMSP-F2 electron precipitation data and averaged values over four second time intervals. Then each model generated a flux value for comparison against these satellite measurements so that Chi-square tests could be performed. Also the mean values for the whole six month period were calculated in order to determine the normalcy of the period tested. It is hoped that the knowledge gained through this endeavor will improve the accuracy of the Magnetospheric Specification Model's forecasts of the fluxes that endanger spacecraft.

Browsing by Author "Freeman, John W., Jr."

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