R-3 Repository :: Browsing by Author "Wolf, Richard A."

Browsing by Author "Wolf, Richard A."

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A case study of the June 4--5, 1991 magnetic storm using the Rice Convection Model
(2000) Garner, Trevor Wynn; Wolf, Richard A.
This dissertation presents one of the most comprehensive computer simulations to date of a geomagnetic storm. During the geomagnetic storm of June 4--5, 1991, five spacecraft took measurements of conditions within the inner magnetosphere, including measurements of the electric and magnetic fields and the particle distribution at several different locations. These data are used to test the theoretical understanding of magnetospheric physics by comparisons to the Rice Convection Model (RCM). The RCM is a first-principles model of the inner magnetosphere that calculates the movement of magnetospheric particles, the currents into and out of the magnetosphere, and the magnetospheric electric field patterns. Furthermore, these comparisons provide answers to some of the pressing questions in magnetospheric physics: How strong does the shielding electric field become during a magnetic storm? Can a self-consistently calculated electric field inject plasma sheet particles to within 3 Earth radii of the Earth? How much of the increase in the ring current is due to the injection of plasma sheet particles and how much is due to the energization of pre-existing particles? The comparisons between the RCM electric field results and the corresponding measurements show very good agreement between the observations and the models, and indicate that a fairly strong electric field develops during the storm. Furthermore, comparisons of RCM calculated particle fluxes and the observations indicate that a self-consistently calculated electric field is able to inject plasma sheet particles deep into the inner magnetosphere, and that this injection is the dominant cause of the increased ring current.
A short study of the plasma sheet in the geomagnetic tail
(1972) Rich, Frederick Joseph; Wolf, Richard A.
This thesis approaches two theoretical problems associated with the plasma sheet in the geomagnetic tail. The first problem is the structure of the plasma sheet. It is approached by considering the plasma sheet as a sheet of collision-less plasma with isotropic gas pressure that is in a state of balance between the macroscopic forces. Only the forces due to the plasma gas pressure and the magnetic field forces are considered. The calculations are done in two dimensions by neglecting any Ysm component of the geomagnetic tail field. The discussion of force balance argues that, unless there is a significant contribution to the forces in the tail from some neglected source, the normal component of field must be on the average positive in all parts of the plasma sheet. A model configuration is computed as an example of force balance and yield the relation Bzo /(dp * Bxx) = 0.72 where Bzo is the normal component of field in the center of the sheet (gammas), dp is the sheet half thickness (Re), and Bxx is the radial gradient of the external field strength Also, the z-variation of the plasma sheet field strength show no presence of a neutral sheet, a thin (~10 to the 3rd km) reversing layer with a field strength of several gammas on either side, within the plasma sheet. Indeed, a neutral sheet may be inconsistent with a force balance plasma sheet unless additions balancing forces or anisotropies of the plasma pressure are considered. The second problem is a study of the effect of a geomagnetic-like neutral sheet, a thin current sheet where ionic, and maybe electronic, motions are non-adiabatic, on plasma. The results of previous studies by Speiser are found to be basically correct but limited. It is shown that the energy, pitch angle and phase angle dependence of the change in pitch angle of ions traveling through the neutral sheet makes it unlikely that the neutral sheet is responsible for direct precipitation of plasma sheet ions into the auroral ionosphere as suggested by Speiser, The effective conductivities of the neutral sheet needed to describe the neutral sheet with MHD are found to be useful but accurate only as a rough estimate of the time effective conductivity which may be a function of position and other parameters.
An experimental study of electron thermal runaway in the lower ionosphere
(1979) Coco, David Stephen; Gordon, William E.; Anderson, Hugh R.; Wolf, Richard A.
An ionospheric heating experiment has been performed at the Arecibo Observatory using the 43 MHz incoherent backscatter radar both as a heater and as a diagnostic. Radar pulses up to 9 msec in length were transmitted with 2.5 MW (2.5 x 1^erg/sec) peak: pulse power yielding a peak power flux of approximately 2. x 1 erg/cm sec at 1 km altitude. A 2 microsecond diagnostic pulse offset in frequency from the heating pulse was used to measure the resultant ohmic heating of electrons in the lower ionosphere. Using a model of electron heating and cooling in the lower ionosphere, the ratio of heated electron temperature to unheated electron temperature is calculated as a function of altitude and heating power flux. This model predicts a beam-averaged electron temperature ratio of about 2.25 at 1 km for a transmitted power of 2. MW, pulse length of 9 msec and an antenna efficiency of 5%. When the predicted beamaveraged electron temperature ratios are compared with the observed, the observed are found to be less than the predicted for all values of transmitted power used. Possible sources of the discrepancy between experiment and theory are discussed.
Analysis of the sudden commencement of February 15, 1967
(1972) Lin, Chong'an; Wolf, Richard A.
A sudden commencement occurred at. 2348 UT on February 15, 1967 when the ATS-1 satellite was 2 hours past local noon at a geocentric distance of 6.6 Re. Plasma was observed by the Suprathermal ion Detector first to flow in the antisolar direction, as expected, but then to flow westward, for about 2 minutes, at about 50 km/sec. Analysis of ground magnetograms suggests that the surprising westward flow at 6.6 R resulted the ionosphere's reaction to the sudden commencement. Beginning about 2 minutes before the start of westward flow at ATS-1,ground magnetometers near the foot of the ATS-1 field line recorded average magnetic-field deflections of about 100Y ,to the northeast. For an assumed height-integrated Hall conductivity of 1 mho, and a standoff distance of 7.2 R derived from Explorer 33 plasma data, the ground magnetic variations imply an electric field that agrees in magnitude and direction with that required to produce the observed flow at ATS-1.
Bifurcation of drift shells near the dayside magnetopause
(2004) Ozturk, M. Kaan; Wolf, Richard A.
The dayside magnetosphere contains a region where the field strength has a local maximum. This region, located just inside the magnetopause around the equatorial plane and between the cusps, has a width of 2--3 Re. When a drift shell with a sufficiently small mirror field intersects this region, it will bifurcate into two branches near local noon, each branch going across one cusp and joining together at the symmetrical local time. The particle then drifts around the Earth over a single branch until it comes back to local noon. In the neighborhood of the bifurcation points, the bounce period tends to infinity, and thus the adiabaticity of the bounce motion is broken there, but not elsewhere. This breaking causes a small but finite jump Delta I in the second invariant. Repeated crossings lead to a random walk in second invariant space, and thus to radial diffusion. We use theory and simulations to determine the magnitude of DeltaI. Our study is limited to static magnetic fields, but it can be extended to general fields. Our results indicate that DeltaI is sensitively dependent on bounce phase at bifurcation, and it can grow significantly for some initial conditions. When the initial second invariant I0 is much larger than the mirror gyroradius rhom, we use separatrix crossing theory. The average of DeltaI over bounce phases is zero, and the rms DeltaI is of the order of rho m. When I0 is comparable to rhom , the equation of bounce motion is approximated as the second Painleve equation, whose asymptotic solutions are used to determine Delta I. In this limit, the rms DeltaI is still O (rhom); however, the average is nonzero, in the form exp(- I0/rhom). Drift-shell bifurcation leads to significant radial diffusion. For MeV electrons, the diffusion coefficient can be several R e per day. Also, because of bifurcation, some quasitrapped particles can remain in the magnetosphere for a finite number of drifts before they leave permanently. Such behavior leads to metastable particles, a new kind of trapping. These results can be useful for radiation-belt modeling efforts.
Calculation of magnetic effects due to a model substorm current system
(1979) Calder, Alexander C.; Wolf, Richard A.
A computer code has been developed for calculating ground magnetic perturbations due to a substorm current system in the magnetosphere. The relevant currents are determined using results of a numerical simulation of the substorm itself devised by R. A. Wolf and collaborators. The magnetic fields due to the currents are calculated numerically using Ampere's law. The substorm simulation and its relation to the current calculation are briefly described. The assumed paths and means of calculating the currents are discussed. The results of the first run are presented and compared with observational data from the AFGL magnetometer array.
Comparison study of ring current simulations with and without bubble injections
(Wiley, 2016) Yang, Jian; Toffoletto, Frank R.; Wolf, Richard A.
For many years, stand-alone ring current models have been successfully producing storm time ring current enhancements without specifying explicit localized transient injections along their outer boundaries. However, both observations and simulations have suggested that the frequent burst flows or bubble injections can contribute substantially to the storm time ring current energy. In this paper, we investigate the difference in the spatial and temporal development of the ring current distribution with and without bubble injections using the Rice Convection Model-Equilibrium. The comparison study indicates that the simulation with bubble effects smoothed out along geosynchronous orbit can predict approximately the same large-scale ring current pressure distribution and electric potential pattern as the simulation with bubble effects included. Our results suggest that the increase of the hot plasma population along geosynchronous orbit can be envisaged as an integrated effect of bubble injections from the near-Earth plasma sheet. However, the observed fluctuations in the plasma population and electric field can only be captured when the mesoscale injections are included in the simulation. We also confirmed again that adiabatic convection of fully populated flux tubes cannot inject the ring current from the middle plasma sheet. The paper provides a justification for using stand-alone ring current models in the inner magnetosphere to simulate magnetic storms, without explicit consideration of bubbles and magnetic buoyancy effects inside geosynchronous orbit.
Coupling of two computational models of the Earth's magnetosphere
(1997) Hojo, Michikazu; Wolf, Richard A.
The first major step has been completed in a long range project to merge the Fedder-Lyon Global 3D magnetohydrodynamic code and the Rice Convection Model (RCM) of the Earth's magnetosphere. Using MHD results as initial and boundary conditions, RCM runs were carried out for three different values of the energy invariant $\lambda$ of the plasma-sheet ions: $\lambda$ = negligibly small as in ideal MHD, $\lambda$ estimated from global MHD results, and $\lambda$ estimated from observations. In the first two runs, the RCM produced thin, well-defined patterns of region-2 magnetic-field-aligned currents shielding the inner magnetosphere from the convection electric field. These results differed substantially from the MHD result, indicating inaccuracy in the MHD code's numerical method when applied to the inner magnetosphere. The third run produced weak shielding and non-classic current patterns, which provide insight into the effect of plasma-sheet temperature on shielding.
Criterion for interchange instability in the plasma sheet
(2008) Xing, Xiaoyan; Wolf, Richard A.
Interchange instability is an important dynamic mechanism in plasma physics and has been advanced as an explanation of a variety of phenomena in the magnetospheric physics. This work derives a new instability criterion for interchange motion in a plasma that connects to a finite-conductivity wall. The new criterion is for a arbitrary magnetic <beta> (ratio between thermal pressure and magnetic pressure averaged within flux tube) system, which contains background shear flow, whereas most classical criteria did not consider all of these conditions. Thus this new result is more appropriate to be applied in a real plasma system like the Earth's plasma sheet, which exhibits a wide range of <beta> values and background shear flow. Based on magnetosphere-ionosphere coupling theory and ideal MHD adiabatic theory in the inner plasma sheet, a theoretical model was constructed in the ionosphere region. A finite boundary layer was set up between two regions of uniform-content flux tubes, and a perturbation on the boundary layer was investigated. Both analytical and numerical approaches are used to study the stability of the plasma configuration. The flux tubes are interchange unstable when the angle between the gradient of flux tube volume, defined as V = dsB , and the gradient of adiabatic specific entropy PV 5/3 is larger than arccos<b> 1lnPV5/3 1ln V/ 21+5<b>/6 . Combining this new criterion with the statistical calculation of the plasma sheet characteristics by using the Tsyganenko magnetic field model (the 1996 version) and the Tsyganenko-Mukai plasma model, it is found that, in the Earth's inner plasma sheet, the angle between the two gradients is typically of the order of 15°, which indicates that the statistical-average Earth's plasma sheet is interchange stable. This result is applicable to the study of interchange instability and plasma transport in the global-MHD and other large-scale magnetosphere simulations, and provides a theoretical base for the study of analogous dynamic processes in the magnetospheres of other planets like Jupiter.
Current Sheet Thinning in the Wake of a Bubble Injection
(Wiley, 2022) Wang, Wenrui; Yang, Jian; Toffoletto, Frank R.; Wolf, Richard A.; Nakamura, Rumi; Cui, Jun
A crucial property of the substorm growth phase is the current sheet thinning, which is often attributed to adiabatic convection. Injecting low-entropy bubbles reduce pressure-balance inconsistencies and restore current sheet thickness to its initial value. Recent observations from Cluster and MMS showed additional thinning of the current sheet in the wake of a transitory bubble injection compared with the configuration before the injection. We employ the Rice Convection Model-MHD coupled code to investigate how the transport of bubbles causes the fast thinning. The simulation results reconstructed the observations' most prominent characteristics. We find more earthward transport of the magnetic flux in the bubble than its wake, therefore depleting magnetic flux and stretching the magnetic field lines there. Interestingly, additional R0-sense field-aligned currents close the enhanced dawn-to-dusk current behind the bubble.
Does chaos matter in the plasma sheet?
(1995) Usadi, Adam Keith; Wolf, Richard A.
Can the average bulk flow of an ensemble of charged particles in Earth's plasma sheet still be described by adiabatic theory even if the ensemble contains a significant number of particles executing non-adiabatic motion? This is part of a broader spectrum of questions which ask if chaotic microscopic processes can be parametrized as macroscopic ones when ensemble averaged. Wolf and Pontius (1993) have shown that at least for a simple 2D, tail-like magnetic field configuration, the average particle drift speed of an appropriately chosen ensemble of particles, including those executing chaotic motion, is given correctly by the simple adiabatic guiding-center drift formula. Here, we extend the proof to 2${1\over2}$D magnetic fields (3 component, 2 spatial dependences) and include the effects of an electric field. The results of numerical test-particle simulations further show that the dispersion of particles about the mean drift speed tends to decrease due to the presence of chaotic particle scattering. Thus, we have shown that the standard way of representing particle transport in the inner magnetosphere, namely the isotropic pitch angle, bounce averaged drift formalism, is valid for the central plasma sheet despite the presence of non-adiabatic particle motion.
Double-adiabatic MHD theory of a thin filament in the geotail and possible applications to bursty bulk flows and substorms
(2004) Ji, Shuo; Wolf, Richard A.
During fast fluid flows in Earth's magnetotail, the plasma distribution function often takes the form of one beam flowing through another, which raises the question of whether Bursty Bulk Flows (BBF's) can reasonably be represented in terms of single fluid magnetohydrodynamics (MHD), either in global MHD codes or in thin-filament theory. An exact kinetic solution is compared with exact fluid solutions for a simplified case of cold, collisionless particles in a pipe, under conditions where there are counter-streaming beams similar to the ones that often occur in Earth's magnetotail. The results from kinetic theory differ from standard fluid theory but are exactly consistent with Chew-Goldberger-Low double-adiabatic fluid theory. Double-adiabatic MHD equations are derived for the motion of a thin filament through a medium. Simulation results are presented for a double-adiabatic filament that starts out with lower gas pressure than nearby flux tubes and also for plasma ejected earthward from a patch of reconnection at X ∼ -25 RE. As in earlier calculations for the isotropic case, in both cases the near-equatorial part of the filament moves rapidly earthward. A compressional shock wave forms in the filament near the equatorial plane and propagates earthward. The near-equatorial region of the filament exhibits characteristics similar to a flow burst, while the behavior far from the equatorial plane resembles that of earthward-streaming plasma-sheet boundary layer. In both cases, the double-adiabatic filament becomes firehose unstable after the shock wave reflects from the earthward boundary of the simulation and propagates back into the tail. The tailward-propagating compressional wave, which brakes the earthward flow in the filament, is thus characterized by strong magnetic fluctuations. Within the context of the Near-Earth-Neutral-Line model of substorms, we suggest that firehose instability might cause the intense magnetic-field fluctuations that are observed in the inner plasma sheet at substorm onset. Additional simulations have been carried out to confirm the robustness of our principal conclusion that fast earthward flows in the Earth's plasma sheet should lead to firehose instability.
Earthward motion of depleted flux tubes in the Earth's plasma sheet: MHD model calculations
(1995) Chen, Chuxin; Wolf, Richard A.
A magnetohydrodynamic (MHD) theory has been developed for the motion of a thin magnetic flux tube through a two-dimensional stationary medium that is in MHD equilibrium. The flux tube is represented as a one-dimensional filament. Simple properties of the computed time development of the filament have been explored analytically, including linear intermediate and slow-mode waves and corresponding MHD shock solutions. Numerical solutions for filaments in the tail display the strong earthward flow and dipolar shapes that are characteristic of bursty bulk flows that are frequently observed in the plasma sheet of the Earth's magnetosphere.
Electrodynamics of the low-latitude ionosphere
(1994) Riley, Peter; Wolf, Richard A.
We have undertaken a study of the low and mid latitude ionospheric electric field pattern, during both magnetospherically quiet and active periods. Our analysis can be conveniently split into two parts. i.In an effort to study the penetration of magnetospheric electric fields to low latitudes, we have compared Jicamarca F-region vertical drifts for 10 radar-observation periods with the auroral boundary index (ABI). The ABI is the latitude of the equatorward edge of the diffuse aurora at local midnight, as estimated from precipitating-electron fluxes measured from DMSP spacecraft. The periods occurred in the interval January 1984 to June 1991 inclusive and each lasted between 2 and 5 days. We focus on periods that occurred in September 1986, March 1990, and June 1991. In the post-midnight sector, where we expect the penetration to be strongest, we found many examples of correlation; specifically, associated with an ionospheric updraft (implying an eastward electric field) is a strong poleward motion of the auroral boundary. However, we also found a significant number of cases where there was little or no correlation. We conclude that there is only mediocre agreement between the observed Sudden Postmidnight Ionospheric Events (SPIEs) and the ABI. These SPIEs have also been compared with other magnetospheric parameters, namely $D\sb{\rm st}$ IMF $B\sb{z}$ and the polar cap potential. $D\sb{\rm st}$ showed significantly better correlation with the SPIEs. We summarize the proposed models for SPIEs and compare their predictions with the data, concluding that no single model can account for all events. While it is clear that some of these SPIEs can be explained in terms of direct penetration of magnetospheric electric fields, we suggest that the remainder may be due to magnetospherically-generated neutral wind effects. ii. We have constructed a model of the low- and mid-latitude potential distribution, applicable for both quiet and active times. We use the Mass-Spectrometer-Incoherent-Scatter (MSIS) model to input the number densities and temperature of the neutral species, and the International reference Ionosphere (IRI) model to input the electron/ion densities and temperatures. As our wind input we use the Horizontal Wind Model (HWM). We find that our model can reproduce the all of the main features of the low latitude ionosphere during quiet times, and supports some of our ideas about magnetospheric penetration during active periods. We use the model to probe the dependency of the low latitude penetration pattern on solar conditions and season and found that the inferred equatorial drifts are relatively insensitive to either. Thus we conclude that ionospheric pre-conditioning is unlikely to play a significant role. On the other hand, the low latitude penetration pattern is strongly dependent on the assumed poleward boundary.
Instantaneous recycling approximation in chemical evolution of galaxies
(1985) Pantelaki, Irini A.; Clayton, Donald W.; Wolf, Richard A.; Dufour, Reginald J.
The purpose of this study is to determine the range of validity of the instantaneous recycling approximation (IRA) in the models for the chemical evolution of galaxies. To achieve this and in order to be able to follow the evolution of the gas mass and the gas metallicity with time without making the instantaneous recycling assumption, I constructed a simple numerical code. This same code was later converted to a form assuming the instantaneous recycling approximation and the two groups of solutions (with and without IRA) were compared. The results show that in the case of a star formation rate which at early times builds up to a maximum and later declines, a physically important departure from the instantaneous recycling approximation is observed in: a) The evolution of the gas mass at late times when the remaining gas mass density becomes a small fraction of the total density and b) the time evolution of the abundances in the interstellar gas of the elements contributed by the low mass stars. A less significant discrepancy between the two solutions (with and without IRA) appears at late times in the cases of the abundances of the elements whose origin are the intermediate and high mass stars.
Interpretation of high speed flows in the plasma sheet
(1994) Chen, Chuxin; Wolf, Richard A.
We propose that the "bursty bulk flows" are "bubbles" in the Earth's plasma sheet. Specifically, they are flux tubes that have lower values of $pV\sp{5/3}$ than their neighbors, where p is the thermal pressure of the particles and V is the volume of a tube containing one unit of magnetic flux. Whether they are created by reconnection or some other mechanism, the bubbles are propelled earthward by a magnetic-buoyancy force, which is related to the interchange instability. Most of the major observed characteristics of the bursty bulk flows can be interpreted naturally in terms of the bubble picture. We propose a new "stratified fluid" picture of the plasma sheet, based on the idea that bubbles constitute the crucial transport mechanism. Results from simple mathematical models of plasma-sheet transport support the idea that bubbles can resolve the pressure-balance inconsistency.
Magneto-hydrostatic equilibrium configurations for the magnetosphere of Uranus
(1984) Ip, Antonio K.; Wolf, Richard A.; Dunning, F. Barry; Baker, Stephen D.
At the time of the Voyager 2 flyby of Uranus, the rotational axis of the planet will be roughly antiparallel to the solar wind flow. If Uranus has a magnetic dipole moment that is approximately aligned with its spin axis, we will have the rare opportunity to observe a ''pole-on'' magnetosphere as discussed qualitatively by Siscoe(1971). Due to the axial symmetry of a pole-on magnetosphere we are able to utilize the Grad-Shafranov theory for calculating a self-consistent magneto-hydrostatic equilibrium model of the magnetosphere of Uranus. In this paper we present an analytic model for the linear case. We also mention an approximation method which enables us to calculate the tail configuration when different relations between the thermal plasma pressure and the vector potential are assumed.
Modeling the dynamics of outer radiation belt electrons
(2000) Naehr, Stephen Michael; Wolf, Richard A.
A computer model has been built to simulate the dynamic evolution of relativistic electrons in the outer radiation belt. The model calculates changes in electron flux due to three mechanisms: (1) fully-adiabatic response of electrons to variations in the magnetic field, (2) time-dependent radial diffusion, parameterized by overall magnetospheric activity, and (3) penetration of new particles into the model via a time-dependent outer boundary condition. Data from Los Alamos geosynchronous satellites, the CRRESELE statistical electron flux model, the Kp index, and the Toffoletto-Hill-Ding magnetic field model are all used to provide realistic, time-dependent inputs to the model. To evaluate the model, a simulation of the radiation belts during the November 3--12, 1993 magnetic storm was generated. Comparison of results to Global Positioning System (GPS) radiation dosimeter data indicates that the model can accurately predict storm-time flux variations for electrons with energies less than 600 keV. Modeled fluxes for higher energy electrons show insufficient enhancement during the recovery phase of the storm, suggesting the existence of an acceleration mechanism other than fully adiabatic variations and radial diffusion.
On the contribution of plasma sheet bubbles to the storm time ring current
(Wiley, 2015) Yang, Jian; Toffoletto, Frank R.; Wolf, Richard A.; Sazykin, Stanislav
Particle injections occur frequently inside 10 Re during geomagnetic storms. They are commonly associated with bursty bulk flows or plasma sheet bubbles transported from the tail to the inner magnetosphere. Although observations and theoretical arguments have suggested that they may have an important role in storm time dynamics, this assertion has not been addressed quantitatively. In this paper, we investigate which process is dominant for the storm time ring current buildup: large-scale enhanced convection or localized bubble injections. We use the Rice Convection Model-Equilibrium (RCM-E) to model a series of idealized storm main phases. The boundary conditions at 14–15 Re on the nightside are adjusted to randomly inject bubbles to a degree roughly consistent with observed statistical properties. A test particle tracing technique is then used to identify the source of the ring current plasma. We find that the contribution of plasma sheet bubbles to the ring current energy increases from ~20% for weak storms to ~50% for moderate storms and levels off at ~61% for intense storms, while the contribution of trapped particles decreases from ~60% for weak storms to ~30% for moderate and ~21% for intense storms. The contribution of nonbubble plasma sheet flux tubes remains ~20% on average regardless of the storm intensity. Consistent with previous RCM and RCM-E simulations, our results show that the mechanisms for plasma sheet bubbles enhancing the ring current energy are (1) the deep penetration of bubbles and (2) the bulk plasma pushed ahead of bubbles. Both the bubbles and the plasma pushed ahead typically contain larger distribution functions than those in the inner magnetosphere at quiet times. An integrated effect of those individual bubble injections is the gradual enhancement of the storm time ring current. We also make two predictions testable against observations. First, fluctuations over a time scale of 5–20 min in the plasma distributions and electric field can be seen in the central ring current region for the storm main phase. We find that the plasma pressure and the electric field EY there vary over about 10%–30% and 50%–300% of the background values, respectively. Second, the maximum plasma pressure and magnetic field depression in the central ring current region during the main phase are well correlated with the Dst index.
Optical emission measurements during beam plasma interactions
(1985) Mantjoukis, George A.; Haymes, Robert C.; Bernstein, William; Wolf, Richard A.; Freeman, John W.
3914 A light intensity profiles were measured with a geometrically scanning photometer during interations of an energetic electron beam with a weakly ionized plasma. Mass spectrometric measurements indicated N^ to be the dominant neutral constituent. The experiments were carried in the very large SESL vacuum facility at the Johnson Space Center which allowed an interaction length of ~ 2 m. Variable current (1-7 ma) and variable energy (5-16 V) beams were injected over a large pitch angle range ( to ~75°) for several applied magnetic field strengths (.89, 1.52, and 2.22 Gauss) over the pressure range .6-3. x 1“® Torr to allow study of the light intensity dependence on the experiment parameters. The photometer measurements were made at a fixed axial position (midway between the electron gun and the collector); overall measurements of the complete beam emission patterns (total light) were made with several low light level TV cameras. The measured 3914 Â intensity profile gives the radial distribution of the relative ionization rate while its integral gives the (relative) total ionization rate independent of geometry. The following important results were obtained 1) At low beam currents (1^ < Ic, the beam current required for BPD ignition) the relative light intensity and beam geometric configuration were consistent with single particle behavior. 2) For I], > Ic (BPD) the geometry of the illuminated region changes drastically and the 3914 total intensity increases by factors of 1-3 indicating the presence of new ionization sources (suprathermal electrons). (a) For beam injection parallel to the magnetic field (zero pitch angle), the radial width (FWHM) of the illuminated region is approximately twice the maximum (anti-node) width for preBPD conditions; it scales approximately as 1/B and E1/2. (b) For non-zero pitch angle injection, the full width of the illuminated region is approximately equal to the diameter of the single particle helix; at large pitch angle injection, the BPD shows significant limb brightening indicating a somewhat hollow configuation. Significant ionization outside the helical dimensions is not observed. (c) For all conditions the total 3914 light intensity during BPD can be fitted to the parabolic relationship QT ~ K Ic1/2 (Ib - Ic)1/2 Thus the total ionization rate increases nonlinearly with I (the nominal power supply return current) and remains proportional to the square root of the threshold current during BPD although I|j » Ic. The consistency of this scaling over the large injection pitch angle range implies that the important plasma processes are Independent of injection pitch angle. (d) The maximum efficiency of energy transfer from the beam to the plasma (as measured by the total ionization rate) occurs when I|j 2 Ic. Typical estimates of this maximum efficiency range from 3-12Z for the 2 meter path length.

Browsing by Author "Wolf, Richard A."

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