• Deactivation and excitation of 0I(6s ⁵S) at above-thermal energies

      Enzweiler, Joseph A. (1982-05)
      Absolute collisional deactivation and excitation cross sections have been measured for a beam of O (⁵P-⁵S°) incident on N₂. The beam energy was varied from 3.95 to 10.65 keV. Cross sections for charge transfer (electron capture) of 0⁺ in N₂ were also measured in the energy range from 2.4 to 24.3 keV. The variation of light intensity 5436 Å (from the 6s-3p transition of the quintet system of atomic oxygen) emitted from the beam as a function of N₂ target pressure was fitted to a beam of kinetic equation to determine deactivation and excitation cross sections. The cross section for deactivation in N₂ decreases from 6.84 x 10⁻¹⁵ cm² at 3.95 keV to 1.5 x 10⁻¹⁵ cm² at 19.65 keV. The cross section for excitation decreases from 3.3 x 10⁻¹⁹ at 3.95 keV to 2.25 x 10⁻¹⁹ cm² at 19.65 keV.
    • Electron transport and optical emissions in the aurora

      Lummerzheim, Dirk; Rees, M. H.; Lee, L. C.; Olson, J. V.; Royer, T. C.; Stamnes, K. (1987)
      A one-dimensional, steady state auroral model is developed based on a linear electron transport calculation. A set of cross sections for electron neutral collisions describing elastic scattering, energy loss, and photon emission is compiled and used in conjunction with a discrete ordinate transport code. Calculated electron intensities are compared with in situ rocket measurements. Auroral optical emissions that result from direct electron impact on neutrals are calculated for synthetic and observed electron spectra. A systematic dependence of the brightness of auroral features on energy flux, characteristic energy, and atmospheric composition is found and parameterized. A method for interpreting the brightness and the ratio of brightnesses of certain auroral emissions in terms of the energy flux, characteristic energy, and relative oxygen density is described. Application of this method to auroral images acquired by nadir viewing instruments aboard a satellite is discussed and the distribution of energy flux, characteristic energy, and ionospheric conductances over the auroral oval is determined. Emissions that are suitable for analysing auroral spectra in terms of the atomic oxygen abundance in the auroral zone are identified.
    • Suprathermal electron tails in a beam-plasma instability

      Hollerbach, Uwe; Swift, W.; Stenbaek-Nielsen, H. C.; Rees, M. H.; Kan, J. R. (1987-09)
      This study investigated the suprathermal electron tails produced in a beam-plasma instability, and their scaling with beam and background densities. A periodic one-dimensional electrostatic simulation was used to study the suprathermal tails. Electrons were treated as particles, and ions were treated as a fluid. The simulation showed that ion dynamics are required for the formation of the suprathermal tails, as expected from the theory of the oscillating two-stream instability. The energy of the suprathermal tails is directly proportional to the beam density, and does not depend strongly on the background density. There is a slight decrease in the energy of the suprathermal tails as the background density increases. A novel numerical effect was also found: a three-plasmon interaction caused by the modification of the Langmuir wave dispersion relation when high-order splines are used as particle shape factors.
    • An anomalous process of fast ionization of a barium shaped charge release

      Xin, Wei; Swift, D. W.; Smith, R. W.; Kan, J. R. (1987-12)
      Fast ionization in excess of the photo-ionization rate appears to only occur when the barium shaped charge release is along the magnetic field direction. This thesis investigates the hypothesis that rapid ionization is caused by electrons heated in an ion cyclotron wave excited by the field-aligned streaming of barium ions through the ambient ionospheric plasma. The seed population of barium ions is assumed to be due to photo-ionization. The number density of barium ions due to photo-ionization is calculated. The plasma dispersion relation is derived based on the assumption of collisionless plasma. The excitation of barium ion cyclotron waves due to the interpenetrating of barium ions through the ambient plasma is investigated. It is proposed that the electrons are heated by the Doppler shifted waves via Landau damping. The Doppler shift is caused by an ambipolar electric field generated by the finite divergence of the injected barium neutrals.
    • Effects of gravity waves on the polar oxygen and hydroxyl airglow

      Viereck, Rodney Allen; Deehr, Charles S.; Degen, V.; Fritts, D. C.; Rees, M. H.; Smith, R. W. (1988)
      The effect of gravity waves on the OH (87 km) and O$\sb2$ (95 km) airglow emissions was examined using spectroscopic airglow data. The data was obtained from Longyearbyen, Svalbard (78$\sp\circ$N) and Fairbanks, Alaska (64$\sp\circ$N) using Ebert-Fastie spectrometers and a system of Meridian Scanning Photometers. The spectrometers scanned in wavelength from 8200A to 8750A which included the airglow emissions from the OH(6-2) Meinel band and the O$\sb2$(0-1) atmospheric band. The analysis was done by fitting a synthetic spectrum to the data and thereby the rotational temperature was calculated as well as the band intensity of each of the emissions. The rotational temperatures were assumed to represent the temperature of the emission region. Gravity waves were assumed to modify the density and temperature of the atmosphere in the region of the airglow emissions. These modifications were measured as fluctuations in the band intensity and rotational temperatures of the two emissions. In order to compare the data with theoretical models, it was necessary to calculate two parameters. The parameter $\eta$ is defined as the ratio of the amplitudes of the fluctuations in intensity and temperature. The other parameter is the phase angle between the fluctuations in intensity and temperature. These parameters were found to vary with wave period. The variations in $\eta$ and phase agreed fairly well, for long period waves, with the most recent models. None of the models agree with the observed values of $\eta$ and phase for short period waves. The second part of this thesis examines the vertical and horizontal wavelengths, the phase speeds, and the propagation directions of several specific gravity wave examples. During a 60 hour period of data taken from Svalbard, three well defined gravity waves were observed. The propagation directions implied a moving source south of the observing station.
    • The quasiparallel collisionless shock wave: A simulation study

      Mandt, Mark Edward; Kan, J. R.; Das, D.; Lee, L. C.; Olson, J. V.; Swift, D. W. (1988)
      The structure of the quasi-parallel collisionless shock wave is studied via a numerical simulation model. The model is compared to observations and theoretical predictions and within its limitations appears to reproduce the true shock structure reasonably well. Three electron equations of state and their effects on the simulation are examined. It is found that only the isotropic-adiabatic electron equation of state yields acceptable results in the simulation at high Mach numbers. The scale lengths of the shock are measured, normalized by the natural scale lengths of the plasma, and plotted as a function of the Alfven Mach number. It is found that the wavelength of the upstream waves follows that predicted for a phase standing whistler quite well and the scalelength of the jump in the magnitude of the magnetic field is generally greater than, but approximately equal to this wavelength. For Alfven Mach numbers $M\sb{A} >$ 2.5, waves are generated in the downstream region. Their wavelength and the scale length of the plasma transition are larger than the natural scale lengths of the plasma. The ion heating is seen to occur in two stages. In the first stage which occurs upstream of the principal shock ramp, the heating can be characterized by a polytropic power law equation of state with an exponent much greater than the isentropic-adiabatic rate of $\gamma$ = 5/3. The second stage of heating which occurs from the principal shock ramp to the downstream region is characterized by an exponent on the order of the isentropic-adiabatic rate. The results show that the ion heating occurs mainly around the principle density jump near the center of the shock transition region, while the increase in entropy takes place mainly in the upstream side of the shock transition region. It is suggested that the ion heating is a consequence of the non-adiabatic scattering of the ions through the magnetic field of the shock and its upstream precursor wave.
    • Brucella suis type 4 in foxes and their role as reservoirs/vectors among reindeer

      Morton, Jamie Kay; Williamson, Francis S. L. (1989)
      Field and laboratory studies were conducted to test the hypotheses that (1) the reindeer/caribou organism, Brucella suis type 4, is incidentally transmitted to reindeer predators such as foxes but does not cause reproductive disease in them, and (2) infected predators such as foxes are terminal hosts and do not serve as reservoirs of infection for reindeer. In field collections, serologic prevalence of brucellosis was similar for male and female foxes (Vulpes vulpes and Alopex lagopus). B. suis type 4 was isolated from female Vulpes and Alopex. No association between reproductive status of foxes and brucellosis infections was observed. Serologic titers in Vulpes experimentally infected by oral exposure to Brucella suis type 4 were detected first by the standard tube and plate agglutination tests which were followed by the buffered Brucella antigen, rivanol, and complement fixation tests. Brucella suis type 4 was isolated from the feces 4 to 6 days post-exposure (PE) and from the oral cavity for as long as 3 weeks PE in Vulpes challenged with 10$\sp9$ or 10$\sp{11}$ colony forming units. Brucella suis type 4 was isolated frequently from regional lymph nodes in the head up to 18 weeks PE, and from only more distant nodes at 22 and 66 weeks PE. Organisms did not localize in the reproductive tract. Clinical effects of brucellosis in Vulpes experimentally-infected were not observed. Pathologic lesions were not detected in the male and non-gravid female reproductive tract. Due to breeding failure, effects of Brucella suis type 4 on the pregnant fox reproductive tract were not determined in experimental infections. Gross and microscopic pathology was limited to lymph nodes. Fox to fox transmission attributed to aerosols from products shed by infected foxes occurred readily. Transmission from Vulpes to lemmings (Dicrostonyx rubricatus) that were exposed to urine from infected fox occurred frequently. Transmission from infected Vulpes to two reindeer (Rangifer tarandus) occurred under conditions of close confinement. Ingestion of organisms passed mechanically in the fox feces was considered the probable source of infection. Fox saliva containing Brucella was also implicated in transmitting the organism through bites or aerosols.
    • A simulation study of magnetic reconnection processes at the dayside magnetopause

      Shi, Yong; Lee, L. C.; Swift, D. W.; Gosink, J.; Gatterdam, R.; Akasofu, S-I. (1989)
      In this thesis, the dayside reconnection processes are studied by using computer simulations. First, the global magnetic reconnection patterns at the dayside magnetopause are studied based on a two-dimensional incompressible magnetohydrodynamic (MHD) code. It is found that multiple X line reconnection may prevail at the dayside magnetopause when the magnetic Reynolds number is large ($>$200). The formation and subsequent poleward convection of magnetic islands are observed in the simulation. The Alfven Mach number of the solar wind, $M\sb{Asw}$, can also change the reconnection patterns. For a large $M\sb{Asw}$, reconnection tends to occur at the higher latitude region. Secondly, the structure of the dayside reconnection layer is studied by a two-dimensional compressible MHD simulation. In a highly asymmetric configuration typical of the dayside magnetopause, the pair of slow shocks bounding the reconnection layer in Petschek's symmetric model is found to be replaced by an intermediate shock on the magnetosheath side and a weak slow shock on the magnetospheric side. In addition, a mechanism for the enhancement of $B\sb y$, which is observed in the magnetopause current layer and magnetic flux tubes, is proposed.
    • Electrostatic ion cyclotron waves in barium injection experiments in space

      Kangas, Kim A.; Swift, D. W.; Stenbaek-Nielsen, H. C.; Kan, J. R. (1989-05)
      Electrostatic ion cyclotron waves are investigated in a charge-generated barium-shaped plasma directed parallel to the earth’s magnetic field. The barium plasma is generated as a result of a barium shape charge release in the upper F₂ region of the ionosphere undergoing photoionization, Using a differential velocity distribution given by Stenbaek-Nielsen et al., [1984], this situation has been modeled based on the condition of collisionless plasma. The instabilities were studied for cases with and without an ambient oxygen ion background. It was concluded that fast ionization in excess of photoionization due to the excitation of electrons by electrostatic ion cyclotron waves was not feasible for the ejection directed along the earth’s magnetic field nor would there be any contribution to Alfven’s critical velocity mechanism if the injection was directed perpendicular to the magnetic field.
    • A simulation study of magnetic reconnection processes at the dayside magnetopause

      Shi, Yong; Lee, L. C.; Akasofu, S-I.; Gatterdam, R.; Gosink, J.; Swift, D. W. (1989-12)
      In this thesis, the day side reconnection processes are studied by using computer simulations. First, the global magnetic reconnection patterns at the dayside magnetopause are studied based on a two-dimensional incompressible magnetohydrodynamic (MHD) code. It is found that multiple X line reconnection may prevail at the dayside magnetopause when the magnetic Reynolds number is large (> 200). The formation and subsequent poleward convection of magnetic islands are observed in the simulation. The Alfvén Mach number of the solar wind, MAsw , cam also change the reconnection patterns. For a large reconnection tends to occur at the higher latitude region. Secondly, the structure of the dayside reconnection layer is studied by a two-dimensional compressible MHD simulation. In a highly asymmetric configuration typical of the dayside magnetopause, the pair of slow shocks bounding the reconnection layer in Petschek’s symmetric model is found to be replaced by an intermediate shock on the magnetosheath side and a weak slow shock on the magnetospheric side. In addition, a mechanism for the enhancement of By, which is observed in the magnetopause current layer and magnetic flux tubes, is proposed.
    • A study of the magnetosphere-ionosphere coupling processes

      Zhu, Lie; Kan, J. R. (1990)
      Magnetosphere-ionosphere (M-I) coupling processes are studied by using numerical modeling. An M-I coupling model of substorms on the ionospheric recombination time scale (tens of seconds) is developed. The model is two-dimensional (2-D) and time-dependent from which several signatures of substorms can be obtained and understood. The model is then extended to northward interplanetary magnetic field (IMF) conditions to study the effects of the M-I coupling on the high-latitude convection. Based on the model results, a mechanism for the origin of distorted two-cell ionospheric convection is proposed. The ionospheric and ground signatures of multiple field-aligned current sheets originating from dayside flux transfer events have been modeled. The interaction between Alfven waves and field aligned potential drops is studied by using a local model.
    • A theory of field-aligned current generation from the plasma sheet and the poleward expansion of aurora substorms

      Yamauchi, Masatoshi; Akasofu, Syun-Ichi (1990)
      This dissertation reports a study of the generation of field-aligned currents in the plasma sheet in terms of magnetosphere-ionosphere coupling. For the study, the plasma sheet and the ionosphere are treated as two-dimensional layers by height integration. In the magnetosphere between them, the Alfven wave transition time through this region is assumed to be zero. The ionospheric momentum is allowed to be transferred to the plasma sheet. Both linear analyses and numerical simulation are performed to study the field-aligned current generation. In the linear analysis, evolution from initial perturbations is studied. Zero order configurations are steady state without field-aligned currents. The field-aligned currents are treated as a perturbed quantity and linearly related with the other perturbed quantities. One result for the linear waves is that the magnetohydrodynamics (MHD) fast mode and Alfven mode are coupled through the ionospheric Hall current. The Hall current causes the dawn-dusk asymmetry: a westward-travelling wave is amplified on the region 1 current system, while an eastward-travelling wave is amplified elsewhere. The expansion phase of the magnetospheric substorm after the onset is numerically simulated on the near-earth plasma sheet. The inner edge of the plasma sheet is taken as the outflow boundary. As the initial condition, an enhanced earthward magnetospheric convection is assumed to cause a finite pressure increase at the inner edge of the plasma sheet. The numerical results are as follows. An MHD fast-mode wave is generated. It propagates tailward accompanied by the field-aligned currents. The wave propagation and the field-aligned currents account for the poleward expansion of the aurora and the region 1 field-aligned current during the expansion phase of the substorm. The region 1 field-aligned currents are linked with the dusk to dawn current on this wave, which is driven by the dynamo mechanism of the wave. The ionospheric Hall current causes asymmetry of the wave, and hence, of the field-aligned current distribution. This asymmetry accounts for the stronger field-aligned current in the premidnight sector.
    • Particle simulations of magnetic field reconnection and applications to flux transfer events

      Ding, Da-Qing; Lee, L. C.; Akasofu, S-I; Hawkins, J. G.; Olson, J. V.; Swift, D. W. (1990)
      Basic plasma processes associated with driven collisionless magnetic reconnection at the Earth's dayside magnetopause are studied on the basis of particle simulations. A two-and-one-half-dimensional (2$1\over2$-D) electromagnetic particle simulation model with a driven inflow boundary and an open outflow boundary is developed for the present study. The driven inflow boundary is featured with a driving electric field for the vector potential, while the open outflow boundary is characterized by a vacuum force free condition for the electrostatic potential. The major findings are as follows. (1) The simulations exhibit both quasi-steady single X-line reconnection (SXR) and intermittent multiple X line reconnection (MXR). The MXR process is characterized by repeated formation and convection of magnetic islands (flux tubes or plasmoids). (2) Particle acceleration in the MXR process occurs mainly in O line regions as particles are trapped within magnetic islands, not in X line regions. The MXR process results in a power law particle energy spectrum of $f(E)\sim E\sp{-4}$. (3) Field-aligned particle heat fluxes and intense plasma waves associated with the collisionless magnetic reconnection process are also observed. (4) When applied to the dayside magnetopause, simulation results show that the MXR process tends to generate a simultaneous magnetic field perturbation on both sides of the dayside magnetopause, resembling the observed features of two-regime flux transfer events (FTEs). (5) An intrusion of magnetosheath plasma bulge into the magnetosphere due to the formation of magnetic islands may lead to the layered structures observed in magnetospheric FTEs. (6) In the current sheet, the enhanced tearing mode instability caused by the driving force applied at the driven inflow boundary creates an energy source at a specific wavenumber range with $k\sb{z}L\sim$ 0.3 in the modal spectrum of the magnetic field $B\sb{x}$ component. An inverse cascade of the modal spectrum of $B\sb{x}$ leads to the formation of the large-scale ordered magnetic island structures observed in the simulations. (7) In addition, the results of a theoretical study show that the tearing mode instability, and hence the magnetic reconnection at the dayside magnetopause, do not exhibit strong dependence on the magnetosheath $\beta$ values.
    • A theoretical study of magnetosphere-ionosphere coupling processes

      Cao, Fei; Kan, J. R.; Akasofu, S-I.; Biswas, N.; Shaw, G.; Swift, D. (1991)
      Magnetosphere and ionosphere are coupled electrodynamically by waves, field-aligned currents and parallel electric fields. Several fundamental coupling processes are addressed in my thesis. It is shown that the Alfven wave is the dominant mode in transmitting field-aligned currents. Therefore, dynamic M-I coupling can be modeled by the Alfven wave bouncing between the ionosphere and the magnetospheric boundaries. The open magnetopause, separating the solar wind and the magnetosphere, behaves like a near perfect reflector to the Alfven wave because of the large solar wind inertia. At the plasma sheet, however, the reflection coefficient may extend over a wide range, depending on the location in the plasma sheet. As the Alfven wave propagates back and forth between the magnetosphere and ionosphere, the field-aligned current density increases dramatically at certain locations, especially near the head of the westward traveling surge, causing potential drops to develop along magnetic field lines. It is found that the existence of parallel potential drops can distort the global convection pattern and limit the upward field-aligned current. The magnetic reconnection at the dayside magnetopause is responsible for enhancing the convection in the magnetosphere, which subsequently propagates toward the ionosphere by the Alfven wave. The patchy and intermittent reconnection at the dayside magnetopause can be initiated by the 3-D tearing instability, leading to the isolated magnetic islands and X-line segments. The nonlinear evolution of tearing in terms of the magnetic island coalescence is also studied.
    • Plasma dynamics of the Earth magnetopause-boundary layer and its coupling to the polar ionosphere

      Wei, Chang-Quan; Lee, Lou-Chuang; Deehr, C. S.; Swift, D. W.; Tape, W. R.; Watkins, B. J. (1991)
      In this thesis, the plasma dynamics of the Earth magnetopause-boundary layer and its coupling to the polar ionosphere are studied by using computer simulations. First, the plasma dynamics and structure of the magnetopause-boundary layer are studied by a two-dimensional incompressible magnetohydrodynamic simulation code. It is found that the Kelvin-Helmholtz instability with driven boundary conditions at the magnetopause can lead to the formation of plasma vortices observed in the magnetopause-boundary layer. In the later stage of development, a density plateau is formed in the central part of the boundary layer. Second, the coupling of plasma vortices formed in the boundary layer to the polar ionosphere is studied based on a magnetosphere-ionosphere coupling model. The finite ionospheric conductivity provides a dragging force to the plasma flow and leads to the decay of plasma vortices in the boundary layer. In the model, the ionospheric conductivity is allowed to be enhanced due to accelerated electrons precipitating in upward field-aligned current regions. The competing effect of the formation and decay of vortices leads to the formation of strong vortices only in limited regions. Several enhanced conductivity regions are formed along the post-noon auroral oval, which may account for the observed auroral bright spots. In addition, the evolution of localized plasma vortices, as well as magnetic flux ropes, along magnetic field lines is studied. The evolution leads to the generation of large-amplitude Alfven waves, which carry field-aligned currents and provide the link for the coupling of plasma vortices and magnetic flux ropes in the magnetosphere to the polar ionosphere.
    • Radiation transport in cloudy and aerosol loaded atmospheres

      Kylling, Arve; Stamnes, Knut; Shaw, Glenn E.; Weeks, Wilford W.; Rees, Manfred H.; Smith, Roger W. (1992)
      The equation for radiation transport in vertical inhomogeneous absorbing, scattering, and emitting atmospheres is derived from first principles. It is cast in a form amenable to solution, and solved using the discrete ordinate method. Based on the discrete ordinate solution a new computationally efficient and stable two-stream algorithm which accounts for spherical geometry is developed. The absorption and scattering properties of atmospheric molecules and particulate matter is discussed. The absorption cross sections of the principal absorbers in the atmosphere, H$\sb2$O, CO$\sb2$ and O$\sb3,$ vary erratically and rapidly with wavelength. To account for this variation, the correlated-k distribution method is employed to simplify the integration over wavelength necessary for calculation of warming/cooling rates. The radiation model, utilizing appropriate absorption and scattering cross sections, is compared with ultraviolet radiation measurements. The comparison suggests that further experiments are required. Ultraviolet (UV) and photosynthetically active radiation (PAR) is computed for high and low latitudes for clear and cloudy skies under different ozone concentrations. An ozone depletion increases UV-B radiation detrimental to life. Water clouds diminish UV-B, UV-A and PAR for low surface albedos and increase them for high albedos. The relative amount of harmful UV-B increases on overcast days. The daily radiation doses exhibit small monthly variations at low latitudes but vary by a factor of 3 at high latitudes. Photodissociation and warming/cooling rates are calculated for clear skies, aerosol loaded atmospheres, and atmospheres with cirrus and water clouds. After major volcanic explosions aerosols change O$\sb3$ and NO$\sb2$ photodissociation rates by 20%. Both aged aerosols and cirrus clouds have little effect on photodissociation rates. Water clouds increase $(\sim$100%) photodissociation rates that are sensitive to visible radiation above the cloud. Solar warming rates vary by 50% in the stratosphere due to changing surface albedo. Water clouds have a similar effect. The net effect of cirrus clouds is to warm the troposphere and the stratosphere. Only extreme volcanic aerosol loadings affect the terrestrial warming rate, causing warming below the aerosol layer and cooling above it. Aerosols give increased solar warming above the aerosol layer and cooling below it.
    • A self-consistent time varying auroral model

      Min, Qilong; Rees, M. H.; Kan, J. R.; Lummerzheim, D.; Piacenza, R.; Stamnes, K. (1993)
      A time dependent model of auroral processes has been developed by self-consistently solving the electron transport equation, the ion continuity equations and the electron and ion energy equations. It is used to study the response of ionospheric and atmospheric properties in regions subjected to electron bombardment. The time history of precipitation events is computed for a variety of electron spectral energy distributions and flux magnitudes. Examples of daytime and night-time aurorae are presented. Precipitating energetic auroral electrons heat the ambient electrons and ions as well as enhancing the ionization rate which increases the ion concentration. The consequences of electric field acceleration and an inhomogeneous magnetic field in auroral electron transport in the topside ionosphere are investigated. Substantial perturbations of the low energy portion of the electron flux are produced: An upward directed electric field accelerates the downward directed flux of low energy secondary electrons and decelerates the upward directed component. Above about 400 km the inhomogeneous magnetic field produces anisotropies in the angular distribution of the electron flux. The effects of the perturbed energy distributions on auroral spectral emission features and on the electron temperature are noted. The response of the Hall and Pederson conductivities to auroral electron precipitation is discussed as a function of the characteristic energy of the spectral distribution.
    • Structure of reconnection layers in the magnetosphere

      Lin, Yu; Lee, Lou-Chuang; Hawkins, J. G.; Sentman, D. D.; Smith, R. W.; Swift, D. W. (1993)
      Magnetic reconnection can lead to the formation of observed boundary layers at the dayside magnetopause and in the nightside plasma sheet of the magnetosphere. In this thesis, the structure of these reconnection layers is studied by solving the one-dimensional Riemann problem for the evolution of a current sheet. Analytical method, resistive MHD simulations, and hybrid simulations are used. Based on the ideal MHD formulation, rotational discontinuities, slow shocks, slow expansion waves, and contact discontinuity are present in the dayside reconnection layer. Fast expansion waves are also present in the solution of the Riemann problem, but they quickly propagate out of the reconnection layer. Our study provides a coherent picture for the transition from the reconnection layer with two slow shocks in Petschek's model to the reconnection layer with a rotational discontinuity and a slow expansion wave in Levy et al.'s model. In the resistive MHD simulations, the rotational discontinuities are replaced by intermediate shocks or time-dependent intermediate shocks. In the hybrid simulations, the time-dependent intermediate shock quickly evolves to a steady rotational discontinuity, and the contact discontinuity does not exist. The magnetotail reconnection layer consists of two slow shocks. Hybrid simulations of slow shocks indicate that there exists a critical number, $M\sb{c}$, such that for slow shocks with an intermediate Mach number $M\sb{I} \ge M\sb{c}$, a large-amplitude rotational wavetrain is present in the downstream region. For slow shocks with $M\sb{I} < M\sb{c}$, the downstream wavetrain does not exist. Chaotic ion orbits in the downstream wave provide an efficient mechanism for ion heating and wave damping and explain the existence of the critical number $M\sb{c}$ in slow shocks.
    • The morphology and electrodynamics of the boreal polar winter cusp

      McHarg, Matthew G.; Olson, J. J. (1993)
      The major result of this thesis is the magnetic signatures of the dayside cusp region. These signatures were determined by comparing the magnetic observations to optical observations of different energy particle precipitation regions observed in the cusp. In this thesis, the cusp is defined as the location of most direct entry of magnetosheath particles into the ionosphere. Optical observations show that the observing station rotates daily beneath regions of different incident energy particles. Typically, the station passes from a region in the morning of high energy particles into a region near magnetic noon of very low energy precipitation, and then returns to a region of high energy precipitation after magnetic noon. A tentative identification of the cusp is made on the basis of these observations. The optical observations also are used to determine the upward field aligned current density, which is found to be most intense in the region identified as the cusp. The magnetic field measurements are found to correlate with the optical measurements. When the characteristic energy is high, the spectrogram shows large amplitude broad band signals. The Pc5 component of these oscillations is right hand polarized in the morning, and left hand polarized in the afternoon. During the time the optics detect precipitation with a minimum characteristic energy, the magnetic spectrogram shows a unique narrow band tone at 3-5 mHz. The occurrence statistics of the magnetic oscillations are compared to DMSP satellite observations of the cusp and low latitude boundary layer. The pulses that make the narrow band tone are found to come in wave trains that are phase coherent. These trains of coherent pulses are found to be separated by phase jumps from adjacent wave trains. These jumps in phase occur when a new field aligned current appears on the equatorward edge of the cusp. This combination of phase coherent wave trains associated with poleward propagating auroral forms which are shown to contain intense field aligned currents may be the signature of newly reconnected flux tubes in the ionosphere.