• A Mechanism For Current Sheet Thinning In The Growth Phase Of Magnetospheric Substorms

      Hall, Fred, Iv; Otto, Antonius (2006)
      The thinning of the near-Earth current sheet during the growth phase of magnetospheric substorms is a fundamental problem of space physics. It is a characteristic of the slow, steady evolution of the magnetosphere during the growth phase, during which the bulk kinetic energy of the solar wind is transformed into and stored as magnetic field energy in the magnetotail lobes. The thin near-Earth current sheet at the end of the growth phase provides the conditions for the onset of the expansion phase, and is fundamentally important to understand the physical mechanism for the onset of the rapid evolution during which the stored energy is released. I propose that current sheet thinning occurs because of the evacuation of a 'magnetic flux reservoir' in the near-Earth magnetotail by convection to replace magnetic flux that is eroded on the dayside by magnetic reconnection. My hypothesis is able to predict basic properties of current sheet thinning, such as the location, temporal evolution, and dynamics of this process. I examined this new mechanism both conceptually and quantitatively. My conceptual considerations enabled the prediction of the location and duration of current sheet thinning. This location is largely independent of the detailed state of the magnetosphere. I examined this mechanism quantitatively through the use of a three-dimensional ideal MHD simulation. I was able to predict the duration of the growth phase by considering the time needed to deplete our proposed 'magnetic flux reservoir.' The simulation demonstrates the global increase of the current density in this reservoir, despite the removal of magnetic flux---which one would otherwise expect to lead to a decrease of current---as well as even greater local amplifications of the current density. The simulation results are even more significant because the model does not include other effects of the real magnetosphere that contribute to a further increase of the tail current. The increase in current density and thinning are found to be consistent with the amount of flux removed from the system. In addition, I have found a new explanation for the very thin bifurcated current sheets that have been reported in recent publications.
    • 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.
    • 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.
    • A simulation study of three-dimensional magnetic reconnection

      Ma, Zhi-Wei; Lee, L. C.; Kan, J. R.; Shaw, G. E.; Smith, R. W.; Hawkins, J. G. (1994)
      The magnetic reconnection process plays an important role in the interaction between the solar wind and the magnetosphere. It leads to the transfer of energy from the solar wind into the magnetosphere. In this thesis, we study three-dimensional (3D) aspects of magnetic reconnection based on magnetohydrodynamic (MHD) simulations. First, we examine the magnetic field topology of magnetic flux ropes formed in multiple X line reconnection (MXR). It is found that the magnetic field topology depends on the relative extent and location of the two neighboring X lines. Magnetic flux ropes with either smooth or frayed ends are obtained in our simulations. For magnetic flux ropes with smooth ends, a major amount of magnetic flux is connected at each end to only one side of magnetopause. Second, the evolution of the core magnetic field in the magnetic flux tube is studied for various magnetic reconnection processes. We find that the 3D cases always lead to a larger enhancement of core field than the corresponding 2D cases since plasma can be squeezed out of the flux tube in the third direction. The MXR process gives rise to a larger increase of the core field than the single X line reconnection process. The core magnetic field can be enhanced to three times the ambient magnetic field strength in the 3D MXR process. Finally, we examine the generation and propagation of Alfven waves and field-aligned currents in the 3D reconnection process. For cases with a zero guide field, it is found that a large portion of the field-aligned currents ($\sim$40%) is located in the closed field line region. Both the pressure gradient term and inertia term contribute to the generation of field-aligned currents. For cases with nonzero guide field, one sense of field-aligned currents is dominant due to the presence of the initial field-aligned current. In these cases, the inertia term makes a major contribution to the redistribution of field-aligned currents. The influence of the initial guide field on the longitudinal shift of the current reversal site is found to be consistent with observations.
    • A study of the link between cloud microphysics and climate change

      Hu, Yong-Xiang; Stamnes, Knut; Musgrave, David; Rees, Manfred; Shaw, Glenn E.; Tsay, Shee-Chee; Watkins, Brenton (1994)
      The climate related cloud radiative properties (fluxes and heating rates) are found to be determined by the second and the third moments of the cloud droplet size distribution. The detailed distribution of the cloud droplet size is unnecessary to obtain for climate purposes. An accurate parameterization of cloud optical properties suitable for climate models is developed. A new radiative-convective model has been developed and used for studying cloud-climate interactions. The energy balance at the Earth-atmosphere interface is treated in a self-consistent manner which avoids artificial tuning. The cloud radiative properties are accurately incorporated and are suitable for sensitivity studies of cloud-radiation-climate interactions. A sensitivity study of role of the cloud microphysical properties in the climate system is performed by studying the impact of cloud radiative forcing on the equilibrium state temperature. The cloud equivalent radius is found to be a very important variable in the climate system. A climate sensitivity study is performed to highlight the important role of the cloud absorption. An adjoint radiative transfer method is developed for use in the cloudy and aerosol-loaded atmospheres. The physical meaning of the adjoint radiative properties are discussed. The method is expected to be useful in climate modeling and remote sensing studies. A preliminary study of the atmospheric irreversibility is performed to elucidate the connection between cloud microphysical properties and the macrophysical direction of global climate. A variational principle which describes the macrophysical character of the climate system is established.
    • 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 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.
    • 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.
    • An investigation of the dynamics of the mesopause: Fabry-Perot observations of winds and temperatures from nightglow emissions

      Conner, James F.; Smith, R. W. (1994)
      This work is a study of the behavior of tidal and planetary waves in the upper-middle atmosphere near the geographic south pole. This is accomplished with a characterization of the dynamic state of these motions. I used ground-based Fabry-Perot Spectrometer (FPS) measurements of the multiple-line, $P\sb1$(2)$\sb{c,d}$, nightglow emissions from the X$\sp2$II band of the neutral OH* molecule. I developed analytical techniques to determine a space and time distribution of spectral amplitudes and phases for the dynamic parameters of kinetic temperature and neutral wind in the OH* layer. Spectral analysis of the variations in this layer indicate the existence of two groups: a planetary wave group (periods of ${\sim}$1-10 days), with eastward phase progression, and a near semi-diurnal group (periods of ${\sim}$8-13 hours), with westward phase progression. Specific periods vary slightly for different years; this is most likely due to remote propagation conditions. Further separation of each group shows the wind oscillations exhibit wave-number one behavior with associated wave-number zero temperature oscillations, (with a few exceptions). The periodicities in the planetary group neutral wind motions are consistent with the model results of Salby, 1984, for propagation to high latitudes through realistic mean flows. The characterization of the dynamics of this layer has led to the discovery of a basic azimuthal asymmetry in the distribution of spectral amplitude for a given oscillation, that is, preferred azimuths. These preferred azimuths appear to be associated with changes in the direction, not the amplitude, of a cross-polar mean wind. This finding, in conjunction with the evanescence of some features, uncovered two cases of planetary wave dissipation. These occur when oscillations attempt to maintain their preferred alignment with a changing mean wind direction resulting in a decay of wind amplitude and a burst of thermal oscillation. Both cases occur at the same time. Coincident with these decays are enhancements in the wind and thermal energy of other, longer period, oscillations which share the same azimuthal preferences. Also coincident is an acceleration of the mean wind.
    • Analysis Of Methods For Solar Wind Propagation From Lagrangian Point L1 To Earth

      Jensen, Poul F.; Bristow, William; Newman, David; Nielsen, Hans; Otto, Antonius; Smith, Roger (2013)
      The Lagrangian point L1 is situated about 1.5 million kilometers sunwards from Earth and provides a unique orbiting point for satellites, placing them constantly upstream in the solar wind, allowing for prediction of solar wind conditions impacting Earth's magnetosphere. Short-term forecasting of geomagnetic activity requires extrapolation of solar wind data from L1 to Earth (typical propagation time around 1 hour), as does any research in interactions between the solar wind and the magnetosphere during intervals when no Earth-orbiting satellites are in the solar wind. To accurately predict propagation delays it is necessary to take the geometry of incoming solar wind structures into account. Estimating the orientation of solar wind structures currently has to be done using single satellite measurements, which will likely remain the case for another decade or more, making it important to optimize single satellite techniques for solar wind propagation. In this study a comprehensive analysis of 8 different single satellite propagation methods was performed, each involving several variable parameters. 4 of these used electric field calculations and had not previously been tested for solar wind propagation. Large amounts of data were propagated from a satellite near L1 to target satellites near Earth for comparison to measured data, using specific test scores to evaluate relative performance between methods and parameter values. Electric field methods worked well for continuous data but did not predict arrival time of discontinuities (abrupt transitions) as accurately as methods based on magnetic field data, one of which delivered the best results on all accounts. This method had also been found to give best results in a previous study, but optimal parameter values were significantly different with the larger data set used here. Propagating 6,926 discontinuities it was found that on average they arrive about 30 seconds later than predicted (about 1% of the propagation time). Barring a systematic error in velocity data or delay calculations the offset suggests an asymmetry in the geometry of solar wind structures. While this idea is physically plausible it was not unambiguously supported by the data.
    • 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.
    • Applications Of A Time-Dependent Polar Ionosphere Model For Radio Modification Experiments

      Fallen, Christopher Thomas; Watkins, Brenton (2010)
      A time-dependent self-consistent ionosphere model (SLIM) has been developed to study the response of the polar ionosphere to radio modification experiments, similar to those conducted at the High-Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska. SCIM solves the ion continuity and momentum equations, coupled with average electron and ion gas energy equations; it is validated by reproducing the diurnal variation of the daytime ionosphere critical frequency, as measured with an ionosonde. Powerful high-frequency (HF) electromagnetic waves can drive naturally occurring electrostatic plasma waves, enhancing the ionospheric reflectivity to ultra-high frequency (UHF) radar near the HF-interaction region as well as heating the electron gas. Measurements made during active experiments are compared with model calculations to clarify fundamental altitude-dependent physical processes governing the vertical composition and temperature of the polar ionosphere. The modular UHF ionosphere radar (MUIR), co-located with HAARP, measured HF-enhanced ion-line (HFIL) reflection height and observed that it ascended above its original altitude after the ionosphere had been HF-heated for several minutes. The HFIL ascent is found to follow from HF-induced depletion of plasma surrounding the F-region peak density layer, due to temperature-enhanced transport of atomic oxygen ions along the geomagnetic field line. The lower F-region and topside ionosphere also respond to HF heating. Model results show that electron temperature increases will lead to suppression of molecular ion recombination rates in the lower F region and enhancements of ambipolar diffusion in the topside ionosphere, resulting in a net enhancement of slant total electron content (TEC); these results have been confirmed by experiment. Additional evidence for the model-predicted topside ionosphere density enhancements via ambipolar diffusion is provided by in-situ measurements of ion density and vertical velocity over HAARP made by a Defense Meteorological Satellite Program (DMSP) satellite.
    • 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.
    • Building Blocks Of Self -Organized Criticality

      Woodard, Ryan; Newman, David (2004)
      Why are we having difficulty developing economical nuclear fusion? How can a squirrel cause a statewide power blackout? How do correlations arise in a random complex system? How are these questions related? This thesis addresses these questions through a study of self-organized criticality (SOC). Among the systems that have been proposed as SOC are confined fusion plasmas, the Earth's magnetosphere and earthquake faults. SOC describes how large-scale complex behavior can emerge from small-scale simple interactions. The essence of SOC is that many dynamical systems, regardless of underlying physics, share a common nonlinear mechanism: local gradients grow until exceeding some critical gradient and then relax in events called avalanches. Avalanches range in size from very small to system-wide. Interactions of many avalanches over long times result in robust statistical and dynamical signatures that are surprisingly similar in many different physical systems. Two of the more well-known signatures are power law scaling of probability distribution functions (PDFs) and power spectra. Of particular interest in the literature for approximately a century are 1/f spectra. I studied the SOC running sandpile model and applied the results to confined and space plasmas. My tools were power spectra, PDFs and rescaled range ( R/S) analysis. I found that SOC systems with random external forcing store memory of previous states in their local gradients and can have dynamical correlations over very long time scales regardless of how weak the external forcing is. At time scales much longer than previously thought, the values of the slope of the power spectra, beta and the Hurst exponent, H, are different from the values found for white noise. As forcing changes, beta changes in the range 0.4 <math> <f> &lap;</f> </math> beta &le; 1 but the Hurst exponent remains relatively constant, H &ap; 0.8. The same physics that produces a 1/f spectrum at strong forcing produces a f -0.4 spectrum at weaker forcing. Small amounts of diffusive spreading added to the two dimensional SOC sandpile greatly decreases the frequency and maximum size of large transport events. More diffusion increases the frequency of large events to values much greater than for systems without diffusion.
    • Characteristic behavior of the dayside aurora in the minutes leading up to substorm onset: evidence for external triggering of substorms by the interplanetary magnetic field

      Andersen, Carl Stephen (2005-12)
      Two meridian scanning photometers, one located in Alaska and one in Svalbard, are used to examine the behavior of the dayside aurora just before the onset of the expansion phase of 61 substorms. In a strong majority of cases (80%), a poleward movement of the dayside aurora is seen in the minutes preceding sub storm onset. For the cases examined where a poleward movement of the dayside aurora is seen, a northward turning of the (generally southward) IMF is usually, but not always, seen. This suggests that for a majority of substorms, the 'trigger' is, or is related to, a northward turning of the IMF which can be seen by the motions of the dayside aurora. The observed movements of the dayside aurora and supposed onset triggers begin, on average, 12 minutes before substorm onset, which is the period before on-set during which nightside 'auroral fading' is known to happen. Two opposite but not necessarily exclusive behaviors of ionospheric convection have also been reported to occur during this period before onset, namely a decrease and/or an increase in polar cap convection velocities. Radar measurements of ionospheric convection are examined for these events but do not show an easily identifiable characteristic behavior.
    • Characteristics of dayside auroral displays in relation to magnetospheric processes

      Minow, Joseph I.; Smith, Roger W. (1997)
      The use of dayside aurorae as a ground based monitor of magnetopause activity is explored in this thesis. The origin of diffuse (OI) 630.0 nm emissions in the midday auroral oval is considered first. Analysis of low altitude satellite records of precipitating charged particles within the cusp show an unstructured electron component that will produce a 0.5-1 kR 630.0 nm emission throughout the cusp. Distribution of the electrons is controlled by the requirement of charge neutrality in the cusp, predicting a diffuse 630.0 nm background even if the magnetosheath plasma is introduced into the magnetosphere in discrete merging events. Cusp electron fluxes also contain a structured component characterized by enhancements in the electron energy and energy flux over background values in narrow regions a few 10's of kilometers in width. These structured features are identified as the source of the transient midday arcs. An auroral model is developed to study the morphology of (OI) 630.0 nm auroral emissions produced by the transient arcs. The model demonstrates that a diffuse 630.0 nm background emission is produced by transient arcs due to the long lifetime of the O$(\sp1D)$ state. Two sources of diffuse 630.0 nm background emissions exist in the cusp which may originate in discrete merging events. The conclusion is that persistent 630.0 nm emissions cannot be interpreted as prima facie evidence for continuous particle transport from the magnetosheath across the magnetopause boundary and into the polar cusp. The second subject that is considered is the analysis of temporal and spatial variations of the diffuse 557.7 nm pulsating aurora in relation to the 630.0 nm dominated transient aurora. Temporal variations at the poleward boundary of the diffuse 557.7 nm aurora correlate with the formation of the 630.0 nm transient aurorae suggesting that the two events are related. The character of the auroral variations is consistent with the behavior of particle populations reported during satellite observations of flux transfer events near the dayside magnetopause. An interpretation of the events in terms of impulsive magnetic reconnection yields a new observation that relates the poleward moving transient auroral arcs in the midday sector to the flux transfer events.
    • Characterization and diagnostic methods for geomagnetic auroral infrasound waves

      Oldham, Justin J.; Szuberla, Curt A. (2015-12)
      Infrasonic perturbations resulting from auroral activity have been observed since the 1950's. In the last decade advances in infrasonic microphone sensitivity, high latitude sensor coverage, time series analysis methods and computational efficiency have elucidated new types of auroral infrasound. Persistent periods of infrasonic activity associated with geomagnetic sub-storms have been termed geomagnetic auroral infrasound waves [GAIW]. We consider 63 GAIW events recorded by the Fairbanks, AK infrasonic array I53US ranging from 2003 to 2014 and encompassing a complete solar cycle. We make observations of the acoustic features of these events alongside magnetometer, riometer, and all-sky camera data in an effort to quantify the ionospheric conditions suitable for infrasound generation. We find that, on average, the generation mechanism for GAIW is confined to a region centered about ~60° longitude east of the anti-Sun-Earth line and at ~77° North latitude. We note furthermore that in all cases considered wherein imaging riometer data are available, that dynamic regions of heightened ionospheric conductivity periodically cross the overhead zenith. Consistent features in concurrent magnetometer conditions are also noted, with irregular oscillations in the horizontal component of the field ubiquitous in all cases. In an effort to produce ionosphere based infrasound free from the clutter and unknowns typical of geophysical observations, an experiment was undertaken at the High Frequency Active Auroral Research Program [HAARP] facility in 2012. Infrasonic signals appearing to originate from a source region overhead were observed briefly on 9 August 2012. The signals were observed during a period when an electrojet current was presumed to have passed overhead and while the facilities radio transmitter was periodically heating the lower ionosphere. Our results suggest dynamic auroral electrojet currents as primary sources of much of the observed infrasound, with modulation of the electrojets due to energetic particle precipitation, dispersion due to coupling with gravity waves, and reflection and refraction effects in the intervening atmosphere all potential factors in the shaping of the waveforms observed.
    • Cloud condensation nuclei

      Ji, Qiang; Shaw, Glenn E.; Stamnes, Knut; Bowling, Sue Ann; Benner, Richard; Kienle, Juergen; Watkins, Brenton (1995)
      In this study the supersaturation spectra of Cloud Condensation Nuclei (CCN) and the size distribution spectra of aerosols were investigated. These studies were conducted because it is believed that atmospheric aerosols, especially CCN, can affect the climate of the Earth. First, the size distributions of aerosols and the number concentrations of CCN were measured at different times in different meteorological airmass systems. The results indicate that the CCN supersaturation spectrum can be calculated from the size distribution of aerosols in only a few cases, suggesting that the direct measurement of CCN is of importance. Second, based on the measurements, a new general equation is proposed to describe the CCN supersaturation spectrum for some types of aerosols. The corresponding equation for CCN size distribution is derived. It is also shown theoretically that, with certain assumptions, the aerosol size distribution in the accumulation mode can be described by a bell-shaped distribution, in agreement with our measurements. The new equations are compared with actual data. Finally, a new method is devised to facilitate the measurement of CCN. The new instrument, which we call the "CCN Remover", separates CCN from the rest of aerosols by activation and removal. Together with a particle sizer, a new CCN measurement system, the CCN Remover Spectrometer, can provide information on both the supersaturation spectrum and the size distribution of CCN. Preliminary laboratory tests show close agreement between measurement results and theoretical predictions. The Remover Spectrometer was successfully tested in the NASA SCAR-B (Smoke, Clouds, and Radiation-Brazil) biomass burning experiment.
    • Cold ions of ionospheric origin observed at the dayside magnetopause and their effects on magnetic reconnection

      Lee, Sun-Hee; 이, 선희; Zhang, Hui; Ng, Chung-Sang; Otto, Antonius; Zong, Qiu-Gang (2015-08)
      Magnetic reconnection at the dayside magnetopause is one of the most important mechanisms that efficiently transfers solar wind particles, momentum, and energy into the magnetosphere. Magnetic reconnection at the magnetopause is usually asymmetric since the plasma and magnetic field properties are quite different in the magnetosphere and the magnetosheath. Cold dense plasma, originating either directly from the ionosphere or from the plasmasphere, has often been observed at the adjacent magnetopause. These cold plasmas may affect reconnection since they modify the plasma properties on the magnetospheric side significantly. This dissertation presents case and statistical studies of the characteristics of the cold ions observed at the dayside magnetopause by using Cluster spacecraft datasets. The plasmaspheric plumes have been distinguished from the ionospheric outows using ion pitch angle distributions. The ionospheric outows feature unidirectional or bidirectional field-aligned pitch angle distributions, whereas the plasmaspheric plumes are characterized by 90° pitch angle distributions. The occurrence rates of the plasmaspheric plumes and ionospheric outows and their dependence on the solar wind/Interplanetary Magnetic Field (IMF) conditions have been investigated. It is found that the occurrence rate of plasmaspheric plume or ionospheric plasma strongly depends on the solar wind/IMF conditions. In particular, plasmaspheric plumes tend to occur during southward IMF while ionospheric outows tends to occur during northward IMF. The occurrence rate of the plasmaspheric plumes is significantly higher on the duskside than that on the dawnside, indicating that the plasmaspheric plumes may lead to a dawn-dusk asymmetry of dayside reconnection. Furthermore, this dissertation investigates the behavior of the cold dense plasma of ionospheric origin during magnetic reconnection at the dayside magnetopause. The motion of cold plasmaspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection near the subsolar magnetopause, the colder ions are simply entrained by E x B drift at high latitudes on the recently reconnected magnetic field lines. This indicates that plasmaspheric ions can sometimes play a very limited role in magnetic reconnection process. Finally, this dissertation examines a controlling factor that leads to the asymmetric reconnection geometry at the magnetopause. It is demonstrated that the separatrix and ow boundary angles are greater on the magnetosheath side than on the magnetospheric side of the magnetopause, probably due to the stronger density asymmetry rather than magnetic field asymmetry at this boundary.