• 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.
    • Active Tectonics In Southern Alaska And The Role Of The Yakutat Block Constrained By Gps Measurements

      Elliott, Julie; Freymueller, Jeff (2011)
      GPS data from southern Alaska and the northern Canadian Cordillera have helped redefine the region's tectonic landscape. Instead of a comparatively simple interaction between the Pacific and North American plates, with relative motion accommodated on a single boundary fault, the margin is made up of a number of small blocks and deformation zones with relative motion distributed across a variety of structures. Much of this complexity can be attributed to the Yakutat block, an allochthonous terrane that has been colliding with southern Alaska since the Miocene. This thesis presents GPS data from across the region and uses it to constrain a tectonic model for the Yakutat block collision and its effects on southern Alaska and eastern Canada. The Yakutat block itself moves NNW at a rate of 50 mm/yr. Along its eastern edge, the Yakutat block is fragmenting into small crustal slivers. Part of the strain from the collision is transferred east of the Fairweather -- Queen Charlotte fault system, causing the region inboard of the Fairweather fault to undergo a distinct clockwise rotation into the northern Canadian Cordillera. About 5% of the relative motion is transferred even further east, causing small northeasterly motions well into the northern Cordillera. Further north, the GPS data and model results indicate that the current deformation front between the Yakutat block and southern Alaska runs along the western side of the Malaspina Glacier. The majority of the ~37 mm/yr of relative convergence is accommodated along a narrow band of thrust faults concentrated in the southeastern part of the St. Elias orogen. Near the Bering Glacier, the tectonic regime abruptly changes as crustal thrust faults give way to subduction of the Yakutat block beneath the western St. Elias orogen and Prince William Sound. This change aligns with the Gulf of Alaska shear zone, implying that the Pacific plate is fragmenting in response to the Yakutat collision. The Bering Glacier region is undergoing internal deformation and may represent the final stage of accretion of the Yakutat block sedimentary layers. Further west, modeled block motions suggest the crust is laterally escaping along the Aleutian forearc.
    • Advancements in seismic tomography with application to tunnel detection and volcano imaging

      Clippard, James Doyle; Christensen, Douglas H.; Pulpan, Hans; Barry, Ronald P.; Harisen, Roger A.; Eichelberger, John C. (1998)
      Practical geotomography is an inverse problem with no unique solution. A priori information must be imposed for a stable solution to exist. Commonly used types of a priori information smooth and attenuate anomalies, resulting in 'blurred' tomographic images. Small or discrete anomalies, such as tunnels, magma conduits, or buried channels are extremely difficult imaging objectives. Composite distribution inversion (CDI) is introduced as a theory seeking physically simple, rather than distributionally simple, solutions of non-unique problems. Parameters are assumed to be members of a composite population, including both well-known and anomalous components. Discrete and large amplitude anomalies are allowed, while a well-conditioned inverse is maintained. Tunnel detection is demonstrated using CDI tomography and data collected near the northern border of South Korea. Accurate source and receiver location information is necessary. Borehole deviation corrections are estimated by minimizing the difference between empirical distributions of apparent parameter values as a function of location correction. Improved images result. Traveltime computation and raytracing are the most computationally intensive components of seismic tomography when imaging structurally complex media. Efficient, accurate, and robust raytracing is possible by first recovering approximate raypaths from traveltime fields, and then refining the raypaths to a desired accuracy level. Dynamically binned queuing is introduced. The approach optimizes graph-theoretic traveltime computation costs. Pseudo-bending is modified to efficiently refine raypaths in general media. Hypocentral location density functions and relative phase arrival population analysis are used to investigate the Spring, 1996, earthquake swarm at Akutan Volcano, Alaska. The main swarm is postulated to have been associated with a 0.2 km$\sp3$ intrusion at a depth of less than four kilometers. Decay sequence seismicity is postulated to be a passive response to the stress transient caused by the intrusion. Tomograms are computed for Mt. Spurr, Augustine, and Redoubt Volcanoes, Alaska. Relatively large amplitude, shallow anomalies explain most of the traveltime residual. No large amplitude anomalies are found at depth, and no magma storage areas are imaged. A large amplitude low-velocity anomaly is coincident with a previously proposed geothermal region on the southeast flank of Mt. Spurr. Mt. St. Augustine is found to have a high velocity core.
    • Alaska Shorefast Ice: Interfacing Geophysics With Local Sea Ice Knowledge And Use

      Druckenmiller, Matthew L.; Eicken, Hajo (2011)
      This thesis interfaces geophysical techniques with local and traditional knowledge (LTK) of indigenous ice experts to track and evaluate coastal sea ice conditions over annual and inter-annual timescales. A novel approach is presented for consulting LTK alongside a systematic study of where, when, and how the community of Barrow, Alaska uses the ice cover. The goal of this research is to improve our understanding of and abilities to monitor the processes that govern the state and dynamics of shorefast sea ice in the Chukchi Sea and use of ice by the community. Shorefast ice stability and community strategies for safe hunting provide a framework for data collection and knowledge sharing that reveals how nuanced observations by Inupiat ice experts relate to identifying hazards. In particular, shorefast ice break-out events represent a significant threat to the lives of hunters. Fault tree analysis (FTA) is used to combine local and time-specific observations of ice conditions by both geophysical instruments and local experts, and to evaluate how ice features, atmospheric and oceanic forces, and local to regional processes interact to cause break-out events. Each year, the Barrow community builds trails across shorefast ice for use during the spring whaling season. In collaboration with hunters, a systematic multi-year survey (2007--2011) was performed to map these trails and measure ice thickness along them. Relationships between ice conditions and hunter strategies that guide trail placement and risk assessment are explored. In addition, trail surveys provide a meaningful and consistent approach to monitoring the thickness distribution of shorefast ice, while establishing a baseline for assessing future environmental change and potential impacts to the community. Coastal communities in the region have proven highly adaptive in their ability to safely and successfully hunt from sea ice over the last 30 years as significant changes have been observed in the ice zone north of Alaska. This research further illustrates how Barrow's whaling community copes with year-to-year variability and significant intra-seasonal changes in ice conditions. Hence, arctic communities that have coped with such short-term variability may be more adaptive to future environmental change than communities located in less dynamic environments.
    • 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.
    • Brine Percolation, Flooding And Snow Ice Formation On Antarctic Sea Ice

      Maksym, Ted; Jeffries, Martin O. (2001)
      Modelling studies of brine percolation, flooding, and snow ice formation on Antarctic sea ice were undertaken to (1) determine the influence of brine transport processes on the salinity, porosity, and stable isotopic composition of snow ice and the underlying ice, (2) explain the range of salinities and isotopic composition observed in ice cores, and to provide a better estimate of the contribution of snow ice to the thickness of the winter pack ice, (3) better understand the microstructural controls on brine percolation and its effects on the properties of sea ice, and (4) understand the effects of meteorological forcing on snow ice formation and development of the ice cover. Snow ice thickness is most dependent on snow accumulation rates. Once snow ice begins to form on a floe, most of the subsequent thickening is due to snow ice formation. Results show that percolation in winter sea ice is most likely an inhomogeneous process. Flooding most likely occurs rapidly through localized regions of high permeability, such as in large, open brine drainage channels or cracks. Simulations of the freezing of a flooded slush layer show that focussing of thermohaline convection may form porous drainage channels in the ice and snow. These channels allow rapid desalination of the slush and exchange of H218O depleted brine with sea water. Significant positive shifts in delta18O are possible in the slush layer. This process can explain the range of delta18O observed in ice cores. Based on these results, a cutoff of delta18O < -2� is recommended for snow ice identification in the Ross, Amundsen, and Bellingshausen seas. Such a cutoff puts the amount of snow ice observed at 6--18% of the ice thickness. Although flooding appears to occur through spatially restricted regions of the ice, the precise nature of the flow and factors controlling onset of percolation are unclear.
    • 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.
    • Climate, seasonal snow cover and permafrost temperatures in Alaska north of the Brooks Range

      Zhang, Tingjun; Osterkamp, T. E.; Bowling, S. A.; Goering, D.; Gosink, J.; Hopkins, D.; Wendler, G. (1993)
      Climatological data, active layer and permafrost measurements, and modeling were used to investigate the response of permafrost temperatures to changes in climate in Alaska north of the Brooks Range. Mean annual air temperature (MAAT) from 1987 to 1991 within about 110 km from the Arctic Coast was ${-12.4}\pm0.3\sp\circ C,$ while the mean annual permafrost surface temperature (MAPST) ranged from ${-9.0}\sp\circ C$ along the coast to ${-5.2}\sp\circ C$ inland. Air temperature changes alone can not explain the permafrost warming from the coast to inland. Measurements show that MAPST are about $3\sp\circ C$ to $6\sp\circ C$ warmer than MAAT in the region. The interaction of local microrelief and vegetation with snow appears to change the insulating effect of seasonal snow cover and may be the major factor which controls the permafrost temperature during the winter and thus the MAPST. Sensitivity analyses show that for the same MAAT conditions, changes in seasonal snow cover parameters can increase or decrease the MAPST about $7\sp\circ C.$ Snowfall was greater during the cold years and less during the warm years and was poorly correlated between stations. These results suggest that the effects of changes in air temperatures on permafrost temperatures historically may also have been modified by changes in snow cover. A numerical model was used to investigate the effect of changes in initial permafrost temperature conditions, MAAT, seasonal snow cover and thermal properties of soils on the permafrost temperatures. Permafrost may have started warming about the same time as the atmosphere did in the late 1800's, and the long term mean surface temperature of the permafrost may have been established prior to this time. Variations in the penetration depth of the warming signal may be related to differences in thermal properties of permafrost. Variations in the magnitude of the permafrost surface warming may be due to the effect of local factors such as soil type, vegetation, microrelief, soil moisture, and seasonal snow cover. The effect of the interaction of vegetation and snow cover may amplify the signal of temperature change in the permafrost.
    • Coseismic Deformation Of The 2001 El Salvador And 2002 Denali Fault Earthquakes From Gps Geodetic Measurements

      Hreinsdottir, Sigrun; Freymueller, Jeffrey T. (2005)
      GPS geodetic measurements are used to study two major earthquakes, the 2001 MW 7.7 El Salvador and 2002 MW 7.9 Denali Fault earthquakes. The 2001 MW 7.7 earthquake was a normal fault event in the subducting Cocos plate offshore El Salvador. Coseismic displacements of up to 15 mm were measured at permanent GPS stations in Central America. The GPS data were used to constrain the location of and slip on the normal fault. One month later a MW 6.6 strike-slip earthquake occurred in the overriding Caribbean plate. Coulomb stress changes estimated from the M W 7.7 earthquake suggest that it triggered the MW 6.6 earthquake. Coseismic displacement from the MW 6.6 earthquake, about 40 mm at a GPS station in El Salvador, indicates that the earthquake triggered additional slip on a fault close to the GPS station. The MW 6.6 earthquake further changed the stress field in the overriding Caribbean plate, with triggered seismic activity occurring west and possibly also to the east of the rupture in the days to months following the earthquake. The MW 7.9 Denali Fault earthquake ruptured three faults in the interior of Alaska. It initiated with a thrust motion on the Susitna Glacier fault but then ruptured the Denali and Totschunda faults with predominantly right-lateral strike-slip motion unilaterally from west to east. GPS data measured in the two weeks following the earthquake suggest a complex coseismic rupture along the faults with two main regions of moment release along the Denali fault. A large amount of additional data were collected in the year following the earthquake which greatly improved the resolution on the fault, revealing more details of the slip distribution. We estimate a total moment release of 6.81 x 1020 Nm in the earthquake with a M W 7.2 thrust subevent on Susitna Glacier fault. The slip on the Denali fault is highly variable, with 4 main pulses of moment release. The largest moment pulse corresponds to a MW 7.5 subevent, about 40 km west of the Denali-Totschunda fault junction. We estimate relatively low and shallow slip on the Totschunda fault.
    • Crustal and upper mantle velocity structure in Alaska

      Searcy, Cheryl Kaye; Christensen, Douglas; Layer, Paul; Stringer, William; Kawasaki, Koji; Stone, David (1996)
      The crustal and upper-mantle velocity structure of Alaska testifies to a complex tectonic framework. Much of the structure and history of this framework remains to be conclusively determined. This thesis presents the results of three independent investigations of velocity structure in Alaska in an attempt to provide some insight into its tectonic development. The first study involved the analysis of receiver functions to determine velocity structure beneath College Station (COL), located in Fairbanks, Alaska. Receiver functions from several back azimuths facilitate a fairly detailed analysis of deep crustal velocity structure beneath COL, including an indication that Moho dips to the northeast. The second study also employed receiver function methods to investigate velocity structure for four temporary three-component seismic stations placed in the Brooks Range. Due to the short deployment of these stations in the Brooks Range only a rough estimate of crustal velocities were obtained. Nevertheless, crustal thickening beneath the Brooks range is clearly indicated by an increase in the depth to Moho. The final study undertaken was a three-dimensional tomographic P-wave velocity inversion for the subduction zone region of south central Alaska. Data for the tomographic inversion consisted of local and teleseismic ray paths. The resulting velocity perturbations indicate a positive velocity anomaly associated with the subducting Pacific plate. Furthermore, the tomographic images clarify physical characteristics of the subducting plate such as structure, thickness, and depth of penetration into the mantle.
    • Crustal Deformation Along The San Andreas Fault And Within The Tibetan Plateau Measured Using Gps

      Chen, Qizhi; Freymueller, Jeffrey T. (2002)
      Using the Global Positioning System (GPS), we study crustal deformation along the San Andreas Fault (SAF) in the San Francisco Bay area and within the Tibetan Plateau, and provide new constraints for the kinematics of these actively deforming plate boundaries. GPS measurements in 1996 and 1997 and Electronic Distance Measuring (EDM) data from the 1970s and 1980s at sites along the SAF in northern California were used to determine the near-fault strain rate and to investigate the slip rate, locking depth, and rheology. We found a pronounced high near-fault shear strain rate that can be explained by a 2-D inhomogeneous model in which a low-rigidity compliant zone concentrates strain near the fault. We suggest that the materials on either side of the fault and the cumulative fault offset play a role in the development of the compliant zone. If such a compliant zone is present but unmodeled, the geodetic estimates of slip rate and locking depth (seismogenic depth) would be biased. This would lead to a miscalculated seismic hazard. Thirteen GPS sites in southern Tibet, surveyed in 1995, 1998 and 2000, were merged with other data from China and Nepal into a single, self-consistent velocity field. The Himalaya and southern Tibet was modeled using a kinematically-consistent block model and elastic dislocation theory. We show a significantly lower convergence rate between India and Eurasia in central Himalaya than that previously estimated. We observe that southern Tibet undergoes non-uniform (spatial) east-west extension with one-half of the extension across the Yadong-Gulu rift. We infer that spatially non-uniform extension in southern Tibet results in variation of the arc-normal convergence rates along the Himalaya, and that the Yarlung-Zangbo suture or adjacent structure may be active as a right-lateral strike slip fault. From 44 GPS sites in the Tibetan Plateau, we show that deformation of Tibet is distributed and strain accumulation is spatially uniform across the entire plateau. We propose a kinematic model for the Tibetan Plateau to be a combination of rigid block motion, pure shear and uniaxial contraction in the direction of about N32�E, comparable to the convergence direction between India and Eurasia.
    • Crustal Deformation In Alaska Measured Using The Global Positioning System

      Fletcher, Hilary Jane; Freymueller, Jeffrey T. (2002)
      Repeat observations using the Global Positioning System at sites on the Earth's surface enable the velocity of those sites to be estimated. These velocity estimates can be used to model the processes of the crust's deformation by faulting and folding. The focus of this study is crustal deformation in Alaska and in particular the region of interior Alaska within 300km of Fairbanks, including the Denali fault; the Fairweather fault and Yakutat block in southern Alaska; and the Semidi region of the Aleutian arc. This deformation is driven by the relentless northwestward motion of the Pacific plate relative to North America. The Yakutat block, an allocthonous terrane located in the 'armpit' of southern Alaska is shown to be moving at neither the Pacific Plate rate nor is it attached to North America. Instead it has a velocity parallel to the Fairweather fault, which means that some offshore structure, possibly the Transition Zone, must accommodate some of the Pacific-North American relative motion. The slip on the Fairweather fault is estimated to be 44 +/- 3 mm/yr with a locking depth of 8 +/- 1 km, which implies a recurrence time of ~80 years for an MS 7.9 earthquake. Using a model of southern Alaska block rotation with the Denali fault as the northern boundary, the slip rate on the McKinley segment of the Denali fault is estimated to be ~6--9 mm/yr for a locking depth of 12 km. Moving to the southwest, data from sites in the Semidi segment of the Alaska subduction zone, between the fully-coupled segment to the northeast and the slipping Shumagin segment to the southwest are studied. This region, which sustained a magnitude 8.2 earthquake in 1938, is determined to be highly coupled and accumulating strain. Finally, all of these pieces are connected in a quantitative model for southern Alaska. This model involves three crustal blocks, the Yakutat block, Fairweather block and southern Alaska block, which lie between North America and the Pacific plate and move relative to these major plates.
    • Crustal Thickness Variation In South Central Alaska: Results From Broadband Experiment Across The Alaska Range

      Veenstra, Elizabeth (2009)
      The Broadband Experiment Across the Alaska Range (BEAAR) was a passive source seismic study in which 36 three-component broadband seismic stations were deployed over a period of 27 months to collect high quality data to study the Alaska Range and perhaps elucidate tectonic processes. The wavetrain of a teleseismic body wave may be interpreted in terms of reflection and transmission of waves converted at discontinuities. The recorded signal may be regarded as a convolution of the source-time function, the receiver function, and the instrument response. A receiver function is the contribution to the seismic waveform recorded at a single station due to the response of local crustal structure, and can be inverted for vertical velocity structure beneath the three-component broadband seismic station. Receiver function analyses reveal typical crust beneath the lowlands north of the Alaska Range is 26 km thick, while beneath the mountains typical crust is 35--45 km thick. Receiver function analysis of ~15,000 teleseismic waveforms recorded by BEAAR broadband seismometers provided over 100 crustal thickness data points. Similarity between crustal thicknesses determined from receiver function analysis and crustal thicknesses predicted from topography assuming Airy isostasy indicate the observed crustal root is sufficient to support the Alaska Range. North of the range, however, the crust is systematically thinner than predicted by simple Airy isostasy. A crustal density contrast of 4.6% across the Hines Creek Fault 2700 kgm-3 to the north and 2830 kgm-3 to the south, explains the observed difference between the crustal thicknesses predicted by simple Airy isostasy and the crustal thicknesses determined by receiver function analysis. Our results indicate that variations in both crustal thickness and density are required to explain the seismic and gravity data. Crustal thicknesses across the central Alaska Range suggest that these mountains are supported by a crustal root developed due to contractional thickening. Crustal thickness data reveal differences in terrane thickness: a thin Yukon-Tanana terrane north of the Hines Creek fault and thicker Kahiltna/Wrangellia terranes to the south. Finally, the pattern of thin crust to the north and thicker crust to the south appears to be modified by late Cenozoic structures such as the Denali fault, with contractional thickening in the Alaska Range, including areas north of the Hines Creek fault in the northern foothills fold and thrust belt. BEAAR crustal thickness data suggest that major faults extend to the base of the crust.
    • Deformation Of Alaskan Volcanoes Measured Using Sar Interferometry And Gps

      Mann, Doerte; Freymueller, Jeffrey (2002)
      Geodetic measurements using the Global Positioning System (GPS) and synthetic aperture radar interferometry (InSAR) show deformation of Okmok, Westdahl, and Fisher volcanoes in the Alaska-Aleutian arc. This thesis shows the variety of deformation signals observed, presents models for the observations, and interprets them in terms of underlying processes. InSAR data show deflation of Okmok caldera during its last eruption in 1997, preceded and followed by inflation of smaller magnitude. Modeling shows that the main deformation source, interpreted as a central magma reservoir, is located at 2.5 to 5.0 km depth beneath the approximate center of the caldera, and 5 km away from the active vent. Mass balance calculations and comparison with the long-term eruptive frequency indicate that Okmok may be supplied with magma continuously from a deep source. GPS measurements between 1998 and 2001 show inflation of Westdahl volcano, with a source located about 7 km beneath the summit. The combined subsurface volume increase measured during the GPS and an earlier InSAR observation period [Lu et al., 2000a] accounts for at least 15% more than the volume erupted from Westdahl in 1991--92, suggesting that an eruption of that size could occur at any time. Neighboring Fisher caldera shows subsidence and contraction across the caldera center that is not related to any eruptive activity. The main mechanisms to explain this deformation are degassing and contractional cooling of a shallow magma body, or depressurization of Fisher's hydrothermal system, possibly triggered by an earthquake in the vicinity of the caldera in 1999. A systematic coherence analysis of SAR interferograms documents the cooling history of the 1997 Okmok lava flow. The flow is incoherent directly after emplacement, but coherence increases as more time has passed since the eruption, and also the shorter the period spanned by the interferogram. Coherence is regained three years after the eruption. This corresponds to the time when the 20 m thick flow has solidified, indicating that flow mobility is the dominant factor degrading coherence on young lava flows. Based on these results, InSAR coherence analysis can be used to derive the minimum thickness of a lava flow.
    • Dependence of the ionospheric convection pattern on the conductivity and the southward IMF

      Shue, Jih-Hong; Kan, J. R.; Akasofu, S.-I.; Biswas, N. N.; Rees, M. H.; Weimer, D. R. (1993)
      Electric field measurements from the DE-2 satellite were used to determine the location of the convection reversal boundary and the potential around this boundary under a combination of interplanetary magnetic field (IMF) and auroral electrojet conditions. The electric potential is obtained by the integration of the electric fields. The convection reversal boundary is defined in this study as where the potential has its absolute maximum and minimum values. The data were sorted into 18 categories according to two levels of the negative IMF $B\sb{z},$ three ranges of IMF $B\sb{y},$ and two substorm phases. The data were fit with both continuous and discontinuous boundaries to get a functional representation of boundary potentials and locations. A simple model is constructed by solving the Laplace's equation in order to illustrate the obtained boundary potentials and locations. The results show that the enhanced electric field in the midnight sector is associated with an intense westward electrojet current. It can also be seen that the convection reversal boundary is found to be discontinuous near midnight. The discontinuous convection reversal boundary on the dayside is related to the merging near dayside cusp region. The discontinuous convection reversal boundary on the nightside is related to the conductivity enhancement. The intrusion of the dawn cell into the dusk cell is due to nonuniformity of the Hall conductivity in the ionosphere. Another model is constructed by solving the current continuity equation with field-aligned current and nonuniform conductivity added. It can be found that a secondary convection reversal, which is detached from the dusk-cell convection reversal, appears in the evening-midnight sector within the polar cap when the IMF $B\sb{y}$ is positive and the conductivity is nonuniform. This convection reversal is attributed to be created by the B $\times$ V dynamo. Also, the inclusion of the region 2 field-aligned current leads to an enhancement of the electric field in the region between the region 1 and region 2 currents.
    • Design And Implementation Of A Relative Plasma Density Probe For The Hex Sounding Rocket Mission

      Johnson, Bruce A.; Hawkins, Joseph (2003)
      This thesis describes the design and implementation of a relative plasma density probe for the Horizontal E-region eXperiment (HEX) sounding rocket mission conducted by the University of Alaska Fairbanks (UAF). The purpose of the plasma probe in the HEX experiment is to confirm the anticipated plasma depletion resulting from the upwelling of ionospheric gases theorized to exist poleward of an active auroral arc. The instrument uses a conducting band wrapped around the circumference of the rocket payload to collect and measure positively charged ions in the vicinity of an auroral arc while following a nearly horizontal trajectory. The probe collectors are biased to repel free thermal electrons while the ions are "swept up" by the cross sectional area of the probe's motion. A separate electrode detects the presence of high-energy electrons within the aurora. The resulting collector currents are measured and transmitted to ground equipment for archiving and later analysis.*<p> *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office; QuickTime; Adobe Acrobat; Windows MediaPlayer or RealPlayer; Internet browser.<p>