Browsing Physics by Subject "Atmospheric sciences"
Now showing items 1-4 of 4
Applications Of A Time-Dependent Polar Ionosphere Model For Radio Modification ExperimentsA 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.
Neural Network Approach To Classification Of Infrasound SignalsAs part of the International Monitoring Systems of the Preparatory Commissions for the Comprehensive Nuclear Test-Ban Treaty Organization, the Infrasound Group at the University of Alaska Fairbanks maintains and operates two infrasound stations to monitor global nuclear activity. In addition, the group specializes in detecting and classifying the man-made and naturally produced signals recorded at both stations by computing various characterization parameters (e.g. mean of the cross correlation maxima, trace velocity, direction of arrival, and planarity values) using the in-house developed weighted least-squares algorithm. Classifying commonly observed low-frequency (0.015--0.1 Hz) signals at out stations, namely mountain associated waves and high trace-velocity signals, using traditional approach (e.g. analysis of power spectral density) presents a problem. Such signals can be separated statistically by setting a window to the trace-velocity estimate for each signal types, and the feasibility of such technique is demonstrated by displaying and comparing various summary plots (e.g. universal, seasonal and azimuthal variations) produced by analyzing infrasound data (2004--2007) from the Fairbanks and Antarctic arrays. Such plots with the availability of magnetic activity information (from the College International Geophysical Observatory located at Fairbanks, Alaska) leads to possible physical sources of the two signal types. Throughout this thesis a newly developed robust algorithm (sum of squares of variance ratios) with improved detection quality (under low signal to noise ratios) over two well-known detection algorithms (mean of the cross correlation maxima and Fisher Statistics) are investigated for its efficacy as a new detector. A neural network is examined for its ability to automatically classify the two signals described above against clutter (spurious signals with common characteristics). Four identical perceptron networks are trained and validated (with >92% classification rates) using eight independent datasets; each dataset consists of three-element (each element being a characterization parameter) feature vectors. The validated networks are tested against an expert, Prof. Charles R. Wilson, who has been studying those signals for decades. From the graphical comparisons, we conclude that such networks are excellent candidate for substituting the expert. Advantages to such networks include robustness and resistance to errors and the bias of a human operator.
Observations Of Metal Concentrations In E-Region Sporadic Thin Layers Using Incoherent-Scatter RadarThis thesis has used incoherent-scatter radar data from the facility at Sondrestrom, Greenland to determine the ion mass values inside thin sporadic-E layers in the lower ionosphere. Metallic positively-charged ions of meteoric origin are deposited in the earth's upper atmosphere over a height range of about 85-120 km. Electric fields and neutral-gas (eg N2, O, O2) winds at high latitudes may produce convergent ion dynamics that results in the re-distribution of the background altitude distribution of the ions to form thin (1-3 km) high-density layers that are detectable with radar. A large database of experimental radar observations has been processed to determine ion mass values inside these thin ion layers. The range resolution of the radar was 600 meters that permitted mass determinations at several altitude steps within the layers. Near the lower edge of the layers the ion mass values were in the range 20-25 amu while at the top portion of the layers the mass values were generally in the range 30-40 amu. The numerical values are consistent with in-situ mass spectrometer data obtained by other researchers that suggest these layers are mainly composed of a mixture or Mg +, Si+, and Fe + ions. The small tendency for heavier ions to reside at the top portion of the layers is consistent with theory. The results have also found new evidence for the existence of complex-shaped multiple layers; the examples studied suggest similar ion mass values in different layers that in some cases are separated in altitude by several km.
Solar Flare Soft X -Ray Irradiance And Its Impact On The Earth's Upper AtmosphereSolar flares dramatically enhance the soft X-ray region of the solar spectrum. The enhancement is more significant than previously thought, and the solar soft X-ray instruments aboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) and Solar Radiation and Climate Experiment (SORCE) satellites have observed more flares than expected. This dissertation presents a state-of-the-art analysis used to determine flare spectra from TIMED and SORCE solar observations. A relationship is established between Geostationary Operational Environmental Satellite (GOES) flare 0.1-0.8 nm irradiances and XPS flare 0.1-2 and 0.1-7 nm irradiances. Solar flares primarily enhance the soft X-ray irradiance in the 0.1-2 nm range, and rapidly modify the energy input to the lower thermosphere. Most of the excess flare 0.1-2 nm irradiance comes from 1-2 nm. Thus, flares deposit a large amount of their energy between 100-110 km. One of the key effects of this energy deposition is to modify nitric oxide (NO), which plays an important role in the energy balance of the thermosphere as it is a source of radiative cooling through infrared emissions. The density of NO is highly variable as a function of time and latitude, and reaches a maximum in the same altitude region where the flare irradiance is absorbed. This dissertation also presents valid comparisons between Student Nitric Oxide Explorer (SNOE) satellite NO observations and those predicted by a photochemical thermospheric model to provide a better understanding of low latitude flare enhanced NO column density. Large flares can deposit the same amount of 0.1-2 and 0.1-7 nm energy to the thermosphere during a relatively short time as the Sun normally deposits in one day. The NO column density doubles as the daily integrated energy to the thermosphere doubles.