Engineering topics

Inan, Umran S.

Publications and source records attributed to Inan, Umran S..

13 records

NASA NTRS2012

High fidelity, radiation tolerant analog-to-digital converters

Techniques for an analog-to-digital converter (ADC) using pipeline architecture includes a linearization technique for a spurious-free dynamic range (SFDR) over 80 deciBels. In some embodiments, sampling rates exceed a megahertz. According to a second approach, a switched-capacitor circuit is configured for correct operation in a high radiation environment. In one embodiment, the combination yields high fidelity ADC (>88 deciBel SFDR) while sampling at 5 megahertz sampling rates and consuming <60 milliWatts. Furthermore, even though it is manufactured in a commercial 0.25-.mu.m CMOS technology (1 .mu.m=12.sup.-6 meters), it maintains this performance in harsh radiation environments. Specifically, the stated performance is sustained through a highest tested 2 megarad(Si) total dose, and the ADC displays no latchup up to a highest tested linear energy transfer of 63 million electron Volts square centimeters per milligram at elevated temperature (131 degrees C.) and supply (2.7 Volts, versus 2.5 Volts nominal).

Wang, Charles Chang-I↗

NASA NTRS1995

Focusing of nonducted whistlers by the equatorial anomaly

Impulsive ELF/VLF electric field bursts observed by the vector electric field instrument (VEFI) on the Dynamics Explorer 2 (DE 2) satellite on almost every crossing of the geomagnetic equator in the evening hours are interpreted as originating in lightning discharges. These signals that peak in intensity near the magnetic equator are observed within 5-20 deg latitude of the geomagnetic equator at altitudes of 300-500 km with amplitudes of the order of approximately mV/m in the 512- or 1024-Hz frequency band of the VEFI instrument. Whistler-mode ELF/VLF wave propagation through a horizontally stratified ionosphere predicts strong attenuation of subionospheric signals reaching the equator at low altitudes. However, ray tracing analysis shows that the presence of the equatorial density anomaly, commonly observed in the upper ionosphere during evening hours, leads to the focusing of the wave energy from lightning near the geomagnetic equator at low altitudes, thus accounting for all observed aspects of the phenomenon. The observations presented here indicate that during certain hours in the evening, almost all the energy input from lightning discharges entering the ionosphere at less than 30 deg latitude remains confined to a small region (in altitude and latitude) near the geomagnetic equator. The net wideband electric field, extrapolated from the observed electric field values in the 512- to 1024-Hz band, can be approximately 10 mV/m or higher. These strong electric fields generated in the ionosphere by lightning at local evening times may be important for the equatorial electrodynamics of the ionosphere.

Sonwalkar, Vikas S.↗

NASA NTRS1995

Heating, ionization and upward discharges in the mesosphere due to intense quasi-electrostatic thundercloud fields

Quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes are found to significantly heat mesospheric electrons and produce ionization and light. The intensity, spatial extent, duration and spectra of optical emissions produced are consistent with the observed features of the Red Sprite type of upward discharges.

Pasko, Victor P.↗

NASA NTRS1995

An interpretation of a mysterious 3.0- to 4.6-kHz emission band observed on Voyager 2 near Neptune

A whistler mode interpretation is provided for the narrowband signal (f approx. 3 - 4.6 kHz, Delta f approx. 200 - 800 Hz) detected by the plasma wave instrument on Voyager 2 during its encounter with Neptune. Our analysis indicates that this signal may have been generated in a limited spatial region and that it propagated to other regions of the Neptunian magnetosphere in the nonducted whistler mode with wave normal vectors lying close to the whistler mode resonance cone. The observed frequency variation of the emission along the Voyager 2 trajectory is consistent with this interpretation. The source location is estimated to be near the magnetic equator at L approx. 4 and dipole longitude of 111 deg W (260 deg W longitude in Neptune coordinate system). The source frequency and bandwidth are estimated to be 3.6 kHz and 300 Hz, respectively. The waves most likely would have been generated by energetic electrons with 2- to 20-keV parallel energy via a gyroresonance mechanism. Our interpretation of the narrowband emissions places the following limits on the Neptunian thermal plasma density and temperature: (1) N(sub e, min) greater than 0.16 el/cu cm for 1.2 R(sub N) less than R less than 5 R(sub N), (2) N(sub e, max) = 597.5/cu cm at R - 1.3 R(sub N), (3) T(sub e, max) less than 500-1000 K at R approx. 5 R(sub N). It is also possible that the weak UV aurora observed near Neptune could have been caused by the precipitation of energetic particles by the narrowband emission as a result of wave particle interactions.

Sonwalkar, Vikas S.↗

NASA NTRS1994

Heating of the nighttime D region by very low frequency transmitters

VLF signals propagating in the Earth-ionosphere waveguide are used to probe the heated nighttime D region over three U.S. Navy very low frequency (VLF,3-30 kHz) transmitters. Ionospheric cooling and heating are observed when a transmitter turns off and on in the course of normal operations. Heating by the 24.0-kHz NAA transmitter in Cutler, Maine, (1000 kW radiated power) was observed by this method in 41 of 52 off/on episodes during December 1992, increasing the amplitude and retarding the phase of the 21.4-kHz NSS probe wave propagating from Annapolis, Maryland, to Gander, Newfoundland, by as much as 0.84 dB and 5.3 deg, respectively. In 6 of these 41 episodes, the amplitude of the 28.5-kHz NAU probe wave propagating from Puerto Rico to Gander was also perturbed by as much as 0.29 dB. The latter observations were unexpected due to the greater than 770 km distance between NAA and the NAU-Gander great circle path. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 850 kW) transmitters was observed serendipitously in data from earlier measurements of the amplitudes of VLF signals propagating in the Earth-ionosphere waveguide. A three-dimensional model of wave absorption and electron heating in a magnetized, weakly ionized plasma is used to calculate the extent nad shape of the collision frequency (i.e., electron temperature) enhancement above a VLF transmitter. The enhancements are annular, with a geomagnetic north-south asymmetry and a radius at the outer half-maximum of the collision frequency enhancement of about 150 km. Heating by the NAA transmitter is predicted to increase the nighttime D region electron temperature by as much as a factor of 3. The calculated changes in the D region conductivity are used in a three-dimensional model of propagation in the Earth-ionosphere wavelength to predict the effect of the heated patch on a subionospheric VLF probe wave. The range of predicted scattered field amplitudes is in general consistent with the observed signal perturbations. Discrepanices in the predictions are attributed to lack of knowledge of the D region electron density profile along the probe wave great circle paths.

Rodriguez, Juan V.↗

NASA NTRS1994

Electron density changes in the nighttime D region due to heating by very-low-frequency transmitters

Modification of the nighttime D region electron density (N(sub e)) due to heating by very-low-frequency (VLF) transmitters is investigated theoretically using a four-species model of the ion chemistry. The effects of a 100 kW, a 265 kW, and a 1000 kW VLF transmitter are calculated for three ambient N(sub e) profiles. Results indicate that N(sub e) is reduced by up to 26% at approximately 80 km altitude over a 1000 kW transmitter.

Rodriguez, Juan V.↗

NASA NTRS1993

A multiple-mode three-dimensional model of VLF propagation in the earth-ionosphere waveguide in the presence of localized D region disturbances

Transient localized D region disturbances, such as those associated with lightning discharges, affect the characteristics of VLF waves propagating in the Earth-ionosphere waveguide. In particular, both phase and amplitude changes in the subionospheric signal can be observed at receiving sites as a result of the wave scattering that takes place in the disturbed region. In the present paper we present a multiple-mode three-dimensional model of VLF propagation in the Earth-ionosphere waveguide in the presence of localized D region disturbances. The model takes into account great circle (GC) propagation paths with realistic ground and ionospheric conductivity changes that result in mode conversion along the path. It is assumed that conductivity changes transverse to the GC paths are negligible except in the vicinity of the D region disturbance and that mode coupling is negligible within the disturbed region. This new model is applied to experimental observations and is found to be in general agreement. The diagnostics potential of the model for characterizing energetic particle precipitation events is discussed.

Poulsen, William L.↗

NASA NTRS1992

D region disturbances caused by electromagnetic pulses from lightning

Attention is given to a simple formulation of the propagation and absorption in a magnetized collisional plasma of EM pulses from lightning which describes the effect of discharge orientation and radiated electric field on the structure and magnitude of heating and secondary ionization in the D region. Radiation from most lightning discharges can heat substantially, but only the most intense (not less than 20 V/m) are likely to cause ionization enhancements not less than 10 percent of the ambient in a single ionization cycle. This dependence on the radiated electric field is modified by the discharge radiation pattern: a horizontal cloud discharge tends to cause larger heating and ionizaton maxima while a vertical return stroke causes disturbances of a larger horizontal extent.

Rodriguez, Juan V.↗

NASA NTRS1992

Ionospheric modification with a VLF transmitter

Detectable heating of the nightime D region by a 28.5 kHz signal is observed in 16 out of 144 cases with events occurring under conditions of moderate to low D region electron densities as represented by the unperturbed VLF signal levels. Three dimensional modeling of the effects of NAU heating on a probe VLF signal predicts values in general agreement with observations and suggests that maximum effects should occur under tenuous D region conditions.

Inan, Umran S.↗

NASA NTRS1992

Thunderstorm coupling to the magnetosphere and associated ionospheric effects

This project deals with the coupling of electromagnetic energy released during a thunderstorm to the magnetosphere and the ionosphere. Both the effects of an individual lightning event as well the aggregate of all the lightning events during a thunderstorm are considered. Energy in the very low frequency (VLF) band can play a variety of roles in the magnetospheric and ionospheric physics: generation of plasmaspheric hiss believed to be responsible for the slot region in the radiation belts, generation of lower hybrid waves that can heat ions in the auroral and subauroral regions, precipitation of energetic electrons, ionospheric heating etc. While these phenomena have been identified, and characterized to some extent, the influence and role of thunderstorm energy on the magnetosphere and ionosphere at a global scale is not known. Only recently, simultaneous high resolution (temporal and spatial) data sets from ground based lightning detectors and space and ground based VLF detectors have become available, and thus it has become possible to raise a question of the kind mentioned above and try to answer it quantitatively. Work on the correlation between individual lightning discharges in a thunderstorm as detected by the lightning network and the whistlers observed on the DE-1 satellite continued during this period. Results are summarized.

Inan, Umran S.↗

NASA NTRS1989

Lightning as an embryonic source of VLF hiss

The characteristics of hiss emissions triggered by lightning-generated whistlers are studied based on observations of the DE-1 satellite obtained between December 28, 1986 and January 18, 1987. The results suggest that these emissions may constitute the dominant wave activity in the 1.5-6-kHz range on L shells of 3.5-3 in the afternoon sector during geomagnetically quiet periods. The lightning-generated waves may play a dominant role in the loss of 0.5-50-keV electrons trapped on these field lines in the afternoon sector, and may also provide a loss mechanism for ring current protons.

Sonwalkar, Vikas S.↗

NASA NTRS1988

Wave normal direction and spectral properties of whistler mode hiss observed on the DE 1 satellite

Hiss is represented by a field distribution function in order to investigate magnetospheric hiss as a spatially and temporally enduring phenomenon. The study takes into account the whistler mode relationships and the linear and spin motion of the satellite. Hiss signals received on September 23, 1983 by the DE-1 electric and magnetic field antennas are analyzed. A wave normal angle of 60 + or - 5 deg with respect to the local geomagnetic field is found near the geomagnetic equator, and wave normal directions from 30-80 deg with respect to the local geomagnetic field are found away from the equator.

Sonwalkar, Vikas S.↗

NASA NTRS1987

Waves and instabilities

The U.S research activities during the mid-1982 to mid-1986 period in the area of plasma waves and instabilities are described. The principal results obtained by satellite observations and theoretical research can be divided into the following six categories: (1) whistler-mode wave-electron interactions, (2) wave-induced particle precipitation, (3) ULF wave phenomena, (4) ion-mode waves and instabilities, (5) auroral kilometric radiation, and (6) ionospheric irregularities and instabilities. In addition, waves and instabilities found in the outer magnetosphere, including the geomagnetic tail, magnetopause, and bow shock regions, are briefly discussed.

Inan, Umran S.↗