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NASA NTRS1978

Seasonal and interannual variations in total ozone revealed by the Nimbus-4 backscattered ultraviolet experiment

The first two years of Backscattered Ultraviolet (BUV) ozone data from the Nimbus-4 spacecraft were reprocessed. The seasonal variations of total ozone for the period April 1970 to April 1972 are described using daily zonal means to 10 deg latitude zones and a time-latitude cross section. In addition, the BUV data are compared with analyzed Dobson data and with IRIS data also obtained from the Nimbus-4 spacecraft. A harmonic analysis was performed on the daily zonal means. Amplitudes, days of peaks, and percentage of variance were computed for annual and semi-annual waves and for higher harmonics of an annual period for the two years. Asymmetries are found in the annual waves in the two hemispheres, with a subtle interannual difference which may be due to changes in the general circulation. A significant semi-annual component is detected in the tropics for the first year, which appears to result from influences of the annual waves in the two hemispheres.

Hilsenrath, E.↗
NASA NTRS1978

Recent advances in analytical satellite theory

Recent work on analytical satellite perturbation theory has involved the completion of a revision to 4th order for zonal harmonics, the addition of a treatment for ocean tides, an extension of the treatment for the noninertial reference system, and the completion of a theory for direct solar-radiation pressure and earth-albedo pressure. Combined with a theory for tesseral-harmonics, lunisolar, and body-tide perturbations, these formulations provide a comprehensive orbit-computation program. Detailed comparisons with numerical integration and observations are presented to assess the accuracy of each theoretical development.

Gaposchkin, E. M.↗
NASA NTRS1979

Identification of limit cycles in multi-nonlinearity, multiple path systems

A method of analysis which identifies limit cycles in autonomous systems with multiple nonlinearities and multiple forward paths is presented. The FORTRAN code for implementing the Harmonic Balance Algorithm is reported. The FORTRAN code is used to identify limit cycles in multiple path and nonlinearity systems while retaining the effects of several harmonic components.

Mitchell, J. R.↗
NASA NTRS1979

Geomagnetic Sq variation at satellite altitudes - Is Sq correction important in Magsat data analysis

By a spherical harmonic expansion of the magnetic potential the geomagnetic Sq variation is shown to have peak-to-peak amplitudes of a few tens of nanotesla at 400 km altitude. It is shown, in particular, that for harmonics of degree n up to about 4, with which Sq can be well represented, the vertical component of the magnetic field at altitudes to about 400 km is virtually the same as on the ground. Even though the NASA's Magnetic Field Satellite (Magsat), scheduled for launch in September 1979, is planned to be placed in a twilight sun-synchronous orbit, Sq over certain areas of the earth can be of comparable magnitude as magnetic anomalies of geological origin sought in the data. Due consideration should be given to Sq as well as to the effects from magnetospheric sources in analyzing the Magsat data. The Sq effect is expected to be large during the period of Magsat operation which is near the predicted solar maximum.

Sugiura, M.↗
NASA NTRS1979

An orbiting infrared interferometer to search for nonsolar planets

This paper proposes an orbiting infrared interferometer with its fringe null centered on a nearby star at a distance of say 10 parsecs. A large planet ('Jupiter') would have an angular separation from the star of about 0.5 arcsec. To have a fringe crest on the planet, a fringe period of 1.0 arcsec is needed and at 40 microns the required baseline is 8 m. Spinning the interferometer about the line of sight to the star results, even with pointing errors, in a relatively slowly varying but strongly suppressed stellar output and a more rapidly varying fringe-like planetary signal rich in higher harmonics. For pointing errors up to .050 arcsec the planet's fourth harmonic greatly exceeds that from the star, thereby relaxing interferometer pointing tolerances. Indeed, it appears that the limiting factor is zodiacal infrared background radiation and not the intense localized stellar flux which can effectively be eliminated by the fringe null of the spinning infrared interferometer.

Macphie, R. H.↗
NASA NTRS1979

Gravity field of Venus - A preliminary analysis

The gravitational field of Venus obtained by tracking the Pioneer Venus Orbiter is examined. For each spacecraft orbit, two hours of Doppler data centered around periapsis were used to estimate spacecraft position and velocity and the velocity residuals obtained were spline fit and differentiated to produce line of sight gravitational accelerations. Consistent variations in line of sight accelerations from orbit to orbit reveal the presence of gravitational anomalies. A simulation of isostatic compensation for an elevated region on the surface of Venus indicates that the mean depth of compensation is no greater than about 100 km. Gravitational spectra obtained from a Fourier analysis of line of sight accelerations from selected Venus orbits are compared to the earth's gravitational spectrum and spherical harmonic gravitational potential power spectra of the earth, the moon and Mars. The Venus power spectrum is found to be remarkably similar to that of the earth, however systematic variations in the harmonics suggest differences in dynamic processes or lithospheric behavior.

Phillips, R. J.↗
NASA NTRS1979

Plasma radiation diagnostics of the primary energy release region in solar flares

The possibility is investigated that the plasma turbulence used in many recent models of the primary energy release and acceleration in solar flares should be detectable by radiation near the fundamental and second harmonic of the plasma frequency. Formulae are derived for fundamental emission due to the combination of ion-acoustic and Langmuir plasma turbulence and for second harmonic emission due to the combination of two Langmuir waves. These results are applied to recent primary energy release and acceleration models which shows that either such radiation should be detectable and possibly distinguishable with suitable microwave interferometers or that its absence places fairly stringent constraints on the possible level of Langmuir or Langmuir and ion-acoustic waves in these models.

Smith, D. F.↗
NASA NTRS1979

Magnetospheric plasma wave research 1975-1978

Research conducted from 1975 through 1978 on magnetospheric plasma waves generated in or passing through the magnetospheres of the earth and Jupiter is reviewed. Attention is given to bow shock whistlers and electrostatic waves, electromagnetic wave packets in the magnetosheath ('lion roars'), nonthermal continuum radiation apparently associated with energetic electrons in the outer radiation zone, electrostatic and magnetic noise and PI2 pulsations in the magnetotail and plasma sheet and polar cusp phenomena. Plasma waves in the auroral zone, including VLF hiss and saucers, electrostatic wave turbulence and kilometric radiation are treated, together with MHD waves and magnetic pulsations, ion cyclotron waves, VLF chorus and hiss and electrostatic electron cyclotron harmonic radiation in and beyond plasmapause, plasmaspheric hiss, whistlers, power line harmonic radiation and controlled wave generation experiments. Observations of plasmasphere and auroral zone phenomena in the ionosphere are discussed, and escaping and trapped plasma waves in the magnetospheres of Jupiter and other planets are considered. Planned future magnetospheric research for the next four years is outlined.

Shawhan, S. D.↗
NASA NTRS1979

Gravitational spectra from direct measurements

A simple rapid method is described for determining the spectrum of a surface field (in spherical harmonics) from harmonic analysis of direct (in situ) measurements along great circle arcs. The method is shown to give excellent overall trends (smoothed spectra) to very high degree from even a few short arcs of satellite data. Three examples are taken with perfect measurements of satellite tracking over a planet made up of hundreds of point masses using (1) altimetric heights from a low-orbiting spacecraft, (2) velocity (range rate) residuals between a low and a high satellite in circular orbits, and (3) range rate data between a station at infinity and a satellite in a highly eccentric orbit. In particular, the smoothed spectrum of the earth's gravitational field is determined to about degree 400(50-km half wavelength) from 1 x 1 deg gravimetry and the equivalent of 11 revolutions of GEOS 3 and Skylab altimetry. This measurement shows that there is about 46 cm of geoid height (rms worldwide) remaining in the field beyond degree 180.

Wagner, C. A.↗
NASA NTRS1980

Observations of electron gyroharmonic waves and the structure of the Io torus

Narrow-banded emissions were observed by the Planetary Radio Astronomy experiment on the Voyager 1 spacecraft as it traversed the Io plasma torus. These waves occur between harmonics of the electron gyrofrequency and are the Jovian analogue of electrostatic emissions observed and theoretically studied for the terrestrial magnetosphere. The observed frequencies always include the component near the upper hybrid resonant frequency, (fuhr) but the distribution of the other observed emissions varies in a systematic way with position in the torus. A refined model of the electron density variation, based on identification of the fuhr line, is included. Spectra of the observed waves are analyzed in terms of the linear instability of an electron distribution function consisting of isotropic cold electrons and hot losscone electrons. The positioning of the observed auxiliary harmonics with respect to fuhr is shown to be an indicator of the cold to hot temperature ratio. It is concluded that this ratio increases systematically by an overall factor of perhaps 4 or 5 between the inner and outer portions of the torus.

Birmingham, T. J.↗
NASA NTRS1980

Reducing the error of geoid undulation computations by modifying Stokes' function

The truncation theory as it pertains to the calculation of geoid undulations based on Stokes' integral, but from limited gravity data, is reexamined. Specifically, the improved procedures of Molodenskii et al. are shown through numerical investigations to yield substantially smaller errors than the conventional method that is often applied in practice. In this improved method, as well as in a simpler alternative to the conventional approach, the Stokes' kernel is suitably modified in order to accelerate the rate of convergence of the error series. These modified methods, however, effect a reduction in the error only if a set of low-degree potential harmonic coefficients is utilized in the computation. Consider, for example, the situation in which gravity anomalies are given in a cap of radius 10 deg and the GEM 9 (20,20) potential field is used. Then, typically, the error in the computed undulation (aside from the spherical approximation and errors in the gravity anomaly data) according to the conventional truncation theory is 1.09 m; with Meissl's modification it reduces to 0.41m, while Molodenskii's improved method gives 0.45 m. A further alteration of Molodenskii's method is developed and yields an RMS error of 0.33 m. These values reflect the effect of the truncation, as well as the errors in the GEM 9 harmonic coefficients. The considerable improvement, suggested by these results, of the modified methods over the conventional procedure is verified with actual gravity anomaly data in two oceanic regions, where the GEOS-3 altimeter geoid serves as the basis for comparison. The optimal method of truncation, investigated by Colombo, is extremely ill-conditioned. It is shown that with no corresponding regularization, this procedure is inapplicable.

Jekeli, C.↗
NASA NTRS1980

Considerations in the placement of phase calibration tones

In the use of tones to calibrate unwanted instrumental phases for very long baseline interferometry experiments, certain problems exist which are related to the placement of these phase calibrator tones. (1) A bias exists in an analytically generated stopping function used during correlation if its frequency satisfies the following condition; f = (f sub s)x(m)/n, where f sub s = sampling frequency, m = any integer, and n = any odd integer. (2) Due to the quantized representation of sine waves in the stopping function, odd harmonics of the fundamental frequencies are generated. Several mechanisms are available through which these harmonics can cause errors in the residual phase extracted from the recorded tones. (3) When multiple tones are injected into a pass band, intermodulation products can occur. The magnitude of these effects is discussed along with strategies designed to avoid them.

Young, L. E.↗
NASA NTRS1979

Southern Hemisphere measurements of the anisotropy in the cosmic microwave background radiation

A recent airborne measurement of the large-angular-scale anisotropy in the cosmic background radiation from the Southern Hemisphere (Lima, Peru) is in essential agreement with previous measurements from the northern hemisphere. The net anisotropy from the combined data can be described by a first-order spherical harmonic (Doppler) anisotropy of amplitude 3.1 plus or minus 0.4 mK with a quadrupole component of less than 1 mK. Additional ground-based measurements of the linear polarization yield an upper limit of l mK, or one part in 3000, at 95% confidence level for the amplitudes of any spherical harmonic through third order.

Smoot, G. F.↗
NASA NTRS1979

Effects of power line radiation into the magnetosphere

Observations of the effects of VLF power line radiation on whistler-mode waves in the magnetosphere are reviewed. High-altitude OGO-3 spectral data reveal evidence of enhanced chorus activity over populated regions starting at harmonics of the power-line frequencies. Low-altitude Ariel 3 measurements of 3.2 kHz noise intensity also indicate an enhancement of VLF activity over populated areas and their conjugates, however the relative importance of power line radiation, whistlers and spontaneous emissions is not known. The low-altitude polar-orbiting OGO-4 satellite also observed noise spectra at the harmonics of power line frequencies over industrial regions. Ground observations from Eights and Siple, Antarctica indicate that power line radiation effects on magnetospheric ducted paths peak at 3 kHz and near dawn, and exhibit a pronounced decrease on Sundays in the conjugate region, when power consumption is at a minimum. Experiments simulating power line radiation effects have also been performed. It is suggested that power line radiation effects magnetospheric activity by lowering the threshold for wave growth, with the localization of VLF sources acting to localize corresponding particle precipitation without necessarily affecting global average precipitation.

Helliwell, R. A.↗
NASA NTRS1980

Electrostatic waves in the Jovian magnetosphere

Observations by the plasma wave receivers on Voyagers 1 and 2 show that a wide variety of electrostatic waves are present within the Jovian magnetosphere and that the Jovian electrostatic waves are for the most part very similar to those observed in the terrestrial magnetosphere. Bands of emission near the upper hybrid resonance frequency in the dayside outer magnetosphere are detected between higher harmonics of the electron gyrofrequency. Inside of about 23 Jupiter radii, electron cyclotron harmonic emissions appear to be durable features of the inner Jovian magnetosphere and are extremely well confined to the Jovian magnetic equator. The cyclotron emissions extend from just above the local electron gyrofrequency to the upper hybrid resonance frequency.

Kurth, W. S.↗
NASA NTRS1979

Development and analysis of a twelfth degree and order gravity model for Mars

Satellite geodesy techniques previously applied to artificial earth satellites have been extended to obtain a high-resolution gravity field for Mars. Two-way Doppler data collected by 10 Deep Space Network (DSN) stations during Mariner 9 and Viking 1 and 2 missions have been processed to obtain a twelfth degree and order spherical harmonic model for the martian gravitational potential. The quality of this model was evaluated by examining the rms residuals within the fit and the ability of the model to predict the spacecraft state beyond the fit. Both indicators show that more data and higher degree and order harmonics will be required to further refine our knowledge of the martian gravity field. The model presented shows much promise, since it resolves local gravity features which correlate highly with the martian topography. An isostatic analysis based on this model, as well as an error analysis, shows rather complete compensation on a global (long wavelength) scale. Though further model refinements are necessary to be certain, local (short wavelength) features such as the shield volcanos in Tharsis appear to be uncompensated. These are interpreted to place some bounds on the internal structure of Mars.

Christensen, E. J.↗
NASA NTRS1980

Analysis of mode-locked and intracavity frequency-doubled Nd:YAG laser

The paper presents analytical and computer studies of the CW mode-locked and intracavity frequency-doubled Nd:YAG laser which provide new insight into the operation, including the detuning behavior, of this type of laser. Computer solutions show that the steady-state pulse shape for this laser is much closer to a truncated cosine than to a Gaussian; there is little spectral broadening for on-resonance operation; and the chirp is negligible. This leads to a simplified analytical model carried out entirely in the time domain, with atomic linewidth effects ignored. Simple analytical results for on-resonance pulse shape, pulse width, signal intensity, and harmonic conversion efficiency in terms of basic laser parameters are derived from this model. A simplified physical description of the detuning behavior is also developed. Agreement is found with experimental studies showing that the pulsewidth decreases as the modulation frequency is detuned off resonance; the harmonic power output initially increases and then decreases; and the pulse shape develops a sharp-edged asymmetry of opposite sense for opposite signs of detuning.

Siegman, A. E.↗
NASA NTRS1980

The position and polarization of Type V solar bursts

Observations of the position and polarization of Type V solar radio bursts and their preceding Type III bursts are presented. The polarization, frequency range, source position, source movement, source size and brightness temperature of the bursts were measured using a 24-220 MHz spectropolarimeter, an 8-8000 MHz spectrograph and a three-frequency radioheliograph. Type V radiation is frequently found to have the opposite sense of circular polarization from that of the preceding Type III burst, with a degree of polarization similar to that of harmonic Type III radiation. A reversal of polarization is not observed when the accompanying Type III burst has no fundamental-harmonic structure, or when the Type V radiation is poorly developed. Possible mechanisms for the reversal are examined, including opposite magnetic field directions in Type III and V bursts, changes in mode coupling and a change in the mode of emission from o-mode for Type III to x-mode for Type V, and conditions needed for the mode change which is considered the most likely mechanism, are determined.

Dulk, G. A.↗