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At least 451 records · Page 25

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NASA NTRS1983

Evidence for quasi-periodic components in Dobson network total ozone records

Power spectrum analysis has been applied to the total ozone time series at each of a sample of Dobson stations with records of more than 15 years in the interval 1957 to 1981. The distributions of strongly periodic or quasiperiodic signals in these spectral estimates, for three spectral estimators and two levels of confidence, show distinct features. These appear at the previously known or suspected periods of 3.5-4.0 years, 27 months, and 21 months as well as the previously unreported period of 11.5 months. Possible geophysical implications are discussed.

Guthrie, P. D.↗

NASA NTRS1983

An analysis of periodicities in the 1470 to 1974 Beijing precipitation record

An analyis of a time series consisting of an annual index of dryness/wetness for the years 1470 to 1974 in Beijing, China is presented. Its power spectrum shows that dominant cycles occur with long periods of the order of 80 years. Cycles with periods of 11 and 22 years are weak or non-existent, but a significant signal at 18.7 years (which is also the period of a component of the lunar tide generating force) is detected. The long term variations in Beijing precipitation appear to lag long term (Gleissberg) variations in solar activity by nearly 75 years. A pattern which spans nearly 150 years in the Beijing record is found to be repeated with notable similarity.

Hameed, S.↗

NASA NTRS1983

Solar oscillations with 13-day period

Reference is made to the solar observations made by Claverie et al. (1982) over a three-month period in the summer of 1981 which show oscillatory velocity with a period of 13.1 days and amplitude of 6.6 m/s. These investigators reject the possibility that they see the Doppler shift from a radial oscillation, because the amplitude is implausibly large. They also do not believe that their signal was induced by solar magnetic fields, since typical mean solar fields are too small. Photo-electric drift-scan measurements of the solar diameter and full-disk magnetograms taken at Kitt Peak National Observatory are examined here for evidence of variations corresponding to the velocity oscillations of the 13.1-day period. An upper limit on radius variations is reported which is a factor of six below the amplitude needed to explain the velocity observations as a radial oscillation. Attention is also given to the possible role of the rotation of large-scale surface magnetic features.

Duvall, T. L., Jr.↗

NASA NTRS1983

Solar wind variations in the 60-100 year period range: A review

The evidence for and against the reality of a solar wind variation in the period range of 60-100 years is reexamined. Six data sets are reviewed; sunspot numbers, geomagnetic variations, two auroral data sets and two (14)C data sets. These data are proxies for several different aspects of the solar wind and the presence or absence of 60-100 year cyclic behavior in a particular data set does not necessarily imply the presence or absence of this variation in other sets. It was concluded that two different analyses of proxy data for a particular characteristic of the heliospheric solar wind yielded conflicting results. This conflict can be resolved only by future research. It is also definitely confirmed that proxy data for the solar wind in the ecliptic at 1 A.U. undergo a periodic variation with a period of approximately 87 years. The average amplitude and phase of this variation as seen in eleven cycles of proxy data are presented.

Feynman, J.↗

NASA NTRS1984

On long-term periodicities in the sunspot record

Sunspot records are systematically maintained, with the knowledge that an 11 year average period exists since about 1850. Thus, the sunspot record of highest quality and considered to be the most reliable is that of cycle eight through the present. On the basis of cycles 8 through 20, various combinations of sine curves were used to approximate the observed R sub MAX values (where R sub MAX is the smoothed sunspot number at cycle maximum). It is found that a three component sinusoidal function, having an 11 cycle and a 2 cycle variation on a 90 cycle periodicity, yields computed R sub MAX values which fit, reasonably well, observed R sub MAX values for the modern sunspot cycles. Extrapolation of the empirical functions forward in time allows for the projection of values of R sub MAX for cycles 21 and 22. For cycle 21, the function projects a value of 157.3, very close to the actually observed value of 164.5. For cycle 22, the function projects a value of about 107. Linear regressions applied to cycle 22 indicate a long-period cycle (cycle duration 132 months). An extensive bibliography on techniques used to estimate the time dependent behavior of sunspot cycles is provided.

Wilson, R. M.↗

NASA NTRS1984

Radial velocity studies of cataclysmic binaries. II - The ultrashort period dwarf nova T Leonis

Spectroscopic and photometric observations of T Leo which reveal the object to be an ultrashort period dwarf nova are presented. In particular, the analysis indicates that T Leo has an orbital period of 84.69936 (plus or minus 0.00068) minutes, which is the third shortest orbital period of any known cataclysmic variable and the shortest known for any U Gem type dwarf nova. These observations enable the placement of upper limits of 0.19 and 0.4 solar mass on the mass of the red dwarf and white dwarf, respectively. The overall energy distribution is relatively flat (f-sub-lambda approximately constant) which is consistent with a low mass accretion rate. The models of Williams and Ferguson (1982) suggest that the mass accretion rate is about 10 to the -10th solar mass/yr. A narrow peak component of H-alpha is observed, the phasing of which cannot be explained by the canonical hot spot model. Possible explanations for its origin are presented and discussed. It is predicted that future observations of T Leo may reveal the object to be a SU UMa type dwarf nova.

Shafter, A. W.↗

NASA NTRS1984

Long-period classical Cepheids - Theory versus observation

New full amplitude models of classical Cepheids having periods longer than 13 days have been calculated, allowing the complete systematics of the observed light and velocity curves of classical Cepheids to be discussed in detail. Assuming that a normal evolutionary mass-luminosity relation is obeyed, the models reproduce such empirical phenomena as the full Hertzsprung progression of light and velocity curves and the gradual shift in phase of the Hertzsprung bump, together with the persistence in phase of the postmaximum shoulder on the light curve. Also reproduced are the retardation of maximum expansion velocity after maximum light, the correlation of light and velocity amplitudes, and the period-amplitude scatter diagram for both the light and the velocity amplitude. The average period predicted for a Cepheid light curve type is noted to be about 40 percent longer than that actually observed.

Carson, T. R.↗

NASA NTRS1983

A 50 day periodicity in the cosmic ray anisotropy

A modulation of the anisotropy of relativistic interplanetary particles with a period of approximately 50 days is discussed. Even though it is not unusual for cosmic ray data to reflect the solar rotation frequency and its harmonics with periods of 25/n days (n being 1, 2, 3, . . .), it is not known how a periodicity of 50 days, which would be a subharmonic, is generated. Since the Cerenkov counter responds to all kinds of relativistic particles, it does not identify the particles that cause the 50 day east-west modulation. Jovian electrons are considered unlikely in that they are most abundant near their source, whereas the modulation has never been reported for ground-based neutron monitors. It is pointed out that since the Cerenkov threshold is somewhat lower than typical neutron monitor cutoffs, it is conceivable that the modulation affects nucleons just in this energy gap. It is also thought that there might be a uniformly distributed component of interplanetary electrons which undergo this peculiar modulation.

Fillius, W.↗

NASA NTRS1984

Quasi-periodic oscillations in a symmetric general circulation model

Observational evidence has been presented for the existence of quasi-periodic fluctuations of the tropical circulation with periods around two weeks and around 40 days. It is expected that an understanding of the mechanisms of these quasi-periodic oscillations in the tropical atmosphere will improve the predictability of the short range climate fluctuations in the tropics. The present study evolved as an outgrowth of an investigation conducted by Goswami et al. (1984). In this investigation remarkable oscillations of the Hadlay circulation for an ocean covered earth were observed. In the current study evidence is presented regarding the episodic behavior of the tropical circulation in general, and the propagation characteristics of these oscillations in the lower atmosphere. Attention is given to the results of six different experiments.

Goswami, B. N.↗

NASA NTRS1984

Wave-wave interactions in the stratosphere - Observations during quiet and active wintertime periods

Smith (1983) has demonstrated that wave-wave interactions among planetary waves 1, 2, and 3 were important during the month of January 1979, a period characterized by extremely large wave 1 amplitudes in the stratosphere. In the present investigation, the same analysis is applied to the period November 1978 through March 1979 with the aim to determine the conditions under which wave-wave interactions were important. Attention is given to enstrophy budget calculations, the wave 1/wave 2 vacillation, a quantitative measure of wave-wave interactions, and examples of wave-wave interactions during several periods. It is found that the vacillation between waves 1 and 2 has no clear relationship to the tropopause forcing as represented by the 300 mb amplitude.

Smith, A. K.↗

NASA NTRS1985

Periodic cometary showers: Real or imaginary?

Since the initial reports in 1980, a considerable body of chemical and physical evidence has been accumulated to indicate that a major impact event occurred on earth 65 million years ago. The effects of this event were global in extent and have been suggested as the cause of the sudden demise or mass extinction of a large percentage of life, including the dinosaurs, at the end of the geologic time period known as the Cretaceous. Recent statistical analyses of extinctions in the marine faunal record for the last 250 million years have suggested that mass extinctions may occur with a periodicity of every 26 to 30 million years. Following these results, other workers have attempted to demonstrate that these extinction events, like that at the end of the Cretaceous, are temporally correlated with large impact events. A recent scenario suggests that they are the result of periodic showers of comets produced by either the passage of the solar system through the galactic plane or by perturbations of the cometary cloud in the outer solar system by a, as yet unseen, solar companion. This hypothesized solar companion has been given the name Nemesis.

Grieve, R. A. F.↗

NASA NTRS1985

Cosmic ray power spectral variations. 3: Solar rotation periodicities

Earlier studies of the periodic changes in cosmic ray intensity by power spectral analysis provided some understanding of the daily variation in terms of ambient power. The solar rotation periodicities are investigated, using daily means of calgary neutron monitor data during 1965 to 1976. Significant peaks with periods of 27 and 13.5 days with varying magnitudes are observed.

Agrawal, S. P.↗

NASA NTRS1985

The modulation features of the long-period cosmic ray variations in connection with the sign change of the general magnetic field of the Sun

On the basis of the model and experimental investigations, the spatial distribution of cosmic ray anisotropy for the different epochs of solar activity is studied. A solution is offered to the anisotropic diffusion equation with regard to the electromagnetic conditions for the periods of minimum and maximum solar activity and cosmic ray particle drift in a regular interplanetary magnetic field. It is shown that the long period changes amplitude and phase of the diurnal variations of cosmic rays is limited not only by convection and diffusion of particles but also by the drift effect before and after the sing change of the general magnetic field of the Sun. The calculated model is compared with the results obtained on the basis of an analysis of the experimental data from the neutron super monitor station, and it is shown that for the periods when the lines of magnetic force of the Sun come from the Northern Hemisphere the phase of the first harmonic diurnal variation is shifted forwards to an earlier time.

Iskra, K.↗

NASA NTRS1984

A 164-day period in gamma-ray bursts from GBS0526-66?

Evidence is presented for a pattern in the recurrence time of gamma ray bursts from GBS0526-66, showing an apparent period of 164 days. It is suggested that this pattern reflects periodic accretion to a neutron star from an eccentric binary companion. It is proposed that this system drives two types of bursts: (1) the relatively frequent but weak recurrent bursts produced by either magnetospheric gating, or thermonuclear runaway, of periodically accreting matter around the neutron star, and (2) the much rarer intense bursts produced by tidally triggered star quakes.

Rothschild, R. E.↗

NASA NTRS1985

Nucleus precession of periodic comet Comas Sola

The nuclear properties of the periodic comet Comas Sola are studied based on a precession model applied previously to the periodic comets Encke, Kopff, and Giacobini-Zinner. The results imply that, for a few revolutions about the sun, Comas Sola was precessing more rapidly than any comet studied to date. An explanation is offered for this behavior in terms of a perturbation in the comet's obliquity shortly after the 1952 passage through perihelion. The equatorial radius of the nucleus is close to 1 km and its rotation period is 1.5-2.3 days, according to the model results. The calculated shape of the nucleus is compared with those of other comets studied using this technique.

Sekanina, Z.↗

NASA NTRS1985

Periodic binary sequences with very good autocorrelation properties

Computer searches were performed using both an 8086 microprocessor and a Cyber 750 mainframe to find repeated binary phase coded waveforms with very good matched and mismatched autocorrelation properties. The best results for every period up to 64 are given. Sequences with optimal peak sidelobes were discovered for each of these periods. These sequences have extensive applications in radar and communications, particularly in situations when there are very unfavorable signal-to-noise ratios. The best sequence of period 64 when processed using a mismatched filter giving no sidelobes has a reduction in the main lobe of less than 0.23 dB.

Tyler, S.↗

NASA NTRS1985

Instability of time-periodic flows

The instabilities of some spatially and/or time-periodic flows are discussed, in particular, flows with curved streamlines which can support Taylor-Gortler vortices are described in detail. The simplest flow where this type of instability can occur is that due to the torsional oscillations of an infinitely long circular cylinder. For more complicated spatially varying time-periodic flows, a similar type of instability can occur and is spatially localized near the most unstable positions. When nonlinear effects are considered it is found that the instability modifies the steady streaming boundary layer induced by the oscillatory motion. It is shown that a rapidly rotating cylinder in a uniform flow is susceptible to a related type of instability; the appropriate stability equations are shown to be identical to those which govern the instability of a boussinesq fluid of Prandtl number unity heated time periodically from below.

Hall, P.↗

NASA NTRS1986

Upwind Navier-Stokes solutions for separated periodic flows

The application of an upwind implicit approximate factorization Navier-Stokes algorithm to highly separated flow is described. Using both the thin-layer and complete forms of the Navier-Stokes equations, the low Reynolds number laminar flow around a circular cylinder with periodic shedding is solved. The effect of grid density, grid extent, and time step on the Strouhal number is shown. Results from both sets of equations agree within the experimental data band. Unsteady, laminar flow computations around inclined plates and separated airfoils are also described. Strouhal numbers agree to within 5 percent of experiments for inclined plates. Differences between the complete equations and the thin-layer approximation for separated periodic flows are discussed. Computations of an impulsively started circular cylinder and airfoil yield time-accurate flowfield shapes in good agreement with experimental flow visualizations. The turbulent computation of an airfoil at a high angle-of-attack is massively separated, but shows no evidence of periodicity.

Rumsey, C. L.↗