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At least 163 records · Page 9

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NASA NTRS1980

Pulsation of late B-type stars

Radial velocity observations of three of the brightest stars in the Pleiades, Alcyone, Maia and Taygeta, made during the course of one night, 25 October 1976, are discussed. All three stars were discovered to be pulsating with periods of a few hours. Analysis of all published radial velocities for each star, covering more than 70 years and approximately 100,000 cycles, has established the value of the periods to eight decimal places, and demonstrated constancy of the periods. However, amplitudes of the radial velocity variations change over long time intervals, and changes in spectral line intensities are observed in phase with the pulsation. All three stars may also be members of binary systems.

Beardsley, W. R.↗

NASA NTRS1980

Pulsation and mass loss in Mira variables

The behavior of pulsation in the outer layers of a typical Mira variable was studied in the adiabatic and isothermal limits. A shock wave propagates outward once per period and the radial velocity obtained from observations of hydrogen emission lines is identified with the velocity of gas in the post shock region. In the adiabatic case, mass loss in the form of a steady stellar wind was produced. In the isothermal case, no continuous mass loss was produced but occasional ejection of shells occur. Pulsation introduced into a star undergoing steady mass loss as a result of radiation pressure acting on grains caused the mass loss rate to increase by a factor of approximately 40, while the terminal velocity of the flow was almost unaltered.

Wood, P. R.↗

NASA NTRS1979

Satellite observations of the spatial extent and structure of Pc 3, 4, 5 pulsations near the magnetospheric equator

Simultaneous observations of Pc 3, 4, 5 pulsations by five satellites in the pre-noon local time sector at and near synchronous orbit are examined. The periods of these simultaneous pulsations are not the same at the different observation points. This difference is attributed to site dependent resonant conditions. The spatial properties of the temporal phenomenon are demonstrated with observations by ISEE-1 and -2 as they pass through oscillations in a spatially limited region. Fundamental and second harmonic standing Alfven waves are observed simultaneously on the same field line. The periods are consistent with model predictions when the measured plasma composition, which by mass consists mainly of singly ionized oxygen, is taken into account.

Singer, H. J.↗

NASA NTRS1980

Soft X-ray pulsations from SS Cygni

Pulsed soft X-rays (0.1 to 0.5 keV) with a period of 9 sec and a pulsed fraction that varies between 0 and 100% were detected from the dwarf nova SS Cygni at the peak of an optical outburst. This detection confirms for the first time the supposed high energy origin of optical pulsations seen in erupting dwarf novae. The pulse shape is remarkably sinusoidal for such a large-amplitude oscillation. The X-ray pulsation observed in this outburst is not coherent, in contrast to previous claims for the related optical oscillations.

Cordova, F. A.↗

NASA NTRS1981

Discovery of X-ray pulsations from 2S 1417-624

X-ray pulsations having a period of 17.64 sec are detected from the hard, variable galactic X-ray source 2S 1417-624. It is noted that the pulsations were found through a Fourier analysis of 40 satellite orbits of data taken with the rotation modulation collimators aboard SAS 3 in July 1978. The pulse profile shows detailed interpulse structure that is similar in character to the pulse profiles of 4U 0900-40 and A0535+26. Significant changes in pulse period are detected over the eight-day observing interval. It is suggested that these changes may be the result of a large intrinsic spin-up rate for the neutron star or from binary orbital motion, with orbital periods longer than approximately 15 days.

Kelley, R. L.↗

NASA NTRS1981

Factors controlling the occurrence of Pc 3 magnetic pulsations at synchronous orbit

The reported investigation has the purpose to extend the study of Pc 3 pulsations at synchronous orbit conducted by Arthur and McPherron (1977). A large data set is used with the goal of finding the factors related to the occurrence of Pc 3 pulsations at synchronous orbit. The results are compared with ground-based observations. The study demonstrates that the diurnal occurrence pattern has a maximum at a prenoon local time and shows a seasonal shift. Among the solar wind parameters, solar wind velocity and the direction of the IMF are found to be dominant factors. Attention is given to the analysis procedure, examples of 2-hour spectra, diurnal occurrence pattern, the correlation between solar wind parameters, and the solar wind stream structure and Pc 3 occurrence.

Takahashi, K.↗

NASA NTRS1981

A motion picture presentation of magnetic pulsations

Using the data obtained from the IMS North American magnetometer network stations at high latitudes, a motion picture was made by a computer technique, describing time changes of Pc5 and Pi3 magnetic pulsation vectors. Examples of pulsation characteristics derived from this presentation are regional polarization changes including shifts of polarization demarcation lines, changes in the extent of an active region and its movement with time.

Suzuki, A.↗

NASA NTRS1981

Search for high frequency pulsations in the onset of the March 5 gamma-ray burst

The onset of the March 5 Gamma-Ray burst was observed with microsecond time resolution by the Monitor Proportional Counter aboard the HEAO-2/Einstein Observatory. The data were analyzed for high frequency periodic pulsations which, if present, would be strong evidence for vibrations of a neutron star following the initial release of energy. Evidence is not found for such pulsations in the data over the period range 29.4737 to 0.0962 ms, and upper limits to the amplitude of 49-70% are set.

Weisskopf, M. C.↗

NASA NTRS1982

Pulsation research during the IMS

After describing the development status of the field of magnetic pulsations in 1975, before the initiation of the International Magnetospheric Study (IMS), attention is given to the IMS's novel observational results and an attempt is made to identify the most effective research methods employed. It is found that the most fruitful work involved small-scale collaboration between a few individuals or a few groups possessing complementary data sets. Consideration is restricted to research on the long period pulsations which can be broadly classified as field line resonances. Recommendations are made for future research efforts.

Hughes, W. J.↗

NASA NTRS1982

Pressure pulsations above turbomolecular pumps

Lange and Singleton (1978) have observed pressure pulses above a turbomolecular pump. They reported that the mean pulse frequency increased with the temperature of the pump cooling water and that the evolved gas was mainly hydrogen. The present investigation takes into account tests conducted with a similar pumping system. The pumping system was equipped with additional pressure-monitoring equipment in order to study these pulsations in more detail. It was found that at least two distinct types of pressure pulsations may be present in a turbomolecular-pumped ultrahigh vacuum system. The random hydrogen pulses are easily eliminated for period of days by changing the cooling water temperature. The cyclic pulses consisting mainly of water vapor are not likely to be a problem in normal experiments.

Danziger, S.↗

NASA NTRS1983

Pulsations in magnetic field and ion flux observed at L = 4.5 on August 5, 1972

Observations made from space (Explorer 45) near L = 4.5 and from magnetic observatories on the ground of the large-amplitude ULF pulsations that followed the unusually strong compression of the earth's magnetosphere associated with the geomagnetic storm of August 1972 are analyzed. A spectral analysis of Explorer 45 magnetometer data indicates a compressional mode oscillation coupled to a transverse mode oscillation. Enhancements in amplitude of a 300-s period wave near 0040 UT August 5 are found to coincide with an intensification of 100- to 1000-Hz magnetic and electric field oscillations and with the appearance of fluxes of energetic ions. It is noted that during this period the ion pitch angle distribution in each available energy channel (24-300 keV) followed a periodic sequence, apparently synchronized with the magnetic pulsations, from normal trapping (highest fluxes near 90 deg and lowest near 0 deg and 180 deg) to a nearly isotropic particle distribution. It is found that during the transitions the particle flux near 90 deg pitch angle was alternately large earthward of the satellite (before isotropy) and larger radially outward from the satellite (after isotropy).

Engebretson, M. J.↗

NASA NTRS1983

Comparison of phase and amplitude structures of Pc 5 pulsations in the morning and afternoon sectors

North American IMS magnetometer network data are used in comparisons of the spatial phase and amplitude structures of the morning and afternoon Pc 5 pulsations, using cross spectral analysis of phase and amplitude readings between different stations. The spatial phase structures are found to be very similar in the morning and afternoon sectors for each of the three components. Amplitudes are larger in the magnetic north than in the magnetic east components for both sectors. Latitudinal amplitude and phase difference variations between the magnetic north and east components exhibit dissimilarities in the morning and afternoon sectors. These results suggest that compressional mode magnetic field oscillations are not observable on the ground in this time sector. The pulsations in both time sectors are instead seen as caused by transverse mode magnetospheric field oscillations.

Saka, O.↗

NASA NTRS1984

Relationship between the IMF magnitude and Pc 3 magnetic pulsations in the magnetosphere

The relationships between the IMF magnitude and pulsation frequencies in the Pc 3-4 range simultaneously observed both at synchronous orbit and at low latitudes on the ground are statistically described. A theoretical discussion is given on how these observations can be interpreted in terms of the characteristic frequency of compressional Pc 3-4 magnetic pulsations in the magnetosphere, based on the well-established ion cyclotron resonance mechanism between magnetosonic mode of low-frequency upstream waves and narrowly reflected ion beams in the earth's foreshock.

Yumoto, K.↗

NASA NTRS1985

Optical and infrared pulsations from the HZ Herculis binary system during the 1983 prolonged X-ray low state

Pulsed infrared and optical flux from HZ Her has been detected during the 1983 prolonged X-ray low state by simultaneous observations with the Lick Observatory Shane 3.1 m and Crossley 91 cm telescopes. The pulsed fluxes in the 1.0-2.5-micron bandpass and the 3200-7500 A bandpass agree in pulse frequency and phase and were measured to be 31 and 13 microJy, respectively. These pulsed fluxes indicate that the reprocessed pulsation spectrum is consistent with optically thin thermal bremsstrahlung radiation with a characteristic temperature of 25,000 K (+25,000; -12,500) which is modulated in intensity. An apparent earth-approaching Doppler velocity of about 65 km/s is observed for these pulsed fluxes; this velocity can be interpreted as pulsed X-ray reprocessing by material in the mass transfer stream between L1 and the outer edge of the accretion disk. Other optical observations made during and before the 1983 prolonged X-ray low state show that the optical pulsations may have been subtly affected by the obscuration responsible for the X-ray low.

Middleditch, J.↗

NASA NTRS1985

Simulation of the westward traveling surge and Pi 2 pulsations during substorms

The westward traveling surge and the Pi2 pulsations are simulated as a consequence of an enhanced magnetospheric convection in a model of magnetosphere coupling. The coupling is characterized by the bouncing of Alfven waves launched by the enhanced convection. The reflection of Alfven waves from the ionosphere is treated in which the height-integrated conductivity is allowed to be highly nonuniform and fully anisotropic. The reflection of Alfven waves from the magnetosphere is characterized by the coefficient Rm, depending on whether the field lines are open or closed. The conductivity in the model is self-consistently enhanced with increasing upward field-aligned current density. The results of the simulation, including the convection pattern, the electrojets, the field-aligned current, the conductivity enhancement, the oscillation of the westward electrojet, and the average speed of the westward surge are in reasonable agreement with the features of the westward traveling surge and the Pi 2 pulsations observed during substorms.

Kan, J. R.↗

NASA NTRS1986

Quasi-periodic pulsations in solar hard X-ray and microwave flares

For more than a decade, various studies have pointed out that hard X-ray and microwave time profiles of some solar flares show quasi-periodic fluctuations or pulsations. Nevertheless, it was not until recently that a flare displaying large amplitude quasi-periodic pulsations in X-rays and microwaves was observed with good spectral coverage and with a sufficient time resolution. The event occurred on June 7, 1980, at approximately 0312 UT, and exhibits seven intense pulses with a quasi-periodicity of approximately 8 seconds in microwaves, hard X-rays, and gamma-ray lines. On May 12, 1983, at approximately 0253 UT, another good example of this type of flare was observed both in hard X-rays and in microwaves. Temporal and spectral characteristics of this flare are compared with the event of June 7, 1980. In order to further explore these observational results and theoretical scenarios, a study of nine additional quasi-periodic events were incorporated with the results from the two flares described. Analysis of these events are briefly summarized.

Kosugi, Takeo↗

NASA NTRS1986

A double layer model for solar X-ray and microwave pulsations

The wide range of wavelengths over which quasi-periodic pulsations have been observed suggests that the mechanism causing them acts upon the supply of high energy electrons driving the emission processes. A model is described which is based upon the radial shrinkage of a magnetic flux tube. The concentration of the current, along with the reduction in the number of available charge carriers, can rise to a condition where the current demand exceeds the capacity of the thermal electrons. Driven by the large inductance of the external current circuit, an instability takes place in the tube throat, resulting in the formation of a potential double layer, which then accelerates electrons and ions to MeV energies. The double layer can be unstable, collapsing and reforming repeatedly. The resulting pulsed particle beams give rise to pulsating emission which are observed at radio and X-ray wavelengths.

Tapping, K. F.↗

NASA NTRS1987

A search for millisecond periodic and quasi-periodic pulsations in low-mass X-ray binaries

The results of a Fourier analysis to detect fast periodic and quasi-periodic pulsations in the X-ray emission from the sources 4U 0614 + 091, 4U 1636 - 536, 4U 1735 - 44, 4U 1820 30, GX 5 - 1, GX 9 + 9, Ser X-1, and Cyg X-2 are presented. This search has been carried out for the first time in the soft energy band (0.2-2.0 keV), using data from the Einstein Observatory high resolution imager instrument. An approximate method of minimizing the decrease in search sensitivity produced by the Doppler shift of the pulse periods due to source orbital motion is discussed. No pulsations have been detected, and upper limits, which depend on the orbital parameters assumed for the sources, are set on the pulsed flux fraction.

Mereghetti, S.↗