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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

An observational search for solar pulsations at periods from 7 to 70 minutes

Results are reported for measurements of the large-scale solar velocity field performed by comparing Doppler shifts from the disk center and limb with the aid of the magnetically insensitive Fe I line at 5123.730 A. Average power spectra are plotted, but no statistically significant periodicities are found at periods between 7 and 70 min. The sensitivities of the present observations and those of Hill et al. (1976) are compared, and the temperature change for an oscillation in the sun's atmosphere is calculated using the HSRA. It is concluded that the statistical significance and solar origin of the peaks in the SCLERA power spectrum are not well established and that the reality of the solar pulsations reported by Hill et al. is questionable.

Dittmer, P. H.↗
NASA NTRS1979

Some simple properties of stellar pulsation modes

Except for the lowest harmonics, small-amplitude stellar pulsation modes possess many simple properties whose evaluation does not require numerical integration of the fourth-order equations of motion. All antinodes tend to have the same total kinetic energy except for those lying near physical or geometric boundaries. However, when kinetic energy per unit volume is considered, order-of-magnitude enhancements are seen in antinodes lying near the center of the star, and factor-of-2 enhancements occur near the polar axis. The nodes are distributed very regularly along the radius. They follow an exponential law in g-regions, and their separation is proportional to the sound travel time in p-regions. A simple graphical procedure is described for surveying the oscillation frequencies of a new stellar model. A precise condition is derived giving the division of energy between radial and angular motion. Another condition gives the fractional contribution to the velocity field of its two sources, the divergence and the curl. Certain simplifying results of weak coupling among the linear modes are briefly described.

Wolff, C. L.↗
NASA NTRS1979

Observations of pulsating X-ray binaries 4U0900-40 and 4U1223-62

A sounding-rocket observation of the pulsating X-ray binaries 4U 0900-40 (Vela X-1) and 4U 1223-62 (GX301-2, Wray 977) over the energy range from 0.5 to 27 keV is reported. No short-time-scale periodicities in the range from 2 msec to 20 sec are found for either source, although both exhibit some minor flaring activity on time scales of a few seconds. The data for Vela X-1 suggest that the time-averaged spectra are complex and that the pulse-resolved spectra are also complex and variable, with significant fluctuations above 10 keV. When compared with other observations at different epochs, the spectral data for 4U 1223-62 confirm large variability in both intensity and spectral characteristics. It is concluded that the intrapulse changes observed in the spectral behavior of Vela X-1 must reflect changes in the surface magnetic fields and the plasma flows near the neutron star.

Ku, W. H.-M.↗
NASA NTRS1980

Light curves for bump Cepheids computed with a dynamically zoned pulsation code

The dynamically zoned pulsation code developed by Castor, Davis, and Davison was used to recalculate the Goddard model and to calculate three other Cepheid models with the same period (9.8 days). This family of models shows how the bumps and other features of the light and velocity curves change as the mass is varied at constant period. The use of a code that is capable of producing reliable light curves demonstrates that the light and velocity curves for 9.8 day Cepheid models with standard homogeneous compositions do not show bumps like those that are observed unless the mass is significantly lower than the 'evolutionary mass.' The light and velocity curves for the Goddard model presented here are similar to those computed independently by Fischel, Sparks, and Karp. They should be useful as standards for future investigators.

Adams, T. F.↗
NASA NTRS1979

The discovery of 38.22 second X-ray pulsations from the vicinity of OAO 1653-40

A 38.22-s X-ray pulsator has been discovered during an 8-hour HEAO 1 pointed observation of the area of sky containing OAO 1653-40. It is found that this pulsing source is not associated with V861 Sco, the proposed optical identification for OAO 1653-40. The spectrum is very hard, the spectral index being about 1.4, with about a 500-eV equivalent width of iron-line emission. No high-energy cutoff is observed.

White, N. E.↗
NASA NTRS1981

Pulsations and oscillations

The observations of the oscillations of the solar atmosphere are a powerful diagnostic tool for research into the structure of the atmosphere from inside the convection zone to the upper chromosphere. Interpretations of the global pulsations or oscillations are presented with emphasis on using these observations to study the structure of the interior of the Sun.

Deubner, F. L.↗
NASA NTRS1981

The statistical character of Pc 4 magnetic pulsations at synchronous orbit

A statistical study of 215 magnetic pulsation events identified on the basis of waveform and period as Pc 4 (45- to 150-s period) observed at synchronous orbit by ATS 6 is reported. The study was designed to be similar to that done with Pc 3 (10- to 45-s period) by Arthur et al. (1977). These Pc 4 are found to occur most often near 1800 local time but are observed at all local times. The dominant period is approximately 100 s, but a secondary peak in the distribution occurred at approximately 53 s. They tend to be nearly linearly polarized, principally transverse to the ambient magnetic field. The Pc 4 are found to divide naturally into two classes (radial and azimuthal) on the basis of the azimuth of the major axis of polarization. These classes divide further into low-frequency (less than or equal to 0.015 Hz) and high-frequency (greater than or equal to 0.015 Hz) classes. These four classes organize the remaining wave characteristics quite well. Division of the Pc 4 events into these classes does not lead to identification of a single source mechanism as it did with Pc 3. However, some correspondence between the classes and various proposed mechanisms does exist.

Arthur, C. W.↗
NASA NTRS1981

Microphysical development of a pulsating cumulus tower - A case study

In-cloud microphysical data collected within a 22-minute period during seven consecutive passes at the -13 C sampling level of a deep (base +22 C) convective cloud provide observational evidence for a secondary ice production mechanism at work in the Florida environment. The observed microphysical characteristics of the convective tower, particularly the spatial distribution and habit of the ice phase relative to the updraft, are consistent with a rime-splintering hypothesis for secondary ice production. It is shown that the cloud's updraft structure is critically important in governing the timing of the ice production by controlling the flux of graupel particles through the critical temperature zone (-3 C to -8 C). The importance of the cloud's pulsation growth dynamics on the microphysics is emphasized, particularly as it relates to rapidly glaciating cumuli.

Keller, V. W.↗
NASA NTRS1982

Midlatitude Pi2 pulsations: AFGL and ISEE magnetometer observations correlated

The ISEE observations of the pi2 magnetic pulsations occuring substorm onset in the inner magnetosphere are discussed. One of these events which was also detected as a pi2 event by the AFGL midlatitude magnetometers is considered. The event occurred when the foot of the ISEE field line was over North America. The ground and satellite signals are remarkably similar: they start and stop at the same time, have the same period and can be correlated cycle by cycle. The waves are detected in the electric field data from ISEE 1 and in the magnetic field data from both ISEE 1 and ISEE 2. Calculation of the Poynting vector at ISEE 1 shows that the energy flowed mainly westward, but that there was also a component towards the nearer (southern) ionospheric foot of the field line. The phases between the various field components measured by ISEE 1 and 2 indicate that this is a standing hydromagnetic oscillation.

Hughes, W. J.↗
NASA NTRS1981

On pulsating cosmic /radio/ noise absorption

It has been proposed that some absorption events measured on riometers are actually due to backscatter of cosmic radio noise by E-region plasma waves (D'Angelo, 1976, 1978; D'Angelo and Mehta, 1980). Assuming that DC or nearly DC absorption is a viable process, it is shown that it may also be operative in producing pulsations in cosmic noise absorption on riometers, with periods ranging from a few seconds to several minutes.

Dangelo, N.↗
NASA NTRS1982

Generation of Alfven waves by deceleration of magnetospheric convection and broadband Pi pulsations

The generation of Alfven waves by the deceleration of magnetospheric convection caused by ionospheric loading effects in the magnetospheric dynamo is considered. A one-dimensional model of that region of the plasma sheet where convection is decelerated due to the dynamo process in the magnetosphere-ionosphere coupling is formulated, and the stability of the region is analyzed in order to derive the growth rate of unstable Alfven waves. The effects of ionospheric damping on unstable Alfven wave packets bounding between hemispheres are estimated. It is found that the overall growth rate is proportional to the height-integrated Pedersen conductivity and the convection speed in the dynamic region, but changes into a damping rate when the Pedersen conductivity is reduced below a specific threshold. The unstable Alfven waves thus generated are also found to contribute to both burstlike and relatively continuous Pi pulsations observed during substorms.

Kan, J. R.↗
NASA NTRS1982

Discovery of 69 ms periodic X-ray pulsations in A0538-66

The detection of X ray pulsations with a period of 69.2126 ms from A0538-66 was done from the Einstein Observatory. A point source was located by the High Resolution Imager and a proportional counter registered a flux of 40 counts/sec over background. A column spectral density of 2 x 10 to the 23/sq cm identified the object as A0538-66. A persistent periodicity of 0.0692126 sec was observed throughout the event. The outbursts were not detected on three other observations of A0538-66, and a best fit to a change of rotation velocity indicated that the period observed was from a binary star with a Doppler shifted signal. The two stars are separated by less than 39 solar radii, with the larger star having a 40 solar radius. Consideration is given to the possibility that the X rays are due to accretion on a neutron star and that the two stars at times share a common envelope.

Skinner, G. K.↗
NASA NTRS1982

A cross spectral analysis of high-latitude Pc 5 pulsations in the morning sector

An analysis of the spatial phase and amplitude structures of the ground Pc 5 pulsations in the morning sector was carried out using data from the North American IMS magnetometer network. Signals in the X, Y, and Z components were found to propagate from noon to the nightside with an apparent phase velocity of 13 km/sec. The phase was determined to vary with latitude, with total latitudinal phase differences of 200 and 90 deg, respectively, for the X and Y components. Corresponding latitudinal phase differences in the vertical components were 550 deg for the Alaska chain, and 360 deg for the Fort Churchill chain. Phase variation in the Fort Churchill chain was attributed to the ionospheric Hall current related to the field line resonance and the earth induction current. The earth induction current in the shallower regions near Fort Yukon is a necessary factor to explain the large variation in the Alaska chain.

Saka, O.↗
NASA NTRS1983

Discovery of 13.5 s X-ray pulsations from LMC X-4 and an orbital determination

X-ray pulsations with a 13.5-sec period have been detected from the 1.4-d X-ray binary LMC X-4. By measuring the apparent pulse period at several binary orbital phases, and assuming the orbit to be nearly circular, the semimajor axis of the orbit is determined to be 30 + or - 5 ly-sec. This result, together with a revised orbital velocity amplitude of 37.9 + or - 2.4 km/sec, and other available information, suffice for the determination of the component masses of the binary system and the radius of the companion star. The mass of the neutron star is found to be 1.6 +1.0 -0.5 solar masses, while the mass, radius, and effective temperature of the companion star indicate that it may be undermassive for its luminosity.

Kelley, R. L.↗
NASA NTRS1983

The rate of occurrence of dayside Pc 3,4 pulsations - The L-value dependence of the IMF cone angle effect

When the angle of the IMF to the earth sun line is 15 deg or less, the occurrence rate of dayside Pc 3,4 pulsations in 7-8 times the average at L values of 2.4-2.8, and 2.2-3.5 times the average at L of 4-4.3. These waves disappear when the IMF is nearly at right angles to the sun-earth line. Such observations are consistent with a source originating in the waves upstream of the subsolar bow shock, which are transported by convection to the magnetopause. There, they couple to oscillations of magnetospheric field lines. Because the magnetospheric plasma's index of refraction decreases with radial distance except at the plasmapause, inwardly propagating waves should be refracted away from the radial direction. To reach low L values, the waves should therefore couple near the stagnation point and propagate nearly radially inwards. The streamline geometry and its connection to the foreshock region is illustrated for various IMF orientations, using a simple approximation to the magnetosheath flow field.

Russell, C. T.↗
NASA NTRS1983

Discovery of 9.3 s X-ray pulsations from 2S 1553-542 and a determination of the orbit

X-ray pulsations with a period of 9.3 s from the transient galactic X-ray source 2S 1553-542 were detected. The data were obtained with SAS 3 during the only known outburst of the source in 1975 June. Pulse-timing analyses of the data, which span an interval of 20 days, show that there were large changes in the pulse period that are readily attributable to binary orbital motion. The inferred orbital period is 30.6 + or - 2.2 days and the X-ray mass function is 5.0 + or - 2.1 solar masses. These orbital parameters, the transient nature of the source, and the probable lack of an early-type giant companion star, lead to the tentative conclusion that 2S 1553-542 is a Be/X-ray binary system.

Kelley, R. L.↗
NASA NTRS1984

Multispacecraft observations of the harmonic structure of Pc 3-4 magnetic pulsations

Harmonically related Pc 3-4 pulsations (7-100 mHz) are observed simultaneously by the three geosynchronous satellites ATS 6, SMS 1, and SMS 2, which are separated 20 deg from one another. At a given instant the frequency of the same harmonic is different from one spacecraft to another and each spacecraft observes a decrease in the fundamental frequency as it moves from morning (15 mHz) to afternoon (10 mHz). This frequency behavior is explained in terms of standing Alfven waves, for which the frequency is determined by the local magnetic field and plasma density. Occurrence of harmonic Pc 3-4 waves only during daytime hours (0400-2000 LT) and their frequency characteristics suggest a broadband energy source located on the dayside. Possible azimuthal wave number m and azimuthal phase velocity V(phi) of the second through fourth harmonics are determined from an unusual interval during which identical harmonic frequencies were observed at SMS 1 and ATS 6. Under the assumption of tailward propagation of constant-phase fronts at the same velocity for all these harmonics, V(phi) of about 1700 km/s is obtained.

Takahashi, K.↗