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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Observations of nonradial p-mode oscillations on the sun

Observations of the solar velocity field with a diode array attached to the echelle spectrograph of a vacuum tower telescope are described which resolve the solar five-minute oscillatory motion into distinct bands of power. Previous observations are discussed which show that the solar five-minute oscillations can be resolved into frequencies having the character of nonradial p-mode eigenfrequencies of the solar envelope. The present observations confirm the observed frequencies and sharpen the previous resolution of the five-minute oscillations into ridges on the (wavenumber, frequency) plane. A comparison with earlier calculations indicates that the theoretical frequencies are in good but not perfect agreement with those observed. It is concluded that the identification of the five-minute oscillations as nonradial p-mode oscillations in the solar envelope is established beyond doubt.

Rhodes, E. J., Jr.↗
NASA NTRS1987

r-mode oscillations in the sun

The relationship between solar r-modes (long-period oscillations dominated by the Coriolis force) and solar activity cycles is investigated statistically. FFT power spectra of the daily Zurich sunspot numbers (R) and of the Greenwich Photoheliographic projected whole sunspot area (A) are obtained for the high-activity years of solar cycles 14 through 21. Both R and A are found to exhibit periods of 22 + or - 1 d, 34 + or - 1.5 d, and 43 + or - 2 d, corresponding to r-modes with (l,m) angular harmonics (2,2), (3,1), and (2,1), respectively.

Blizard, Jane B.↗
NASA NTRS1987

Nonradial g-mode oscillations in X-ray bursting neutron stars

The oscillation spectrum of nonradial g-modes in X-ray bursting neutron stars has been studied. The pulsation periods are found to be sensitive to the envelope temperature and range from about 15 ms to about 50 ms for the l = 1 g(1) mode during the X-ray burst. From a quasi-adiabatic stability analysis it is likely that a spectrum of l-pole g-modes is unstable due to the epsilon-mechanism associated with rapid alpha captures. As the thermal structure of the envelope of the neutron star changes on time scales less than 0.2 s during the rise of the X-ray burst, the oscillations are expected to be quasi-coherent during this phase. The calculated period derivatives are large during the burst and are about 1 ms/s. The pulsations are short-lived and are most likely to be seen in the immediate vicinity of the burst peak. Finally, the possible relevance of nonradial g-mode pulsations to the recently discovered quasi-periodic oscillations observed in a number of X-ray sources is discussed.

Mcdermott, P. N.↗
NASA NTRS1991

Measurements of high-frequency solar oscillation modes

The spatial-temporal spectrum of solar oscillations exhibits modelike structure at frequencies above the nominal photospheric acoustic cutoff of about 5.3 mHz. The linewidth and frequency of these features are measured as functions of degree from high-quality spectra obtained from observations made at the geographic South Pole. From 5.3 to 6.5 mHz the linewidths are relatively constant with a value of about 70 microHz, approximately one-half the frequency difference between modes of the same degree but successive values of radial order number. This width is larger than can be accounted for by simple considerations of the leakage of trapped acoustic waves. The frequencies of the high-frequency modes adhere to a simple dispersion law if one uses a substantially larger effective phase shift that applies at lower frequencies. The frequency variation of this phase shift changes markedly above the acoustic cutoff frequency.

Duvall, T. L., Jr.↗
NASA NTRS1983

Detection of solar gravity mode oscillations

An analysis of solar velocity data obtained at the Stanford Solar Observatory shows the existence of solar global oscillations in the range 45 to 105 microHz (160 to 370 minutes). These oscillations are interpreted as internal gravity modes of degree l = 1 and l = 2. A good estimate of the order of the modes has also been made. Previously announced in STAR as N84-11086

Delache, P.↗
DOE OSTI2011

THEORETICAL p-MODE OSCILLATION FREQUENCIES FOR THE RAPIDLY ROTATING {delta} SCUTI STAR {alpha} OPHIUCHI

A rotating, two-dimensional stellar model is evolved to match the approximate conditions of {alpha} Oph. Both axisymmetric and nonaxisymmetric oscillation frequencies are computed for two-dimensional rotating models which approximate the properties of {alpha} Oph. These computed frequencies are compared to the observed frequencies. Oscillation calculations are made assuming the eigenfunction can be fitted with six Legendre polynomials, but comparison calculations with eight Legendre polynomials show the frequencies agree to within about 0.26% on average. The surface horizontal shape of the eigenfunctions for the two sets of assumed number of Legendre polynomials agrees less well, but all calculations show significant departures from that of a single Legendre polynomial. It is still possible to determine the large separation, although the small separation is more complicated to estimate. With the addition of the nonaxisymmetric modes with |m| {<=} 4, the frequency space becomes sufficiently dense that it is difficult to comment on the adequacy of the fit of the computed to the observed frequencies. While the nonaxisymmetric frequency mode splitting is no longer uniform, the frequency difference between the frequencies for positive and negative values of the same m remains 2m times the rotation rate.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2002

Detection of Rotational Sequences for Global Oscillation Modes inside the Sun

A very simple mathematical sequence is detected in a half century of thermal radio flux from the Sun. Since the only known physical cause of the sequence is global oscillations trapped in the nonconvecting solar interior, g-modes and probably r-modes are active. If so, their rotation frequencies are detected and some previously reported difference frequencies are confirmed with high confidence. All angular harmonics for 2 less than or = l less than or = 7 are detected as well as some others up to the limit l less than or = 14 resolvable by the observations (a Fourier spectrum of the 10.7 cm flux time series). The mean sidereal rotation of the nonconvecting interior is 428.2 nHz as averaged by g-modes and 429.8 nHz by the r-modes, indicating that g-mode energy is a bit more centrally concentrated. Helioseismology measures such rotation rates near 0.36R (R = solar radius), so the global modes would have about half their kinetic energy above and below that level. This, and the known log(r) energy dependence of most modes implies that these oscillations are significantly reflected near 0.18R, the same level at which sound speed measurements display a maximum departure from theoretical models.

Wolff, Charles L.↗
NASA NTRS2007

Soliton Properties of Coupled g-Mode Oscillations

Several features typical of solitons are also exhibited by stellar g-modes when coupled into "sets" that have unique rotation rates relative to the star. Enhanced nuclear burning due to weakly nonlinear amplitudes in a small portion of the stellar core holds each set together against dispersion. As the nonlinear regions of each set rotate past each other they have a complex interaction (shown in a video), yet emerge from this with their original wave forms. Other similarities with solitons we mentioned, including the physical origin of a phase shift. in longitude due to the interaction. These similarities suggest that a fully nonlinear derivation of g-mode sets may be able to show that their large amplitude regions approach true solitons.

Wolff, Charles L.↗
NASA NTRS1984

The Resonant Count Diagram and Solar g Mode Oscillations

Evidence is provided to support the hypothesis that, because of the particular frequency separations of the solar g modes, resonant three-wave interactions stimulate only a selected few g modes. A resonant count diagram was obtained by plotting the total number of possible resonant three-wave interactions or a given beat frequency against the inverse of the beat frequency (the beat period), within a given frequency tolerance. The 1 = 1, 2, 3, 4 g modes calculated by Christensen-Dalsgaard, Gough and Morgan (1979) for a standard model of the Sun were used. The diagram has a significant peak at 160 minutes as well as other peaks at longer periods. The g modes that Delache and Scherrer (1983) tentatively identified from the Crimea-Stanford data were also plotted. These modes were found to correspond with the other peaks in the diagram. This coincidence between the observed g modes and the peaks in the resonant count diagram suggest that the observed g modes do owe their observability to resonant three-wave interactions.

Guenther, D. B.↗
NASA NTRS1988

Observations of F- and P-Mode Oscillations of High Degree (500 Is Less Than L Is Less Than 2500) in Quiet and Active Sun

Spectra (l - v diagrams) from high resolution observations taken at the Vacuum Tower Telescope (NSO/Sunspot) are presented. The raw data are CCD images taken through the SOUP narrowband filter in Fe I 5576 A. Four filtergrams spaced through the spectral line are combined to form velocity movies. Spectra for 80 min of data with 0.5 to 1.5 arcsec resolution are presented for the entire field-of-view and for quiet and magnetic (plage) subregions. Ridges f and p1 to p5 are evident in velocity spectra, extending to l = 2500 (f), l = 1800 (p1), and l = 1200 (p2). Much less power is seen in the magnetic region than in the quiet Sun. Three-dimensional Fourier filtering shows that oscillation velocity amplitude drops sharply at the boundary of the active region for each family of modes considered.

Tarbell, T. D.↗
NASA NTRS1984

Detection of Solar Gravity Mode Oscillations

An analysis of solar velocity data obtained at the Stanford Solar Observatory has shown the existence of solar global oscillations. The oscillations are in the range 45 to 105 micro Hz (160 to 370 minutes) and are interpreted as internal gravity modes of degree l=1 and l=2.

Scherrer, P. H.↗
NASA NTRS1989

Solar g-mode oscillations: Comparison of SMM-ACRIM and ground-based observations

Progress was made in access to data and in developing programs for its analysis. The difficulties in completing the work in the planned time can be traced to several factors. The correction of the Stanford oscillation using gridded intensity data was not successful. It was concluded that due to poor continuity of the 1985 and 1986 data due to clouds, that a joint analysis with the ACRIM data (best solar oscillation data to date) on the summer 1987 observations should be performed. The 1988 Stanford oscillation data are being examined and the cross comparison of the ACRIM spectrum with the Standford spectrum for 1987 in the g-mode regime will shortly begin.

Scherrer, Philip H.↗
NASA NTRS1993

Interactions among convection, magnetic fields and p-mode oscillations in the sun

Two papers on different aspects of the excitation and damping of solar oscillations were accepted for publication in the Astrophysical Journal. The first paper evaluates the rate at which turbulent convection feeds energy into individual p-modes. It is shown that stochastic excitation by turbulent convection provides a satisfactory fit to the product of the mode energies and linewidths. A somewhat surprising conclusion is that entropy fluctuations are about an order of magnitude more significant than are fluctuations of the Reynolds stress in exciting p-modes. However, entropy fluctuations cannot excite f-modes. This may account for the relatively low energies of the f-modes compared to those of the p-modes. The second paper explores the role of scattering of acoustic modes by turbulent velocity fluctuations. Scattering of a mode is concentrated near the top of its acoustic cavity. Because the turbulence has low Mach number, scattering couples modes having similar frequencies. Its net effects are to increase the linewidths of all modes and to transfer energy from p-modes to f-modes. Scattering is likely to be the dominant source for the linewidths of p-modes. In particular, it may account for the unexpectedly large linewidths measured for low frequency modes. Copies of preprints of the two papers referred to above are attached. The remainder of the report is devoted to a description of unpublished results.

Goldreich, Peter↗
NASA NTRS1988

Frequencies of solar p-mode oscillations

A list is presented of frequencies that can be used as a basis for helioseismic investigations of the average structure of the solar interior as a function of depth. The list includes measurements of frequencies of p-mode multiplets covering the l range from 4 to 99. Two different data sets are employed: one based on Doppler shift measurements made in 1985 at the Big Bear Solar Observatory and another based on intensity measurements made in 1981 at the geographic South Pole. Frequencies from the two data sets are compared, and systematic frequency differences are found that range from less than 0.1 microHz at low values of l to about 0.6 microHz at l = 99; the uncertainty is + or - 0.1 microHz.

Duvall, T. L., Jr.↗
NASA NTRS1991

Observations of intermediate- and high-degree p-mode oscillations during sunspot cycles 21 and 22

Extensive time series of resolved solar images have been available for helioseismological investigations since the late 1970's. The paper presents examples of the frequencies, power levels, modal energies, and widths of solar intermediate-degree and high-degreee p-modes obtained at the Mt. Wilson Observatory's 60-Foot Solar Tower Telescope during the rising phase of the current sunspot cycle. It is shown that the inclusion of frequency splittings from the high-degree p-modes has made it possible to demonstrate that the sun's internal equatorial angular velocity is not constant with radius but varies systematically with radius throughout the solar convection zone and below.

Rhodes, E. J.↗
NASA NTRS1985

Observations of low-degree P-mode oscillations in 1984

Analysis of Stanford differential velocity observations has been extended through the 1984 observing season. Excellent quality observations were obtained in 1984 on 38 days in a 49 day interval from June 20th through August 7th. The power spectrum of this data has been examined and improved frequency determinations have been made for p-modes of degree 2 through 5 and order 5 through 34. Of special interest are the modes of the lower orders, n ranging from 5 to 10, which have not been identified previously.

Henning, H. M.↗
NASA NTRS1986

Observations of low-degree p-mode oscillations in 1984

Analysis of Stanford differential velocity observations has been extended through the 1984 observing season. Excellent quality observations were obtained in 1984 on 38 days in a 49 day interval from June 20th through August 7th. The power spectrum of this data has been examined and improved frequency determinations have been made for p-modes of degree 2 through 5 and order 5 through 34. Of special interest are the modes of the lower orders, n ranging from 5 to 10, which have not been identified previously.

Henning, Harald M.↗