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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 NTRS1999

The Photospheric Convection Spectrum

Spectra of the cellular photospheric flows are determined from observations acquired by the MDI instrument on the SOHO spacecraft. Spherical harmonic spectra are obtained from the full-disk observations. Fourier spectra are obtained from the high-resolution observations. The p-mode oscillation signals and instrumental artifacts are reduced by temporal filtering of the Doppler data. The resulting spectra give power (kinetic energy) per wavenumber for effective spherical harmonic degrees from 1 to over 3000. The spectra show distinct peaks representing granules and supergranules but no distinct features at wavenumbers representative of mesogranu;es or giant cells. The observed cellular patterns and spectra are well represented by a model that only includes granules and supergranules.

Hathaway, D. H.↗
NASA NTRS2002

Coronal Heating and the Increase of Coronal Luminosity with Magnetic Flux

We present the observed scaling of coronal luminosity with magnetic flux in a set of quiet regions. Comparison of this with the observed scaling found for active regions suggests an underlying difference between coronal heating in active regions and quiet regions. From SOHO/EIT coronal images and SOHO/MDI magnetograms of four similar large quiet regions, we measure L(sub corona) and Phi(sub total) in random subregions ranging in area from about four supergranules [(70,000 km)(exp 2)] to about 100 supergranules [(0.5 R(sub sun))(exp 2)], where L(sub corona) is the luminosity of the corona in a subregion and Phi(sub total) is the flux content of the magnetic network in the subregion. This sampling of our quiet regions yields a correlation plot of Log L(sub corona) vs Log Phi(sub total) appropriate for comparison with the corresponding plot for active regions. For our quiet regions, the mean values of L(sub corona) and Phi(sub total) both increase linearly with area (simply because each set of subregions of the same area has very nearly the same mean coronal luminosity per unit area and mean magnetic flux per unit area), and in each constant-area set the values of L(sub corona) and Phi(sub total) 'scatter' about their means for that area. This results in the linear least-squares fit to the Log ((L (sub corona)), vs Log ((Phi (sub total)) plot having a slope somewhat less than one. If active regions mimicked our quiet regions in that all large sets of same-area active regions had the same mean coronal luminosity per unit area and same mean magnetic flux per unit area, then the least-squares fit to their Log((L (sub corona)) vs Log((Phi (sub total)) plot would also have a slope of less than one. Instead, the slope for active regions is 1.2. Given the observed factor of three scatter about the least-squares linear fit, this slope is consistent with Phi(sub total) on average increasing linearly with area (A) as in quiet regions, but L(sub corona) on average increasing as the volume (A(exp 1.5)) of the active region instead of as the area. This possibility is reasonable if the heating in active regions is a burning down of previously-stored coronal magnetic energy rather than a steady dissipation of energy flux from below as expected in quiet regions.

Moore, R. L.↗
NASA NTRS1971

Intensity oscillation in H alpha-fine structures

Using a new technique of directly measuring the intensity variation from the 16mm time-lapse filtergram movies taken in the blue wing and in the line center of H-alpha, periodic intensity oscillations were found in the center of H-alpha supergranulation network, in rosette centers, and in plage granules. The oscillatory period of intensity in the network is of the order of 170 + or - 44 seconds while in regions of stronger magnetic field, such as in plages and in rosettes, the period was found to be longer, on the order of 300 + or - 50 seconds. It is suggested that observed intensity oscillation in the rosette center is related to the shooting out of dark mottles from rosettes. Oscillatory intensity fluctuations were also observed in the sunspot umbra.

Bhatnagar, A.↗
NASA NTRS1972

Intensity oscillation in H-alpha fine structure.

Using a new technique of directly measuring the intensity variation from the 16 mm time-lapse filtergram movies taken in the blue wing and in the line center of H-alpha, we found periodic intensity oscillations in the center of H-alpha supergranulation network, in rosette centers and in plage granules. The oscillatory period of intensity in the network is of the order of 170 plus or minus 44 seconds while in regions of stronger magnetic field, such as in plages and in rosettes, the period was found to be longer, on the order of 300 plus or minus 50 seconds. It is suggested that observed intensity oscillation in the rosette center is related to the shooting out of dark mottles from rosettes. Oscillatory intensity fluctuations have been also observed in the sunspot umbra.

Bhatnagar, A.↗
NASA NTRS1973

A search for periodic structure in solar 2 cm microwave radiation

A power spectral analysis of 285 hr of 2 cm microwave intensity data showed no statistically significant ( 96% confidence) periodicities in the frequency range 1 to 15 mHz. No correlation was found between 2 cm periodicities and solar activity in H alpha, X-ray, and several microwave frequencies. A small shift of power toward higher frequencies in the power spectrum of the 2 cm data was found to be correlated with solar H alpha and X-ray activity. Using the statistical properties of power spectra, an expression for the ratio of the minimum detectable peak-to-peak to ambient temperature at chromospheric heights may be derived. Applied to a model for oscillation bursts in quiescent supergranules, and using the most significant results of experiments to detect the microwave periodicities, this expression yields an upper limit of approximately .0015.

Sentman, D. D.↗
NASA NTRS1973

High spatial resolution photographs of the sun in L alpha radiation.

Photographs of the sun in predominantly L alpha radiation (centered at 1215.67 A) with 3-sec spatial resolution were taken from an Aerobee rocket shortly after fourth contact by the moon on the eclipse day of July 10, 1972. This preliminary reporting of the results describes the instrument and shows two of the photographs taken. The supergranulation is manifest, and active regions and filaments are well resolved over the entire disk. Densitometer traces across the disk are presented, giving the flux incident on the earth from active regions, cell boundaries, and filaments.

Prinz, D. K.↗
NASA NTRS1973

Thermally driven mass flows in the convection zone of the sun

A formulation of the fluid dynamics of convective regions is developed which leads to an analytical description of the solar rotation, the Evershed flow, and the supergranulation. The starting point of the present formulation is the mixing length picture of convective equilibrium, but the earlier point mass model for convective molecules is replaced here by a model with both inertia and intrinsic moment of inertia. This extension introduces three rotational degrees of freedom into the dynamics of individual convective molecules, which enter into the dynamical equations for a mixing length fluid in the form of a separate vector field which we term the spin field. It is shown that for convective molecules having a spherically symmetric mass distribution, the spin field is proportional to the local vorticity.

Dijkhuis, G. C.↗
NASA NTRS1976

Hydraulic concentration of magnetic fields in the solar photosphere. III - Fields of one or two kilogauss

Detailed analysis of weak and strong lines suggests that the magnetic fields in isolated intense flux tubes in supergranule boundaries in the solar photosphere may be as large as 2000 gauss. This paper is a concise systematic review of hydrodynamic effects that might compress a magnetic field to great intensity. The properties of force-free fields are reviewed to show that they do not contribute to concentration of magnetic fields, in spite of the popular notion to the contrary. Of the seven effects considered, it is concluded that only cooling of the gas within the field can produce the high field densities inferred from observation. It is shown that inhibition of convection appears not to possess the necessary qualitative cooling features and that overstability, generating transverse hydromagnetic waves - essentially Alfven waves - is the only way to account for the cooling and field intensification.

Parker, E. N.↗
NASA NTRS1976

The interplanetary magnetic field and energetic electrons from Jupiter

The reported four-month 'season' for observation of Jovian electron events at AU is shown to be a plausible consequence of field-line random walk in the solar wind. Values of the field-line random-walk coefficient deduced earlier in connection with observations of solar protons, photospheric supergranulation, and the interplanetary magnetic-field power spectrum, lead to an approximate value for the spread of Jovian electrons at 1 AU, which corresponds to an approximate four-month duration of events. This model is consistent with a point source at Jupiter.

Jokipii, J. R.↗
NASA NTRS1976

The quiet sun chromospheric network observed from Skylab

The paper analyzes spectra of a supergranulation cell interior and cell boundary obtained near the solar center at wavelengths between 1200 and 1560 A with a normal-incidence spectrograph aboard Skylab. Absolute intensities, relative intensities, and profiles are given for selected optically thin and optically thick lines over the cell interior, the boundary, and intermediate positions; the results are compared with spectra obtained at the limb. Characteristic lengths along the line of sight are derived for the Si III emitting region, and these are compared with the predictions of Gabriel's (1975) model. It is concluded that the present data are representative of a fairly typical cell interior and boundary, that nonthermal motions are isotropic and the same for the interior and boundary, and that the electron-density ratio between the interior and the boundary is about a factor of two or less.

Feldman, U.↗
NASA NTRS1976

Hydraulic concentration of fields in the solar photosphere. IV - Evolution of fields near equipartition

It has previously been inferred that small isolated flux tubes appearing in supergranule boundaries are compressed to 1500 gauss or more. This paper considers whether some dynamic condition within a flux tube exists which provides both stability and a 'mechanical advantage' so that a small force over a small period of time can accomplish the enormous compression from the weak-field to the strong-field state. It is found that the equipartition solutions to the hydromagnetic equations apparently may have the desired property of permitting an infinitesimal external pressure to convert a gentle flow of gas along a weak field into a very intense field through a succession of equipartition states. An illustrative example is presented, and field compression by convective forces is analyzed.

Parker, E. N.↗
NASA NTRS1977

A possible edge effect in enhanced network

K-line observations of enhanced network taken with the NASA/SPO Multichannel Spectrometer on September 28, 1975, in support of OSO-8 are discussed. The data show a correlation between core brightness and asymmetry for spatial scans which cross enhanced network boundaries. The implications of this result concerning mass flow in and near supergranule boundaries are discussed.

Jones, H. P.↗
NASA NTRS1978

The solar XUV He I and He II emission lines. I - Intensities and gross center-to-limb behavior

The center-to-limb variation of the He II 304- and 256-A lines and He I 584- and 537-A lines is derived for different solar features, but averaged over the chromospheric supergranulation structure. The general trend is for limb brightening in quiet-sun regions, limb neutrality in unipolar magnetic regions (UMR), and limb darkening in polar coronal holes. The center-to-limb behavior in these optically thick emission lines indicates collisional excitation and decreasing transition-region temperature gradients with respect to optical depth in the sequence quiet sun to UMR to coronal hole.

Mango, S. A.↗
NASA NTRS1978

Short term evolution of coronal hole boundaries

The evolution of coronal hole boundary positions on a time scale of approximately 1 day is studied on the basis of an examination of all coronal holes observed by Skylab from May to November 1973. It is found that a substantial fraction (an average of 38%) of all coronal hole boundaries shifted by at least 1 deg heliocentric in the course of a day. Most (70%) of these changes were on a relatively small scale (less than 3 times the supergranulation cell size), but a significant fraction occurred as discrete events on a much larger scale. The large-scale shifts in the boundary locations involved changes in X-ray emission from these areas of the sun. There were generally more changes in the boundaries of the most rapidly evolving holes, but no simple relationship between the amount of change and the rate of hole growth or decay.

Nolte, J. T.↗
NASA NTRS1978

Ephemeral region flares and the diffusion of the network

Flarelike events which are frequently observed in H-alpha in apparently quiet regions of the solar disk can be in all cases identified with bipolar ephemeral regions (ER) on magnetograms. These events represent the H-alpha counterpart of X-ray bright point flares. Statistically, this phenomenon is associated with the proximity of the bipolar features to the supergranulation network in the sense that an ER is likely to flare during its lifetime if the distance to the nearest network element is less than or equal to its own pole separation.

Marsh, K. A.↗
NASA NTRS1979

The height variation of supergranular velocity fields determined from simultaneous OSO 8 satellite and ground-based observations

Results are reported for simultaneous satellite and ground-based observations of supergranular velocities in the sun, which were made using a UV spectrometer aboard OSO 8 and a diode-array instrument operating at the exit slit of an echelle spectrograph attached to a vacuum tower telescope. Observations of the steady Doppler velocities seen toward the limb in the middle chromosphere and the photosphere are compared; the observed spectral lines of Si II at 1817 A and Fe I at 5576 A are found to differ in height of formation by about 1400 km. The results show that supergranular motions are able to penetrate at least 11 density scale heights into the middle chromosphere, that the patterns of motion correlate well with the cellular structure seen in the photosphere, and that the motion increases from about 800 m/s in the photosphere to at least 3000 m/s in the middle chromosphere. These observations imply that supergranular velocities should be evident in the transition region and that strong horizontal shear layers in supergranulation should produce turbulence and internal gravity waves.

November, L. J.↗
NASA NTRS1979

The ecology of prominences

The paper discusses the roles of prominences in the solar scheme. Attention is given to classifications and the ways in which prominences exist: hydrostatic support, ballistic support, and magnetic support. In the case of ballistic support, surges are differentiated from sprays which involve the ejection of material that is already above the solar surface. Discussion also covers filimets and fibrils and the conditions for their appearance. It is proposed that most flares originate in prominence instabilities. In addition supergranulation is covered, noting the network is not seen on the boundary of unipolar regions. It is concluded that prominences play a critical role in flares and field reconnection, and the evolution of solar magnetic fields.

Zirin, H.↗
NASA NTRS1980

Linear modes of convection in the solar envelope

The structure of the solar convective envelope calculated according to mixing-length theory is not consistent with the transport of the convective heat flux by any superposition of the fundamental linear convective modes. If the amplitudes of these modes can be estimated by consideration of the most important nonlinear terms in the equations of motion, it becomes possible to find solar models which are consistent with mode transport of the heat through the convective zone. These models may be characterized by a mixing length which varies from point to point through the envelope. The linear modes in such models show peaks in velocity at certain preferred length scales, but these scales do not necessarily correspond to those of the solar motions, such as the supergranulation.

Bogart, R. S.↗