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At least 793 records · Page 44

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NASA NTRS1992

Upper limit on the steady emission of the 2.223 MeV neutron capture gamma-ray line from the sun

A search for steady emission of the 2.223 MeV gamma-ray line arising from the direct capture reaction 1H(n, gamma)2H is presented on the basis of an analysis of SMM data. The upper limits for continuous 2.223 MeV gamma-ray line emission from the active and inactive sun, as established in the present work, are about two orders of magnitude less than previously published results. These findings for inactive periods also suggest that small flares (or a continuous acceleration mechanism) are unlikely to be the source of the heating of the quiet solar corona. It is concluded that the power in accelerated nuclei during inactive periods falls short of the coronal heating requirement by about four orders of magnitude unless there is a large and as yet unobserved population of nuclei at energies below 1 MeV. The energy release in other forms associated with the acceleration process falls short of the heating requirement by about two orders of magnitude if this energy release is in the same proportion to the power in accelerated nuclei as is estimated from observed flares.

Harris, M. J.↗

NASA NTRS1993

Radial velocity observations of the sun at night

The ability to detect planets orbiting stars has been evaluated through solar-spectrum Doppler shift measurements for 5 years, using the sunlit surface of the moon to furnish nighttime access to the solar spectrum integrated over the solar disk as though the sun were being observed at stellar distance. These lunar observations have indicated that the Doppler shift of the integrated solar photosphere disk in violet absorption lines has varied less that +/- 4 m/sec over the 1987-1992 interval studied.

Mcmillan, R. S.↗

NASA NTRS1993

Numerical simulation of the subsolar magnetopause current layer in the sun-earth meridian plane

The formation and stability of the magnetopause current layer near the subsolar point in the sun-earth meridian plane are examined using a 2D electromagnetic particle simulation. For the case of zero IMF, the simulation results show that the current layer remains stable and is essentially the same as in the 1D simulation. The width of the current layer is given by the electron-ion hybrid gyroradius which is much smaller than the ion gyroradius. The current layer is found to remain stable for the northward IMF as well. As in the 1D simulation, the jump of the magnetic field at the current layer for the northward IMF remains small. For the southward IMF, collisionless magnetic reconnection is found to develop, leading to the formation of magnetic islands and density peaking within the current layer.

Okuda, H.↗

NASA NTRS1992

Observations of the electron density spectrum near the sun during spectral broadening transients

We present high-time resolution spectral broadening data for several transients caused by propagating interplanetary disturbances observed inside 20 solar radii. The results show that the shape and level of the ambient near-sun electron density spectrum undergoes abrupt and substantial change during passage of the disturbance. The steepening of the spectrum to near the Kolmogorov value suggests fully-developed inertial-range turbulence in the wake of the interplanetary disturbance.

Woo, R.↗

NASA NTRS1991

Imaging the sun in hard X-rays - Spatial and rotating modulation collimators

Several approaches to imaging hard X-rays emitted from solar flares have been proposed or are planned for the nineties including the spatial modulation collimator (SMC) and the rotating modulation collimator (RMC). A survey of current solar flare theoretical literature indicates the desirability of spatial resolutions down to 1 arcsecond, field of views greater than the full solar disk (i.e., 32 arcminutes), and temporal resolutions down to 1 second. Although the sun typically provides relatively high flux levels, the requirement for 1 second temporal resolution raises the question as to the viability of Fourier telescopes subject to the aforementioned constraints. A basic photon counting, Monte Carlo 'end-to-end' model telescope was employed using the Astronomical Image Processing System (AIPS) for image reconstruction. The resulting solar flare hard X-ray images compared against typical observations indicated that both telescopes show promise for the future.

Campbell, Jonathan W.↗

NASA NTRS1993

Evidence for a fundamental period of the sun and its relation to the 154 day complex of periodicities

We have analyzed the longitude distributions of major flares observed in the 1955-1991 interval, referring them to coordinate systems rotating about axes tilted with respect to the rotation axis of the solar envelope. We find that the longitude distribution exhibits the largest modulation in the coordinate system with the following parameters: rotation period, 25.50 days; tilt angle of the rotation axis, 40 deg; tilt direction, toward the position of the earth on December 4 in its orbit around the sun. We interpret this as being due to an obliquely rotating structure (or a wave pattern rotating about an oblique axis) which has two exciters. We identify the period of 25.50 days as the fundamental period of the hypothetical 'clock' proposed by Bai and Sturrock (1991). The periods of the subharmonics are 51.0, 76.5, 102.0, 127.5, and 153.0 days, in agreement with periodicities found from analysis of flare occurrence times.

Bai, T.↗

NASA NTRS1993

Sun-perturbed Earth-to-moon transfers with ballistic capture

A method is described for constructing a new type of low energy transfer trajectory from the Earth to the moon leading to ballistic capture. This is accomplished by utilizing the nonlinear Earth-moon-sun perturbations on a point mass in three dimensions. The interaction of the gravitational fields of the bodies defines transition regions in the position-velocity space where the dynamic effects on the point mass tend to balance. These are termed weak stability boundaries. The transfer is obtained by the use of trajectories connecting the weak stability boundaries. It uses approximately 18 percent less Delta-V than the Hohmann transfer to insert a spacecraft into a circular orbit about the moon. The use of this transfer has recently been demonstrated by Japan's Hiten spacecraft, which arrived at the moon on October 2, 1991. Application of the transfer method is also made to the Lunar Observer Mission.

Belbruno, Edward A.↗

NASA NTRS1992

The maintenance of the sun's differential rotation and its temporal variations

We use the horizontal momentum balance equation to infer the strength of the meridional circulation (MC) and Reynolds stresses (RS) at the sun's surface from the observed properties of the differential rotation (DR). Both MC and RS are important for maintaining the equatorial acceleration. The results indicate that the average value of MC is about 1.1 m/s, with circulation directed towards the poles in both the hemispheres, and the average value of RS is 3.6 10 exp 7 sq cm2/s, with transport of angular momentum directed towards the equator in both hemispheres, this latter in good agreement with observations. With the above values of MC and RS, we integrate the momentum equation in time, starting from a state of rigid rotation, to investigate the competitive role of MC and RS in producing the presently observed average DR. Our results show that DR is consistent with observations only if both MC and RS have opposite effects, with the strengths given above, in order to balance the viscous torque.

Chiu, H.-Y.↗

NASA NTRS1993

Observations of time variation in the sun's rotation

Observations of solar p-mode frequency splittings obtained at Big Bear Solar Observatory in 1986 and during 1988-90 reveal small (about 1 percent) changes in the sun's subsurface angular velocity with solar cycle. An asymptotic inversion of the splitting data yields the latitude dependence of the rotation rate and shows that the largest changes in the angular velocity, about 4 nanoHz, occurred between 1986 and the later years, at high (about 60 deg) solar latitudes. Earlier helioseismic observations suggest that solar cycle changes in the ratio of magnetic to turbulent pressure in the solar convection zone are large enough to account for the magnitude of the observed angular velocity variations, but a detailed model of the phenomenon does not exist.

Woodard, M. F.↗

NASA NTRS1994

Multi-band automatic sun and sky scanning radiometer system for measurement of aerosols

A weather resistant automatic scanning Sun photometer system is assessed and demonstrated as practical for measurements of aerosol concentrations and properties at remote sites. Interfaced with a transmitter using the Geostationary Data Collection System (GDCS), the data are processed in near real time. The processing allows a time dependence of the aerosols and water vapor and an ongoing assessment of the health and calibration of the instruments. The system's automatic data acquisition, transmission, and processing offer immediate application to atmospheric monitoring and modeling on a regional to global scale and validation of satellite retrievals. It is estimated that under normal circumstances the retrieved aerosol optical thickness has a network wide accuracy of +/- 0.02 from 340 nm to 1020 nm, water vapor +/- 0.2 cm and size distribution from 0.1 to 3 micrometers.

Holben, B. N.↗

NASA NTRS1994

Use of acoustic wave travel-time measurements to probe the near-surface layers of the Sun

The variation of solar p-mode travel times with cyclic frequency nu is shown to provide information on both the radial variation of the acoustic potential and the depth of the effective source of the oscillations. Observed travel-time data for waves with frequency lower than the acoustic cutoff frequency for the solar atmosphere (approximately equals 5.5 mHz) are inverted to yield the local acoustic cutoff frequency nu(sub c) as a function of depth in the outer convection zone and lower atmosphere of the Sun. The data for waves with nu greater than 5.5 mHz are used to show that the source of the p-mode oscillations lies approximately 100 km beneath the base of the photosphere. This depth is deeper than that determined using a standard mixing-length calculation.

Jefferies, S. M.↗

NASA NTRS1994

Theoretical properties of Omega-loops in the convective zone of the Sun. 1: Emerging bipolar magnetic regions

It is proposed that the observed 500 G intensity of the magnetic fields emerging through the surface of the Sun can be understood from the Bernoulli effect in the upwelling Omega-loops of magnetic field. It is also proposed that the inferred 10(exp 5) G azimuthal flux bundles below the base of the convective zone can be understood as a consequence of the large-scale buoyancy associated with the upwelling fluid in and around the rising Omega-loop. The process fits in naturally with the Babcock-Leighton form of the solar alpha-omega-dynamo. The emerging Omega-loop implies the coherence of the upwelling all the way from the bottom of the convective zone, enhancing the convective heat transport to account for the observed variation of the solar irradiance by about two parts in 10(exp 3).

Parker, E. N.↗

NASA NTRS1995

Theoretical properties of Omega-loops in the convective zone of the sun. 2: The origin of enhanced solar irradiance

The brightness of the Sun is observed to vary by 0.2% with the 11 yr activity cycle. We suggest that the brightness increase with magnetic activity is a direct result of the enhanced heat transport provided by the vertical wakes of the successive Omega-loops of magnetic field that produce the activity. The dynamics of a vertical wake in a stratified atmosphere is relatively complicated, but the observed rapid succession of emergence of Omega-loops at a localized site in the middle of a bipolar active region can be understood only if an established concentrated updraft extends up from the bottom of the convective zones. It is these updrafts that we suggest are the major cause of the brightness enhancement. The next paper examines some of the dynamical characteristics of vertical wakes.

Parker, E. N.↗

NASA NTRS1994

The Sun as a probe of varying G

In order to explore the ability of helioseismology and features of solar models to test the constancy of the gravitational constant G during the last 4.5 Gyr of solar evolution, we have constructed a grid of evolutionary sequences for solar models under the assumptions that G varies with time, and have explored the sensitivity of their nonradial acoustic mode oscillation spectra to G variability. All final models satisfy the standard constraints for the present Sun and, except for the variation in G, were constructed under the assumptions of a standard solar model. When compared with the observed solar p-mode spectrum, our models definitely rule out beta greater than 0.4 and beta less than -0.4, where G(t) proportional to t(exp -beta) over the last 4.5 Gyr. These limits can be tightened to rule out absolute value of beta greater than 0 (0.1) by the use of other solar observables. For nonmonotonic variation in G this suggests that long-timescale variations greater than 0 (5%) in G are ruled out over the last 4.5 Gyr. Future prospects for improving the sensitivity of helioseismic tests of a varying G are also discussed. Finally, we explore the sensitivity of the predicted solar neutrino flux to varying G.

Demarque, P.↗

NASA NTRS1994

Physical and optical properties of the Pinatubo volcanic aerosol: Aircraft observations with impactors and a Sun-tracking photometer

As determined in situ by impactor samplers flown on an ER-2 at 16.5- to 20.7-km pressure altitude and on a DC-8 at 9.5- to 12.6-km pressure altitudes, the 1991 Pinatubo volcanic eruption increased the particle surface area of stratospheric aerosols up to 50-fold and the particle volume up to 2 orders of magnitude. Particle composition was typical of a sulfuric acid-water mixture at ER-2 altitudes. Ash particles coated with sulfuric acid comprised a significant fraction of aerosol at DC-8 altitudes. Mie-computed light extinction increased up to 20-fold at midvisible and greater than 100-fold at near-IR wavelengths. The optical thickness measured through the aerosol layer by an autotracking Sun photometer aboard a DC-8 aircraft at 10.7- to 11.3-km pressure altitudes shows a spectral shape that is similar to the Mie-calculated spectral extinction at ER-2 altitudes. Surface area distributions calculated by inversion of spectral optical depth measurements show characteristics that are similar to the mean surface area distribution resulting from 35 in situ measurements.

Pueschel, R. F.↗

NASA NTRS1994

The Sun as a Variable Star: Solar and Stellar Irradiance Variations; Colloquium of the International Astronomical Union, 143rd, Boulder, CO, Jun. 20-25, 1993

Variations in solar and stellar irradiances have long been of interest. An International Astronomical Union (IAU) colloquium reviewed such relevant subjects as observations, theoretical interpretations, and empirical and physical models, with a special emphasis on climatic impact of solar irradiance variability. Specific topics discussed included: (1) General Reviews on Observations of Solar and Stellar Irradiance Variability; (2) Observational Programs for Solar and Stellar Irradiance Variability; (3) Variability of Solar and Stellar Irradiance Related to the Network, Active Regions (Sunspots and Plages), and Large-Scale Magnetic Structures; (4) Empirical Models of Solar Total and Spectral Irradiance Variability; (5) Solar and Stellar Oscillations, Irradiance Variations and their Interpretations; and (6) The Response of the Earth's Atmosphere to Solar Irradiance Variations and Sun-Climate Connections.

Pap, Judit M.↗

NASA NTRS1995

Element Abundance Variations in the Sun and Heliosphere; Meeting E2.1 of the COSPAR Scientific Assembly, 30th, Hamburg, Germany, July 11-21, 1994

A conference on advances in space research related to element abundance variations in the sun and heliosphere produced related papers. The areas addressed in these papers included abundance variations in the solar wind, element abundances in the solar corona, iron abundance in the solar photosphere and corona, iron and calcium abundances during solar flares, helium abundance in the solar corona, element abundances in the solar interior, energetic particle abundances, fluxes of low-energy ions, and solar models. The primary source of data was space-based measurements.

Bely-Dubau, F.↗

NASA NTRS1995

Theoretical properties of Omega-loops in the convective zone of the Sun. 3: Extended updrafts

It was pointed out in an earlier paper that the continuing emergence of Omega-loops at localized sites on the surface of the Sun indicates a continuing updraft at those sites. The updraft evidently extends all the way from the base of the convective zone to somewhat near (approximately 10(exp 9)cm) the surface. We pointed out that such updrafts enhance the convective heat transport to the surface, accounting for a major part of the increased solar brightness or irradiance during times of solar activity. The problem is to work out, as much as possible, the dynamical nature of the extended updrafts, initiated as the wakes of successive rising Omega-loops and driven therafter by the convective forces. The question is, does the updraft take on a long-lived columnar form of its own, or does it never devlop beyond a sequence of rising wakes, resembling beads on a string? The dynamics of a columnar updraft is complicated by both the large Reynolds number and the strong stratication of the atmosphere, and by a total lack of direct observational information. Extended slender updrafts are not a spontaneous occurrence in numerical simulations of thermal convection in a stratified atmosphere, although slender concentrated downdrafts commonly occur. This paper examines several aspects of a columnar updrft in a convective atmosphere under various idealized circumstances to investigate to what extent that state can be maintained against the diminishing vorticity and expansion in the updraft. It appears that the successive passage of Omega-loops from the bottom to the top of the convective zone is an essential feature of the continuing existence of the updraft.

Parker, E. N.↗