Engineering topics

Wang, Y.-M.

Publications and source records attributed to Wang, Y.-M..

33 records · Page 2

NASA NTRS1991

Magnetic field configurations associated with fast solar wind

The implications are considered of the observed inverse correlation between solar wind speed at earth and the expansion rate of the sun-earth flux tube as it passes through the corona. It is found that the coronal expansion rate depends critically on the large-scale photospheric field distribution around the footpoint of the flux tube. The smallest expansions occur in tubes that are rooted near a local minimum in the field. This suggests that the fastest wind streams originate from regions where large coronal holes are about to break apart and from the facing edges of adjacent like-polarity holes, whose field lines converge as they transit the corona. Predictions are made which follow from the above ideas.

Sheeley, N. R., Jr.↗

NASA NTRS1990

Magnetic flux transport and the sunspot-cycle evolution of coronal holes and their wind streams

The relationships between magnetic flux transport from active regions and the formation and evolution of coronal holes are examined through numerical simulations. The model utilized is based on the assumption that coronal holes represent open field regions, and that the solar-wind speed at 1 AU is universely correlated with the divergence rate of the coronal field. The evolution of coronal holes and wind streams during 1980 - 1990 is discussed, along with flux transport and the evolution of open field regions, and focus is placed on declining, rising, and maximum phases. It is concluded that supergranular diffusion spreads active region flux over the solar surface and wipes out pockets of mixed polarity, thus creating unipolar areas containing open field lines; differential rotation spreads flux in longitude and it combines with diffusion to create axisymmetric polar holes from the original active-region fields; and meridional flow accelerates the decay of low-latitude holes by carrying flux to midlatitudes.

Wang, Y.-M.↗

NASA NTRS1990

Latitudinal distribution of solar-wind speed from magnetic observations of the sun

Empirical studies suggest a close relationship between the solar-wind speed near the earth and the magnetic structure of the solar corona. The correlation can be used to infer the latitudinal distribution of wind speed at different phases of the sunspot cycle, and to identify the sources of fast, high-latitude wind streams such as those that might be encountered by the Ulysses spacecraft on its journey toward the solar poles during 1992-1995.

Wang, Y.-M.↗

NASA NTRS1990

Solar wind speed and coronal flux-tube expansion

The hypothesis that the solar wind speed at 1 AU and the rate of magnetic flux-tube expansion in the corona are inversely correlated is shown to be consistent with observations extending over the last 22 years. This empirical relationship allows the daily wind speeds at earth to be predicted from a current-free extrapolation of the observed photospheric field into the corona. The narrow boundaries of high-speed wind streams are attributed to steep gradients in the flux-tube expansion rates at the edges of coronal holes. When a heliospheric current sheet is included in the model, it is found that the flux tubes near the hole axis, although diverging more slowly than those near the hole boundary in the corona, have undergone the greatest net expansion at 1 AU, an effect consistent with the low densities within high-speed streams.

Wang, Y.-M.↗

NASA NTRS1989

Evolution of the sun's polar fields during sunspot cycle 21 - Poleward surges and long-term behavior

Longitudinally averaged observations of the photospheric field during 1976-1986 are analyzed using a flux transport model. The way in which source eruptions, supergranular diffusion, and meridional flow collaborate to produce strong, highly concentrated polar fields near sunspot minimum is clarified as follows: (1) widespread eruptions of individual bipolar magnetic regions, with their leading polarity flux equatorward of their trailing polarity flux, collectively establish a large-scale separation of polarities in latitude; (2) the low-latitude, leading polarity flux diffuses across the equator and merges with its opposite hemisphere counterpart; and (3) meridional flow carries the resulting surplus of trailing polarity flux to the poles, and concentrates it there against the spreading effect of diffusion. Episodic 'surges' of flux to the poles are induced by fluctuations in the source eruption rate. Simulations indicate that relatively weak, trailing polarity surges may occur even in a steady flow field. However, in order to account for the giant surges of alternating polarity and the resulting oscillations in the polar field strength observed during 1980-1982, both accelerated flow and enhanced eruption rates are required.

Wang, Y.-M.↗

NASA NTRS1989

Magnetic flux transport on the sun

Although most of the magnetic flux observed on the sun originates in the low-latitude sunspot belts, this flux is gradually dispersed over a much wider range of latitudes by supergranular convective motions and meridional circulation. Numerical simulations show how these transport processes interact over the 11-year sunspot cycle to produce a strong 'topknot' polar field, whose existence near sunspot minimum is suggested by the observed strength of the interplanetary magnetic field and by the observed areal extent of polar coronal holes. The required rates of diffusion and flow are consistent with the decay rates of active regions and with the rotational properties of the large-scale solar magnetic field.

Wang, Y.-M.↗

NASA NTRS1989

The effect of newly erupting flux on the polar coronal holes

Chromospheric network enhancements that occur along the edges of the polar coronal holes immediately after sunspot minimum are studied. It is shown that these enhancements accompany the eruption of the first large high-latitude bipolar magnetic regions of the new sunspot cycle, and that these eruptions must have encountered relatively concentrated polar fields whose strengths decrease rapidly equatorward of about 60 deg latitude. The helium observations are compared with current-free magnetic field calculations, and it is found that the enhanced helium network occurs where relatively strong fields at high latitude become connected to newly erupted bipolar magnetic regions.

Sheeley, N. R.↗

NASA NTRS1989

Implications of a strongly peaked polar magnetic field

Using the flux-transport equation in the absence of sources, the relation between a highly peaked polar magnetic field and the poleward meridional flow that concentrates it is studied. If the maximum flow speed v(m) greatly exceeds the effective diffusion speed, then the field has a quasi-equilibrium configuration in which the poleward convection of flux via meridional flow approximately balances the equatorward spreading via supergranular diffusion. Recent measurements would then give v(m) of about 7 m/s.

Sheeley, N. R., Jr.↗

NASA NTRS1989

Average properties of bipolar magnetic regions during sunspot cycle 21

The statistical properties of some 2700 bipolar magnetic regions (BMRs) with magnetic fluxes equal to or greater than 3 x 10 to the 20th Mx, which erupted during 1976-1986, are examined. The analysis shows the following: (1) the average flux per BMR declined between 1977 and 1985; (2) the average tilts of BMRs relative to the east-west line increase toward higher latitudes; (3) weaker BMRs had larger root-mean-square tilt angles than stronger BMRs at all latitudes; and (4) over the interval 1976-1986, BMRs with their leading poles equatorward of their trailing poles contributed a total of 4 times as much flux as BMRs with inverted tilts, but the relative amount of flux contributed by BMRs with inverted or zero tilts increased as the sunspot cycle progressed.

Wang, Y.-M.↗

NASA NTRS1988

The solar origin of long-term variations of the interplanetary magnetic field strength

Using simple models for the coronal field structure, the spacecraft observations of the photospheric field during sunspot cycle 21 were extrapolated for the purpose of modeling quantitatively the long-term behavior of the IMF during the sunspot cycle 21. Results were compared with the measurements of the radial component of the IMF at earth. The results indicate that the solar source of the IMF can be represented to a first approximation by the dipole component of the photospheric field, whose axis is nearly perpendicular to the ecliptic plane around sunspot minimum, but tilts more strongly toward it around sunspot maximum. It was also found that the average radial IMF strength varies with heliographic latitude; around sunspot minimum, the radial IMF is expected to be roughly twice as strong above the sun's poles as near the ecliptic plane. The average strength of the photospheric field above latitude 55 deg is about 10 G around sunspot minimum.

Wang, Y.-M.↗

NASA NTRS1988

The quasi-rigid rotation of coronal magnetic fields

Spherical harmonic analysis and numerical simulations are used to study the rotational properties of the coronal magnetic field under the assumption that it can be approximated by a current-free extension of the photospheric field. It is found that the rotation rate in the outer corona is determined, principally, by coronal filtering, the global averages of the photospheric rotation rate, and ongoing source eruptions. The present model is able to account for observationally inferred rotational properties. It is suggested that the coronal rotation rate accelerates gradually due to the equatorward migration of sunspots, and that the 27-day equatorial period is approached toward sunspot minimum as the decaying photospheric flux becomes localized near the equator.

Wang, Y.-M.↗

NASA NTRS1988

Mechanisms for the rigid rotation of coronal holes

The rotation of coronal holes is modeled in terms of a current-free coronal magnetic field in which holes are the footpoint locations of open field lines. It is shown that ongoing field-line reconnection allows the holes to rotate quasi-rigidly with their outer-coronal extensions. The results suggest that a strong axisymmetric field component may be responsible for the prolonged rigid rotation of large meridional holes during the declining phase of the sunspot cycle.

Nash, A. G.↗

NASA NTRS1987

The origin of rigidly rotating magnetic field patterns on the sun

Using analytical calculations and numerical simulations, it is shown that a meridional component of magnetic-flux transport will offset the shearing effect of differential rotation and give rise to rigidly rotating patterns of large-scale magnetic field. The nonaxisymmetric field attains a striped polarity pattern which rotates rigidly like a barber pole while its individual small-scale flux elements rotate at the differential rate of the latitudes they are crossing. On the sun, the meridional transport is provided by supergranular diffusion possibly assisted by a small poleward flow. New sources of flux retard this process and exclude the rigid rotation from the sunspot belts until well into the declining phase of the sunspot cycle. This mechanism accounts for a number of heretofore unexplained phenomena including the tendency for coronal holes to rotate rigidly during the declining phase of the sunspot cycle.

Sheeley, N. R., Jr.↗

NASA NTRS1977

Observations of Hercules X-1 with SAS-3 during 1975 July

X-ray pulsations from Her X-1 with energies between 0.1 and 30 keV were observed for four days with the SAS-3 satellite, during the 1975 July-August ON state of the source. The existence of a strong flux between 0.1 and 0.4 keV, with pulsations that are out of phase with those above 1 keV, is confirmed. A pulsed flux in the 19-30 keV band was discovered. The average fractional rate of change in pulse period between 1972 and 1975 was about 3 x 10 to the -6th/yr, and the absolute value of the average fractional rate of change in orbital period during the same interval was not greater than 5 x 10 to the -7th/yr

Joss, P. C.↗

NASA NTRS1972

A model for the polar transition layer and corona for November 1967.

A model for the chromospheric-coronal transition layer and lower corona has been constructed for the south polar region. EUV observations acquired by the Harvard OSO-4 experiment in the fall of 1967 were used in the analysis. The observations can be explained with a simple model consisting of two types of regions. One region has a temperature-density structure similar to that in models developed for typical equatorial quiet areas. The other region has a corona in which the temperature and density are a factor of about 2 lower and the chromospheric-coronal temperature gradient is less steep by a factor of 4.

Withbroe, G. L.↗