Engineering Papers⌕ Search

Engineering topics

Sheeley, N. R., Jr.

Publications and source records attributed to Sheeley, N. R., Jr..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Properties of metre-wavelength solar bursts associated with coronal mass ejections

An investigation is made to determine the relationship between a coronal mass ejection (CME) and the characteristics of associated metre-wave activity. It is found that (1) the CME width and leading edge velocity can be highly influential in determining the intensity, spectral complexity and frequency coverage of both type II and continuum burst; (2) the presence of a CME is possibly a necessary condition for the production of a metric continuum event and (3) metric continuum bursts as well as intense, complex type II events are preferentially associated with strong, long lasting soft X-ray events.

Robinson, R. D.↗
NASA NTRS1986

Solwind observations of coronal mass ejections during 1979-1985

Coronal observations have been processed for parts of each year during the interval 1979-1985. Around sunspot maximum, coronal mass ejections (CMEs) occurred at the rate of approximately 2 per day, and had a wide range of physical and morphological properties. During the recent years of relatively low sunspot number, CMEs occurred at the rate of only 0.2 per day, and were dominated by the class of so-called streamer blowout. These special CMEs maintained a nearly constant occurrence rate of roughly 0.1 per day during the entire interval.

Sheeley, N. R., Jr.↗
NASA NTRS1986

The origin of the 28- to 29-day recurrent patterns of the solar magnetic field

Numerical simulations of the sun's mean line-of-sight magnetic field suggest an origin for the 28- to 29-day recurrent patterns of the field and its associated interplanetary phenomena. The patterns are caused by longitudinal fluctuations in the eruption of new magnetic flux, the transport of this flux to mid latitudes by supergranular diffusion and meridional flow, and the slow rotation of the resulting flux distributions at the 28- to 29-day periods characteristic of those latitudes.

Sheeley, N. R., Jr.↗
NASA NTRS1986

The decay of the mean solar magnetic field

The effects that differential rotation and a hypothetical meridional flow would have on the evolution of the sun's mean line-of-sight magnetic field as seen from earth have been analyzed. By winding the large-scale field into strips of alternating positive and negative polarity, differential rotation causes the mean-field amplitude to decay and the mean-field rotation period to acquire the value corresponding to the latitude of the surviving unwound magnetic flux. For a latitudinally broad two-sector initial field such as a horizontal dipole, the decay is rapid for about 5 rotations, and slow with a t exp-1/2 dependence thereafter. If a poleward meridional flow is present, it will accelerate the decay by carrying the residual flux to high latitudes where the line-of-sight components are small. The resulting decay is exponential with an e-folding time of 0.75 yr (10 rotations) for an assumed 15 m/s peak meridional flow speed.

Sheeley, N. R., Jr.↗
NASA NTRS1985

Simulations of magnetic-flux transport in solar active regions

The evolution of several observed solar active regions is simulated by solving a transport equation for magnetic flux at the photosphere. The rates of rotation, meridional flow, and diffusion of the flux are determined self-consistently in the calculations. The findings are in good quantitative agreement with previous measures of the rotation rate and diffusion constant associated with photospheric magnetic fields. Although the meridional velocities are consistent in direction and magnitude with recently reported poleward flows, relatively large uncertainties in the velocity determinations make this result inconclusive.

Devore, C. R.↗
NASA NTRS1985

Coronal mass ejections - 1979-1981

An overview of observations of coronal mass ejections (CMEs) during the interval March 28, 1979 through December 31, 1981 is presented. An introduction is first provided to the instrument used in the observations, the method of identifying CMEs, the concepts of CME structural classes, CME importance categories, and the distribution of mass ejections among these classes and categories. The properties of CMEs are given for all mass ejections regardless of their structural classes or importance categories, and then for each class and category. After a brief discussion of the instrument duty cycle, the occurrence rate is presented for all CMEs and for major CMEs, for fast CMEs only, and finally for equatorial CMEs only. The results are compared to those obtained previously.

Howard, R. A.↗
NASA NTRS1985

Energetic protons from a disappearing solar filament

A solar energetic (E 50 MeV) particle (SEP) event observed at 1 AU began about 15000 UT on 1981 December 5. This event was associated with a fast coronal mass ejection observed with the Solwind coronagraph on the P78-1 satellite. No metric type 2 or type 4 burst was observed, but a weak interplanetary type 2 burst was observed with the low frequency radio experiment on the International Sun-Earth Explorer-3 satellite. The mass ejection was associated with the eruption of a large solar quiescent filament which lay well away from any active regions. The eruption resulted in an H alpha double ribbon structure which straddled the magnetic inversion line. No impulsive phase was obvious in either the H alpha or the microwave observations. This event indicates that neither a detectable impulsive phase nor a strong or complex magnetic field is necessary for the production of energetic ions.

Kahler, S. W.↗
NASA NTRS1985

Simulations of the mean solar magnetic field during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, the evolution of the photospheric magnetic field during 1976-1984 was computed and the corresponding evolution of the mean line-of-sight field as seen from earth was derived. A good, but imperfect, agreement was obtained between the observed mean field and the field computed for a nominal choice of flux transport parameters. The response of the computed mean field to variations in the transport parameters and the source properties was determined. The results suggest that the mean-field evolution is a random-walk process with dissipation. New eruptions of flux produce the random walk, and together differential rotation, meridional flow, and diffusion provide the dissipation. The net effect of each new source depends on its strength and orientation and on the time elapsed before the next eruption.

Sheeley, N. R., Jr.↗
NASA NTRS1985

White-light and radio sounding observations of coronal transients

A concerted search for coronal transients was conducted with the 'Solwind' coronagraph during the solar occultations of the two Helios spacecraft in October/November 1979. The polarization angle and bandwidth of the linearly polarized S-band downlink signal were monitored at the three 64-m tracking stations of the NASA Deep Space Network to determine coronal Faraday rotation and spectral broadening. A one-to-one correspondence could be established between abrupt disturbances in the two signal parameters and the passage of a white-light transient through the signal ray path from spacecraft to earth. The white-light morphology and the additional information provided by the radio sounding coverage are presented for each of the five distinct events recorded. Although no specific example could be observed in sufficient detail in both white light and Faraday rotation to derive the small-scale magnetic structure, some qualitative descriptions of the orientation and rough estimates of the magnitude of the transient magnetic field could be made.

Bird, M. K.↗
NASA NTRS1985

The frequency of long-duration solar X-ray events

The rate of occurrence of coronal and interplanetary phenomena during the solar sunspot cycle was studied based on nearly continuous satellite data for long-duration events in the period 1969-1982. The data were obtained using the NRL SOLRAD and the NOAA GOES spacecraft for each year in the interval. The duration of the X-ray events was defined as the time for the logarithm of the 1-8 A flux to return to approximately 10 percent of the pre-event level. The total number of X-ray events is plotted next to the annual sunspot number for the interval 1969-1982. It is shown that the long duration of X-ray events was not related to the sunspot cycle, particularly events of 6 hr or more.

Koomen, M. J.↗
NASA NTRS1985

High-resolution X-ray spectra of solar flares. VII - A long-duration X-ray flare associated with a coronal mass ejection

It has been recognized that very long duration X-ray events (lasting several hours) are frequently associated with coronal mass ejection. Thus, Sheeley et al. (1983) found that the probability of the occurrence of a coronal mass ejection (CME) increases monotonically with the X-ray event duration time. It is pointed out that the association of long-duration, or long-decay, X-ray events (LDEs) with CMEs was first recognized from analysis of solar images obtained by the X-ray telescopes on Skylab and the Naval Research Laboratory (NRL) slitless spectroheliograph. Recently high-resolution Bragg crystal X-ray spectrometers have been flown on three spacecraft, including the Department of Defense P78-1 spacecraft, the NASA Solar Maximum Mission (SMM), and the Japanese Hinotori spacecraft. In the present paper, P78-1 X-ray spectra of an LDE which had its origin behind the solar west limb on November 14, 1980 is presented. The obtained data make it possible to estimate temperatures of the hottest portion of the magnetic loops in which the emission arises.

Kreplin, R. W.↗
NASA NTRS1985

A comparison of solar helium-3-rich events with type II bursts and coronal mass ejections

The acceleration process for energetic particles in He-3-rich events and for particles in normal-abundance events are compared. A list of 66 He-3-rich events observed with the Goddard Space Flight Center particle detector on ISEE 3 is presented, and it is shown that these events are not statistically associated with either of the two common signatures of normal-abundance events, metric type II and coronal mass ejections. This result indicates that enhanced abundance events may be produced only in the impulsive phases of flares, while normal abundance events are produced in subsequent flare shock waves.

Kahler, S.↗
NASA NTRS1985

Doppler scintillation observations of interplanetary shocks within 0.3 AU

It is pointed out that more definitive shock velocity observations near the sun are needed for an improved determination of the extent of shock deceleration from the sun to earth. Woo and Armstrong (1981) have demonstrated the use of radio scattering and scintillation observations, using spacecraft signals, for measuring interplanetary shock waves near the sun. Woo and Armstrong provided the first near-sun profiles of solar wind speed and electron density fluctuation for a shock wave produced by a solar flare. The present investigation has the objective to demonstrate the use of Doppler or phase scintillations for monitoring and observing interplanetary shocks. It is also shown that Doppler noise, a parameter which is routinely observed and recorded by the NASA Deep Space Network, is essentially a measure of Doppler scintillations.

Woo, R.↗
NASA NTRS1985

Coronal mass ejections and interplanetary shocks

A comparison between Solwind observations of coronal mass ejections (CME's) and Helios 1 observations of interplanetary shocks during 1979-1982 indicates that 72 percent of the shocks were associated with large, low-latitude mass ejections on the nearby limb. Most of the associated CME's had speeds in excess of 500 km/s, but some of them had speeds in the range 200-400 km/s. An additional 26 percent of the shocks may have been associated with CME's, but we were less confident of these associations because the sizes and locations of the CME's did not seem appreciably different from those of the numerous CME's without Helios shocks. Only 2 percent of the shocks clearly lacked CME's. As the average level of sunspot activity declined during 1982, the shock frequency also declined, but the observed shocks and some of their associated CME's had unusually high speeds well in excess of 1000 km/s.

Sheeley, N. R., Jr.↗
NASA NTRS1984

Associations between coronal mass ejections and solar energetic proton events

A comparison between proton events and coronal mass ejections (CMEs) based on nearly three years of observations around the recent maximum of solar activity is presented. Peak proton fluxes are found to correlate with both the speeds and the angular sizes of the associated CMEs. It is shown that CME speeds do not significantly correlate with CME angular sizes, so that peak proton fluxes are correlated with two independent CME parameters. With larger angular sizes, CMEs are more likely to be loops and fans rather than jets and spikes and are more likely to intersect the ecliptic.

Kahler, S. W.↗
NASA NTRS1984

Characteristics of flares producing metric type II bursts and coronal mass ejections

An attempt is made to study the origin of coronal shocks by comparing several flare characteristics for two groups of flares: those with associated metric type II bursts and coronal mass ejections (CMEs) and those with associated metric type II bursts but no CMEs. CMEs accompany about 60 percent of all flares with type II bursts for solar longitudes greater than 30 deg, where CMEs are well observed with the NRL Solwind coronagraph. H-alpha flare areas, 1-8 A X-ray fluxes, and impulsive 3-cm fluxes are all statistically smaller for events with no CMEs than for events with CMEs. It appears that both compact and large mass ejection flares are associated with type II bursts. The events with no CMEs imply that at least many type II shocks are not piston-driven, but the large number of events of both groups with small 3 cm bursts does not support the usual assumption that type II shocks are produced by large energy releases in flare impulsive phases. The poor correlation between 3 cm burst fluxes and the occurrence of type II bursts may be due to large variations in the coronal Alfven velocity.

Kahler, S.↗
NASA NTRS1984

The concentration of the large-scale solar magnetic field by a meridional surface flow

Analytical and numerical solutions to the magnetic flux transport equation in the absence of new bipolar sources of flux are calculated for several meridional flow profiles and a range of peak flow speeds. It is found that a poleward flow with a broad profile and a nominal 10 m/s maximum speed concentrates the large-scale field into very small caps of less than 15 deg half-angle, with average field strengths of several tens of gauss, contrary to observations. A flow which reaches its peak speed at a relatively low latitude and then decreases rapidly to zero at higher latitudes leads to a large-scale field pattern which is consistent with observations. For such a flow, only lower latitude sunspot groups can contribute to interhemispheric flux annihilation and the resulting decay and reversal of the polar magnetic fields.

Devore, C. R.↗