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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Prediction of solar induced currents and effects on power transmission systems in central Canada

The auroral-electrojet zone covers three quarters of Manitoba and consequently, solar storms strongly affect transmission lines. Harmonics are generated at transformers due to the saturation of their cores by induced currents, and the level of harmonics produced may cause malfunction of control relays, and yield unacceptable distortions in normal ac waveforms. The expected effects of long ac transmission systems were studied with emphasis on a 500 kv line to be built from Winnipeg to Minneapolis-St. Paul. Spectral analysis of induced current records from Manitoba Hydro's LaVerendrye station and magnetograms from IMS stations in Manitoba were used along with results of Campbell's work on the Alaskan pipeline induction problem (1978) in order to predict periodic and surge currents. It is concluded that the surge currents will produce significant levels of harmonics and corresponding operating problems during magnetic storms.

Goddard, W. R.↗
NASA NTRS1979

Review of selected geomagnetic activity indices

Magnetic activity indexes are reviewed. Classifications of magnetograms from single observatories and the global range of potential associated with the equivalent currents which could have produced the variations monitored at a large array of recording sides are addressed. Principal magnetic activity indexes discussed include: the auroral electrojet index and its associated indexes (AU, AL and AO) useful for auroral zone studies; the Kp, ap, aa and am indexes which are measures of midlatitude geomagnetic activity; and the Dst index of magnetic activity recorded at low latitudes. It is concluded that geomagnetic activity indexes are useful in studies of the interaction between solar activity, the interplanetary magnetic field and solar wind, the magnetosphere, ring current, field aligned currents, and ionospheric currents.

Allen, J. H.↗
NASA NTRS1979

Evidence for a peak in the number of isolated Type III bursts prior to large solar flares

Evidence for a broad maximum in the number of isolated metric-wave Type III bursts prior to the large H alpha solar flares observed between May 1973 and February 1974 is reported. The time distribution of isolated Type III bursts within 12.6 h of each of the 111 solar flares of importance 1 or greater recorded during the period is shown to peak approximately 5 h before the time of flare maximum, most markedly in the case of bursts with projected positions near the solar flare position. The examination of H alpha, X-ray and magnetogram data in the vicinity of the flare implies a purely coronal storage and release mechanism for the flare energy. The peak of Type III burst activity is thus suggested as a good indicator of coronal energy input and storage near the time of a large solar flare, although not a reliable flare predictor.

Jackson, B. V.↗
NASA NTRS1980

X-ray bright points and the solar cycle

Soft X-ray filtergrams show the presence of large numbers of small closed regions of coronal emission. These features, called X-ray bright points, correspond to small short-lived regions of emerging magnetic flux. X-ray data for 1970 to 1978 shows that the number of X-ray bright points appears to be anticorrelated with traditional activity indices such as sunspot number. A comparison of X-ray data with KPNO magnetograms shows that to within a factor of 2 the average total amount of magnetic flux emerging over the full sun is constant through the entire period of observation. The solar cycle therefore appears to be an oscillation in the wavenumber distribution of emerging flux than of the total quantity of the magnetic flux produced.

Golub, L.↗
NASA NTRS1981

Pulsations in total columnar electron content

Radio signals from the ATS 6 beacon received at Boulder reveal small-amplitude, quasi-sinusoidal fluctuations with periods in the range of 10 to 50 s. Visual comparisons of these data (116 events for October 1974 to April 1975) shows a good correspondence with simultaneous geomagnetic pulsations at Boulder in two thirds of the cases for which Boulder magnetograms were available, but they do not necessarily correspond with magnetic pulsations on ATS 6. Spectral analyses, by the method of maximum entropy, were made on sample records. The principal results are the following: (1) The occurrence of the pulsations is higher on magnetically disturbed days. (2) The maximum likelihood of occurrence is around 2100 UT (1400 LT). (3) The dominant spectrum peaks of the radio fluctuations and geomagnetic field on the ground generally coincide. Cases are found also in which temporal characteristics of the spectra are similar. These results indicate a close association of the radio fluctuations with the Pc 3-4 type pulsations of the geomagnetic field on the ground. It is suggested that the radio fluctuations originate mainly in the F region of the ionosphere, while some of them could be due to plasmapause effects.

Okuzawa, T.↗
NASA NTRS1981

The corona associated with solar filaments

Spectroheliograms in high temperature ions such as Fe XV show the existence of both filaments that are brighter than the ambient corona and filaments that are darker than the ambient corona. The relationship of the filaments to photospheric magnetograms is described, and a possible physical mechanism to explain the differences is discussed.

Chapman, R. D.↗
NASA NTRS1981

VLA observations of solar active regions at 6 cm wavelength

Synthesized maps of two solar active regions obtained from observations with the Very Large Array (VLA) with 9-arcsec resolution are presented. The most intense sources in these regions are found to be associated with filamentary structures and magnetic neutral lines as shown in H-alpha and photospheric magnetograms. These sources are not located directly over sunspots in disagreement with earlier observations. EUV and X-ray observations have suggested that similar structures should be visible at cm wavelengths around but, outside of, sunspots if the magnetic field is sufficiently strong. These results are consistent with the locations of hot (greater than 1,000,000 K) plasmas in active regions expected from generalization based on optical photographs. Given the sizes of the radio sources, the volume emission measures of soft X-rays observed from OSO-8 rule out the possibility of thermal bremsstrahlung being of any significance, as far as the 6 cm emission is concerned. Therefore, gyroresonance absorption process is the most likely cause of 6 cm emission from these sources, and its likelihood is enhanced by the magnetic field geometry that is known to exist over filaments and neutral lines.

Kundu, M. R.↗
NASA NTRS1981

Measurement of solar radius changes

Results of daily photometric measurements of the solar radius from Mt. Wilson over the past seven years are reported. Reduction of the full disk magnetograms yields a formal error of 0.1 arcsec in the boustrophedonic scans in the 5250.2 A FeI line. 150 scan lines comprise each observation; 1,412 observations were made from 1974-1981. Measurement procedures, determination of the scattered light of the optics and the atmosphere, and error calculations are described, noting that days of poor atmospheric visibility are omitted from the data. The horizontal diameter of the sun remains visually fixed while the vertical component changes due to atmospheric diffraction; error accounting for thermal effects, telescope aberrations, and instrument calibration are discussed, and results, within instrument accuracy, indicate no change in the solar radius over the last seven years.

Labonte, B. J.↗
NASA NTRS1982

The magnetic flux in the quiet sun network

Direct magnetic measurements are used to confirm that the Ca II K line emission from the quiet sun network does not vary with the 11 year cycle (White and Livingston, 1981). As the K emission intensity is correlated with magnetic field strength, magnetic flux measurements are valid for comparison. Data were taken from the full-disk Mount Wilson daily magnetograms, and the aperture was decreased from 17.5 to 12.5 during the interval 1970 through 1980. Measurements are restricted to the latitude zones centered on + or - 1.7 degrees. The mean total flux in the + or - 15.3 degree zones increased by a factor of 10 between activity minimum and maximum, and the quiet sun flux shows no variation, thus confirming White and Livingston's (1981) result. In order for the amplitude of the quiet sun magnetic flux to remain constant, a balance between increasing and decreasing factors must exist, and the primary factor in this balance is the rate of destruction of the quiet sun flux.

Labonte, B. J.↗
NASA NTRS1982

The three-dimensional structure of atmospheric magnetic fields in two active regions

The magnetic field above two unrelated active regions on September 11-12, 1974 has been studied by means of magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and H-alpha. In C I, originating low in the photosphere, the fields are strong and sharply defined, by contrast with Ca II and H-alpha, where they are very diffuse. The similar diffuseness of Fe I is due to the spreading of the field with height to form nearly horizontal magnetic canopies over regions free of field at lower levels. Within a region between two small sunspots some 140 Mm apart, the canopy height is typically 300-400 km, and it is about 150-250 km within a small superpenumbra. It is hypothesized that the chromospheric fibrils taken to delineate the field configuration are not due primarily to lateral variations in field, but rather to differences in density or excitation of gas across the lines of force.

Giovanelli, R. G.↗
NASA NTRS1983

An investigation of coronal active region loop structures using AS&E rocket X-ray images

Simultaneous high spatial resolution observations at 6 cm in soft X-rays, in photospheric magnetograms, and in optical filtergrams were used to compare the most intense sources of centimetric emission in two active regions to coronal loops, sunspots, chromospheric structures, and photospheric magnetic fields. Results show that the majority of the bright microwave components are not associated with sunspots or X-ray emission. A nonthermal mechanism appears necessary to explain the brightest microwave components, discrete regions of continuous particle acceleration may be common in active regions. Studies of the plasma parameters of selected loops imply that the radio emission is consistent with gyro-resonance absorption at the third and fourth harmonic, at least from part of each loop. Results are presented for: (1) X-ray and microwave observations of active regions; (2) comparison of coronal holes observed in soft X-rays and Hel 10830 A spectrosheliograms; and (3) the reappearance of polar coronal holes and the evolution of the solar magnetic field.

Webb, D. F.↗
NASA NTRS1982

Observations of the longitudinal magnetic field in the transition region and photosphere of a sunspot

The Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission spacraft has observed for the first time the longitudinal component of the magnetic field by means of the Zeeman effect in the transition region above a sunspot. The data presented here were obtained on three days in one sunspot, have spatial resolutions of 10 arcsec and 3 arcsec, and yield maximum field strengths greater than 1000 G above the umbrae in the spot. The method of analysis, including a line-width calibration feature used during some of the observations, is described in some detail in an appendix; the line width is required for the determination of the longitudinal magnetic field from the observed circular polarization. The transition region data for one day are compared with photospheric magnetograms from the Marshall Space Flight Center. Vertical gradients of the magnetic field are compared from the two sets of data; the maximum gradients of 0.41 to 0.62 G/km occur above the umbra and agree with or are smaller than values observed previously in the photosphere and low chromosphere.

Henze, W., Jr.↗
NASA NTRS1983

On possible correlations in the photospheric magnetic field

A correlation analysis is performed upon the rate of change of the photospheric magnetic field in the north and south hemispheres of the sun. Both active region logs and data obtained from the daily Mount Wilson magnetograms are employed, and the analysis is performed for all available days, as well as for days in which active regions are near central meridian. The result of the analysis shows no correlation between these changes and it is concluded that local convective turbulence is more important than dynamo processes with regard to the appearance of individual active regions.

Wallenhorst, S. G.↗
NASA NTRS1983

Naked sunspots

Naked sunspots are spots seen in H-alpha to be devoid of associated plage. In magnetograms and K-line little if any opposite polarity field is found, and in soft X ray images a blank appears in the region of the spot. In almost all cases studied in which naked spots resulted the spot groups had emerged in unipolar regions of the same polarity as the naked spot. At least half of the naked spots are associated with coronal holes. The naked spots are long-lived and show rotation rates close to the Newton-Nunn curve. Most of the naked spots had bright rims in H-alpha, and the one spot observed to disappear left no trace in the background magnetic field. These spots may be a means by which separation of p from f magnetic polarity occurs.

Liggett, M.↗
NASA NTRS1983

Magnetic rotation of the solar photosphere

Magnetograms made at Mt. Wilson Observatory from January 1967 to May 1982 are crosscorrelated in 34 latitude strips at 1-4-day increments to determine the rotation of magnetic features in the solar photosphere. The data are smoothed by averaging corresponding correlations and calculating rotation from the displacement of the averaged-correlation maximum; the usefulness and validity of this procedure are discussed. No significant time variation or field dependence is found for the period of the observations, at least to the accuracy of the calculated means (variance of from about 2 m/sec at low latitudes to about 10 m/sec near the poles). The rotation function omega at solar latitude phi is shown to be 2.902 0.464 sin sq phi - 0.328 sin to the 4th phi microrad/sec, in agreement with the Mt. Wilson Doppler profile near the poles and with the sunspot determination of Newton and Nunn (1951) at sunspot latitudes, where the Doppler estimate is about 30 m/sec slower.

Snodgrass, H. B.↗
NASA NTRS1983

X-ray and microwave observations of active regions

Coordinated high-resolution (1-3 arcsec) observations of two active solar regions (H 421 and H 419) on November 16, 1979, are reported: soft-X-ray filtergrams from a sounding rocket flight, VLA total-intensity and circular-polarization microwave (6-cm) radio maps, KPNO full-disk photospheric magnetograms, and BBSO H-alpha data. The images were converted to 4.8-arcsec/mm-scale transparencies and coaligned on the basis of sunspot positions for comparison. The two active regions are characterized in detail, and intensity, size, and polarization data for the brightest microwave components (BMC) are listed. It is found that 19 of the 32 BMC are farther than 5 arcsec from any sunspot, and that X-ray-emitting structures only rarely correspond to sunspots, or BMC. About one third of the BMC are located at the feet or legs of coronal loops smaller than about 50,000 km. The limitations implied by these obervations for proposed thermal-bremsstrahlung, thermal-gyro-resonance, and nonthermal microwave-emission mechanisms are discussed.

Webb, D. F.↗
NASA NTRS1983

The sun at 1.4 GHz - Intensity and polarization

1.4 GHz radio pictures of the sun are presented which were made on 1981 September 26 using the VLA with a resolution of 40 arcsec. Features observed include active regions, limb brightening, coronal holes and filament channels; all correspond well with features seen in H-alpha or He 10830 A, and can be explained as relative enhancements or depressions of free-free brmsstrahlung from the corona and upper transition region. The degree of circular polarization of the radiation from active regions ranges up to 0.2 and the sense corresponds, with a few exceptions, with the polarity of the photospheric magnetograms, even though the 1.4 GHz radiation arises from the corona at a height of 10 to 50 Mm. The polarization is in the sense of the x-mode and is highest near the edges of active regions, whereas the brightness is highest (maximum of 2.2 x 10 to the 6th K) near the line of zero polarization.

Dulk, G. A.↗
NASA NTRS1983

Solar oscillations with 13-day period

Reference is made to the solar observations made by Claverie et al. (1982) over a three-month period in the summer of 1981 which show oscillatory velocity with a period of 13.1 days and amplitude of 6.6 m/s. These investigators reject the possibility that they see the Doppler shift from a radial oscillation, because the amplitude is implausibly large. They also do not believe that their signal was induced by solar magnetic fields, since typical mean solar fields are too small. Photo-electric drift-scan measurements of the solar diameter and full-disk magnetograms taken at Kitt Peak National Observatory are examined here for evidence of variations corresponding to the velocity oscillations of the 13.1-day period. An upper limit on radius variations is reported which is a factor of six below the amplitude needed to explain the velocity observations as a radial oscillation. Attention is also given to the possible role of the rotation of large-scale surface magnetic features.

Duvall, T. L., Jr.↗