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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Properties of the large- and small-scale flow patterns in and around AR 19824

A series of magnetograms spanning just over 5 days and covering about 4 x 5 arcmin has been used to trace the photospheric motions of 170 concentrations of magnetic flux tubes in and around the decaying active region 19824. A persistent large-scale flow pattern superposed on small-scale random motions of both polarities is noted whose overall pattern resembles the flow of a river around a sand bar, and whose long-term evolution suggests a persistence of several months. The observed motions of concentrations of flux tubes are corrected for the large-scale flow in order to study their random motions.

Schrijver, C. J.↗
NASA NTRS1990

Hallmarks of the magnetic field in the solar atmosphere - Structure, evolution, heating, and flaring

Recent observations of the solar magnetic field and its effects on the solar atmosphere are discussed, with an emphasis on large-scale active regions and their implications for the fine-scale magnetic structure and for activity in the so-called quiet regions. Sample magnetograms, sunlight images, H-alpha images, X-ray images, and spectroheliograms are presented and characterized in detail, and the form and action of the magnetic field in flares are considered. It is pointed out that simultaneous observations of all levels (from the photosphere to the corona) at 100-km (about 100-marcsec) resolution are needed to see the extent of fields looping into the corona and understand their structure and activity; large space-based observatories would be required.

Moore, Ronald L.↗
NASA NTRS1991

The magnetic network location of explosive events observed in the solar transition region

Compact short-lived explosive events have been observed in solar transition region lines with the High-Resolution Telescope and Spectrograph (HRTS) flown by the Naval Research Laboratory on a series of rockets and on Spacelab 2. Data from Spacelab 2 are coaligned with a simultaneous magnetogram and near-simultaneous He I 10,380 -A spectroheliogram obtained at the National Solar Observatory at Kitt Peak. The comparison shows that the explosive events occur in the solar magnetic network lanes at the boundaries of supergranular convective cells. However, the events occur away from the larger concentrations of magnetic flux in the network, in contradiction to the observed tendency of the more energetic solar phenomena to be associated with the stronger magnetic fields.

Porter, J. G.↗
NASA NTRS1991

Short-term changes in solar oscillation frequencies and solar activity

It is shown that the frequencies of solar rho-mode oscillations change significantly over periods as short as one month. These changes correlate significantly with variations in the strength of surface solar activity as measured by the average, over the sun's visible surface, of the magnitude of the line-of-sight magnetic field component from magnetograms. The frequency and mean magnetic variations are found to obey a linear relationship. It is seen that the mean frequency shift at any time depends on the history of solar activity over an interval of, at most, several months prior to the measurement and conclude that the dominant mechanism of the frequency shift is correlated with surface magnetic activity.

Woodard, M. F.↗
NASA NTRS1991

Formation of the infrared emission lines of Mg I in the solar atmosphere

A non-LTE radiative transfer investigation of the emission lines is conducted at 7 and 12 microns using a realistic atomic model for neutral magnesium. An average quiet sun atmospheric model is used to calculate emission-line profiles that resemble the observed ones, i.e., broad absorption troughs with narrow central emission, and significant limb brightening. The charge exchange rates are found to be significant, but the effects of high-n coupling between Mg and Mg(+) together with radiative low-n transitions are of greater importance. It is confirmed that the emission cores are formed no higher than the temperature minimum region, and that the emission is caused by non-LTE effects rather than by the chromospheric temperature rise. It is inferred from the model calculations that the line core is sensitive to magnetic fields located almost 400 km above those measured in ordinary magnetograms; the gas pressure decreases 20-fold between these two heights.

Chang, E. S.↗
NASA NTRS1991

Determination of the coronal magnetic field from vector magnetograph data

A new algorithm was developed, tested, and applied to determine coronal magnetic fields above solar active regions. The coronal field above NOAA active region AR5747 was successfully estimated on 20 Oct. 1989 from data taken at the Mees Solar Observatory of the Univ. of Hawaii. It was shown that observational data can be used to obtain realistic estimates of coronal magnetic fields. The model has significantly extended the realism with which the coronal magnetic field can be inferred from observations. The understanding of coronal phenomena will be greatly advanced by a reliable technique, such as the one presented, for deducing the detailed spatial structure of the coronal field. The payoff from major current and proposed NASA observational efforts is heavily dependent on the success with which the coronal field can be inferred from vector magnetograms. In particular, the present inability to reliably obtain the coronal field has been a major obstacle to the theoretical advancement of solar flare theory and prediction. The results have shown that the evolutional algorithm can be used to estimate coronal magnetic fields.

Mikic, Zoran↗
NASA NTRS1991

Evidence for the interaction of large scale magnetic structures in solar flares

By modeling the observed vertical magnetic field of an active region AR 2372 by the potential field of an ensemble of magnetic dipoles, the likely location of the separatrices, surfaces that separates cells of different field line connectivities, and of the separator which is the intersection of the separatrices, is derived. Four of the five off-band H-alpha kernels of a flare that occurred less than 20 minutes before obtaining the magnetogram are shown to have taken place near or at the separatrices. These H-alpha kernels are connected by field lines that pass near the separator. This indicates that the flare may have resulted from the interaction in the separator region of large scale magnetic structures.

Mandrini, C. H.↗
NASA NTRS1992

Decimetric solar type U bursts - VLA and Phoenix observations

Observations of type U bursts, simultaneously detected by the VLA at 1.446 GHz and by the broadband spectrometer Phoenix in the 1.1-1.7 GHz frequency band on August 13, 1989 are reported. Extrapolations of the coronal magnetic field, assuming a potential configuration, indicate that the VLA 20 cm source demarcates an isodensity level. The source covers a wide angle of diverging magnetic field lines whose footpoints originate close to a magnetic intrusion of negative polarity into the main sunspot group of the active region with dominant positive polarity. The centroid of the 20-cm U-burst emission, which corresponds to the turnover frequency of the type U bursts and remains stationary during all U bursts, coincides with the apex of extrapolated potential field lines at a height of about 130,000 km. It is demonstrated that the combination of radio imaging and broadband dynamic spectra, combined with the magnetic field reconstruction from magnetograms, can constrain all physical parameters of a magnetic loop system.

Aschwanden, Markus J.↗
NASA NTRS1992

High latitude solar magnetic fields

Kitt Peak magnetograms are used to measure polar magnetic fields. The polar mean absolute field increases at the same time as the polar mean field decreases. That is, the polar mean absolute field varies in phase with solar activity, in contrast to the out of phase variation of the mean polar field. It is found that the polar fields have a large bipolar component even at solar minimum, with a magnitude equal to that found at low latitudes outside the active latitude bands.

Murray, Norman↗
NASA NTRS1992

A regularization method for extrapolation of solar potential magnetic fields

The mathematical basis of a Tikhonov regularization method for extrapolating the chromospheric-coronal magnetic field using photospheric vector magnetograms is discussed. The basic techniques show that the Cauchy initial value problem can be formulated for potential magnetic fields. The potential field analysis considers a set of linear, elliptic partial differential equations. It is found that, by introducing an appropriate smoothing of the initial data of the Cauchy potential problem, an approximate Fourier integral solution is found, and an upper bound to the error in the solution is derived. This specific regularization technique, which is a function of magnetograph measurement sensitivities, provides a method to extrapolate the potential magnetic field above an active region into the chromosphere and low corona.

Gary, G. A.↗
NASA NTRS1992

Evolution of vector magnetic fields and the August 27 1990 X-3 flare

Vector magnetic fields in an active region of the sun are studied by means of continuous observations of magnetic-field evolution emphasizing magnetic shear build-up. The vector magnetograms are shown to measure magnetic fields correctly based on concurrent observations and a comparison of the transverse field with the H alpha fibril structure. The morphology and velocity pattern are examined, and these data and the shear build-up suggest that the active region's two major footprints are separated by a region with flows, new flux emergence, and several neutral lines. The magnetic shear appears to be caused by the collision and shear motion of two poles of opposite polarities. The transverse field is shown to turn from potential to sheared during the process of flux cancellation, and this effect can be incorporated into existing models of magnetic flux cancellation.

Wang, Haimin↗
NASA NTRS1992

The solar oscillations investigation: Michelson Doppler imager

The Solar Oscillations Investigation (SOI) developed the Michelson Doppler Imager (MDI) to investigate the properties of the solar interior using the tools of helioseismology and of the photosphere and corona using more conventional techniques. The fundamental goal is to understand the Sun by determining its structure and observing its dynamics. The basic observables, velocity, intensity, and magnetic field, are computed on board from up to twenty 1024 by 1024 filtergrams made each minute. Subsequent analysis will extend the region to be explored downward into the solar interior and upward into the corona. While the instrument is dedicated to producing an uninterrupted series of helioseismology data, several magnetograms will be made each day and special eight hour campaigns are being developed to address specific scientific questions, some in coordination with other SOHO (Solar and Heliospheric Observatory) instruments.

Hoeksema, J. Todd↗
NASA NTRS1992

Observations of plasma sheet expansion at substorm onset, R = 15 to 22 Re

We have used a large number of auroral magnetograms to identify four isolated substorms and estimate their onset times. At the onsets, ISEE-1 was in the vicinity of magnetic midnight at radial distances of 15.6 to 21.8 Re and very near the outer boundary of the plasma sheet. We find that, for each event, the plasma sheet expanded, and the magnetic field dipolarized at the inferred onset time. Our most definitive event occurred while ISEE was at a geocentric radial distance of 21.8 Re. This result conflicts with previous understanding, though further verification of the result is required. Our observations show very similar characteristics to those observed at synchronous orbit, and they are consistent with an extension of a portion of the substorm current wedge to the radial distance of the satellite. If this explanation is correct, ISEE must have been within the longitude range of the substorm current wedge at the onsets.

Lyons, L. R.↗
NASA NTRS1992

Coronal Magnetic Structures Observing Campaign. II - Magnetic and plasma properties of a solar active region

Simultaneous soft X-ray, microwave, and photospheric magnetic field observations were taken during the Coronal Magnetic Structures Observing Campaign. The plasma electron temperatures and emission measures determined from the X-ray data are used to predict the intensity and structure of the thermal bremsstrahlung emission at 20 and 6 cm. Comparing these predictions with the microwave observations, it is found that the 20 cm structure is very similar to that expected from the X-rays, but a substantial amount of the 6 cm emission was resolved out. The predicted 20 cm brightness temperatures are higher than the observed, requiring cool absorbing material (not greater than 500,000 K) between the hot X-ray plasma and the observer. The absorption mechanism in the cool plasma at 20 cm is most likely thermal bremsstrahlung, requiring coronal magnetic fields as high as 150 G. 'Coronal Magnetograms', made by extrapolating the photospheric longitudinal field using the Sakurai code, show that appropriate values of the total field are reached at heights of 6000-10,000 km above the photosphere (at many but not all locations).

Schmelz, J. T.↗
NASA NTRS1992

Polarimetry and spectroscopy of a simple sunspot. I - On the magnetic field of a sunspot penumbra

We investigate the magnetic field structure of a medium sized sunspot using high resolution magnetograms and spectrograms and derive a relationship between the brightness of penumbral structures and the inclination of the magnetic field. The field inclination to the spot normal is larger in the dark structures than in the bright ones. We show that the field strength does not vary between dark and bright structures. At the inner penumbral boundary the field strength is 2000 Gauss and about 1000 Gauss at the outer penumbral edge. The line-of sight component of the material flow decreases rapidly within one arcsecond at the photospheric boundary of the spot.

Schmidt, W.↗
NASA NTRS1992

On the inclination of photospheric solar magnetic fields

We analyze the Kitt Peak magnetograms and find that the distribution function of magnetic field strengths is given by a power law, with an index that depends on the mean absolute magnetic flux. We also find that weak (measured) magnetic fields - but not strong ones - are significantly inclined to the vertical. We argue that this inclination is produced by convective motions at the top of the convection zone.

Murray, Norman↗
NASA NTRS1992

The X flare of 1991 November 15 - Coordinated Mees/Yohkoh observations

This is a preliminary report on two unique new results from coordinated observations at Mees Solar Observatory and Yohkoh of the X1.5 flare of November 15, l991 using vector magnetograms, H-alpha imaging spectra, X-ray images, and X-ray spectra. First, we find a close spatial relationship between H-alpha redshifts and X-rays from a flare loop and its footpoints at a time of large X-ray blueshifts. Second, we find that impulsive-phase hard X-rays originate in regions that are new, but not coincident with, the peaks of the vertical electrical current density distribution in AR 6919.

Canfield, Richard C.↗
NASA NTRS1992

The reversal of the solar polar magnetic fields. IV - The polar fields near sunspot maximum

The evolution of solar polar magnetic fields between Carrington rotations 1815 and 1834 is described using magnetic data from the Mount Wilson Observatory and the National Solar Observatory. The observations are compared with simulations using the flux transport equation. It is shown that the evolution of the polar field cannot be reproduced accurately by simulations of the diffusion and poleward drift of the emerging active regions at sunspot latitudes. Histograms of the distribution of the field intensities derived from daily magnetograms are presented which indicate that flux emerges at high latitudes and contributes to the evolution of the magnetic patterns.

Murray, N.↗