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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

FIP, FIT or MAD? Analysis of High Signal-to-Noise ASCA Spectra of Coronal Stars

ASCA (Advanced Satellite for Cosmology and Astrophysics) and EUVE (Extreme Ultraviolet Explorer) spectra of active late-type stars imply that Fe and other medium-Z elements may be 2-10 times less abundant in the coronae of these stars than in their photo-spheres (the MAD effect). These deficiencies may be related to the solar FIP (First Ionization Potential) effect, in which Fe and other low First Ionization Potential elements appear enriched in the solar corona over their photospheric values. The FIP effect is time variable. As part of this proposal, the K0-2 III star, 29 Draconis, was observed in X rays with the ASCA spacecraft in order to measure the coronal abundances of this star at three different stellar longitudes over its 31-day rotation cycle. The goal of the observations was to learn whether coronal abundances, and hence coronal magnetic structure, vary across the surface of 29 Draconis in phase with the motion of dark star-spots across its disk. A second task included in this project was a systematic reanalysis of 18-20 deep exposures of active coronal stars, which were extracted from the ASCA public archives. New thermal models were computed for each spectrum in order to derive coronal metal abundances for each star. The goal of this survey was to search for possible trends in coronal abundance with various stellar parameters such as rotation, chromospheric activity levels at ultraviolet and optical wavelengths, or evolutionary stage.

Simon, Theodore↗
NASA NTRS1974

Solar coronal holes as sources of recurrent geomagnetic disturbances

Observations of the solar corona by Oso 7 have been used in a superposed epoch analysis to study the relationships between classes of coronal features and geomagnetic activity. Both bright coronal regions and regions of less than average brightness were investigated. It was found that for the period from January 1972 through January 1973, a significant enhancement in geomagnetic activity occurred 2-3 days after central meridian passage of large coronal holes that extended to within 5 deg of the solar subearth point when they were on the meridian. Large coronal holes appear to satisfy the requirements for 'M regions' which were hypothesized to be responsible for recurrent geomagnetic disturbances (Bartels, 1934). If solar wind high-speed streams originate preferentially in these regions, their velocity at the base of the corona will be substantially higher than that expected from an axisymmetric solar wind model.

Neupert, W. M.↗
NASA NTRS2017

Variation in Coronal Activity from Solar Cycle 24 Minimum to Maximum Using Three-Dimensional Reconstructions of the Coronal Electron Density from STEREO/COR1

Three-dimensional electron density distributions in the solar corona are reconstructed for 100 Carrington rotations (CR 2054 - 2153) during 2007/03 - 2014/08 using the spherically symmetric method from polarized white-light observations with the inner coronagraph (COR1) onboard the twin Solar Terrestrial Relations Observatory (STEREO). These three-dimensional electron density distributions are validated by comparison with similar density models derived using other methods such as tomography and a magnetohydrodynamics (MHD) model as well as using data from the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO)-C2. Uncertainties in the estimated total mass of the global corona are analyzed based on differences between the density distributions for COR1-A and -B. Long-term variations of coronal activity in terms of the global and hemispheric average electron densities (equivalent to the total coronal mass) reveal a hemispheric asymmetry during the rising phase of Solar Cycle 24, with the northern hemisphere leading the southern hemisphere by a phase shift of 7 - 9 months. Using 14 CR (approx. equal to 13-month) running averages, the amplitudes of the variation in average electron density between Cycle 24 maximum and Cycle 23/24 minimum (called the modulation factors) are found to be in the range of 1.6 - 4.3. These modulation factors are latitudinally dependent, being largest in polar regions and smallest in the equatorial region. These modulation factors also show a hemispheric asymmetry: they are somewhat larger in the southern hemisphere. The wavelet analysis shows that the short-term quasi-periodic oscillations during the rising and maximum phases of Cycle 24 have a dominant period of 7 - 8 months. In addition, it is found that the radial distribution of the mean electron density for streamers at Cycle 24 maximum is only slightly larger (by approx. equal to 30%) than at cycle minimum.

Wang, Tongjiang↗
NASA NTRS1972

The 1969 solar occultation of the Crab Nebula pulsar.

Results of an occultation experiment carried out on June 14-15, 1969, when the Crab Nebula pulsar passed within 80 min of arc from the center of the sun. For a time interval of two months around this event comparisons are made of the reception times of pulses at frequencies of 111.5, 196.5, and 430 MHz for the purpose of determining the contribution of the solar corona to the integrated electron density along the total path length to the pulsar. For impact parameters between 5 and 12 solar radii the measured integrated densities are not incompatible with those that can be derived from a spherically symmetric Baumbach-Allen model. The systematic differences in density obtained at each observation frequency can be explained in terms of refraction effects.

Rankin, J. M.↗
NASA NTRS1978

Plasma waves near the sun: Advances possible with a solar probe

A review is presented of current experimental and theoretical knowledge of plasma waves in the solar wind, with comments on the scientific importance of obtaining plasma wave measurements in the region near the sun with the solar probe. The waves discussed include the acoustic waves and shocks which are thought to be the primary source for heating the solar corona, Alfven waves, ion-cyclotron waves, whistler-mode turbulence, ion-acoustic waves, and electron plasma oscillations associated with solar radio emissions. A discussion is presented of the types of measurements which would be needed to study these waves on the solar probe, the constraints imposed on the spacecraft and the research and development which would be needed to provide the necessary instrumentation.

Gurnett, D. A.↗
NASA NTRS1986

The composition of solar energetic particles

The present measurements of elemental abundances in 15 large solar energetic particle events confirm the existence of two major effects: systematic differences between solar energetic particle abundances and photospheric abundances that are approximately correlated with first ionization potential (implying that the solar corona is the likely source population for these particles), and an enhancement of heavy ion abundances relative to a baseline of solar energetic particle abundances whose magnitude increases with rising atomic number. These data suggest the possibility that the degree of heavy ion enhancement has some correlation with spectral slope, and confirm that the aforementioned effects are not due to time or energy variations within individual events. The maximum column density above the solar acceleration region is less than 0.1 g/sq cm, on the basis of these data.

Mcguire, R. E.↗
NASA NTRS1994

Coronal abundances and their variation

This contract supports the investigation of elemental abundances in the solar corona, principally through analysis of high-resolution soft x-ray spectra from the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission. The goals of the study are a characterization of the mean values of relative abundances of elements accessible in the FCS data, and information on the extent and circumstances of their variability. This report is a summation of the data analysis and reporting activities which occurred during the first ten months of the contract, 15 Jun. 1993 to 15 Apr. 1994.

Saba, Julia L. R.↗
NASA NTRS1994

Coronal abundances and their variation

This contract supports the investigation of elemental abundances in the solar corona, principally through analysis of high-resolution software X-ray spectra from the Flat Crystal Spectrometer on NASA's Solar Maximum Mission. The goals of the study are a characterization of the mean values of relative abundances of elements accessible in the FCS data, and information on the extent and circumstances of their variability. This report is a summation of the data analysis and reporting activities which occurred since the last report, submitted two months early, in April 1994, to facilitate evaluation of the first year's progress for contract renewal. Hence this report covers the period 15 April 1994 - 15 December 1994. A list of publications resulting from this research is included.

Saba, Julia L. R.↗
NASA NTRS1996

Coronal Abundances and Their Variation

This contract supported the investigation of elemental abundances in the solar corona, principally through analysis of high-resolution soft X-ray spectra from the Flat Crystal Spectrometer on NASA's Solar Maximum Mission. The goals of the study were a characterization of the mean values of relative abundances of elements accessible in the FCS data, and information on the extent and circumstances of their variability. This is the Final Report, summarizing the data analysis and reporting activities which occurred during the period of performance, June 1993 - December 1996.

Saba, Julia L. R.↗
DOE OSTI2020

Electron temperature of the solar wind

Significance The problem of solar wind heating is one of the long-standing fundamental problems of space plasma physics. We propose that the electron component of the solar wind plasma is heated by electrons streaming from the hot solar corona and slowly losing their energy due to weak Coulomb collisions. Our first-principle kinetic derivation predicts the scaling of the solar wind electron temperature with the heliocentric distance, which is in good agreement with observations. Our theory reveals the role of the non-Maxwellian electron distribution function in the heating mechanism, and it may be valuable for interpreting the results of in situ satellite measurements.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

On solar cosmic rays

The report summarizes the major results obtained over the last four years in the following areas: (1) transport in interplanetary space which includes solar particle observations combined with theoretical efforts to develop models which simulate the dynamics of the event; (2) transport in the solar corona and release of particles in the interplanetary medium; and (3) characteristics of source spectra and acceleration models. Modelling the propagation of solar cosmic rays through interplanetary space and establishing the magnitude and the radial and energy dependence of the diffusion coefficient K are considered. An important result due to statistical observation is that the coronal propagation must be independent of or weakly dependent on both rigidity and energy. Increasing evidence is found for prolonged injection at the sun.

Van Hollebeke, M. A. I.↗
NASA NTRS1999

Large-Scale Coronal Heating from the Solar Magnetic Network

In Fe 12 images from SOHO/EIT, the quiet solar corona shows structure on scales ranging from sub-supergranular (i.e., bright points and coronal network) to multi- supergranular. In Falconer et al 1998 (Ap.J., 501, 386) we suppressed the large-scale background and found that the network-scale features are predominantly rooted in the magnetic network lanes at the boundaries of the supergranules. The emission of the coronal network and bright points contribute only about 5% of the entire quiet solar coronal Fe MI emission. Here we investigate the large-scale corona, the supergranular and larger-scale structure that we had previously treated as a background, and that emits 95% of the total Fe XII emission. We compare the dim and bright halves of the large- scale corona and find that the bright half is 1.5 times brighter than the dim half, has an order of magnitude greater area of bright point coverage, has three times brighter coronal network, and has about 1.5 times more magnetic flux than the dim half These results suggest that the brightness of the large-scale corona is more closely related to the large- scale total magnetic flux than to bright point activity. We conclude that in the quiet sun: (1) Magnetic flux is modulated (concentrated/diluted) on size scales larger than supergranules. (2) The large-scale enhanced magnetic flux gives an enhanced, more active, magnetic network and an increased incidence of network bright point formation. (3) The heating of the large-scale corona is dominated by more widespread, but weaker, network activity than that which heats the bright points. This work was funded by the Solar Physics Branch of NASA's office of Space Science through the SR&T Program and the SEC Guest Investigator Program.

Falconer, David A.↗
NASA NTRS1999

Large-Scale Coronal Heating from "Cool" Activity in the Solar Magnetic Network

In Fe XII images from SOHO/EIT, the quiet solar corona shows structure on scales ranging from sub-supergranular (i.e., bright points and coronal network) to multi-supergranular (large-scale corona). In Falconer et al 1998 (Ap.J., 501, 386) we suppressed the large-scale background and found that the network-scale features are predominantly rooted in the magnetic network lanes at the boundaries of the supergranules. Taken together, the coronal network emission and bright point emission are only about 5% of the entire quiet solar coronal Fe XII emission. Here we investigate the relationship between the large-scale corona and the network as seen in three different EIT filters (He II, Fe IX-X, and Fe XII). Using the median-brightness contour, we divide the large-scale Fe XII corona into dim and bright halves, and find that the bright-half/dim half brightness ratio is about 1.5. We also find that the bright half relative to the dim half has 10 times greater total bright point Fe XII emission, 3 times greater Fe XII network emission, 2 times greater Fe IX-X network emission, 1.3 times greater He II network emission, and has 1.5 times more magnetic flux. Also, the cooler network (He II) radiates an order of magnitude more energy than the hotter coronal network (Fe IX-X, and Fe XII). From these results we infer that: 1) The heating of the network and the heating of the large-scale corona each increase roughly linearly with the underlying magnetic flux. 2) The production of network coronal bright points and heating of the coronal network each increase nonlinearly with the magnetic flux. 3) The heating of the large-scale corona is driven by widespread cooler network activity rather than by the exceptional network activity that produces the network coronal bright points and the coronal network. 4) The large-scale corona is heated by a nonthermal process since the driver of its heating is cooler than it is. This work was funded by the Solar Physics Branch of NASA's office of Space Science through the SR&T Program and the SEC Guest Investigator Program.

Falconer, D. A.↗
NASA NTRS2013

Fine-Scale Fluctuations in the Corona Observed with Hi-C

The High Resolution Coronal Imager(HiC) flew aboard a NASA sounding rocket on 2012 July11 and captured roughly 345 s of high spatial and temporal resolution images of the solar corona in a narrowband 193 Angstrom channel. We have analyzed the fluctuations in intensity of Active Region11520.We selected events based on a lifetime greater than 11s (twoHiC frames)and intensities greater than a threshold determined from the average background intensity in a pixel and the photon and electronic noise. We find fluctuations occurring down to the smallest timescale(~11s).Typical intensity fluctuations are 20% background intensity, while some events peaka t100%the background intensity.Generally the fluctuations are clustered in solar structures, particularly the moss.We interpret the fluctuations in the moss as indicative of heating events. We use the observed events to model the active region core.

Winebarger, Amy↗
NASA NTRS1974

High-energy particles associated with solar flares

High-energy particles, the so-called solar cosmic rays, are often generated in association with solar flares, and then emitted into interplanetary space. These particles, consisting of electrons, protons, and other heavier nuclei, including the iron-group, are accelerated in the vicinity of the flare. By studying the temporal and spatial varation of these particles near the earth's orbit, their storage and release mechanisms in the solar corona and their propagation mechanism can be understood. The details of the nuclear composition and the rigidity spectrum for each nuclear component of the solar cosmic rays are important for investigating the acceleration mechanism in solar flares. The timing and efficiency of the acceleration process can also be investigated by using this information. These problems are described in some detail by using observational results on solar cosmic rays and associated phenomena.

Sakurai, K.↗
NASA NTRS1985

Designing a micro-spacecraft

Planetary spacecraft design could move toward less complex probes which would cost less then previous highly instrumented missions. The goal then becomes to fly more frequent missions and use commercial, proven hardware to ameliorate development costs. A commonality would be kept in place from spacecraft to spacecraft, with upgrades being introduced only to meet specific objectives or take advantage of advances in commercial hardware. Mission costs are in large part determined by spacecraft mass, so instrumentation must be miniaturized, i.e., the concept of a micro-satellite. A design study for the Cosimi project, which would feature placing a spacecraft on the far side of the solar corona to broadcast radio signals to earth, demonstrates the feasibility of a 20 cm diam rocket and integrated instruments for performing low-cost solar physics experiments. It is concluded, however, that current program start-ups will continue to maximize the mass and instrumentation of spacecraft.

Burke, J. D.↗
NASA NTRS1988

Simultaneous VLA-satellite observations of the sun

The simultaneous use of VLA and satellite-borne telescopes to observe the quiescent and active corona with comparable resolution in time and space is discussed. VLA data at 90-cm specify closed and open magnetic stuctures in the low solar corona, providing information on the initiating source of coronal mass ejections in satellite coronagraphs. The study of quiescent coronal loops from combined soft X-ray and 20-cm VLA data is considered. The used of combined VLA-satellite data to investigate magnetic interaction and particle acceleration before and during flare is discussed.

Lang, Kenneth R.↗
NASA NTRS1996

Solar coronal structure: A comparison of NSO/SP ground-based coronal emission line intensities and temperatures with Yohkoh SXT and WSO magnetic data

The large-scale structure of the solar corona is investigated using synoptic maps produced from Fe XIV (530.3 nm), Fe X (637.4 nm) and Ca XV (569.4 nm) data obtained at National Solar Observatory (NSO/SP), Yohkoh/Soft X-ray Telescope (SXT) X-ray data and Wilcox Solar Observatory (WSO) 'source surface' maps. We find that the Fe XIV data are an excellent proxy for spatially-average Yohkoh/SXT data. Isolated emission features and large-scale structures are nearly identical in SXT and Fe XIV maps. In addition, coronal holes and other low-emission regions are very similar. Synoptic temperature maps, calculated from the Fe X/Fe XIV ratio, show a tendency for the highest temperatures to occur where the large-scale magnetic fields change polarity at high latitudes, while lower-latitude features, including active regions, have lower apparent tempertures. Regions of enhanced temperature generally follow the helisopheric current sheet (HCS) as defined by the WSO maps. Further, emission in Ca XV (formed at T is approximately equal to 3 MK), generally occurs only over low-latitude regions that are bright in both FE X (T approximately equal to 1 MK) and Fe XIV (T approximately equal to 2 MK). Thus, there is evidence for low (approximately equal to 1 MK), moderate (approximately equal to 2 MK) and high (approximately 3 MK) temperatures in close proximity in the low corona.

Altrock, R. C.↗