58 Matching Results

Search Results

Advanced search parameters have been applied.

A Statistical Study of Hard X-Ray Solar Flares

Description: The results of a statistical study of hard x-ray solar flares are presented in this dissertation. Two methods of analysis were used, the Diffusion Entropy (DE) method coupled with an analysis of the data distributions and the Rescaled Range (R/S) Method, sometimes referred to as "Hurst's method". Chapter one provides an introduction to hard x-ray flares within the context of the solar environment and a summary of the statistical paradigms solar astronomers currently work under. Chapter two presents the theory behind the DE and R/S methods. Chapter three presents the results of the two analysis methodologies: most notably important evidence of the conflicting results of the R/S and DE methods, evidence of a Levy statistical signature for the underlying dynamics of the hard x-ray flaring process and a possible separate memory signature for the waiting times. In addition, the stationary and nonstationary characteristics of the waiting times and peak intensities, are revealed. Chapter four provides a concise summary and discussion of the results.
Date: December 2001
Creator: Leddon, Deborah L.
Partner: UNT Libraries

On the Mechanism of Generation of Magneto-Hydrodynamic Whirl Rings in the Interior of the Sun and Their Relation to Sunspots, Faculae, Prominences, and Flares

Description: Abstract: "A possible mechanism for the generation of magneto-hydrodynamic whirl rings near the center of the sun is described. The organic relation of the reflection of these whirl rings at the surface of the sun to the cooling of sunspots, orientation, and polarity sequence of sunspots, prominences, flares, and faculae is delineated."
Date: March 29, 1955
Creator: Bostick, W. H.
Partner: UNT Libraries Government Documents Department

Science & Technology Review September 2005

Description: This month's issue has the following articles: (1) The Pursuit of Fusion Energy--Commentary by William H. Goldstein; (2) A Dynamo of a Plasma--The self-organizing magnetized plasmas in a Livermore fusion energy experiment are akin to solar flares and galactic jets; (3) How One Equation Changed the World--A three-page paper by Albert Einstein revolutionized physics by linking mass and energy; (4) Recycled Equations Help Verify Livermore Codes--New analytic solutions for imploding spherical shells give scientists additional tools for verifying codes; and (5) Dust That.s Worth Keeping--Scientists have solved the mystery of an astronomical spectral feature in interplanetary dust particles.
Date: July 19, 2005
Creator: Aufderheide III, M B
Partner: UNT Libraries Government Documents Department

SPACE VEHICLE SHIELDING STUDIES. CALCULATIONS OF THE ATTENUATION OF A MODEL SOLAR FLARE AND MONOENERGETIC PROTON BEAMS BY ALUMINUM SHIELDS

Description: Using the straight-ahead approximation, nucleon-meson cascade calculations have been carried out for a typical proton flare spectrum incident on a shield and for approximately monoenergetic incident proton beams. The shield material considered has approximately the properties of Al. The results from the monoenergetic beams may be used to obtain shielding information for any incident proton spectrum. (auth)
Date: January 23, 1963
Creator: Alsmiller, R.G. Jr. & Murphy, J.E.
Partner: UNT Libraries Government Documents Department

Magnetic reconnection in sheared solar magnetic arcades

Description: The evolution of solar magnetic arcades is investigated with the use of MHD simulations imposing resistivity on sheared magnetic fields. It is found that there is a critical amount of shear, over which magnetic reconnection can take place in an arcade-like field geometry to create a magnetic island. The process leading to reconnection cannot be solely attributed to a tearing instability, but rather to a reactive evolution of the magnetic arcade under resistivity. The natures of the arcade reconnection are governed by the spatial pattern of resistivity. A fast reconnection with a small shock angle can only be achieved when the diffusion region is localized. In this case, a highly collimated reconnection outflow can tear the plasmoid into a pair, and most of principal features in solar eruptive processes are reproduced.
Date: December 31, 1996
Creator: Choe, G.S.
Partner: UNT Libraries Government Documents Department

A model solar flares and their homologous behavior

Description: A model describing physical processes of solar flares and their homologous behavior is presented based on resistive MHD simulations of magnetic arcade evolution subject to continuous shear-increasing footpoint motions. It is proposed in the model that the individual flaring process encompasses magnetic reconnection of arcade field lines, generation of magnetic islands in the magnetic arcade, and coalescence of magnetic islands. When a magnetic arcade is sheared, a current sheet is formed and magnetic reconnection can take place to form a magnetic island. A continuing increase of magnetic shear can trigger a new reconnection process and create another island in the underlying arcade below the magnetic island. The newborn island rises faster than the preceding island and merges with it to form one island. Before merging with the upper island is completed, the newborn island exhibits two different phases of rising motion: the first phase with a slower rising speed and the second phase wit h a faster rising speed. This is consistent with the Yohkoh observation by Ohyama and Shibata (1998) of X-ray plasma ejecta motion. The first phase, in which reconnection of line-tied field in the underlying arcade is important, can be regarded to be related with the preflare phase. In the second phase, the island coalescence takes place, which creates an elongated current sheet below and enhances the reconnection rate of the line-tied arcade field. This phase is interpreted as the impulsive phase or the flash phase of flares. The obtained reconnection electric field is large enough to accelerate electrons to an energy level higher than 10 keV, which is necessary for observed X-ray emissions. After merging of the islands is completed, magnetic reconnection continues in the current sheet under the integrated island for rather a long period, which can be considered as the main phase of flares. ...
Date: January 27, 2000
Creator: Choe, G.S. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Simulation studies of acceleration of heavy ions and their elemental compositions; IFSR--755

Description: By using a one-dimensional, electromagnetic particle simulation code with full ion and electron dynamics, we have studied the acceleration of heavy ions by a nonlinear magnetosonic wave in a multi-ion-species plasma. First, we describe the mechanism of heavy ion acceleration by magnetosonic waves. We then investigate this by particle simulations. The simulation plasma contains four ion species: H, He, O, and Fe. The number density of He is taken to be 10% of that of H, and those of O and Fe are much lower. Simulations confirm that, as in a single-ion-species plasma, some of the hydrogens can be accelerated by the longitudinal electric field formed in the wave. Furthermore, they show that magnetosonic waves can accelerate all the particles of all the heavy species (He, O, and Fe) by a different mechanism, i.e., by the transverse electric field. The maximum speeds of the heavy species are about the same, of the order of the wave propagation speed. These are in good agreement with theoretical prediction. These results indicate that, if high-energy ions are produced in the solar corona through these mechanisms, the elemental compositions of these heavy ions can be similar to that of the background plasma, i.e., the corona.
Date: July 1, 1996
Creator: Toida, Mieko & Ohsawa, Yukiharu
Partner: UNT Libraries Government Documents Department

A NOTE ON IMPORTANCE FUNCTIONS FOR THE SHIELDING OF MANNED SPACE VEHICLES

Description: The tissue dose calculations of Kinney were used to calculate several importance functions associated with the dose from primary and secondary particles in the tissue due to penetration of an aluminum shield by a solar-flare proton spectrum. Secondary particles produced in the shield were not considered. (auth)
Date: March 1, 1964
Creator: Alsmiller, R.G. Jr.
Partner: UNT Libraries Government Documents Department

SPACE VEHICLE SHIELDING STUDIES. PART II. THE ATTENUATION OF SOLAR FLARES BY ALUMINUM SHIELDS

Description: Using the straight-ahead approximation, nucleon-meson cascade calculations were carried out for several solarflare proton spectra incident on a shield. The shield material has approximately the properties of aluminum. Both spherical-shell and slab geometries are considered. (auth)
Date: January 24, 1964
Creator: Alsmiller, R.G. Jr. & Murphy, J.E.
Partner: UNT Libraries Government Documents Department

Solar Energetic Particle Spectrum on 13 December 2006 Determined by IceTop

Description: The IceTop air shower array now under construction at the South Pole as the surface component of the IceCube neutrino telescope (Achterberg et al. 2006) detected an unusual near-solar-minimum Ground Level Enhancement (GLE) after a solar flare on 13 December 2006. Beginning at 0220 UT, the 4B class flare occurred at solar coordinates S06 W24, accompanied by strong (X3.4) X-ray emission and type II and IV radio bursts. The LASCO coronagraph on the SOHO spacecraft observed a halo CME launch from the Sun at {approx} 0225 UT with speed estimated to be {approx} 1770 km/s. We have begun (Bieber et al. 2007) a comprehensive analysis of the propagation of solar energetic particles in this event. However the focus of this Letter is the new and unique ability of IceTop to derive the energy spectrum of these particles in the multi-GeV regime from a single detector with a well defined viewing direction. When completed, IceTop will have approximately 500 square meters of ice Cherenkov collecting area arranged in an array of 80 stations on a 125 m triangular grid to detect air showers from one PeV to one EeV. Each station consists of two, two meter diameter tanks filled with ice to a depth of 90 cm. Tanks are instrumented with two Digital Optical Modules (DOM) operated at different gain settings to provide appropriate dynamic range to cover both large and small air showers. Each DOM contains a 10 inch photomultiplier and an advanced readout system capable of digitizing the full waveform. For historical reasons, the two discriminator counting rates recorded in each DOM are termed SPE (Single Photo Electron), and MPE (Multi Photo Electron). In the present analysis the SPE threshold corresponds approximately to 20 photoelectrons (PE), and the MPE threshold to 100 PE. Due to the high altitude (2835m) ...
Date: October 11, 2008
Creator: Collaboration, IceCube & Klein, Spencer
Partner: UNT Libraries Government Documents Department

The sun as a variable star: solar and stellar irradiance variations. Final report

Description: The main objective of this Colloquium was to review the most recent results on the observations, theoretical interpretations, empirical and physical models of the variations observed in solar and stellar irradiances, as well as on Sun-climate connections. The Colloquium was divided into six sessions as defined by the key topics. Included for each session were the 36 invited talks and 110 contributed poster papers. A special session of the Solar Electromagnetic Radiation Study for Solar Cycle 22 (SOLERS22) was held on June 25, 1993, where the five working groups discussed their progress and future plans on measuring the absolute value of solar total and spectral irradiances and studying their temporal variations. Papers on both theoretical models and solar irradiance observations have shown that the solar energy output changes on different time scales: the short-term (from minutes to months) variations are related to surface modulations mainly caused by the evolution of active regions, the solar cycle related long-term variations are directly linked with the evolution of magnetic fields over the activity cycle, while the secular variations over centuries are associated with long-term modulations.
Date: December 31, 1994
Creator: Pap, J.M.
Partner: UNT Libraries Government Documents Department

Solar flare mechanism based on magnetic arcade reconnection and island merging

Description: The authors propose a model describing physical processes of solar flares based on resistive reconnection of magnetic field subject to continuous increase of magnetic shear in the arcade. The individual flaring process consists of magnetic reconnection of arcade field lines, generation of magnetic islands in the magnetic arcade, and coalescence of magnetic islands. When a magnetic arcade is sheared (either by foot point motion or by flux emergence), a current sheet is formed and magnetic reconnection can take place to form a magnetic island. A continuing increase of magnetic shear can trigger a new reconnection process and create a new island in the under lying arcade below the magnetic island. The new born island rises faster than the preceding island and merges with it to form one island. Before completing the island merging process, the new born island exhibits two phases of rising motion: a first phase with a slower rising speed and a second phase with a faster rising speed. The flare plasma heating occurs mainly due to magnetic reconnection in the current sheet under the new born island. The new born island represents the X-ray plasma ejecta which shows two phases of rising motion observed by Yohkoh [Ohyama and Shibata (1997)]. The first phase with slower new born island rising speed corresponds to the early phase of reconnection of line-tied field in the underlying current sheet and is considered as the preflare phase. In the second phase, the island coalescence takes place, and the underlying current sheet is elongated so that the line-tied arcade field reconnection rate is enhanced. This phase is interpreted as the impulsive phase or the flash phase of flares. The obtained reconnection electric field is large enough to accelerate electrons to an energy level higher than 10 keV, which is necessary for observed hard ...
Date: June 15, 2000
Creator: Chen, C.Z. & Choe, G.S.
Partner: UNT Libraries Government Documents Department

Fundamental Magnetofluid Physics Studies on the Swarthmore Spheromak Experiment: Reconnection and Sustainment

Description: The general goal of the Magnetofluids Laboratory at Swarthmore College is to understand how magnetofluid kinetic energy can be converted to magnetic energy as it is in the core of the earth and sun (the dynamo problem) and to understand how magnetic energy can be rapidly converted back to kinetic energy and heat as it is in solar flares (the magnetic reconnection problem). Magnetic reconnection has been studied using the Swarthmore Spheromak Experiment (SSX) which was designed and built under this Junior Faculty Grant. In SSX we generate and merge two rings of magnetized plasma called spheromaks and study their interaction. The spheromaks have many properties similar to solar flares so this work is directly relevant to basic solar physics. In addition, since the spheromak is a magnetic confinement fusion configuration, issues of formation and stability have direct impact on the fusion program.
Date: July 31, 2001
Creator: Brown, M.R.
Partner: UNT Libraries Government Documents Department

Gamma Ray Bursts And Data Challenge One: Searching GRB in One Week of Simulated GLAST LAT Data

Description: GLAST (Gamma-ray Large Area Space Telescope) is a gamma-ray astronomy mission that will be launched in mid 2007. The main instrument is the LAT (Large Area Telescope), a pair conversion telescope with sensitivity in the range 20 MeV-300 GeV. Data Challenge One (DC1) was the simulation of one week of observation of the entire gamma-ray sky by the LAT detector. the simulated data was similar to the real data, which allowed for the development of scientific software. In this paper they present the GRB simulations and the detection algorithms developed by the GLAST GRB and Solar Flare Science Team.
Date: February 22, 2006
Creator: Longo, F.; Omodei, N.; Band, D.; Bonnell, J.T.; Brigida, M.; Cohen-Tanugi, J. et al.
Partner: UNT Libraries Government Documents Department

A Model of Solar Flares Based on Arcade Field Reconnection and Merging of Magnetic Islands

Description: Solar flares are intense, abrupt releases of energy in the solar corona. In the impulsive phase of a flare, the intensity of hard X-ray emission reaches a sharp peak indicating the highest reconnection rate. It is often observed that an X-ray emitting plasma ejecta (plasmoid) is launched before the impulsive phase and accelerated throughout the phase. Thus, the plasmoid ejection may not be an effect of fast magnetic reconnection as conventionally assumed, but a cause of fast reconnection. Based on resistive magnetohydrodynamic simulations, a solar flare model is presented, which can explain these observational characteristics of flares. In the model, merging of a newly generated magnetic island and a pre-existing island results in stretching and thinning of a current sheet, in which fast magnetic reconnection is induced. Recurrence of homologous flares naturally arises in this model. Mechanisms of magnetic island formation are also discussed.
Date: December 12, 2001
Creator: Choe, G.S. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Ion acceleration and direct ion heating in three-component magnetic reconnection

Description: Ion acceleration and direct ion heating in magnetic reconnection are experimentally observed during counterhelicity merging of two plasma toroids. Plasma ions are accelerated up to order of the Alfen speed through contraction of the reconnected field-lines with three-components. The large increase in ion thermal energy (from 10 eV up to 200 eV) is attributed to the direct conversion of the magnetic energy into the unmagnetized ion population. This observation is consistent with the magnetohydrodynamic and macro-particle simulations.
Date: March 1, 1996
Creator: Ono, Y.; Yamada, M. & Akao, T.
Partner: UNT Libraries Government Documents Department

Formation of current sheets in magnetohydrostatic atmospheres (MHS)

Description: It is demonstrated that a 2-D magnetic field configuration in a magnetohydrostatic equilibrium without any null point can be deformed into a configuration with current sheets, i.e., tangential discontinuities, either by temperature change or by footpoint displacement. The magnetohydrostatic solutions by Low, which have a quadrupolar field geometry, are chosen as our initial configurations. When the whole atmosphere is uniformly heated, the expansion of plasma is more effective in the outer flux tubes than in the inner ones. The expanding plasma pushes out the field lines in each bipolar region so that a current sheet of a finite length is formed where the lines from each region come into contact. The resulting pressure profile at the base has pressure maxima at the center of each bipolar regions. The smooth equilibrium solution with the same pressure distribution contains and X-point. If the pressure is initially higher in the outer tubes than in the inner ones, cooling of the atmosphere can also lead to current sheet formation. As the pressure scale height decreases by cooling, the magnetic field pressure dominates the plasma pressure in the upper part of the flux tubes. The subsequent expansion of field lines creates a tangential discontinuity. If resistivity is considered in this weak equilibrium state, magnetic reconnection results in a new Kippenhahn-Sch{umlt u}ITER type field configuration with a magnetic island. It is expected that a prominence can stably reside within the magnetic island. When the field footpoints undergo a shearing motion with a continuous shearing profile, a current sheet can be reformed beyond a critical amount of shear. Our results suggest that the formation of a current sheet and the subsequent magnetic reconnection can be ubiquitous in the solar atmosphere. The resulting field configurations are quite favorable for prominence formation.
Date: December 31, 1996
Creator: Choe, G.S. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Composition and energetics of solar flare particle events measured by satellites, 1989--1991

Description: The Synchronous Orbit Particle Analyzer (SOPA), on board the satellite 1989-046 and others, has detected ions from carbon through nickel at energies from 2 to 50 MeV in the great solar energetic particle events of the current solar cycle. Energetic protons from the same events have been detected by the Charged Particle Analyzer (CPA) on board the satellite 1984-129 and others. We present here a collection of data from these various instruments that includes events of 1989, 1990, and 1991. We demonstrate the association of the events detected by the satellites with solar flares, and examine local solar wind features that in some instances alter the flux. We determine the ionic composition of the events, and compare these compositions among the various events and with those events, and compare these compositions among the various events and with those found in events of previous solar cycles. We obtain time-histories of the energetic particle fluxes, resolved both by ion species and by energy range. These detailed histories are of use, in conjunction with other data, in determining the parameters of the acceleration region.
Date: January 1, 1991
Creator: Gisler, G.R.; Belian, R.D.; Cayton, T.E. & Reeves, G.D.
Partner: UNT Libraries Government Documents Department

A simple model for the stellar analogy of compact solar flares

Description: We have developed a simple point'' model to describe the average thermodynamical properties of a compact flare loop as a function of time during the flare decay phase. The model includes thermal conduction, chromospheric evaporation, and radiative losses; moreover, it assumes lateral (gas + magnetic) pressure balance with the background corona at all times. For the case of a low-{beta} plasma (rigid flux tube), detailed 1-D hydrodynamical simulations are available in the literature for comparison; we show that the temporal variation of average loop properties predicted by the point model are in good agreement with these numerical simulations for a loop with the same energy input. 1 ref., 1 fig.
Date: January 1, 1989
Creator: Kopp, R.A.; Poletto, G. (Los Alamos National Lab., NM (USA) & Osservatorio Astrofisico di Arcetri, Florence (Italy))
Partner: UNT Libraries Government Documents Department

High-resolution measurements, line identification, and spectral modeling of K{alpha} transitions in Fe XVIII-XXV

Description: The iron K{alpha} emission spectrum covering the wavelength region from 1.840 to 1.940 {Angstrom} is analyzed. Measurements are made with a high-resolution Bragg crystal spectrometer on the Princeton Large Torus (PLT) tokamak for plasma conditions which closely resemble those of solar flares. A total of 40 features are identified consisting of either single or multiple lines from eight charge states in iron, Fe XVIII through Fe XXV, and their wavelengths are determined with an accuracy of 0.1--0.4 m{Angstrom}. Many of these features are identified for the first time. In the interpretation of our observations we rely on model calculations that determine the ionic species abundances from electron density and temperature profiles measured independently with non-spectroscopic techniques and that incorporate theoretical collisional excitation and dielectronic recombination rates resulting in the excitation of the 1s2s{sup r}2p{sup s} configurations. The model calculations also include the effect of diffusive ion transport. Good overall agreement between the model calculations and the observations is obtained, which gives us confidence in our line identifications and spectral modeling capabilities. The results are compared with earlier analyses of the K{alpha} emission from the Sun. While many similarities are found, a few differences arise from the somewhat higher electron density in tokamak plasmas (10{sup 13} cm{sup {minus}3}), which affects the fine-structure level populations of the ground states of the initial ion undergoing electron-impact excitation or dielectronic recombination. We also find that several spectral features are comprised of different transitions from those reported in earlier analyses of solar data.
Date: November 1, 1992
Creator: Beiersdorfer, P.; Phillips, T.; Jacobs, V. L.; Hill, K. W.; Bitter, M.; von Goeler, S. et al.
Partner: UNT Libraries Government Documents Department

Collective acceleration in solar flares

Description: Solar flare data are examined with an eye to seeing if they suggest collective acceleration of ions. That, in fact, seems to be the case. The collective acceleration mechanism of Gershtein is reviewed and the possibilities of the mechanism are discussed.
Date: November 1, 1993
Creator: Barletta, W.; Sessler, A. M.; Xie, M.; Gershtein, S. S.; Krishan, V. & Reiser, M.
Partner: UNT Libraries Government Documents Department