34 Matching Results

Search Results

Advanced search parameters have been applied.

Coronal structure inferred from remote sensing observations

Description: Remote-sensing observations of the Sun and inner heliosphere are reviewed to appraise our understanding of the mix of the mechanisms that heat the corona and accelerate the solar wind. An assessment of experimental uncertainties and the basic assumptions needed to translate measurables into physical models, reveals very large fundamental uncertainties in our knowledge of coronal structure near the Sun. We develop a time-dependent, filamentary model of the extended corona that is consistent with a large number of remote sensing observations of the solar atmosphere and the solar wind.
Date: September 1, 1996
Creator: Feldman, W.C.
Partner: UNT Libraries Government Documents Department

Physical nature of the low-speed solar wind

Description: In situ observations indicate that the low-speed wind is highly variable. It commonly originates on open field lines that thread coronal streamers in the vicinity of the magnetic equator, but transient ejections are also a source of low-speed flows on occasion. Close to the Sun a large flow shear probably is common at the interface between low- and high-speed flows. Near solar activity minimum low-speed flows are confined to a narrow band 40-45{degree} wide centered roughly on the solar equator, but near solar maximum low-speed flows may dominate at all heliographic latitudes.
Date: September 1, 1996
Creator: Gosling, J.T.
Partner: UNT Libraries Government Documents Department

A density-temperature description of the outer electron radiation belt during geomagnetic storms

Description: Electron flux measurements from 7 satellites in geosynchronous orbit from 1990-2007 are fit with relativistic bi-Maxwellians, yielding a number density n and temperature T description of the outer electron radiation belt. For 54.5 spacecraft years of measurements the median value ofn is 3.7x10-4 cm-3 and the median value ofT is 142 keY. General statistical properties of n, T, and the 1.1-1.5 MeV flux J are investigated, including local-time and solar-cycle dependencies. Using superposed-epoch analysis triggered on storm onset, the evolution of the outer electron radiation belt through high-speed-steam-driven storms is investigated. The number density decay during the calm before the storm is seen, relativistic-electron dropouts and recoveries from dropout are investigated, and the heating of the outer electron radiation belt during storms is examined. Using four different triggers (SSCs, southward-IMF CME sheaths, southward-IMF magnetic clouds, and minimum Dst), CME-driven storms are analyzed with superposed-epoch techniques. For CME-driven storms an absence of a density decay prior to storm onset is found, the compression of the outer electron radiation belt at time of SSC is analyzed, the number-density increase and temperature decrease during storm main phase is seen, and the increase in density and temperature during storm recovery phase is observed. Differences are found between the density-temperature and the flux descriptions, with more information for analysis being available in the density-temperature description.
Date: January 1, 2009
Creator: Borovsky, Joseph E; Cayton, Thomas E & Denton, Michael H
Partner: UNT Libraries Government Documents Department

Temporal observations of the lambda5303 emission line profile during the 74 minute totality from the Concorde SST at the 30 June 1973 total solar eclipse: preliminary intensity variations above an active region

Description: From international astronomical union symposium on coronal disturbances; Surfers Paradise, Australia (3 Sep 1973). Apparatus was designed for installation on the Concorde SST to provide high resolution emission line profiles at the June 30, 1973 total solar eclipse. A prime objective was to obtain profiles that could be used to detect the coronal response to the 5 min periodicities observed in the solar photosphere. Stability of the sky scattered light, drastic reduction of seeing effects enhanced the value of the exceptional opportunity. Preliminary results are discussed for a single region on the limb, above an active sunspot group. (auth)
Date: August 28, 1973
Creator: Liebenberg, D.H. & Hoffman, M.M.
Partner: UNT Libraries Government Documents Department

Spatial evolution of 26-day recurrent galactic cosmic ray decreases: Correlated Ulysses COSPIN/KET and SOHO COSTEP observations

Description: In December 1995 the Ulysses spacecraft was at a radial distance of 3 AU from the Sun and 60{degree} northern heliographic latitude. To that time the Solar and Heliospheric Observatory (SOHO) started its mission. On board of both spacecraft particle sensors are measuring electrons, protons and helium nuclei in the MeV to GeV energy range. In early 1996 the counting rates of several hundred MeV galactic cosmic rays at Ulysses and at SOHO (Earth orbit) were modulated by recurrent cosmic ray decreases (RCRDs). The RCRDs at SOHO were found to be associated with a corotating interaction region (CIRs). A Lomb (spectral) analysis was performed on the galactic cosmic ray flux from February 1996 to June 1996. Surprisingly, the most probable frequency is {approximately} 28 days and not 26 or 27 days, corresponding to one solar rotation. The amplitude of the RCRDs is {approximately} 2.3% on both spacecraft. The variation in the solar wind speed shows the same periodicities and is anticorrelated to the variation in the cosmic ray flux. In contrast to the RCRDs the amplitude found in the solar wind speed is four times larger at WIND (120 km/s) than at Ulysses (32 km/s). The solar wind proton density and magnetic field strength yielded no significant periodicities, neither at Ulysses nor at WIND. Comparing the RCRDs with coronal hole structures observed in the FE XIV line, they found that a single coronal hole close to the heliographic equation can account for the RCRDs observed simultaneously at Ulysses and SOHO. The coronal hole boundaries changed towards lower Carrington longitudes and vanished slowly. The changes of the boundaries during the investigated period could explain a 28 day periodicity.
Date: December 31, 1998
Creator: Heber, B.; Bothmer, V. & Droege, W.
Partner: UNT Libraries Government Documents Department

Transient aspects of stream interface signatures

Description: Although stream interfaces are steady-state, corotating boundaries between slow and fast solar wind, their signatures are sometimes associated with transient features. Here the authors illustrate two modes of association: interfaces trailing interplanetary coronal mass ejections (ICMEs) at 1 AU and interfaces within ICMEs in the range 4--5 AU. The former are readily understood as boundaries between transient slow wind and steady-state fast wind, where the ICMEs add variability to the interface signatures. The latter are puzzling and may be related to evolution of interfaces.
Date: June 1, 1999
Creator: Crooker, N.U.; Shodhan, S.; Forsyth, R.J.; Burton, M.E.; Gosling, J.T.; Fitzenreiter, R.J. et al.
Partner: UNT Libraries Government Documents Department

The effects of coronal mass ejection on galactic cosmic rays in the high latitude heliosphere: Observations from Ulysses` first orbit

Description: During its first solar orbit the Ulysses spacecraft detected several coronal mass ejections (CMEs) at high heliographic latitudes. The authors present first observations on the effects of these high latitude CMEs on galactic cosmic rays (GCRs) using measurements from the Kiel Electron Telescope (KET) which is part of the Cosmic Ray and Solar Particle Investigation (COSPIN) experiment, the Los Alamos SWOOPS (Solar Wind Observations Over the Poles of the Sun) experiment and the magnetic field experiments. They find the passage of these CMEs over the spacecraft to be associated with short term decreases of GCR intensities The relatively weak shocks in these events, driven by the CMEs` over-expansion, had no strong influence on the GCRs. The intensity minimums of GCRs occurred on closed magnetic field lines inside the CMEs themselves as indicated by bidirectional fluxes of suprathermal electrons. Short episodes of intensity increases of GCRs inside CMEs at times when the bidirectional fluxes of suprathermal electrons disappeared, can be interpreted as evidence that GCRs can easily access the interior of those CMEs in which open magnetic field lines are embedded.
Date: October 1997
Creator: Bothmer, V.; Heber, B.; Kunow, H.; Mueller-Mellin, R.; Wibberenz, G.; Gosling, J. T. et al.
Partner: UNT Libraries Government Documents Department

Effects of the 5 October 1996 CME at 4.4 AU: Ulysses observations

Description: The authors present observations from Ulysses associated with a large coronal mass ejection (CME) that lifted off the west limb of the Sun on 5 October, 1996. The study focuses on the effects of the interplanetary counterpart of the CME on the energetic particle populations at the location of Ulysses, in particular the effect on the sequence of corotating enhancements that had been observed prior to its arrival. They conclude that, despite its large spatial extent, the CME caused no permanent deformation of the heliospheric current sheet.
Date: September 1, 1997
Creator: Marsden, R.G.; Desai, M.I.; Sanderson, T.R.; Forsyth, R.J. & Gosling, J.T.
Partner: UNT Libraries Government Documents Department

Interference of solar-probe inherent atmosphere with in-situ observations

Description: The solar corona is the source of solar wind that leads to the existence of the heliosphere and plays a crucial role in solar terrestrial phenomena. A comprehensive understanding of these phenomena can be provided only by directly measuring ion and electron velocity distributions, plasma waves, and fluxes of energetic particles. The problem presented by the inherent atmosphere of a spacecraft moving in the vicinity of the sun (4--20)R{sub {circle_dot}} and its influence on in-situ measurements of the solar corona plasma is the key to the realization and success of any solar probe mission. Models are developed to study and evaluate the effect of the inherent atmosphere on the in-situ measurements of future solar probes.
Date: January 1, 1998
Creator: Hassanein, A.; Alekseev, V.A.; Konkashbaev, A.I.; Konkashbaev, I.K. & Nikandrov, L.B.
Partner: UNT Libraries Government Documents Department

Magnetic topologies of coronal mass ejection events: Effects of 3-dimensional reconnection

Description: New magnetic loops formed in the corona following coronal mass ejection, CME, liftoffs provide strong evidence that magnetic reconnection commonly occurs within the magnetic ``legs`` of the departing CMEs. Such reconnection is inherently 3-dimensional and naturally produces CMEs having magnetic flux rope topologies. Sustained reconnection behind CMEs can produce a mixture of open and disconnected field lines threading the CMES. In contrast to the results of 2-dimensional reconnection. the disconnected field lines are attached to the outer heliosphere at both ends. A variety of solar and solar wind observations are consistent with the concept of sustained 3-dimensional reconnection within the magnetic legs of CMEs close to the Sun.
Date: September 1, 1995
Creator: Gosling, J.T.
Partner: UNT Libraries Government Documents Department

New insights on geomagnetic storms from observations and modeling

Description: Understanding the response at Earth of the Sun's varying energy output and forecasting geomagnetic activity is of central interest to space science, since intense geomagnetic storms may cause severe damages on technological systems and affect communications. Episodes of southward (Bz<O) interplanetary magnetic field (IMF) which lead to disturbed geomagnetic conditions are associated either with coronal mass ejections (CMEs) and possess long and continuous negative IMF Bz excursions, or with high speed solar wind streams (HSS) whose geoeffectiveness is due to IMF Bz profiles fluctuating about zero with various amplitudes and duration. We show examples of ring current simulations during two geomagnetic storms representative of each interplanetary condition with our kinetic ring current atmosphere interactions model (RAM), and investigate the mechanisms responsible for trapping particles and for causing their loss. We find that periods of increased magnetospheric convection coinciding with enhancements of plasma sheet density are needed for strong ring current buildup. During the HSS-driven storm the convection potential is highly variable and causes small sporadic injections into the ring current. The long period of enhanced convection during the CME-driven storm causes a continuous ring current injection penetrating to lower L shells and stronger ring current buildup.
Date: January 1, 2009
Creator: Jordanova, Vania K
Partner: UNT Libraries Government Documents Department

In situ measurement requirements for a solar probe

Description: The authors present the rationale and in situ measurement requirements for a near-Sun mission intended to answer the central questions of the heating of the corona and the acceleration of the solar wind. These conclusions are based on panel discussions and presentations at the Marlboro workshop. They have in mind not a minimum mission, but rather one that is constrained but feasible within the current mass and telemetry rate restrictions. To distinguish between thermal, wave-driven, and microflare-driven models, the measurements must determine wave levels in a broad range of frequencies, resolve fine-scale structures, find the energetic particle content and its variations, and determine the bulk properties of a few species with detailed distributions for at least electrons and protons. They find that the in situ measurements needed to answer the main questions are similar to those proposed previously (magnetic field, plasma, high-energy particles, and plasma wave instruments) but without neutron and dust experiments. Telemetry and mass constraints will be significant but should not be prohibitive.
Date: September 1, 1996
Creator: Roberts, D.A. & Gosling, J.T.
Partner: UNT Libraries Government Documents Department

Strongly nonlinear magnetosonic waves and ion acceleration

Description: The electromagnetic fields associated with a nonlinear compressional Alfven wave propagating perpendicular to an external magnetic field of arbitrary strength are derived. For the strongly magnetized and high phase velocity case relevant for ion acceleration to high energies, we show that the electric field increases proportionally only to the external magnetic field O (B{sub ext}[in T] MV/cm) and the electrostatic potential increases with the square root of the ion-to-electron mass ratio {radical}M{sub i}/m{sub e}.
Date: November 1, 1997
Creator: Rau, B. & Tajima, T.
Partner: UNT Libraries Government Documents Department

Influence of solar-probe inherent atmosphere on in-situ observations

Description: The solar corona is the source of the solar wind, which is responsible for the heliosphere and plays a crucial role in solar/terrestrial phenomena. A comprehensive understanding of these phenomena can be established only by directly measuring ion and electron velocity distributions, plasma waves, and fluxes of energetic particles near the sun. The problem resulting from the inherent atmosphere of a spacecraft moving in the vicinity of the sun and the influence of this atmosphere on in-situ measurements of the solar corona plasma is key to the realization and success of any solar probe mission. To evaluate the influence of the probe-inherent atmosphere on in-situ observations, the authors have developed comprehensive radiation hydrodynamic models. The physics of plasma/probe/vapor interaction are also being developed in a self-consistent model to predict the effect of probe inherent atmosphere on in-situ measurements of corona parameters during solar flares. Interaction of the ionized atmosphere with the ambient natural plasma will create a turbulent shock wave that can affect in-situ measurements and must be taken into account in designing the spacecraft and its scientific components.
Date: August 1, 1998
Creator: Hassanein, A.; Konkashbaev, A.I.; Konkashbaev, I.K. & Nikandrov, L.B.
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

Experimental study of ion heating and acceleration during magnetic reconnection

Description: This dissertation reports an experimental study of ion heating and acceleration during magnetic reconnection, which is the annihilation and topological rearrangement of magnetic flux in a conductive plasma. Reconnection is invoked often to explain particle heating and acceleration in both laboratory and naturally occurring plasmas. However, a simultaneous account of reconnection and its associated energy conversion has been elusive due to the extreme inaccessibility of reconnection events, e.g. in the solar corona, the Earth's magnetosphere, or in fusion research plasmas. Experiments for this work were conducted on MRX (Magnetic Reconnection Experiment), which creates a plasma environment allowing the reconnection process to be isolated, reproduced, and diagnosed in detail. Key findings of this work are the identification of local ion heating during magnetic reconnection and the determination that non-classical effects must provide the heating mechanism. Measured ion flows are sub-Alfvenic and can provide only slight viscous heating, and classical ion-electron interactions can be neglected due to the very long energy equipartition time. The plasma resistivity in the reconnection layer is seen to be enhanced over the classical value, and the ion heating is observed to scale with the enhancement factor, suggesting a relationship between the magnetic energy dissipation mechanism and the ion heating mechanism. The observation of non-classical ion heating during reconnection has significant implications for understanding the role played by non-classical dissipation mechanisms in generating fast reconnection. The findings are relevant for many areas of space and laboratory plasma research, a prime example being the currently unsolved problem of solar coronal heating. In the process of performing this work, local measurements of ion temperature and flows in a well-characterized reconnection layer were obtained for the first time in either laboratory or observational reconnection research. Furthermore, much progress was made in understanding the reconnection process itself.
Date: January 28, 2000
Creator: Hsu, S.C.
Partner: UNT Libraries Government Documents Department

Laboratory astrophysics: Measurements of n = n{prime} to n = 2 line emission in Fe{sup 16+} to Fe{sup 23+}

Description: One of the dominant forms of astronomical line emission in the 6 {angstrom} to 18 {angstrom} spectral region is line emission produced by n = n{prime} to n = 2 transitions in Fe{sup 16+} to Fe{sup 23+} (i.e., Fe L-shell n-2 line emission). Using the Lawrence Livermore National Laboratory electron beam ion trap (EBIT) facility, the authors have carried out a number of measurements designed to address astrophysical issues concerning Fe L-shell line emission. Desired ions are produced and trapped using the nearly monoenergetic electron beam of EBIT. Trapped ions are collisionally excited and the resulting X-ray line emission detected using Bragg crystal spectrometers. They have recently completed a line survey of Fe L-shell 3-2 line emission. The line survey will allow a more reliable accounting of line blending in astronomical spectra. They have now begun a series of broadband, high resolution line ratio measurements. These measurements are designed to benchmark atomic calculations used in astronomical plasma emission codes and also for comparison with X-ray spectral observations of astronomical objects. Initial measurements have been carried out in Fe{sup 23+}. Preliminary results agree with distorted wave calculations to within 20% and better.
Date: March 5, 1996
Creator: Savin, D.W.; Beiersdorfer, P.; Lopez-Urrutia, J.C.; Decaux, V.; Liedahl, D.A.; Reed, K.J. et al.
Partner: UNT Libraries Government Documents Department


Description: It is suggested that corpuscular radiation is hydrodynamic expansion of the solar corona which is called solar wind. Arguments are presented for this expansion of the solar corona in terms of motion of the comet tail aurorae production, magnetic storms, and cosmic-ray effects on earth. The corona is taken to be isotherrnal out to an undefined distance (in multiples of the radius of the base of the corona) and from there it expands adiabatically until supersonic velocities are reached at infinity. The solar wind is considered to continue after five astronomical units on the spiralling solar magnetic field. Applications of the solar wind hypothesis to solar sailing are discussed. (C.J.G.)
Date: November 1, 1959
Creator: Parker, E.N.
Partner: UNT Libraries Government Documents Department

Solar wind flows associated with hot heavy ions

Description: Solar wind heavy ion spectra measured with the Vela instrumentation have been studied with the goal of determining the solar origins of various solar wind structures which contain anomalously high ionization states. Since the ionization states freeze-in close to the sun they are good indicators of the plasma conditions in the low and intermediate corona. Heavy ion spectra from three different periods throughout the solar cycle have been analyzed. These data are consistent with freezing-in temperatures ranging from approx. 1.5 x 10/sup 6/ K to higher than 9 x 10/sup 6/. The spectra indicating hot coronal conditions occur in roughly 1/7 of all measurements and almost exclusively in postshock flows (PSFs), nonshock related helium abundance enhancements (HAEs), or noncompressive density enhancements (NCDEs). The PSFs and HAEs are both probably interplanetary manifestations of solar flares. The observation of several flare-related HAEs which were not preceded by an interplanetary shock suggests that the flare-heated plasma can evolve into the solar wind without producing a noticeable shock at 1 AU. The NCDEs with hot heavy ions differ from the PSF-HAEs in several ways implying that they evolve from events or places with lower temperatures and less energy than those associated with the flares, but with higher temperatures and densities than the quiet corona. Active regions, coronal mass ejections, and equatorial streamers are possible sources for the NCDEs with spectra indicating hot coronal conditions. These events owe their enhanced densities to coronal processes as opposed to interplanetary dynamical processes. Models of the solar wind expansion demonstrate how some NCDEs can have extreme, nonequilibrium ionization distributions.
Date: May 1, 1980
Creator: Fenimore, E.E.
Partner: UNT Libraries Government Documents Department

Application of the MHD energy principle to magnetostatic atmospheres

Description: We apply the MHD energy principle to the stability of a magnetized atmosphere which is bounded below by much denser fluid, as is the solar corona. We treat the two fluids as ideal; the approximation which is consistent with the energy principle, and use the dynamical conditions that must hold at a fluid-fluid interface to show that if vertical displacements of the lower boundary are permitted, then the lower atmosphere must be perturbed as well. However, displacements which do not perturb the coronal boundary can be properly treated as isolated perturbations of the corona alone.
Date: November 1, 1984
Creator: Zweibel, E.G.
Partner: UNT Libraries Government Documents Department

Disconnection of open coronal magnetic structures

Description: We have examined the Solar Maximum Mission coronagraph/polarimeter observations for evidence of magnetic disconnection of previously open magnetic structures and a number of likely examples have been found. Probable coronal disconnections typically appear as pinching off of helmet streamers followed by the release and outward acceleration of a large U or V-shaped structures. The observed sequence of events is consistent with reconnection across the heliospheric current sheet between previously open magnetic field regions, and the creation of a detached magnetic structure which is open to interplanetary space at both ends. Sunward of the reconnection point, coronal disconnection events would return previously open magnetic flux to the Sun as closed field arches. Here we (1) describe one clear disconnection event (1 June 1989); (2) examine the results of a limited survey of disconnection events; and (3) discuss the potential importance of coronal disconnections for maintaining flux in interplanetary space. 7 refs., 3 figs.
Date: January 1, 1991
Creator: McComas, D.J.; Phillips, J.L.; Hundhausen, A.J. & Burkepile, J.T.
Partner: UNT Libraries Government Documents Department

Large-scale electric fields resulting from magnetic reconnection in the corona

Description: The method of Forbes and Priest (2-D model) is applied to the large two-ribbon flare of July 29, 1973, for which both detailed H..cap alpha.. observations and magnetic data are available. For this flare the ribbons were long, nearly straight, and parallel to each other, and the 2-D model for the coronal field geometry may be adequate. The temporal profile E(t) is calculated and indicates that reconnection sets in at the beginning of the decay phase. From this time the electric field grows rapidly to a maximum value of about 2 V/cm within just a few minutes. Thereafter the decreases monotonically with time. 14 refs., 1 figs. (WRF)
Date: January 1, 1986
Creator: Kopp, R.A. & Poletto, G.
Partner: UNT Libraries Government Documents Department