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  • 1
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    Numerical simulation of solar coronal magnetic fields (1990)
    In:  CASI
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: Many aspects of solar activity are believed to be due to the stressing of the coronal magnetic field by footpoint motions at the photosphere. The results are presented of a fully spectral numerical simulation which is the first 3-D time dependent simulation of footpoint stressing in a geometry appropriate for the corona. An arcade is considered that is initially current-free and impose a smooth footpoint motion that produces a twist in the field of approx 2 pi. The footprints were fixed and the evolution was followed until the field relaxes to another current-free state. No evidence was seen for any instability, either ideal or resistive and no evidence for current sheet formation. The most striking feature of the evolution is that in response to photospheric motions, the field expands rapidly upward to minimize the stress. The expansion has two important effects. First, it suppresses the development of dips in the field that could support dense, cool material. For the motions assumed, the magnetic field does not develop a geometry suitable for prominence formation. Second, the expansion inhibits ideal instabilities such as kinking. The results indicate that simple stearing of a single arcade is unlikely to lead to solar activity such as flares or prominences. Effects are discussed that might possibly lead to such activity.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: NASA-TM-103402 , NAS 1.15:103402 , AD-A227283 , NRL-MR-6715
    Format: application/pdf
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  • 2
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    A bubblelike coronal mass ejection flux rope in the solar wind (1990)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: A resolution to the question of whether coronal mass ejections are loops or bubbles is proposed and applied to the geometrical analysis of a solar wind event detected at 1 AU by ISEE 1 and 3. The discontinuity orientations, the size determined by time of passage, and the magnetic cloud signature are fit into the topology of a flux rope loop distorted by expansion into a thick rope with comparable dimensions in both the ecliptic and meridional planes. The looped rope fills a bubblelike cavity, thus preserving both types of proposed coronal mass ejection geometries. Other interesting features of the data include an apparent separation by the rope core of bidirectionally streaming protons in the leading section from electrons in the trailing section, possible vortical flow within the magnetic cloud, and a well-defined filamentary structure behind the shock.
    Schlagwort(e): SOLAR PHYSICS
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
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    The Third Solar Wind Conference: A summary (1974)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: The Third Solar Wind Conference consisted of nine sessions. The following subjects were discussed: (1) solar abundances; (2) the history and evolution of the solar wind; (3) the structure and dynamics of the solar corona; (4) macroscopic and microscopic properties of the solar wind; (5) cosmic rays as a probe of the solar wind; (6) the structure and dynamics of the solar wind; (7) spatial gradients; (8) stellar winds; and (9) interactions with objects in the solar wind. The invited and contributed talks presented at the conference are summarized.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: NASA-CR-141322 , PUBL-1354-51-REV , Mar 25, 1974 - Mar 29, 1974; Pacific Grove, CA; United States
    Format: application/pdf
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  • 4
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    Solar flares and avalanches in driven dissipative systems (1993)
    In:  Other Sources
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    Publikationsdatum: 2019-08-28
    Beschreibung: The contention of Lu and Hamilton (1991) that the energy release process in solar flares can be understood as avalanches of many small reconnection events is further developed. The dynamics of the complex magnetized plasma of solar active regions is modeled with a simple driven dissipative system, consisting of a vector field with local instabilities that cause rapid diffusion of the field. It is argued that the avalanches in this model are analogous to solar flares. The distributions of avalanches in this model are compared with the solar flare frequency distributions obtained from ISEE 3/ICE satellite observations. Quantitative agreement is found with the energy, peak luminosity, and duration distributions over four orders of magnitude in flare energy, from the largest flares down to the completeness limit of the observations. It is predicted that the power-law solar flare frequency distributions will be found to continue downward with the same logarithmic slopes to an energy of about 3 x 10 exp 25 ergs and duration of about 0.3 s, with deviations from power-law behavior below these values.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 412; 2; p. 841-852.
    Format: text
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  • 5
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    Solar cycle variations in the neutral exosphere inferred from the location of the Venus bow shock (1990)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: Solar UV and EUV varies significantly during the solar cycle. Pioneer Venus can measure this variation both directly and indirectly. A direct measure of the EUV is obtained from the photoelectron current of the Langmuir probe when the spacecraft is in the solar wind. The indirect measure is by monitoring the location of the Venus bow shock. The UV and EUV both heat the upper atmosphere and ionize it. When solar activity is high, the upper atmosphere should be ionized more rapidly. This effect adds a greater number of planetary ions to the magnetosheath plasma as it flows by Venus. It is this increase in mass flow that causes the Venus bow shock to move away from its solar minimum location. Pioneer Venus has now monitored the location of the bow shock for an entire solar cycle. The bow shock location is well correlated with the variation in EUV flux as measured by the Langmuir probe. The bow shock is farther from Venus than expected from the sunspot number or 10.7 cm solar radio flux, indicating that solar UV radiation may be even stronger at the present time than would be predicted from the relationships determined during the previous solar cycle.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: Advances in Space Research (ISSN 0273-1177); 10; 5 19
    Format: text
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  • 6
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    Statistical study of the correlation of hard X-ray and type III radio bursts in solar flares (1990)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: A large number of hard X-ray events which occurred during the maximum of solar cycle 21 have been analyzed in order to study their correlation with type III bursts. It is found that the distribution of occurrences of hard X-ray bursts correlated with type III radio bursts is significantly different from the distribution of all hard X-ray bursts. This result is consistent with the assumption that the hard X-ray and type III intensities are somewhat correlated. A bivariate distribution function of the burst intensities is fitted to the data and is used to determine that the typical ratio of X-ray intensity to type II intensity is about 10 and that the ratio of the number of X-ray producing-electrons to type III-producing electrons is about 1000. Three models which have been proposed to explain the relation between the accelerated hard X-ray and type III-producing electrons are examined in the context of these observations.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 358; 644-653
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
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    Solar cycle 21 effects on the Interplanetary Magnetic Field and related parameters at 0.7 and 1.0 AU (1993)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter at about 0.7 AU and IMP 8 at 1.0 AU are used to compare the long-term behavior of the IMF at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle (as compared to solar maximum or minimum), slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvenic and magnetosonic Mach numbers, and a remarkably consistent sector structure. Differences include the temporal behavior of the high-field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the heliosphere.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 98; A4; p. 5559-5572.
    Format: text
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  • 8
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    The Kelvin-Helmholtz instability in photospheric flows - Effects of coronal heating and structure (1993)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-08-27
    Beschreibung: A series of hydrodynamic numerical simulations has been used to investigate the nonlinear evolution of driven, subsonic velocity shears under a range of typical photospheric conditions. These calculations show that typical photospheric flows are susceptible to the Kelvin-Helmholtz instability (KHI), with rapid nonlinear growth times that are approximately half of a typical granule lifetime. The KHI produces vortical structures in intergranule lanes comparable to a typical fluxule radius; this is precisely the correct scale for maximum power transfer to the corona.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 403; 2; p. 769-779.
    Format: text
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  • 9
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    Solar wind three; Proceedings of the Third Conference, Pacific Grove, Calif., March 25-29, 1974 (1974)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: Papers are presented relating to the study of solar abundances, the evolution of the solar wind, solar corona dynamics, solar wind interactions, and stellar winds. Some of the topics covered include the nuclear composition of cosmic rays, the solar wind as deduced from lunar samples, spectral characteristics of flares, numerical simulation of interplanetary shock ensembles, radial gradient of solar wind velocity from 1 to 5 AU, the enhancement of solar wind fluctuations with scale size near the proton gyroradius, solar wind-Venus interaction observed from magnetic field experiment on Mariner 10, and binary stellar winds. Individual items are announced in this issue.
    Schlagwort(e): SOLAR PHYSICS
    Materialart: Solar wind three; Third Conference; Mar 25, 1974 - Mar 29, 1974; Pacific Grove, CA
    Format: text
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