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  • 1
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The instabilities associated with a high Mach number perpendicular shock are reexamined in light of recent enhanced understanding of the Earth's bow shock. The insights provided by both the ISEE observations and hybrid simulations are reviewed and subsequently incorporated into the instability analyses. The discussion of the instabilities is divided according to their location in the shock layer. In the regions in front of and at the shock transition the cross-field instabilities are subdivided into low frequency modes (e.g. ion-ion streaming, kinetic cross-field streaming, drift lower hybrid instabilities) and high frequency modes (electron cyclotron drift, ion sound and electron whistler instabilities). Further downstream various ion ring-like and anisotropy driven instabilities are considered. In each case the instability analysis is reviewed and recent developments are emphasized. Implications of these results concerning the wave signatures and plasma heating and acceleration are also discussed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The objective of the University of Maryland ISTP theory project is the development of the analytical and computational tools, which, combined with the data collected by the space and ground-based ISTP sensors, will lead to the construction of the first causal and predictive global geospace model. To attain this objective a research project composed of four complementary parts is conducted. First the global interaction of the solar wind-magnetosphe re system is studied using three-dimensional MHD simulations. Appropriate results of these simulations are made available to other ISTP investigators through the Central Data Handling Facility (CDHF) in a format suitable for comparison with the observations from the ISTP spacecrafts and ground instruments. Second, simulations of local processes are performed using a variety of non-MHD codes (hybrid, particle and multifluid) to study critical magnetospheric boundary layers, such as the magnetopause and the magnetotail. Third, a strong analytic effort using recently developed methods of nonlinear dynamics is conducted, to provide a complementary semi-empirical understanding of the nonlinear response of the magnetosphere and its parts to the solar wind input. The fourth part will be conducted during and following the data retrieval and its objective is to utilize the data base in conjunction with the above models to produce the next generation of global and local magnetospheric models. Special emphasis is paid to the development of advanced visualization packages that allow for interactive real time comparison of the experimental and computational data. Examples of the computational tools and of the ongoing investigations are presented.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 2504-2517 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the interaction of collisionless shocks. The interaction between pairs of sub- and supercritical shocks are studied for a wide range of parameters relevant to both the solar and interplanetary plasma. These parameters include the shock Mach number, the electron and ion plasma betas, and the angle between the shock normal and the magnetic field. In the collision of two subcritical quasiperpendicular shocks, the shock electromagnetic field structures pass through each other, but do not carry the plasma associated with them. When unequal shocks collide, there is no evidence of the contact discontinuity predicted by fluid theory. In the collision of supercritical quasiperpendicular shocks, significant acceleration of ions occurs. This acceleration arises when ions reflected at one shock encounter the electric fields of the other shock and takes place in a number of distinct phases. For a wide range of parameters, the maximum ion energy is roughly an order of magnitude larger than the ram kinetic energy of the colliding shocks. This ion energization will subsequently manifest itself as a high ion to electron temperature ratio in the doubly shocked plasma. Finally, it was found that the values of the Mach number and angle between the shock normal and magnetic field play a significant role in the collisions. For shock geometries significantly different from perpendicular, ions from each oncoming shock can escape upstream and undergo considerable acceleration.
    Type of Medium: Electronic Resource
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  • 4
    Publication Date: 2019-07-17
    Description: As the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft continue their penetration into the outer heliosphere, more attention has been focused on the nature of the solar wind interaction with the Very Local Interstellar Medium (VLISM). Since the initial pioneering concepts of Davis in 1955 and Parker in the early 1960's both in situ and remote measurements have led to various constraints that do not fit well into a coherent picture. To provide a context for these various observable constraints, we have adapted an explicitly time-dependent, explicitly three-dimensional magnetohydrodynamic (MHD) code to simulate the dependence of the heliospheric configuration and interaction with the VLISM on the properties of the external medium. The code also allows us to study temporal variations brought about by both short- and long-term changes in the solar wind and/or VLISM properties. We will discuss some of the initial results from this new effort and implications for the distances inferred to the termination shock and heliopause boundary. In particular, we will consider the effect of the Very Local Interstellar Magnetic Field (VLIMF) on the configuration and compare it with inferences from observations of outer heliosphere cosmic rays and the Very Low Frequency (VLF) outer heliospheric radio emissions.
    Keywords: Solar Physics
    Type: ; 53
    Format: text
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  • 5
    Publication Date: 2011-08-18
    Description: A comprehensive description is given of the Jovian MS magnetic fields, and explanations of these phenomena are proposed. While emphasizing Voyager 1 and 2 magnetic field observations and their relations to the plasma observations, it is also shown that the same phenomena are present in the Pioneer 10 magnetic field data. An unusually high occurrence of nearly north or south fields is observed in the outbound MS, especially in the vicinity of the MP. It is noted that the outbound MS fields and their variations tend to occur in a plane parallel to the local MP, according to large scale MP models.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 86; Sept. 30
    Format: text
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  • 6
    Publication Date: 2019-07-12
    Description: The formation mechanisms of collisionless shocks in solar flare plasmas are investigated. The priamry flare energy release is assumed to arise in the coronal portion of a flare loop as many small regions or 'hot spots' where the plasma beta locally exceeds unity. One dimensional hybrid numerical simulations show that the expansion of these 'hot spots' in a direction either perpendicular or oblique to the ambient magnetic field gives rise to collisionless shocks in a few Omega(i), where Omega(i) is the local ion cyclotron frequency. For solar parameters, this is less than 1 second. The local shocks are then subsequently able to accelerate particles to 10 MeV in less than 1 second by a combined drift-diffusive process. The formation mechanism may also give rise to energetic ions of 100 keV in the shock vicinity. The presence of these energetic ions is due either to ion heating or ion beam instabilities and they may act as a seed population for further acceleration. The prompt acceleration of ions inferred from the Gamma Ray Spectrometer on the Solar Maximum Mission can thus be explained by this mechanism.
    Keywords: SOLAR PHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 189; 1-2
    Format: text
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  • 7
    Publication Date: 2019-07-12
    Description: The interaction between collisionless shocks is examined from a kinetic viewpoint using computer simulations of shock collisions spanning the range of ambient plasma and shock parameters typical of the interplanetary medium. It is found that the interaction between two subcritical, perpendicular shocks is largely fluidlike with no evidence for accelerated particles. The electromagnetic fields associated with each shock, but not the plasmas, pass through each other during the collision. This interaction is due to self-consistently generated electric fields which adjust to the value needed to bring to rest the two incoming streams. The interaction between two supercritical perpendicular collisionless shocks is similar to the subcritical case, in that the electromagnetic fields associated with each shock are transmitted whereas particles are not. However, the interaction is also highly dynamic with substantial acceleration of ions. Increasing the Mach number gives rise to an increasing number of energetic particles.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids (ISSN 0031-9171); 30; 2504-251
    Format: text
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  • 8
    Publication Date: 2019-08-16
    Description: In this study we examine observations made by AMPTE/CCE of energetic ion bursts during seven substorm periods when the satellite was located near the neutral sheet, and CCE observed the disruption cross-tail current in situ. We compare ion observations to analytic calculations of particle acceleration. We find that the acceleration region size, which we assume to be essentially the current disruption region, to be on the order of 1 R(sub E). Events exhibiting weak acceleration had either relatively small acceleration regions (apparently associated with pseudobreakup activity on the ground) or relatively small changes in the local magnetic field (suggesting that the magnitude of the local current disruption region was limited). These results add additional support for the view that the particle bursts observed during turbulent current sheet disruptions are due to inductive acceleration of ions.
    Keywords: Geophysics
    Type: NASA/CR-95-207163 , NAS 1.26:207163 , Paper-94GL03384 , Geophysical Research Letters (ISSN 0094-8534); 22; 5; 627-630
    Format: application/pdf
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  • 9
    Publication Date: 2019-07-12
    Description: The early time interaction of an artifically injected lithium cloud with the solar wind is simulated with a one-dimensional hybrid code. Simulation results indicate that the lithium cloud presents an obstacle to the solar wind flow, forming a shock-like interaction region. Several notable features are found: (1) The magnetic field is enhanced up to a factor of about 6, followed by a magnetic cavity downstream. (2) Solar wind ions are slowed down inside the lithium cloud, with substantial upstream reflection. (3) Most of the lithium ions gradually pick up the velocity of the solar wind and move downstream. (4) Intense and short-wavelength electric fields exist ahead of the interaction region. (5) Strong electron heating occurs within the lithium cloud. (6) The convection electric field in the solar wind is modulated in the interaction region. The simulation results are in remarkable agreement with in situ spacecraft measurements made during lithium releases in the solar wind by the AMPTE (Active Magnetospheric Particle Tracer Explorers) Program.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 91; 1333-133
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  • 10
    Publication Date: 2019-08-27
    Description: Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983/84 and 1992/3. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approximately 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approximately 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approximately 115 AU.
    Keywords: SOLAR PHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 16; 9; p. (9)303-(9)306
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