SH33A-1187 INVITED 1340h
On the assumptions commonly made for the evaluation of the compression, speed, normal orientation of a collisionless magnetohydrodynamic (MHD) shock, and their limitations
The magnetohydrodynamic Rankine-Hugoniot conditions for a shock are reviewed. The requirements for the computation of the fast forward interplanetary shock parameters and their uncertainties are presented. The underlying assumptions used in the evaluation of the shock parameters are discussed. The applied technique is a combination of the "pre-averaged" magnetic-coplanarity, velocity-coplanarity, and the Abraham-Schrauner-mixed methods (1). It is shown that within acceptable limits this technique generally gave good results, with some exceptions. Arguments are presented that a nearby-located MHD discontinuity would cause the failure of the applied technique. Examples of that situation are presented. In these cases the shock normal is identified with the help of data from multiple spacecraft located within hundreds of earth radii distance from each other. Here the assumption is that within hundreds of earth radii distances the shock curvature is negligible. (This assumption appears to be valid for locally-driven transient shocks.) (1) Berdichevsky et al., J. Geophys. Res., 105, 27,289-27,314, 2000; Errata in J. Geophys. Res., 106, 25,133, 2001
SH33A-1188 1340h
CME Shock and Sheath Structures Relevant to Particle Acceleration
Most high-energy solar energetic particles (SEPs) are believed to be accelerated at shock waves driven by coronal mass ejections (CMEs). The acceleration process strongly depends on the shock geometry, and the structure of the sheath that forms behind the shock. In an effort to understand the structure and time evolution of such CME-driven shocks and their relevance to particle acceleration, we investigate the interaction of a fast CME with the ambient solar wind by means of a three-dimensional (3-D) numerical ideal magnetohydrodynamics (MHD) model. Our global steady-state coronal model possesses high-latitude coronal holes and a helmet streamer structure with a current sheet near the equator, reminiscent of near solar minimum conditions. Fast and slow speed solar wind flow at high and low latitude respectively and the Archimedian spiral geometry of the interplanetary magnetic field is reproduced by solar rotation. Within this model system, we drive a CME to erupt by the introduction of a Gibson-Low magnetic flux rope that is embedded in the helmet streamer in an initial state of force imbalance. The flux rope rapidly expands and is ejected from the corona with maximum speeds in excess of 1000 km/s driving a fast-mode shock from the inner corona to a distance of 1 astronomical unit (AU). We find that the ambient solar wind structure strongly affects the evolution of the CME-driven shocks causing deviations of the of the fast-mode shocks from their expected global configuration. These deflections lead to substantial compressions of the plasma and magnetic field in their associated sheath region. The sudden post-shock increase in magnetic field strength on low latitude field lines is found to be effective for accelerating particles to the GeV range.
SH33A-1189 INVITED 1340h
Particle Injection and Acceleration at Shock in Interplanetary Space: Kinetic Simulations
Shocks in interplanetary space are known to be an important site for particle acceleration. Because of high-resolution data, the Earth's bow shock used to be a source for detailed information of shock structure and particle injection and particle acceleration. However, recent interplanetary missions such as SOHO, WIND and ACE provided excellent data of interplanetary shocks and provided new insights and enriched the knowledge of the structure and dynamic of interplanetary discontinuities and their injection and acceleration properties. Kinetic simulations are very successful tool for studying the microphysics of collisionless shocks, as well as wave-particle interactions. Processes associated with injection and acceleration of various ion populations such as the solar wind and pickup ions have been investigated in relation to shock parameters and shock topology. The purpose of this talk is to review from the point of view of kinetic simulations aspects of ion reflection, wave-particle interaction, and ion injection and acceleration of shocks in interplanetary space.
SH33A-1190 INVITED 1340h
Energetic Particles Associated with the Interplanetary Shock of 25 September 2001
We report on the elemental abundances of energetic particles associated with an interplanetary shock which passed the Advanced Composition Explorer (ACE) spacecraft at $\sim$ 20:05 UT on 25 September 2001. The shock is associated with a solar X-ray event (X2.6) at S16E23 on 24 September at $\sim$ 10:38 UT. This corresponds to an average speed of 1232 km/sec to reach 1 AU. Heavy elements above 14 MeV/nucleon were observed from He to Fe. The time-intensity profile for He is that of a classic Energetic Storm Particle event, i.e. there is a shock-associated increase riding on top of a solar particle event profile. This increase is $\sim$ 12 hours wide and peaks $\sim$ 6 to 8 hours after the shock arrival. What is unusual about this event is that the time-intensity profiles of O and Fe are considerably different from the He profile. This event illustrates some of the difficulties in relating observations to current models of shock acceleration. This research was supported by NASA at the Goddard Space Flight Center, the California Institute of Technology (under Grant NAG5-12929), and the Jet Propulsion Laboratory.
SH33A-1191 1340h
GEOTAIL Observation of Interplanetary Shock-Magnetic Hole Interaction on 25 September 2001: An overview
Magnetic holes (MHs) in the solar wind are isolated depressions in the magnitude of the interplanetary magnetic field, which have attracted recent observational and theoretical attentions. On 25 September 2001, we observed an interaction event between an interplanetary shock (IPS) and a MH using the GEOTAIL data (plasma and energetic particles, magnetic and magnetic fields, and plasma waves) as well as those from the other ISTP spacecraft (ACE and WIND). What we observed were, (1) within the MH the solar wind plasma density and temperature respectively showed a decrease and increase keeping the pressure balance condition with the dropping magnetic pressure, (2) the solar wind velocity showed a significant decrease in the region surrounding the MH, (3) the nonthermal ions were trapped within the MH structure, and (4) in the MH there were enhancement of magnetic turbulence in the frequency range of 0.1-8 Hz, while no obvious enhancement was seen in the frequency range above $\sim$10 Hz. We will discuss physical implications of these observations. In addition, we are trying to deduce the evolution history of the MH-IPS system from the multi-satellite data.
SH33A-1192 1340h
Formation of an Accelerated Ion Population by Electromagnetic, non-MHD,Finite-size, Fast-mode Shocks.
Solar Energetic Particles form high energy tails which affect the geophysical environment and impose a significant hazard for manned space exploration.An important site for their energization is related to propagating shocks,which assume a variety of shapes and often appear in tandem. Although most of the numerical investigations address the energization mechanisms through diffusion-like approach, basically it is (a) kinetic in nature, (b) occurs often over small spatial scales, and (c) is very sensitive to the details of the shock fields and seed population. We construct a self-consistent, non-MHD, electromagnetic configuration, made from a solitary shock or combined from two propagating shocks, perform an explicit integration of particle trajectories with detailed focus on their initial conditions, and investigate the plausibility of thermal source for the energized population. It is found that:(1) the energization of ions for low Mach numbers ($\sim2$) as observed for emerging shocks close to Sun depends crucially on the narrow size of the shock, (2) the gyration phase and the obliqueness of the particle flow direction with respect to the shock normal determines the seed population;(3) the acceleration by shock surfing becomes a prerequisite for scattering,reflection and additional energization due to multiple shock crossing.
SH33A-1193 1340h
Modeling Interactions of Coronal Mass Ejections in the Lower Heliosphere
We present a three-dimensional (3D) numerical ideal magnetohydrodynamics (MHD) model of interacting coronal mass ejections (CMEs). The simulations are based on a 3D model of a CME propagating through a bimodal solar wind characteristic of solar minimum. The model of the solar corona contains high-latitude coronal holes and a helmet streamer structure with a current sheet at the solar equator. The CMEs are driven by a 3D Gibson-Low flux rope superimposed onto the steady-state corona. Following the propagation of the first CME, the solar wind is disturbed and the second CME is launched into a less dense heliosphere. Its characteristics are different, and, notably, it shows less deceleration while propagating. Different cases corresponding to different delays between the two CME initiations have been simulated. Each simulation is performed for ten hours after the launch of the second CME. Physics based AMR has been used to capture both ejecta and associated shocks. Physical quantities are derived for each case and compared with the case of an identical CME propagating in the heliosphere without the existence of the previous CME. We have also produced synthetic white-light coronograph images of the CMEs.
SH33A-1194 1340h
Coronal Mass Ejections and Magnetic Clouds Modeled as MHD Bounded States
Multiple loops can be seen in the solar corona before the onset of a coronal mass ejection (CME), during and after the event. We apply multi-toroidal configurations to model CMEs and their interplanetary counterparts - magnetic clouds. In the laboratory, plasma confinement is often achieved by conducting metal walls which introduce elastic forces to maintain equilibrium. Such walls, carrying electric surface currents, usually are taken as boundaries where the magnetic field is truncated to provide finite energy for the configuration. The idea of MHD bounded states as solutions with continuous magnetic field and finite magnetic energy was put forward in 1975 [1]. Such solutions describe a single toroid (ground state) and multiple toroids (excited states) [1],[2]. We analyze noncircular cross sections of such toroids and compare the components of the magnetic field vector with in situ observations in interplanetary magnetic clouds. We present Ulysses spacecraft observations in support of our multi-tube model for interplanetary magnetic clouds based on bounded state MHD configurations with axial and helical symmetry [3],[4]. The interaction of CMEs with the global coronal field will also be discussed. In our presentation, we stress the difference in boundary conditions for magnetic configurations in laboratory and space plasmas. [1] Osherovich, V.A., `On an equilibrium of an MHD config-uration with axial symmetry 1', Soln Dann, 5, p. 70, 1975. [2] Osherovich, V.A. and Lawrence, J.K., `Elaboration of the new magnetohydrostatic sunspot theory (Double return flux model)', Sol. Phys., 88, p. 117, 1983. [3] Krat, V.A. and Osherovich, V.A., `Note on the asymmetry of bipolar sunspot groups', Solar Phys., 59, pp. 43-47, 1978. [4] Osherovich, V.A., Fainberg, J. and Stone, R.G., `Multi-tube model for interplanetary magnetic clouds', Geophys. Res. L., 26(3), pp. 401-404, 1999.
SH33A-1195 1340h
Mass Transport by Heliospheric Energetic Neutral Atoms
Energetic neutral atoms (ENAs) are recognized as a powerful tool for remote probing of hot space plasmas in distant regions. The plasmas at the heliospheric boundary where the expanding solar wind meets the surrounding local interstellar medium are believed to produce ENAs. The heliospheric ENAs originating in the heliospheric sheath between the termination shock and the heliopause and reaching the inner heliosphere also provide an important and heretofore-unaccounted source of atomic hydrogen in the sun's vicinity. We show that these ENAs are a major contributor to the density of interplanetary hydrogen at heliocentric distances < 1 AU and could dominate in the downwind (interstellar wind) region under typical solar and interstellar conditions. Mass transport in the heliosphere by ENAs may become especially important for determining the origin of the inner source of neutral particles in the sun's vicinity and for characterization of the three-dimensional solar wind flow by imaging in extreme ultraviolet.
SH33A-1196 1340h
A New Procedure of Shock Fittings to the MHD Rankine-Hugoniot Relations
In study the MHD shock-discontinuity interactions, it is essential to use accurate shock parameters in the analysis. These shock parameters are derived from the Rankine-Hugoniot (RH) relations based on observed solar wind plasmas and magnetic fields on both sides of the shock. In this study, we present a novel procedure of shock fitting. To minimize the difficulties due to the uncertainty of the shock frame reference and fluctuations of the vector quantities of solar wind velocity and magnetic field, we choose only the scalar parameters $m$, $\theta_{Bn}$ and $y$ to fit the RH shock jump relations, where $m=B_2/B_1$ and $y=n_1/n_2$ are the ratios of magnetic field strength and solar wind proton number density across the shock, and "1" and "2" refer to up- and down-stream states of the shock, respectively. The $\theta_{Bn}$ is the angle between the shock normal and upstream magnetic field, $\bf {B_1}$ derived from the shock coplanetary theorem. With these three scalar parameters, we propose a procedure to derive a set of best-fit values of $m$, $\theta_{Bn}$, $y$, $\beta_1$ and $\beta_2$ to the RH relations, where beta is the ratio of solar wind thermal energy over magnetic energy. The physical parameters of the shock are also calculated from the best-fit values. Shocks with anisotropic temperatures, heat flow and additional momentum flux due to waves and/or turbulence on both sides of the shock are also included in our analysis. Applications of this procedure will be given in this study.
SH33A-1197 1340h
The unusual signatures at and near 2018 UT (Wind in situ observations) of the travelling, fast forward interplanetary shock on September 25, 2001
We show that at 2018 UT on September 25, 2001, a velocity discontinuity of [-300, -200, -200] kms-1 (in GSE coordinates) passes Wind. However, the downstream shock candidate region appears to be far from thermal equilibrium. Henceforth it is our purpose to show that it is not a shock discontinuity amenable to the Rankine-Hugoniot based techniques. A possible shock related thermalization (at Wind) appears approximately 90 seconds later, at 2019:31UT, behind a magnetic hole showing a large rotational discontinuity. Timing using Wind and other spacecraft located tens to hundreds of earth radii apart allows the preliminary assessment of an overall shock normal. In the context of Sun-Earth connections, the disturbance appears related to the Sept 24,1030UT halo-CME [EIT observed a CME starting in AR9632 located at [S18; E27] at 09:12 UT], which was moving at 2240 kms-1 in the plane of the sky. In most events we put special value on the identification of the plasma density by using the electron plasma thermal line. However, there is a loss of detection of the plasma line between 2010 and 2016 UT. This shock is important, because strong particle and wave intensities appear to be associated with it. Candidate compressional waves are identified upstream of the shock. Their presence suggests that they are an additional source of turbulence. This is so, because the compressional waves are likely being created by the shock accelerated particles and subsequently overtaken by the shock, probably, in an almost continuous process.
SH33A-1198 1340h
EXPLORING THE EVIDENCE OF PARTICLE-WAVE COUPLING IN THE DISTANT UPSTREAM ION-FORESHOCK
We discuss the far ion foreshock for cases when the interplanetary magnetic field (IMF) is approximately parallel to the supersonic motion of the solar wind plasma. Several events have been identified of an enhanced ultra low frequency wave activity in the presence of an ion foreshock composed by (1) a beam (low energy <4keV), (2) a transverse population to the IMF ("intermediate" energy, from 5 to 40 keV), and (3) an isotropic population ("high" energy end, from 40 to 140 keV). This observations took place up to 250 RE upstream of the Earth (Berdichevsky et al., 1999). Here we study the frequency content, the polarization and the particle-wave coupling using a cross-spectrum analysis. We do a qualitative analysis of the origin of this wave-activity using the linear theory of plasma instabilities and explore the possibility of trapping of these strongly scattered particle fluxes. In this study we present results for observations on June 11, July 13, and Aug 25, 1995. For the study we use sampling times of waves and electron data of 12 sec or higher resolution energetic particle data. Solar wind plasma and magnetic field are collected by the MFI, SWE, WAVES, and 3DP instruments on Wind, and MGF and EPIC in GEOTAIL spacecraft. The spectral technique used is based on a set of Intrinsic Modes resulting from an Empirical Decomposition Method. Berdichevsky D. et al., 1999, J. Geophys. Res., 104, No A1, 463
SH33A-1199 1340h
Relaxation of flux ropes and magnetic reconnection in the Reconnection Scaling Experiment at LANL
Magnetic reconnection and plasma relaxation are studied in the Reconnection Scaling Experiment (RSX) with current carrying plasma columns (magnetic flux ropes). Using plasma guns, multiple flux ropes ($B_\theta \leq 100$ Gauss, $L=90 $ cm, $r\leq3$ cm) are generated in a three-dimensional (3D) cylindrical geometry and are observed to evolve dynamically during the injection of magnetic helicity. Detailed evolution of electron density, temperature, plasma potential and magnetic field structures is reconstructed experimentally and visible light emission is captured with a fast-gated, intensified CCD camera to provide insight into the global flux rope dynamics. Experiments with two flux ropes in collisional plasmas and in a strong axial guide field ($B_z / B_\theta > 10$) suggest that magnetic reconnection plays an important role in the initial stages of flux rope evolution. During the early stages of the applied current drive ($t\leq 20 $ $\tau_{Alfv\acute{e}n}$), the flux ropes are observed to twist, partially coalesce and form a thin current sheet with a scale size comparable to that of the ion sound gyro-radius. Here, non-ideal terms in a generalized Ohm's Law appear to play a significant role in the 3D reconnection process as shown by the presence of a strong axial pressure gradient in the current sheet. In addition, a density perturbation with a structure characteristic of a kinetic Alfv\'{e}n wave is observed to propagate axially in the current layer, anti-parallel to the induced sheet current. Later in the evolution, when a sufficient amount of helicity is injected into the system, a critical threshold for the kink instability is exceeded and the helical twisting of each individual flux rope can dominate the dynamics of the system. This may prevent the complete coalescence of the flux ropes.