Control of Suprathermal Electron Flux by Solar Wind Dynamics
The high variability of the intensity of suprathermal electron flux in the solar wind is usually ascribed to the high variability of sources on the Sun. Here we demonstrate that a substantial amount of the variability arises instead from solar wind dynamics in stream interaction regions, where fast wind runs into slow wind and creates a pressure ridge at the interface. Superposed epoch analysis centered on stream interfaces in 26 interaction regions previously identified in Wind data reveal a twofold increase in 250-eV flux (integrated over pitch angle). This result is understood in terms of the magnetic field compression there. If suprathermal electrons spiraling along field lines are viewed as beads along strings, then increasing the density of the strings through compression will also increase the density of the beads. An intriguing aspect of the result is that while the field strength peaks at the stream interface, as expected, the peak in electron flux lags by ∼1.5 hours. Although this lag may not be statistically significant, its sense is consistent with the systematic transport of open magnetic flux via interchange reconnection at coronal hole boundaries predicted by the global footpoint circulation model of L. A. Fisk and colleagues.
Comparison of Interplanetary Ion and Electron Source Regions
We consider similarities and differences in particle dropout features between energetic ion and suprathermal electron data in interplanetary particle events associated with impulsive flares at the Sun. Dropouts are thought to be caused by spacecraft encounters with magnetic field lines that are not connected to the particle source or, in a small minority of cases, by encounters with distinct boundaries in the solar wind itself. By examining dropouts of the first type, we relate differences in the dropouts between the ion and electron data to the source regions. Electron and ion dropouts are observed to be coincident in about half of events, suggesting electron and ion source regions have some offset at least half the time. However, for the particle events observed within ICMEs, the electron and ion dropouts are always coincident, suggesting that the particle acceleration regions for ions and electrons in these events are cospatial.
Variations in the Spectral Slopes of Interplanetary Data
Inferences on turbulence in interplanetary plasmas commonly depend on the slope of the power spectrum. We have studied the slopes of spectra of the interplanetary magnetic field and charged particles from the HISCALE and EPAM detectors on Ulysses and ACE and find that it is systematically nonstationary. As an example, when the spectra are estimated on time blocks varying between three hours and one day, the average slope on the ACE GSE By component is close to -5/3, but fluctuations about this average are not random. Using 1-minute data, the slopes made from three hour data blocks offset by one hour gives a new time series. Power spectra of these series have strong peaks that are probably gravity modes. Here we extend these calculations to vector--valued data. The eigenvalues of the spectral matrices have similar characteristics to the individual components but the eigenvectors, that describe relative delay and orientation, point to several distinct families of modes.
Simulation study of energetic electron bursts from reforming shocks
Collisonless shocks redistribute energy, heating and accelerating electrons responsible for various plasma waves and emissions in the upstream and downstream shock region. In this paper we study numerically the electron dynamics in time-dependent shock fields generated by an one-dimensional multiscale hybrid code, and steady state model shocks by tracing the exact test particle electron trajectories. It is shown that the upstream energetic electron bursts are produced cyclically at the shock reformation period in the time- dependent reforming shocks providing the upstream shock Mach number and plasma beta is high and low enough, respectively. Observation of the upstream electron distribution functions shows time-varying loss cone structures and beam features. In contrast to the reforming shocks, a continous electron beam is formed by the reflected electrons in upstream of steady state model shocks. Parameter study shows that the upstream incoming electrons can be reflected non-uniformly or continuously depending on the shock parameters. Bursty energetic electron events take place when the plasma beta is low (β ≤ 0.4) and the shock Mach number is high (MA≥ 6). In contrast, continuously reflected electrons are observed for low beta (β ≤ 0.4), low Mach number (MA ≤ 4) shocks, even when the shock is reforming, because the changes in shock fields are relative small. The electron burst events disappear and the observed upstream electron distribution function contours are steady-state. A continuous electron beam is formed which is qualitative the same as the beam from steady-state shocks. Increasing the plasma beta (providing the shock is still reforming) has minor effects on the upstream electron beam features. This calculation demonstrates that shock nonstationarity may lead to major changes in electron distributions and associated plasma waves upstream and downstream depending on the upstream shock parameters, probably requiring modification to the steady state shock model used for predicting foreshock radio emission.
Heavy Ions as a Probe of Solar Wind MHD Turbulence in SEP Events
Heavy ions, having a broad range of charge to mass ratios, offer a powerful tool to understand the process of transport in Solar Energetic Particle (SEP) events. Earlier works [Cohen et al. 1999, 2005, Mason et al. 2006] have suggested that certain ordering exists for the spectral forms and time intensity profiles and such orderings are closely related to the power spectrum of the interplanetary solar wind MHD turbulence. In this work, we extend our earlier work and examine the relationship between the power law index (q) of the interplanetary power spectrum and the (A/Q) dependence of the observed time intensity profile. In particular, we separate the time intensity profile into an initial peak phase and a later decay phase and study how the shape of each phase may depend on q and (Q/A). Knowledge of such a dependence will make it possible to align the time intensity profiles of different heavy ions having different energies. Seeking such an alignment from observations then provides an effective way to probe the solar wind MHD turbulence. The model used here is based on a Monte-Carlo approach that solves the focused transport equation via a stochastic differential equation method. The particle-turbulence interaction is described using an extended Quasi Linear Theory. Some advantages of using a Monte-Carlo technique are that it makes possible to easily investigate the effect of an radial-dependent diffusion coefficient and can treat a moving shock relatively easily. Results and implications from our modeling are shown and discussed.
The interaction of magnetic discontinuities with collisionless shocks
We address the physics of the interaction of magnetic discontinuities (primarily current sheets) with collisionless shocks. Past studies have shown that at the intersection point of a current sheet of a particular magnetic polarity reversal and a shock, a region of hot plasma is formed, known as a hot-flow anomaly (or also a hot diamagnetic cavity). We expand on the earlier studies of the physics of hot-flow anomalies by examining a wider range of current-sheet / shock orientations. Among other things, we find that when the current sheet is oblique to the shock-normal direction, a large fraction of low-energy ions are accelerated to very high energies (up to 100 times the ram energy of the plasma in some cases). This may help explain some recent spacecraft observations both in the solar wind near 1 AU, and also in the outer heliosphere by the Voyagers. In addition, because there are large number of discontinuities in the solar wind, it may be that the acceleration of ions at the intersection point of these structures and propagating interplanetary shocks may be an efficient means of accelerating particles and producing suprathermal ions in the heliosphere. We will discuss the basic physics of this process and present recent relevant observations.
Test Particle Acceleration in Kinematic MHD Models of Collapsing Magnetic Traps
During solar flares a large number of charged particles are accelerated to high energies, but the exact mechanism responsible for this is still unclear. Acceleration in collapsing magnetic traps is one of the mechanisms that has been proposed. In this contribution we discuss test particle acceleration in analytic kinematic MHD models for collapsing magnetic traps. Particle orbits are calculated using the guiding centre approximation. A few illustrative examples of collapsing trap models will be presented together with some preliminary studies of particle orbits and the efficiency of the energy gain process.
Plasma in the Heliosheath
Voyagers 1 and 2 continue to explore the heliosheath. We present results from the first 18 months of Voyager 2 plasma observations in the heliosheath. We find that the speeds observed by V1 and V2 are very different, with V2 observing radial speeds about twice those observed at V1. This difference is in the opposite direction of that predicted by models. Densities have slowly decreased across the sheath, probably due to a decrease in solar wind flux throughout the heliosphere. Temperatures are also decreasing as is the variation in these parameters. Non-radial flows observed at the heliosheath are much larger parallel to the solar equatorial plane than perpendicular plane, suggesting that termination shock is more extended in the equatorial plane and smaller at the poles.