SH53A-01 INVITED
Stochastic Charged Particle Acceleration in the Heliosphere
We will describe our work on stochastic acceleration in compressional turbulence, which naturally leads to power law velocity distributions with the unique spectral index of -5 (Fisk and Gloeckler (2006), ApJL 640, L79-L82), and extend it to the acceleration of suprathermal and low-energy (< few MeV) particles at shocks. Redistribution of energy by stochastic acceleration, from the core population consisting of particles with speeds less than Vo (about twice the solar wind speed, Vsw) to the tail particles with speed above Vo, should occur both upstream and downstream from shocks. However, the core particles are heated non- adiabatically when crossing a shock, which raises more of them above the threshold for injection into the tail than is the case in the turbulence upstream of the shock, and this will yield an enhanced suprathermal tail downstream. We derive a simple expression for the pressure jump, as a function of the shock Mach number, of the tail particles in crossing the shock, and compare this with observations of solar wind, suprathermal particles and pickup ions upstream and downstream of shocks observed in the heliosphere with the SWICS instrument on Ulysses.
SH53A-02 INVITED
Energetic Particle Intensities Produced at Shocks with Surface Ripples
Observations of traveling interplanetary shocks and energetic particle intensities have revealed a large variety of shock structures and associated energetic storm particle (ESP) events. Very few events agree with the complete set of predictions made by the diffusive shock acceleration theory. In fact the most common type of particle intensity enhancement associated with the passage of a shock is that with irregular time intensity profiles, i.e. multiple intensity bursts observed before and/or after the shock. Fluctuations of both the solar wind plasma where the shock travels and/or of the shock surface determine the main features of these ESP events with irregular variations of intensities and angular distributions. We present observations of one of these events where the pre-shock magnetic field was relatively steady. We show that ripples on the shock surface are responsible for the observational characteristics of the associated ESP event. Different trajectories of the spacecraft with respect to a given shock ripple result in ESP events of diverse nature.
SH53A-03
MeV Ion Anisotropies in the Vicinity of Interplanetary Shocks
The anticipated signatures of interplanetary shock acceleration to be found in energetic ion anisotropies in the vicinity of interplanetary shocks include near-isotropic particle distributions consistent with of diffusive shock acceleration, "pancake" distributions indicative of shock drift acceleration, and flow reversals suggestive of a particle acceleration region passing by the observing spacecraft. In practice, while clear examples of these phenomena exist, more typically, particle anisotropies near interplanetary shocks show considerable variation in time and space, both in individual events and from event to event. We investigate the properties of MeV/n ions in the vicinity of a number of interplanetary shocks associated with the largest energetic particle events of solar cycle 23, and previous cycles, including their intensity-time profiles, anisotropies, and relationship with local solar wind structures, using observations from the IMP 8, ISEE-3, Helios 1 and 3 spacecraft. The aim is to help to understand the role of shocks in major solar energetic particle events.
SH53A-04 INVITED
Deviations From the Predictions of Standard Diffusive Shock Acceleration
The mechanism for the acceleration of energetic charged particles remains one of the most important problems of space physics. Diffusive shock acceleration, now almost three decades old, has many attractive features and has become the most popular mechanism. However, it is becoming increasingly clear that in situ observations of energetic particles and shock waves often show very poor agreement with the basic theoretical predictions. Often the energetic-particle enhancements are not observable or occur at a time displaced from the shock passage. Observations from two or more spacecraft show different energetic-particle profiles on the same propagating shock. Statistical studies show poor agreement between the spectrum of energetic particles and the parameters of the shock. Finally, energetic electrons are often observed, but the injection of electrons into diffusive shock acceleration is not understood. One obvious explanation for these discrepancies is that diffusive shock acceleration is not the accelerator of the energetic particles. However, viable alternative mechanisms are so far lacking. Another possibility, diffusive shock acceleration in the presence of broad-band turbulent fluctuations in the upsream fluid will be shown to be a promising solution.
SH53A-05 INVITED
Outstanding Puzzles in Shock Acceleration
The association between shocks and energetic particles in the heliosphere is well established. In addition, the theory of diffusive shock acceleration (DSA) has been viewed as a success in describing many features of this association in "events" ranging from planetary bow shocks, to interplanetary traveling shocks and corotating interaction region (CIR) shocks in the solar wind. Nevertheless, a challenging array of observations seems to contradict the predictions of DSA. Several of these contradictions have been listed in the session description. This paper addresses some of these contradictions. They result because our applications of the theory are usually restricted to simplifying assumptions, such as a planar stationary configuration and injection from a single pool of low-energy particles, which are usually not warranted. In addition, the fundamental assumptions of DSA are often violated in Nature, where phase-space distribution functions are not nearly isotropic and spatial diffusive transport is inadequate or difficult to specify. Finally, the process of particle injection at the shock from low energies into the process of shock acceleration, which dictates to a large extent ion composition, is not described by the theory of DSA. These inadequacies appear to account for (most of) the contradictions. Clearly generalizations to the theory of DSA are required.
SH53A-06
Shock Acceleration with a Focused Transport Approach
Observations by Voyager 1 reveal accelerated energetic ion spectra with large, magnetic-field-aligned particle anisotropies upstream of the termination shock, as well as highly anisotropic intensity spikes when the spacecraft crossed the shock. Similar features are observed at nearly-perpendicular interplanetary shocks. These features do not agree with simple diffusive shock acceleration theory based on the standard cosmic-ray transport equation which is limited to near isotropic particle distributions. It will be shown that shock acceleration with the focused kinetic transport equation, which is not limited to small anisotropies, is a viable alternative, while still retaining many of the familiar transport mechanisms associated with standard cosmic-ray transport theory (the focused transport equation is essentially the zero gyro-radius limit of the drift kinetic equation including pitch- angle scattering). We discuss how the focused transport equation contains all the physics associated with "scatter-free" shock drift acceleration theory, but since it also contains pitch-angle scattering of particles by small- scale turbulence, it can describe both shock drift acceleration with scattering and first order Fermi shock acceleration without requiring small anisotropies. On this basis we will show with simulations that the above- mentioned observations can be explained naturally. It will also be illustrated (i) that the lack of cross-field transport inherent in the focused transport equation does not exclude the acceleration of unaccelerated pickup ions at a nearly perpendicular shock when random variations in the field angle (shock obliquity) are included in the focused transport model, and (ii) that time-dependent shocks inevitably lead to accelerated spectra with more than a single power law. We argue that anomalous cosmic-ray intensity peak beyond the shock is due to further heating in the heliosheath, either by stochastic acceleration from turbulence or by adiabatic compression of the solar wind by the local interstellar flow.
SH53A-07
Pitch-angle distributions and anisotropies of energetic particles at the termination shock: theory vs. Voyager observations
We analyze and model plasma-frame pitch-angle distributions of low-energy accelerated ions observed by Voyager 1 in the immediate vicinity of the termination shock (TS). In view of limited applicability of Parker's transport equation to highly beamed upstream populations we developed a novel technique employing a three- moment expansion of the focused transport equation in Legendre harmonics. For the first time high temporal resolution LECP measurements are combined with Voyager magnetometer data to elucidate the dependence of the three Legendre harmonics (intensity, first-order anisotropy, and second-order anisotropy) on particle energy. Our analysis of pitch-angle and spatial distributions indicate that the observed low-energy populations were accelerated by a highly oblique shock wave (i.e., the TS). We show that measured intensities peak sharply at the shock crossing consistent with theoretical predictions of adiabatic reflection off a highly oblique shock. We show how derived first- and second-order anisotropies can be used to deduce the local obliquity of the TS at the time of crossing. The model predicts upstream anisotropies as high as 100% for a nearly perpendicular TS, whereas downstream distributions are nearly isotropic in all cases.
SH53A-08
Energetic electron bursts from a reforming High Mach Number collisionless shock
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. 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. This calculation demonstrates that shock nonstationarity will lead to major changes in electron distributions and associated plasma waves upstream and downstream, probably requiring modification to the steady state shock model used for predicting foreshock radio emission.