Session GM1: Mini-conference on Shock Acceleration in Space and Astrophysical Plasmas I

Solar Energetic Particles -- A Radiation Hazard to Humans and Hardware in Space

During large solar energetic particle (SEP) events the intensity of $>$30 MeV protons in nearby interplanetary space can increase by a million times over the steady intensity of galactic cosmic rays, creating a radiation hazard to both humans and hardware in space. With NASA now committed to sending astronauts to the Moon and possibly on to Mars, outside the protective cover of the Earth's magnetosphere, interest in understanding and forecasting large SEP events has taken on a new sense of urgency. The past solar maximum included four of the top ten SEP events of the space era. Fortunately, the array of spacecraft now in interplanetary space has provided greatly improved measurements of the composition and energy spectra of accelerated ions, leading to fresh insights into the nature of these events. The largest SEP events are accelerated by coronal and interplanetary shocks driven by coronal mass ejections (CMEs) traveling at $>$2000 km/sec. Although shock acceleration is ubiquitous in nature, its efficiency is highly variable, making it difficult to forecast the onset and evolution of large SEP events. This talk will describe the radiation hazards associated with the largest SEP events, discuss their frequency of occurrence, consider a worst-case SEP event, and describe how the radiation risks can be mitigated.   

Three-Dimensional Global Simulations of CME-Driven Shock Waves

Fast Coronal Mass Ejection (CME) events are characterized by a sudden release of energy and mass from the solar conrona which naturally cause the generation of powerful interplanetary shock waves of a speed ranging from 500 to 3500 km/s. These shock waves are large-scale disturbances to the interplanetary medium that accelerate particles found in gradual solar energetic particle (SEP) events and supra- thermal ions. The acceleration process depends strongly on shock speed and geometry which both exhibit significant temporal and spatial variation. Quantitative studies using global three-dimensional simulations allow to find and predict these complicated magnetic field structures as they propagate from the low corona to 1 AU. Furthermore, we are able to distinguish shock compressed solar wind plasma from CME ejecta in synthetic white-light images which are consistent with coronagraph observations. Capturing such shock is a necessary step in building a quantitative model of SEP acceleration and transport that can be used to forecast and mitigate the radiation hazards for spacecraft.   

In-situ observations of collisionless shocks: A review of CLUSTER satellite observations of the Earth’s bow shock

Of all the ways in which a magnetized collisionless plasma shock differs from its neutral fluid counterpart, one of the most striking is the fact that at a collisionless shock, particles are observed to stream from the shock back into the upstream region. The combination of the backstreaming particles together with the upstream core plasma distribution is subject to a number of instabilities that lead to the generation of waves and subsequent wave-particle interactions, all forming an integral part of the shock. The Cluster mission, launched in 2000 is a constellation of four identical spacecraft in polar orbit around the Earth. It is designed to resolve spatial and temporal variations in space plasmas, in particular at the Earth’s bow shock. Major results from the mission thus far relating to shocks are described. In particular, observations of shock structure, the cross-shock electric field and various ion beam wave instabilities are presented. Finally, we consider the open questions relating to collisionless shock physics, in particular concentrating on time-dependence introduced by variations in the upstream conditions, and describe the challenges facing the next generation of space missions.   

Kinetic Alfven Waves and Electron Physics in Oblique Slow Shocks

Particle-in-cell simulations are used to examine kinetic Alfven waves and electron physics in very oblique slow shocks and in slow shocks with moderate shock angles under low electron beta conditions. In these shocks, the downstream has a parallel electron temperature anisotropy, as observed in slow shocks in space. The anisotropy results from electron heating through Landau resonance in the parallel electric fields of obliquely propagating kinetic Alfven waves (KAW) excited by ion-ion streaming. In the shock ramp, spiky structures occur in density and electron parallel temperature, where the ion parallel temperature decreases due to the reduction of the ion backstreaming speed. The electron and ion dynamics in the KAW fields found in slow shocks are further confirmed with results from simulation and linear Vlasov theory of ion-ion streaming interaction without the shock. PIC or a hybrid method with a more sophisticated electron fluid model are required to accurately model the dissipative and acceleration processes in these slow shocks.   

The Acceleration of Energetic Particles at and Near the Solar Wind Termination Shock

The Voyager 1 (V1) spacecraft crossed the solar wind termination shock on 16 December 2004 at a distance of 94 AU from the Sun, marking the first time that a spacecraft has entered the heliosheath. For 2.5 years prior to that crossing, beams of low-energy particles, highly variable in intensity, were observed to be streaming along the magnetic field line connecting the shock to V1. The energy spectrum of H from $\sim $50 keV to $\sim $20 MeV in both the upstream and downstream regions of the shock shows evidence of two components consistent with a two-stage injection and acceleration process at the termination shock. At higher energies, observations of anomalous cosmic rays (ACRs) yielded a big surprise. The intensity of ACRs, which originate as interstellar neutral atoms that drift into the heliosphere and become ionized in the solar wind, was predicted to peak at the shock where they were thought to be accelerated. However, the ACR He spectrum did not peak at the termination shock, indicating that the source of ACRs was elsewhere than where V1 crossed the shock. However, the ACR intensity unexpectedly increased as V1 continued downstream of the shock as though V1 was approaching the source region. At the same time, Voyager 2 (V2) is nearing the termination shock and is expected to cross during the next year. Together the V1 and V2 observations should help clarify where and how particles are accelerated in the outer heliosphere.   

Solar Energetic Particles as a Laboratory for the Study of Shock Physics

Shocks driven by fast coronal mass ejections (CMEs) are generally believed to be the dominant accelerators in large, gradual solar energetic particle (SEP) events. A key challenge for this notion has been the highly variable spectral and compositional characteristics above a few tens of MeV per nucleon. Although this variability is a daunting problem, there are also high levels of correlation among the variable factors, which provide compelling clues to their origin. I will review observations from ACE, Wind and other spacecraft that have yielded these clues. I will also review recent efforts to understand this high-energy variability in terms of the interplay of two factors: evolution in the shock-normal angle as the shock moves outward from the Sun; and a compound seed population, typically comprising at least suprathermals from the corona (or solar wind) and suprathermals from flares. A simple analytical implementation of these ideas has been shown to semi-quantitatively account for a wide range of SEP phenomenology, including facets that have been known but unexplained for more than 20 years. These simple calculations also highlight a number of questions where we should be able to use SEP observations to deepen our understanding of shock acceleration.