Notes on Magnetohydrodynamics of Magnetic Reconnection in Turbulent Media

Astrophysical fluids have very large Reynolds numbers and therefore turbulence is their natural state. Magnetic reconnection is an important process in many astrophysical plasmas, which allows restructuring of magnetic fields and conversion of stored magnetic energy into heat and kinetic energy. Turbulence is known to dramatically change different transport processes and therefore it is not unexpected that turbulence can alter the dynamics of magnetic field lines within the reconnection process. We shall review the interaction between turbulence and reconnection at different scales, showing how a state of turbulent reconnection is natural in astrophysical plasmas, with implications for a range of phenomena across astrophysics. We consider the process of magnetic reconnection that is fast in magnetohydrodynamic (MHD) limit and discuss how turbulence—both externally driven and generated in the reconnecting system—can make reconnection independent on the microphysical properties of plasmas. We will also show how relaxation theory can be used to calculate the energy dissipated in turbulent reconnecting fields. As well as heating the plasma, the energy dissipated by turbulent reconnection may cause acceleration of non-thermal particles, which is briefly discussed here.     

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.     

Global Monitoring for Environment and Security: data policy considerations

Data policy is an important element of the initiative on Global Monitoring for Environment and Security (GMES), which aims to establish a European capacity for the provision and use of operational information for monitoring and management of the environment and civil security by 2008. This paper documents the data policies of the variety of information providers likely to be involved in GMES, identifies the obstacles and draws conclusions on ways to improve data policy coherence. It addresses the main policy issues associated with Earth observation data, as well as with other types of environmental data.     

Combining MHD and kinetic modelling of solar flares

Solar flares are explosive events in the solar corona, representing fast conversion of magnetic energy into thermal and kinetic energy, and hence radiation, due to magnetic reconnection. Modelling is essential for understanding and predicting these events. However, self-consistent modelling is extremely difficult due to the vast spatial and temporal scale separation between processes involving thermal plasma (normally considered using magnetohydrodynamic (MHD) approach) and non-thermal plasma (requiring a kinetic approach). In this mini-review we consider different approaches aimed at bridging the gap between fluid and kinetic modelling of solar flares. Two types of approaches are discussed: combined MHD/test-particle (MHDTP) models, which can be used for modelling the flaring corona with relatively small numbers of energetic particles, and hybrid fluid-kinetic methods, which can be used for modelling stronger events with higher numbers of energetic particles. Two specific examples are discussed in more detail: MHDTP models of magnetic reconnection and particle acceleration in kink-unstable twisted coronal loops, and a novel reduced-kinetic model of particle transport in converging magnetic fields.     

The Solar Magnetic Field as a Coronal Hole Extension Forms: Effects of Magnetic Helicity and Boundary Conditions

An analytical solution is presented for linear force fields within a spherical shell, representing the solar corona. Allowing for a global magnetic helicity, we find magnetic fields over the entire corona with realistic inner boundary conditions obtained from transformation and extrapolation of photospheric magnetograms and considering alternative outer boundary conditions. Such fields are found for the well known coronal hole extension event of August 1996. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microphysics of Cosmic Plasmas: Hierarchies of Plasma Instabilities from MHD to Kinetic

In this article, we discuss the idea of a hierarchy of instabilities that can rapidly couple the disparate scales of a turbulent plasma system. First, at the largest scale of the system, L, current carrying flux ropes can undergo a kink instability. Second, a kink instability in adjacent flux ropes can rapidly bring together bundles of magnetic flux and drive reconnection, introducing a new scale of the current sheet width, ?, perhaps several ion inertial lengths (δ _i ) across. Finally, intense current sheets driven by reconnection electric fields can destabilize kinetic waves such as ion cyclotron waves as long as the drift speed of the electrons is large compared to the ion thermal speed, v _D ?v _i . Instabilities such as these can couple MHD scales to kinetic scales, as small as the proton Larmor radius, ρ _i .     

All terrain ground collision avoidance and maneuvering terrainfollowing for automated low level night attack

The development of an all-terrain ground collision avoidance system (GCAS) in conjunction with a maneuvering terrain-following (TF) system is discussed. Both systems use a digital terrain database. These systems are presented concurrently because of their complementary mission role. The unprecedented pilot interaction available with the automatic systems provides a means to improve combat survivability significantly. The pilot is able to execute high rate turns, evasive maneuvers, and inverted ridge crossings while following the terrain contour. Safety is maintained in day, night and weather by the GCAS. The combination of these systems with an automated attack system, automatic target handoff system, night vision system, and a route planner to provide a lethal night attack capability is examined. Piloted simulation and preliminary flight test results are presented     

A Cosmic Ray Signature of Equatorial Coronal Holes

The evolution of open field regions on the Sun over the last cycle is illustrated by observations of coronal holes in SOHO EIT images. The development of a large equatorial coronal hole near solar minimum is discussed, indicating the processes which led to the appearance of open field regions at low latitude. The observed cosmic ray signature is presented and interpreted in terms of the passage of the Earth through the streamer belt, which at this time had become distorted by the coronal hole and associated active region. The times when such equatorial coronal holes might be expected to directly influence cosmic ray counts in this way are seen to be limited to the approach to solar minimum, around minimum and the approach to maximum. This revised version was published online in August 2006 with corrections to the Cover Date.