INSTITUTET FÖR RYMDFYSIK	UPPSALA
Swedish Institute of Space Physics	(59°50.272′N, 17°38.786′E)

17th Cluster workshop

17th Cluster workshop is taking place in Uppsala 12-15 May 2009. Workshop homepage

Workshop is organize by Swedish Institute of Space Physics (IRF) and Centre for Dynamical Processes and Structure Formation (CDP). Below are listed the invited presentations that are of broad interest and are open for everybody to attend.

Place: University main building, Lecture Hall IX

Invited presentations

• 09:00 A. Brandenburg (Nordita, Sweden). Source and fate of the Sun's magnetic helicity
• 10:15 coffee
• 10:45 H. Ji (Princeton University, USA). Study of Electron-Scale Dissipations During Magnetic Reconnection
• 14:00 K. Kusano (JAMSTEC, Japan). The trigger mechanism of magnetic reconnection in solar eruption
• 14:30 G. Howes (University of Iowa, USA). Kinetic Turbulence in Weakly Collisional Plasmas
• 15:00 coffee

• 09:00 Y. Omura (RISH, Kyoto University, Japan). Nonlinear particle dynamics associated with electrostatic solitary waves and whistler-mode chorus emissions in the magnetosphere

Abstracts

A. Brandenburg
Nordita, Sweden

Source and fate of the Sun's magnetic helicity

Magnetic helicity has long been known to be an interesting diagnostics of solar wind turbulence. Its presence has also been detected on the solar surface. Although this quantity is notoriously noisy, there seems to be reasonable agreement that the Sun sheds negative magnetic helicity from its northern hemisphere and positive from its southern. The net flux is estimated to be about 10^46 Mx^2 per 11 year cycle. In my talk I will review new theoretical developments connected with magnetic helicity. No net magnetic helicity can be produced, owing to its conservation property. However, positive and negative magnetic helicities can be separated in space and scale, and magnetic helicity can also be lost through the surface. The latter is likely to happen through coronal mass ejections that can sustain a stream of magnetic helicity that extends far into the solar wind. The solar wind may still be able to reveal details abouts its source through its composition into field components of different sign and scale. In spite of gauge problems, the small scale component may best be identified through the density of linkages -- a concept that can be put on a rigorous basis.

H. Ji
Princeton University, USA

Study of Electron-Scale Dissipations During Magnetic Reconnection

Magnetic reconnection efficiently converts magnetic field energy to particle kinetic energy by breaking field lines and thus altering their topologies in magnetospheric plasmas as well as solar and more distant astrophysical plasmas. Despite its disruptive influences on the large-scale structures of these plasmas, the crucial topological changes and their associated dissipation take place only within thin current layers. The modern collisionless models predict that ions exhaust through a thick, ion-scale layer while mobile electrons leave through a thin, electron-scale layer, allowing for efficient release of magnetic energy. While ion layers have been frequently detected in space and studied in detail in the laboratory, the existence of electron layers near the X-line, and whether their associated dissipation results predominantly from laminar 2D or turbulent 3D dynamics, is still an open question. Here we report the first definite evidence for electron layers near the X-line [1] and the 3D nature of their dissipation in a reconnecting laboratory plasma [2]. The measured electron layers display properties strikingly similar to predictions by 2D full particle simulations [3], including their geometrical shape and insensitivity to ion mass, but disagree on the electron layer thickness. The electron layers are 3-5 times thicker in the laboratory than in simulations, and as a consequence, the widely hypothesized 2D effects due to electron nongyrotropic pressure are shown to be largely insufficient to explain the observed reconnection rates. These results effectively rule out all known 2D mechanisms operative in the simulation model as a main dissipation mechanism, suggesting that 3D effects, which include wave-particle interactions [4], are responsible for electron-scale dissipation during fast reconnection. Comparisons with space observations and possible future collaborative research will be discussed. [1] Y. Ren et al., PRL 101, 085003 (2008). [2] H. Ji et al., GRL 35, L13106 (2008). [3] S. Dorfman et al., PoP 15, 102107 (2008). [4] Y. Wang, R. Kulsrud, and H. Ji, PoP 15, 122105 (2008).

K. Kusano
JAMSTEC, Japan

The trigger mechanism of magnetic reconnection in solar eruption

Solar eruptions, which arise as flares and coronal mass ejection (CME), are the most energetic phenomena in our solar system. While magnetic reconnection is widely believed to be a key driver of solar eruptions, the initiation mechanism and the physical condition for the onset of them are not yet well understood. The objective of this study is to clarify how the multi-scale interaction in the solar corona is involved in the trigger process of solar eruptions. In the solar corona, there is vast gap between the geometrical scale of active region, in which magnetic free energy is loaded, and the diffusion scale of magnetic reconnection. Hence, in order to account for the onset process, we have to understand how the magneto-hydrodynamic (MHD) activity driven by the magnetic free energy creates a small scale structure through some nonlinearity. In this paper, first, we review several flare models from the view point of reconnection theory. Second, we propose a new model of flare onset, in which the mutual feed-back between two different reconnections can cause the explosive commencement of solar flare. The new model is examined in terms of the three-dimensional MHD simulations and the observation of the major flare occurred in the active region NOAA 10930 on Dec. 13, 2006. Finally, the results of our data-driven simulation of this eruptive event are shown, and the issues of reconnection as a multi-scale dynamics are addressed.

G. Howes
University of Iowa, USA

Kinetic Turbulence in Weakly Collisional Plasmas

Plasma is a ubiquitous form of matter in the universe, nearly always found to be both magnetized and turbulent. One must understand this behavior to interpret the observations of many astronomical environments, including the galactic interstellar medium, accretion flows around stars and black holes, and the solar wind streaming outward from our Sun. In such systems, at the large scales at which the turbulence is typically driven, the dynamics can often be adequately described as a fluid. Nonlinear interactions then drive a cascade of turbulent energy to ever smaller length scales, ultimately reaching the small scales at which the turbulence is dissipated. These dissipative scales are often smaller than the particle mean free path, requiring a kinetic description of the turbulent dynamics and the dissipation mechanism. This transfer of turbulent energy through an inertial range from the driving scale to dissipative scales in a kinetic plasma followed by the conversion of this energy into heat is a fundamental plasma physics process. In this talk, I will present a theoretical model of this kinetic turbulent cascade. The first nonlinear simulations of the kinetic turbulent cascade at the scale of the ion Larmor radius show good qualitative agreement with observations of turbulence in the solar wind. Progress in our understanding of kinetic turbulence necessarily requires a combined effort of theoretical modeling, nonlinear numerical simulations, and observational constraints from the turbulent solar wind.

Y. Omura
RISH, Kyoto University, Japan

Nonlinear particle dynamics associated with electrostatic solitary waves and whistler-mode chorus emissions in the magnetosphere

Recent spacecraft observations with high time resolution and large scale computer simulations with realistic models have revealed nonlinear nature of waves in space plasmas. Two different kinds of wave phenomena have been identified as nonlinear coherent waves that cannot be described by conventional linear and quasi-linear theories in which spectra of constant frequency waves with random phases are assumed. One is electrostatic solitary waves (ESW), and the other is whistler-mode chorus emissions. These wave phenomena are quite different in appearance, but they have the common ground for nonlinear dynamics of resonant electrons. ESW are generated by electron beam instabilities driven by beam electrons accelerated by shocks or induced electric field along a static magnetic field. ESW are primarily longitudinal electrostatic waves moving along a static magnetic field line, and they are observed in various regions of the magnetosphere including the plasma sheet boundary layer and auroral regions. ESW have coherent phase variation localized in space and time, and may not be described properly by frequency spectra. Whistler-mode chorus emissions, on the other hand, are transverse electromagnetic waves with coherent phase variation that appears typically as rising tones in frequency spectra. They are generated by whistler-mode instability driven by temperature anisotropy of energetic electrons trapped near the magnetic equator. Recent simulation studies show that a coherent chorus element starts to grow from a threshold amplitude with a progressively rising frequency near the magnetic equator, and it propagates away from the equator along the static magnetic field with the growing wave amplitude. In both wave phenomena, nonlinear wave trappings of energetic electrons play essential roles in generating the waves. ESW are associated with formation of electrostatic electron holes, which is a kind of BGK mode formed along a static magnetic field. The number of resonant electrons trapped in a wave potential is relatively smaller than that of untrapped electrons surrounding the wave potential, resulting in an electron hole. Likewise, whistler-mode chorus emissions are generated by formation of an electromagnetic electron hole in the three-dimensional velocity phase space through cyclotron resonance. Nonlinear motions of resonant electrons forming these electron holes are described by the same pendulum equation. In the case of chorus emissions, the inhomogeneity due to both frequency variation and spatial gradient of the magnetic field contribute to additional nonhomogeneous term in the pendulum equation, playing a vital role in formating a resonant current that gives rise to wave growth. Because of the frequency variation accompanying the nonlinear wave growth at the equator, a chorus element can interact with electrons with a wide range of energy from several keV to a few MeV at the equator. During the generation process we find that a fraction of resonant electrons are energized very efficiently by special forms of nonlinear wave trapping called relativistic turning acceleration (RTA) and ultra-relativistic acceleration (URA). Particle energization by nonlinear wave trapping is a universal acceleration mechanism that can be effective in space and cosmic plasmas that contain a magnetic mirror geometry.