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Timetable (MRTW01)

Magnetic Reconnection in the Sun and the Magnetosphere

Monday 9th August 2004 to Tuesday 10th August 2004

Monday 9th August 2004
09:00 to 09:30 The topology of three-dimensional reconnection

The process of magnetic reconnection in the absence of neutral points is analysed with respect to the topology of the magnetic field. It is shown that the domain of magnetic flux undergoing reconnection comprises regions of different reconnective behaviour. There are regions where the process generates only a certain type of slippage of plasma with respect to the field lines, as well as regions where the magnetic field is reconnected in a way similar to the classical two-dimensional reconnection. It is shown that this behaviour is consistent with the small but non-vanishing production of magnetic helicity in this case. An interpretation of the rate of reconnection for the different regions gives further insights into the complex process of three-dimensional reconnection.

09:30 to 10:00 Singularities in 3D-Euler \& MHD INI 1
10:00 to 10:15 Kinematic 3D reconnection at nulls INI 1
10:15 to 10:30 MHD wave propagation in the neighbourhood of 2D null point

The nature of fast magnetoacoustic and Alfvén waves is investigated in a zero beta plasma. This gives an indication of wave propagation in the low beta solar corona. It is found that for a two-dimensional null point, the fast wave is attracted to that point and the front of the wave slows down as it approaches the null point, causing the current density to accumulate there and rise rapidly. Ohmic dissipation will extract the energy in the wave at this point. This illustrates that null points play an important role in the rapid dissipation of fast magnetoacoustic waves and suggests the location where wave heating will occur in the corona. The Alfvén wave behaves in a different manner in that the wave energy is dissipated along the separatrices. For Alfvén waves that are decoupled from fast waves, the value of the plasma beta is unimportant. However, the phenomenon of dissipating the majority of the wave energy at a specific place is a feature of both wave types.

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10:30 to 10:45 G Abel (British Antarctic Survey)
Fractal reconnection at the Earth's magnetopause \& associated ionospheric convection

Large scale properties of reconnection structures on the magnetopause can be explained successfully by simple models incorporating laminar magnetosheath flow with antiparallel reconnection. However, such models are inconsistent with the highly turbulent nature of the magnetosheath flow adjacent to the magnetopause. This presentation proposes a fractal reconnection model that resolves this contradiction by replacing the laminar magnetosheath flow with a turbulent flow which has realistic levels of fluctuation. The resultant fractal reconnection structures preserve the large scale behaviour of simpler models, consistent with ground based observations, but have small scale fluctuations consistent with those observed in situ by spacecraft. We also present observations of evidence for scale-free fluctuations in ionospheric convection associated with dayside reconnection. This scale free behaviour may well arise as a consequence of the fractal reconnection model proposed.

11:30 to 12:00 Overview of collisionless reconnection theory

Particle-in-cell simulations in 3D are used to explore the physics of collisionless magnetic reconnection. The presence of a moderate guide field does not alter significantly the reconnection rate but does drastically change the detailed dynamics of the reconnection region. Away from the neutral line an electron beam feature (produced by a finite parallel electric field) produces turbulence in the ion plasma frequency range; the region near the X line, however, remains quiet with respect to wave turbulence. Futher issues affecting reconnection such as the roles of external driving and the normal magnetic field component will be considered.

12:00 to 12:30 M Hesse ([NASA, Goddard])
The physics of the reconnection diffusion region

We present an analysis based on particle-in-cell simulations and kinetic theory of the electron dissipation region in component merging. Specifically, we will derive scaling laws of the electron demagnetization scale based on electron nongyrotropy effects, which continue to play the dominant role even in the presence of a guide field. We will compare our results to those of other recent and on-going investigations, and derive a general expression of the reconnection electric field for guide-field magnetic reconnection. The role of electron heat flux will be discussed explicitly.

12:45 to 13:00 Hall magnetic reconnection in 2D \& 3D

Numerical results of two and three dimensional magnetic reconnection in the Hall limit (L < c/wpi where c/wpi ion inertial length) are presented. Two dimensional Hall magnetohydrodynamic (MHD) simulations are used to determine the magnetic reconnection rate in the Hall limit. The simulations are run until a steady state is achieved for four initial current sheet thicknesses: L = 1,5,10, and 20 c/wpi It is found that the asymptotic (i.e., time independent) state of the system is nearly independent of the initial current sheet width. Specifically, the Hall reconnection rate is weakly dependent on the initial current layer width and is ~ 0.1 V_A0B_0 where V_A0 and B_0 are the Alfven velocity and magnetic field strength in the upstream region. Moreover, this rate appears to be independent of the scale length on which the electron `frozen-in' condition is broken (as long as it is < c/wpi). The 3D reconnection process is initiated with a magnetic field perturbation localized along the current channel in a reversed field plasma configuration. The perturbation induces a magnetic wave structure that propagates opposite to the current, and leads to the asymmetric thinning of the plasma layer, strong plasma flows in the direction of the current, and rapid magnetic reconnection. The propagating wave structure is a Hall phenomenon associated with magnetic field curvature. The results are applied to reconnection processes in space.

14:30 to 15:00 The role of the Kelvin-Helmholtz instability in magnetic reconnection

Magnetic topology plays an important role in the global dynamics of high temperature plasmas. Within the ideal MHD plasma description, two plasma elements that are initially connected by a magnetic field line remain connected at any subsequent time. This condition introduces a topological linking between plasma elements that is preserved during the ideal plasma evolution. Magnetic linking constraints the plasma dynamics by making configurations with lower magnetic energy, but different topological linking, inaccessible. Magnetic field line reconnection partially removes these constrains by allowing the field lines to decouple locally from the plasma motion and to reknit in a different net of connections. In collisionless magnetic field line reconnection the decoupling between the magnetic field and the plasma motion occurs because of the current limitation due to the finite electron inertia (in the fluid limit) or to thermal effects (in the kinetic plasma description). However, in the absence of dissipation, the plasma response both in the fluid and in the kinetic electron treatment admits generalized linking conditions that in a two-dimensional configuration are preserved during the process of magnetic reconnection in the form of Lagrangian invariants.

Here we compare the analytical and numerical results obtained recently [1,2] in the study of the nonlinear development of magnetic reconnection in the fluid and in the drift-kinetic limits of the electron response and establish a clear link between these two regimes by showing that the (two) fluid Lagrangian invariants and the (infinite number of) drift-kinetic Lagrangian invariants evolve in time in an analogous fashion: in both cases the growth and saturation of the magnetic island is accompanied by their spatial mixing in the reconnection plane. In particular we show that in the cold electron fluid limit the pattern of current layers formed within the magnetic island in the nonlinear phase of the reconnection process is subject to the onset of a secondary instability of the Kelvin Helmholtz type which leads to a turbulent redistribution of the current layers and to the development of long lived fluid vortices inside the magnetic island.

[1] E. Cafaro, et al., Phys. Rev. Lett, 80, 4430 (1998); D. Grasso, et al., Phys. Rev.Lett., 86, 5051 (2001); D. Del Sarto, et al., Phys. Rev. Lett., 91, 235001 (2003). [2] T. Liseikina, et al., Phys. Plasmas, in press (2004).

15:00 to 15:30 BK Shimavoggi ([Central Florida])
Critical exponents and universality in fully developed turbulence

Electron-inertia effects on the magnetic reconnection induced by perturbing the boundaries of a slab of plasma with a magnetic neutral surface inside are considered. Cases with the boundaries perturbed at rates slow or fast compared with the hydromagnetic evolution rate are considered separately. When the boundaries are perturbed at a rate slow compared with the hydromagnetic evolution rate and fast compared with the resistive evolution rate a current sheet forms at the magnetic neutral surface which then disappears via exponential damping and diffusion and reconnection takes place. On the other hand, when the boundaries are perturbed at a rate fast compared with the hydromagnetic evolution rate, there is no time for the current sheet formation and reconnection to take place [1]. References: [1] N. Al-Salti and B. K. Shivamoggi: Phys. Plasmas, vol.10, p. 4271, (2003).

16:00 to 16:15 Parameter study of ion acoustic resistivity in collisionless plasmas

In order for magnetic reconnection to proceed, there must be some mechanism which can violate the ideal MHD condition in the diffusion region and allow plasma to diffuse across the magnetic field. One of the candidates for breaking the ideal MHD condition is resistivity or anomalous transport due to wave-particle interactions between unstable waves and the ambient plasma. We propose that in the presence of strong currents, ion-acoustic waves will be driven unstable and can provide significant values of resistivity. We present results from a parameter study of the resistivity due to this instability, concentrating on similar ion and electron temperature ratios. This parameter regime is difficult to study analytically, and so we use self-consistent Vlasov simulations to study the plasma response to the ion-acoustic instability. Our results show that the resistivity is very strongly dependent on the current present to drive the waves unstable.

16:15 to 16:30 Nonlinear evolution of the ion-acoustic instability INI 1
16:30 to 17:00 Impulsive reconnection dynamics INI 1
Tuesday 10th August 2004
09:00 to 09:30 Observational evidence for magnetic reconnection in the solar corona

Magnetic reconnection has long been identified as the source of impulsive energy release in solar flares. The process is also proposed as the energy source for other less energetic phenomena e.g. jets, transient active region brightenings. Examples of its role in heating large coronal structures have been proposed while it could also be involved in supplying energy to the corona through the continuous occurrence of nano-flaring. However conclusive and quantitative observational evidence of its ocurrence has been difficult to establish. In this talk, the available observational evidence will be reviewed and prospects for future advances examined.

09:30 to 10:00 C Parnell ([St Andrews])
Reconnection \& coronal heating

The role of driven reconnection in the heating of the background corona and small-scale events such as nanoflares, microflares and bright points will be the focus of the talk. Clearly, the dynamics of the magnetic carpet play a major role in all these phenomena. Furthermore, from detailed tracking of the solar magnetic carpet, estimates have been made as to the time it takes to completely reconnect all the magnetic field in the corona, i.e. estimates of the coronal reconnection recycle time have been found. These estimates suggest that, through motions of the magnetic fragments alone, the corona flux can be reconnected in 3 hours whereas, if emergence and cancellation are included, the flux is recycled in 1.4 hours.

10:00 to 10:15 Space- and time-dependent heating of solar coronal loops

We are investigating how space and time dependant heating of solar coronal loops affects their observable properties.

To make progress with the problem of how the solar corona is heated we need to understand how different heating mechanisms can be distinguished in observations. Loops outline the structure of the magnetic field and show where plasma is densest and may be strongly heated (hence luminous). Thus through observations of loops and the interpretation of those observations we can find clues to the heating mechanisms. Different heating mechanisms can produce different forms of spatially varying heating.

We simulate the loop in one dimension with a hydrodynamic scheme, applicable to loops shorter than the scale height. We apply three spatial profiles for heating, at frequencies ranging over four orders of magnitude. The heating variation drives waves and flows in the loop plasma. Spatial heating regimes produce clearly distinct relations between temperature and frequency, which can be characterised quite simply. These relations could be compared to observations to constrain heating models.

10:15 to 10:30 A critical test of reconnection theories on the magnetopause

The ionospheric signatures of reconnection are conditioned by the location of reconnection regions on the magnetopause, and by spatial and temporal variations in the associated reconnection electric field. These factors in turn depend on the underlying physics of reconnection. This talk will describe our work on using ionospheric signatures of reconnection, observed by SuperDARN radars, to distinguish between competing reconnection theories. This work is based on modelling results which provide a critical test for theories of magnetopause reconnection under certain seasonal and solar wind conditions. The implications of our results for the large-scale physics of reconnection will be discussed.

10:30 to 10:45 Anti-parallel merging \& component reconnection: role in magnetospheric dynamics

Magnetic reconnection is a key process in magnetospheric dynamics. In the presence of nonzero IMF By component, magnetic neutral points are formed near the cusp region or at the flunks. The relative role of almost anti-parallel merging near neutral points vs. component reconnection at the subsolar flow stagnation point is a matter of ongoing discussions. To address this problem we employ a combination of small-scale and meso-scale MHD simulations with nongyrotropic corrections to the magnetic induction equation and global MHD simulations with adaptive mesh refinement. For high resolution meso-scale simulations we modified the ideal MHD approach by allowing the ion fluid to be nongyrotropic. We demonstrated that the reconnection rate is controlled by ion nongyrotropic behavior near the reconnection site. For small-scale geometry comparison with the results of hybrid and particle simulations will be presented. The presence of large guide field reduce ion nongyrotropy effects and slow down the reconnection rate. We will also use global MHD simulation code BATSRUS to model dayside magnetopause dynamics after IMF turning from an initial northward orientation to IMF clock angles 90 < theta < 180. Analysis of the reconnected magnetic flux budget showed that reconnection occurs along the extended area at magnetopause surface. We found that magnetopause surface become unstable and demonstrated formation of plasma bubbles and flux ropes (FTEs).

11:30 to 12:00 Some open problems for magnetic reconnection in solar flares

Following a brief overview of current flare models and the role of reconnection suggested for them, I will discuss a few open problems, which appear to be promising items for further research, in relation to the observations. These include the following questions.

(1) Does a termination shock in the downward reconnection outflow form?

(2) What is the nature of the hot, hazy structure that has been observed in a few flares above the post-flare loop arcade? What is the nature of the dark downflows typically observed in them?

(3) Can we explain the large numbers of individual hard X-ray spikes typically observed in the impulsive phase of flares by ``impulsive bursty'', i.e. resistive, reconnection?

(4) Does reconnection actually produce the waves supposed by models of stochastic particle acceleration (e.g. Miller et al. 1997, JGR 102, 14,631)?

(5) Does reconnection in a current sheet formed above erupting flux play an important role in the flare process? Which typical observational consequences do we expect?

12:00 to 12:30 Active-region magnetic structures \& their perturbations by flares

The coronal structure of a solar active region consists of magnetic fields originating mostly in subphotospheric current systems. Except for loops with higher density (and therefore higher temperature, except for prominences) this coronal structure is invisible at X-ray wavelengths. In general the plasma beta is low and the Alfven speed is large, especially between the bright loops. Eruptive flares (CMEs) disrupt parts of this magnetic field, forming temporary sources of solar wind. They also launch blast waves that are have recently become detectable in soft X-rays and other wavelengths (in addition to the traditional Moreton wave and meter-wave type II bursts). The blast waves, in the best cases, can be imaged directly in soft X-rays near the flare core at the onset of the eruption. We find that type II bursts commonly occur (in 12/28 cases) in conjunction with the TRACE "kink mode" loop oscillations catalogued by Schrijver, Aschwanden, and Title. The oscillating loops occur in those parts of the active region not participating in the CME (recognized as "dimming" in the low corona). I review the observations of blast waves and kink-mode oscillations and discuss the implications for the eruption process.

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12:30 to 12:45 Topology of magnetic breakout INI 1
14:30 to 15:00 Exploring the nature of magnetic reconnection in solar transients

Magnetic reconnection in the solar atmosphere is universally accepted to be the primary process allowing transient solar activity, such as flares and CMEs. Direct and unequivocal observational signatures of reconnection on the Sun are outwith current instrument capabilities. However, by combining theoretical ideas with high temporal, spatial and spectral resolution data over a number of wavelengths, some of the expected consequences of reconnection can be tested. Conversely, by adopting reconnection as a working hypothesis the wealth of data relating to the spatial and temporal evolution of flares and CMEs can serve to constrain the conditions under which the reconnection must occur. In this paper we will discuss some recent work where theoretical ideas are coupled to detailed observations to gain insight into the (assumed) reconnection process.

15:00 to 15:30 Anti-parallel versus component reconnection at the Earth's magnetopause

Magnetic reconnection between the interplanetary magnetic field (IMF)

and the geomagnetic field is the important if not dominant process for mass, energy and momentum transfer from the Earth's magnetosheath to the magnetosphere. While reconnection is generally recognized as important, the details of the process are still under investigation and remain poorly understood.

The anti-parallel reconnection hypothesis states that reconnection

occurs at or near the region of magnetic field lines of exactly opposite polarity. However, recent observations have pointed to significant reconnection rates in regions with less than anti-parallel field share. For this component reconnection hypothesis, share angle as small as 50 degree between the magnetosheath and magnetospheric magnetic fields participating in reconnection have been observed. This presentation will review the current evidence for anti-parallel and component reconnection and how to remotely track the reconnection site.

16:00 to 16:30 Magnetic reconnection at the Dayside Terrestrial Magnetopause

The physics of magnetic reconnection at Earth's dayside magnetopause is addressed. We present data from three-dimensional resistive magnetohydrodynamics (MHD) simulations, demonstrating that when the plasma resistivity is spatially uniform and constant in time, magnetic reconnection occurs at the subsolar point via the flux pileup mechanism. When the interplanetary magnetic field (IMF) is due south, the dependence of the magnetic pileup on the Lundquist number is consistent with that predicted by simple analytical solutions of the resistive MHD equations (see, for example, Sonnerup and Priest, J. Plasma Phys., 14, 283, 1975). Thus, one expects the reconnection rate in such resistive MHD models to stall at a critical value of the Lundquist number, making it unlikely that such models will reproduce Dungey's open magnetosphere in the high Lundquist number limit (i.e., Petschek's slow shock mechanism does not appear to be relevant in such models). We present a possible solution of this flux pileup saturation problem: if the spatial scale of the stagnation point flow is comparable to the ion inertial length, then Hall electric fields can permit the magnetic flux pileup to saturate before the reconnection begins to stall. If time permits, we will discuss -- in the context of resistive MHD simulations -- the dependence of the geometry of dayside magnetopause reconnection on the IMF orientation.

16:30 to 17:00 W Baumjohann ([Graz])
Bifurcated \& thin current sheets: CLUSTER results

Bifurcated and/or thin current sheets in the Earth’s magnetotail have been predicted theoretically for the past couple of years. However, observational evidence, especially for bifurcated current sheets, was scarce. Data from the four Cluster spacecraft proved to be ideal to analyze the highly dynamic tail current sheet. It exhibits both thin current monolayers as well as bi furcated sheets, under quiet conditions as well as close to the reconnection site.

17:00 to 17:15 Walen & slow-mode shock analyses applied to high-speed flows of the near-Earth magnetotail

Observed changes in the high-speed magnetotail flow direction from earthward to tailward and vice versa have been interpreted as a reconnection X-line passing by the spacecraft. Here we analyze several such events using Cluster observations from the near-Earth magnetotail. We investigate to what extent tailward and earthward flows satisfy the Walen condition and discuss whether tailward flows are more likely to do so due to the obstacle posed by the inner magnetosphere to earthward flows. The hypothesis that tailward flows would satify the Walen relation more readily than earthward flows across Petschek-type slow-mode shocks will be examined based on the Rankine-Hugoniot shock jump conditions. These results suggest that X-lines may form closer to the Earth than X = -20 Re.

17:15 to 17:30 Plasma depletion at the high latitude dayside magnetopause: Cluster observations INI 1
University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons