SM22A-01 INVITED
In-Situ Observations of Magnetic Reconnection and Associated Energetic Particle Acceleration in Near-Earth Space
Magnetic reconnection is considered to play an important role for the acceleration of energetic particles in laboratory plasmas, planetary magnetospheres and solar corona. Despite of much observational evidence that energetic particles are produced in reconnection regions, the acceleration mechanisms are yet poorly understood. The near-Earth space is an excellent laboratory to study the acceleration mechanisms since multi-spacecraft coordinated measurements of energetic particles and electromagnetic fields in reconnection regions are available in-situ. Here we present Cluster spacecraft observations in a thin reconnecting current sheet in the magnetotail and discuss the acceleration mechanisms of ~ 100 keV electrons. We also present Cluster observations in the terrestrial magnetosheath to show evidence of reconnection in turbulent plasma and discuss associated particle acceleration.
SM22A-02
Observations of Turbulence Generated by Magnetic Reconnection
Cluster observations of fluctuating electric and magnetic fields within a magnetic reconnection ion diffusion region (guide field ~ 0) in the Earth's magnetotail are presented. It is shown that in the inertial subrange, the electric and magnetic fluctuations both followed a -5/3 power law; at higher frequencies, the spectral indices were -1 and -8/3 respectively. The associated dispersion relation is examined, and is found to be consistent with fast mode/whistler waves. Lower hybrid waves, which could be enhanced by whistler mode conversion, are also observed. The anomalous resistivity associated with these waves is examined, and whilst it is thought that it does not significantly modify the reconnection rate, the waves could play a role in particle acceleration.
SM22A-03
Particle acceleration by fluctuating electric fields
Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the primary candidate mechanisms for releasing non-potential energy from magnetized plasmas. A collisionless magnetic reconnection scenario could provide both the energy release mechanism and the particle accelerator in flares. We studied particle acceleration consequences from fluctuating (in-time) electric fields superposed on an X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic generic features of dynamic reconnection, or the reconnective dissipation of a linear disturbance. Time evolution of thermal particle distributions are obtained by numerically integrating particle orbits. A range of frequencies of the electric field is used, representing a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and often have energy distributions of bimodal form. Protons are accelerated to gamma-ray producing energies and electrons to and above hard X-ray producing energies in timescales of less than 1 second. The acceleration mechanism could be applicable to all collisionless plasmas.
SM22A-04
Experiments on 3D Evolution of Spontaneous Magnetic Reconnection
We conduct experiments in the Versatile Toroidal Facility (VTF) at MIT to study magnetic reconnection in the strong guide-field regime. A set of coils inside the vacuum chamber is used to drive the reconnection by effectively pulling their currents away from the x-line. By Faraday's Law, a toroidal electric field is induced. The plasma responds initially with steady reconnection, but then after 10 tA (where the Alfven time is tA = L/vA, L is the machine size and vA is the Alfven velocity) there is a spontaneous burst of fast reconnection [1]. Although the external drive is axisymmetric, we observe non-axisymmetry (3D effects) in the plasma's response: the reconnection event starts at one toroidal angle and then propagates around the machine in one Alfven time (10 μ s). Our diagnostics include multiple arrays of magnetic and Langmuir probes at multiple toroidal angles. These diagnostics allow us to measure the 3D properties of this low- collisionality reconnection and identify an external interchange mode as the principle trigger of the event. [1] J. Egedal et al, Phys. Rev. Lett. 98, 015003 (2007)
SM22A-05
Two Rotational Discontinuities in Collision: An Alternate Interpretation to the Suggestion of Widespread "Magnetic Reconnection" in the Solar Wind
With a general non-dissipative model of two interacting rotational discontinuities (RDs) with opposing phase velocities along B we have reproduced the recently reported suite of signatures interpreted as those of collisionless magnetic reconnection in the solar wind. This model has been derived analytically and also reinforced with novel hybrid simulations of the interaction. Nearly a complete degeneracy is thus illustrated between the experimental evidence used to support the solar wind reconnection interpretation and the highly probable passing of two RDs through one another in the solar wind plasma that is often modeled as a "sea" of Alfvenic turbulence. The recovered morphology includes (i) the characteristic Walen signature with opposing signs, (ii) the Hall signatures, (iii) the ion phase space beams, (iv) the inferred "reconnection" rate from the non-zero tangential electric field and (v) counter-streaming jets. There is already evidence in the solar wind from B-V correlations that Alfvenic power is propagating in both directions relative to the radial and hence magnetic field direction. From this fact it is clear that the likelihood of the reconnection interpretation of the oppositely signed B-V correlations goes like the product of two probabilities P(a)P(b): P(a) for the inference that the shears are opposing RD layers and P(b) that assesses the probability that an unmeasured electron current scale layer enabling collisionless reconnection is attached to the observed MHD patterns. The present paper explains the observation only assuming that P(a) is a high likely in the Alfven turbulence already documented in the interplanetary medium. None of the presented evidence is compelling that P(b) is appreciable. Thus, by Occam's razor the two RD interaction (without attending reconnection) interpretation is the more likely interpretation of the space data, pending further differentiation of these two possibilities.
SM22A-06
Observations of 3-D Reconnection and Dynamics of Electron Scale Thin Current Sheets with Small Satellite Separation
A variety of spacecraft separation distances, together with different constellation orientations, are important to fully investigate neutral sheet dynamics and the complex geophysical phenomena that occur there. Beginning on June 20, 2007, two of the four Cluster satellites were in orbit in a formation with only 17km separation. The new orientation, with two spacecraft very close together, provided an excellent opportunity to study thin neutral sheets and to investigate the micro- and meso-scale dynamics of critical magnetospheric phenomena. In this talk, we will concentrate on several reconnection events in magnetotail region with small separation distances and high resolution fields, particles and waves data. We will show the results from a study of dynamics and structure of thin current sheets and the structure of the 3-D magnetic null on the electron scale. We also will investigate the related particle dynamics, the characteristics of waves and plasma flows in the vicinity of the reconnection site.
SM22A-07
Analysis of ESWs within reconnection diffusion region and slow-shock in the near-earth magnetotail: Geotail observations
ESWs are usually observed in the plasma sheet boundary layer in the magnetotail, and in the vicinity of the magnetic reconnection diffusion region. Here we report the observation of ESWs in the vicinity of the core region of a magnetic reconnection X-null in the near-earth tail. The intricate X-null structure includes the twisted magnetic field Bz on the earthward-side in the X-Y plane and a slow-shock on the tailward side. From the WaveForm Capture (WFC) measurement, wave turbulence has been analyzed in the close vicinity of X- null region. Results confirm ESWs exist in the region very close to the core of the X-null, which is suggested to be a generation source of such kind of solitary structure. The ESWs are observed both in the upstream and downstream of the slow-shock in the core of X-null region, and are also observed in the +Y and -Y side of the twisted null structure. The opposite polarization of the ESW in the upstream and downstream of the slow-shock around the X-null can be used as a tracer to derive the source region of the ESWs associated with tail reconnection. The ESWs' spatial structure is re-constructed by assuming the perfect synchronous bi- polar waveform in the direction parallel to the ambient field. The ESW can propagate along (parallel to or anti-parallel to) the ambient magnetic field after its generation. The amplitude of ESWs weakens as the distance from the X-null region increases and thus the electron hole is suggested to die out. The spatial structure of ESWs around the vicinity of the X-null can bring a new light to the ESWs' evolution and even to the energy release process during reconnection.