M. V. Goldman

Nonlinear two‐stream instabilities as an explanation for auroral bipolar wave structures

The evolution of two counter-streaming electron beams is shown by means of 2-D kinetic simulations to lead to electron distributions and coherent localized bipolar plasma wave structures with features similar to those measured by the FAST satellite in the auroral ionosphere. Electrostatic whistler waves are generated at later times when the bipolar structures begin to lose coherence and break up in the dimension transverse to the geomagnetic field.

Evolution of electron phase-space holes in 3D

Electron phase-space holes are regions of de- pletedelectrondensitycommonlygeneratedduringthenon- linear stage of the two-stream instability. Recently, bipolar electric eld structures | a signature of electron holes | havebeenidentiedintheacceleration regionof theauroral ionosphere. This paper compares the evolution of electron holes in 2-D and 3-D using massively-parallel PIC simula- tions. In 2-D, the holes decay after hundreds of plasma periods while emitting electrostatic whistler waves. In the 3-Dsimulations,electronholesalsogounstableandgenerate whistlers but, due to physical processes not present in 2-D, energy flows out of the whistlers and into highly perpen- dicular lower hybrid modes. As a result of this dierence, 3-D holes do not decay as far as 2-D holes. The dierences between 2-D and 3-D evolution may have important impli- cationsforholelongevityandwavegenerationintheauroral ionosphere.

Scales of guide field reconnection at the hydrogen mass ratio

We analyze the signatures of component reconnection for a Harris current sheet with a guide field using the physical mass ratio of hydrogen. The study uses the fully kinetic particle in cell code IPIC3D to investigate the scaling with mass ratio of the following three main component reconnection features: electron density cavities along the separatrices, channels of fast electron flow within the cavities, and electron phase space holes due to the Buneman instability in the electron high speed channels. The width and strength of the electron holes and of the electron cavities are studied up the mass ratio proper of hydrogen, considering the effect of the simulation box size, and of the boundary conditions. The results compare favorably with the existing data from the Cluster and Themis missions and provide quantitative predictions for realistic conditions to be encountered by the planned magnetospheric multiscale mission.

Bipolar electric field signatures of reconnection separatrices for a hydrogen plasma at realistic guide fields

In preparation for the MMS mission we ask the question: how common are bipolar signatures linked to the presence of electron holes along separatrices emanating from reconnection regions? To answer this question, we conduct massively parallel simulations for realistic conditions and for the hydrogen mass ratio in boxes larger than considered in similar previous studies. The magnetic field configuration includes both a field reversal and a out of plane guide field, as typical of many space situations. The guide field is varied in strength from low values (typical of the Earth magnetotail) to high values comparable to the in plane reconnecting field (as in the magnetopause). In all cases, along the separatrices a strong electron flow is observed, sufficient to lead to the onset of streaming instabilities and to form bipolar parallel electric field signatures. The presence of bipolar structures at all guide fields allows the control of the MMS mission to consider the presence of bipolar signatures as a general flag of the presence of a nearby reconnection site both in the nightside and in the dayside of the magnetosphere.

Ion reflection and acceleration near magnetotail dipolarization fronts associated with magnetic reconnection

Dipolarization fronts (DFs) are often associated with the leading edge of earthward bursty bulk flows in the magnetotail plasma sheet. Here multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations are used to show that a spatially limited region of counterpropagating ion beams, whose existence is not evident in either the plasma moments or the electric field, is observed on the low-density side of DFs. The THEMIS magnetic field data are used to establish appropriate comparison cuts through a particle-in-cell simulation of reconnection, and very good agreement is found between the observed and simulated ion distributions on both sides of the DF. Self-consistent back tracing shows that the ion beams originate from the thermal component of the preexisting high-density plasma into which the DF is propagating; they do not originate from the inflow region in the traditional sense. Forward tracing shows that some of these ions can subsequently overtake the DF and pass back into the high-density preexisting plasma sheet with an order-of-magnitude increase in energy; this process is distinct from other ion reflection processes that occur directly at the DF. The interaction of the reconnection jet with the preexisting plasma sheet therefore occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence of this study is the conclusion that reconnection jets are not simply fed by plasma inflow across the separatrices but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.

Magnetospheric Multiscale Satellites Observations of Parallel Electric Fields Associated with Magnetic Reconnection.

We report observations from the Magnetospheric Multiscale satellites of parallel electric fields (E_{∥}) associated with magnetic reconnection in the subsolar region of the Earths magnetopause. E_{∥} events near the electron diffusion region have amplitudes on the order of 100  mV/m, which are significantly larger than those predicted for an antiparallel reconnection electric field. This Letter addresses specific types of E_{∥} events, which appear as large-amplitude, near unipolar spikes that are associated with tangled, reconnected magnetic fields. These E_{∥} events are primarily in or near a current layer near the separatrix and are interpreted to be double layers that may be responsible for secondary reconnection in tangled magnetic fields or flux ropes. These results are telling of the three-dimensional nature of magnetopause reconnection and indicate that magnetopause reconnection may be often patchy and/or drive turbulence along the separatrix that results in flux ropes and/or tangled magnetic fields.

On Multiple Reconnection X-lines and Tripolar Perturbations of Strong Guide Magnetic Fields

We report new multi-spacecraft Cluster observations of tripolar guide magnetic field perturbations at a solar wind reconnection exhaust in the presence of a guide field B-M. which is almost four ti ...

EVIDENCE OF MAGNETIC FIELD SWITCH-OFF IN COLLISIONLESS MAGNETIC RECONNECTION

The long-term evolution of large domain particle-in-cell simulations of collisionless magnetic reconnection is investigated following observations that show two possible outcomes for collisionless reconnection: toward a Petschek-like configuration or toward multiple X points. In the present simulation, a mixed scenario develops. At earlier time, plasmoids are emitted, disrupting the formation of Petschek-like structures. Later, an almost stationary monster plasmoid forms, preventing the emission of other plasmoids. A situation reminiscent of Petschek’s switch-off then ensues. Switch-off is obtained through a slow shock/rotational discontinuity compound structure. Two external slow shocks (SS) located at the separatrices reduce the in-plane tangential component of the magnetic field, but not to zero. Two transitions reminiscent of rotational discontinuities (RD) in the internal part of the exhaust then perform the final switch-off. Both the SS and the RD are characterized through analysis of their Rankine–Hugoniot jump conditions. A moderate guide field is used to suppress the development of the firehose instability in the exhaust.

Study of electric and magnetic field fluctuations from lower hybrid drift instability waves in the terrestrial magnetotail with the fully kinetic, semi-implicit, adaptive multi level multi domain method

The newly developed fully kinetic, semi-implicit, adaptive multi-level multi-domain (MLMD) method is used to simulate, at realistic mass ratio, the development of the lower hybrid drift instability (LHDI) in the terrestrial magnetotail over a large wavenumber range and at a low computational cost. The power spectra of the perpendicular electric field and of the fluctuations of the parallel magnetic field are studied at wavenumbers and times that allow to appreciate the onset of the electrostatic and electromagnetic LHDI branches and of the kink instability. The coupling between electric and magnetic field fluctuations observed by Norgren et al. [“Lower hybrid drift waves: Space observations,” Phys. Rev. Lett. 109, 055001 (2012)] for high wavenumber LHDI waves in the terrestrial magnetotail is verified. In the MLMD simulations presented, a domain (“coarse grid”) is simulated with low resolution. A small fraction of the entire domain is then simulated with higher resolution also (“refined grid”) to capture ...

Buneman instability in a magnetized current-carrying plasma with velocity shear

Buneman instability is often driven in magnetic reconnection. Understanding how velocity shear in the beams driving the Buneman instability affects the growth and saturation of waves is relevant to turbulence, heating, and diffusion in magnetic reconnection. Using a Mathieu-equation analysis for weak cosine velocity shear together with Vlasov simulations, the effects of shear on the kinetic Buneman instability are studied in a plasma consisting of strongly magnetized electrons and cold unmagnetized ions. In the linearly unstable phase, shear enhances the coupling between oblique waves and the sheared electron beam, resulting in a wider range of unstable eigenmodes with common lower growth rates. The wave couplings generate new features of the electric fields in space, which can persist into the nonlinear phase when electron holes form. Lower hybrid instabilities simultaneously occur at k||/k⊥~me/mi with a much lower growth rate and are not affected by the velocity shear.