Magnetic reconnection is a fundamental physical process in plasmas whereby stored 40 magnetic energy is converted into heat and kinetic energy of charged particles. 41Reconnection occurs in many astrophysical plasma environments and in laboratory 42 plasmas. Using very high time resolution measurements, NASA's Magnetospheric 43 2 Multiscale Mission (MMS) has found direct evidence for electron demagnetization and 44 acceleration at sites along the sunward boundary of Earth's magnetosphere where the 45 interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) 46 observed the conversion of magnetic energy to particle energy, (ii) measured the electric 47 field and current, which together cause the dissipation of magnetic energy, and (iii) 48identified the electron population that carries the current as a result of demagnetization 49 and acceleration within the reconnection diffusion/dissipation region. 50 51 Introduction 52

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Kinetic 1D simulations reveal that a weak density depression in a current-carrying plasma can lead to the formation of a strong potential ramp (double layer). The ramp and plasma turbulence it creates share many features with recent particle and field measurements in the auroral ionosphere. An electron beam accelerated by the ramp produces a series of propagating electron phase-space holes via a spatial two-stream instability. Electron heating associated with the formation and merging of these holes is found to influence the subsequent evolution of the potential ramp.

Abstract.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.

The primary target of the Magnetospheric MultiScale (MMS) mission is the electron-scale di usion layer around reconnection sites. Here we study where these regions are found in full three-dimensional simulations. In two dimensions the sites of electron di usion, defined as the regions where magnetic topology changes and electrons move with respect to the magnetic field lines, are located near the reconnection site. But in three dimensions we find that the reconnection exhaust far from the primary reconnection site also becomes host to secondary reconnection sites. Four diagnostics are used to demonstrate the point: the direct observation of topology impossible without secondary reconnection, the direct measurement of topological field line breakage, the measurement of electron jets emerging from secondary reconnection regions, and the violation of the frozen-in condition. We conclude that secondary reconnection occurs in a large part of the exhaust, providing many more chances for MMS to find itself in the right region to hit its target.T he primary focus of the planned Magnetospheric MultiScale (MMS) Mission 1 is the identification and in situ study of electron-scale regions where magnetic reconnection develops (http://mms.gsfc.nasa.gov/science.html). Magnetic reconnection 2 is believed to be the engine of many space and astrophysical processes where magnetic energy is stored over relatively long times to be released suddenly in bursts of kinetic energy. The mission planning and the thinking of the community has been in large part informed by the two-dimensional (2D) picture that has emerged from decades of simulations based on the classic field reversal configuration, where two regions of oppositely directed magnetic fields reconnect at one central location called the x-point. There magnetic field lines break and form new connections.This paradigm has been tremendously successful and from 2D simulations has found confirmation in many laboratory experiments 3 and in situ space observations 4 . When going from two to three dimensions, the model still remains valid if one can assume the presence of a region where the variations along the out-of-plane directions are small. In this case, extended reconnection regions develop, retaining a 2D-like configuration over significant widths in the out-of-plane direction 5,6 . But 3D effects can drastically alter the structure of a reconnection site 7 , with the possibility of inherently 3D configurations even in fast kinetic reconnection 8 .We think it is imperative to ask the question as to how these new 3D discoveries impact the execution of the MMS mission. The mission was designed with the two-nested-box vision 9 : around the reconnection site the ions become decoupled from magnetic field lines in a larger region, whereas the electrons continue to move along with the field lines until a smaller inner region is reached. This scheme is etched into the minds of every researcher working in reconnection and appears in the place of honour in the science section of the MMS web...