Secondary reconnection sites in reconnection-generated flux ropes and reconnection fronts

Abstract

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...

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

“…One possibility is patchy reconnection, which can develop magnetic islands in 2D or flux ropes if a guide field, even small, is present. Turbulence in the magnetosheath plasma, a common characteristic, may also cause patchy reconnection or tangled B. Alternatively, flux ropes or tangled B may be generated from turbulence resulting from the reconnection process or its outflow as has been observed in 3D simulations [25,26].…”

“…The B fluctuations suggest that a flux rope or a tangled magnetic field topology that is associated with magnetic reconnection process. Larger-scale flux ropes have been observed [35,36] and simulated [25].…”

“…The 3D topology of magnetic reconnection is not as well understood; reconnection can drive turbulence, which can lead to complex magnetic topologies [e. g., 25,26]. In addition, turbulence in the magnetosheath plasma can introduce irregular boundary conditions, which also can result in complex magnetic topologies.…”

“…This letter concentrates on a particular type of E || events, large-amplitude and unipolar, that are observed in conjunction with complex magnetic field structures. We hypothesize that these E || events represent secondary reconnection [25]. Patchy reconnection or turbulence along the separatrix can result in flux ropes and tangled magnetic fields, which, ultimately must resolve or untangle.…”

Magnetic Null Points in Kinetic Simulations of Space Plasmas

“…The physical units in the code are normalized to the corresponding plasma parameters: proton inertial length d i , proton plasma frequency pi w , and proton mass m i ; hence, the magnetic field unit is m e i pi w . The iPic3D code has been applied to a variety of models: "classical" magnetic reconnection Lapenta et al 2010), three-dimensional magnetic reconnection (Vapirev et al 2013;Lapenta et al 2015), magnetic reconnection at null points (Olshevsky et al 2015a), lunar magnetic anomalies , andplanetary magnetospheres (Peng et al 2015c). Recently introduced two-way coupling of the iPic3D with the BATS-R-US fluid model (Daldorff et al 2014) opens even broader perspectives for, e.g., full-scale models of planetary magnetospheres.…”