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There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that comes the need to better shield society against space weather by improving forecasts, environmental specifications, and infrastructure design. We recognize that much progress has been made and continues to be made with a powerful suite of research observatories on the ground and in space, forming the basis of a Sun-Earth system observatory. But the domain of space weather is vastextending from deep within the Sun to far outside the planetary orbits -and the physics complex -including couplings between various types of physical processes that link scales and domains from the microscopic to large parts of the solar system. Consequently, advanced understanding of space weather requires a coordinated international approach to effectively provide awareness of the processes within the Sun-Earth system through observation-driven models. This roadmap prioritizes the scientific focus areas and research infrastructure that are needed to significantly advance our understanding of space weather of all intensities and of its implications for society. Advancement of the existing system observatory through the addition of small to moderate state-of-the-art capabilities designed to fill observational gaps will enable significant advances. Such a strategy requires urgent action: key instrumentation needs to be sustained, and action needs to be taken before core capabilities are lost in the aging ensemble. We recommend advances through priority focus (1) on observation-based modeling throughout the Sun-Earth system, (2) on forecasts more than 12 hrs ahead of the magnetic structure of incoming coronal mass ejections, (3) on understanding the geospace response to variable solar-wind stresses that lead to intense geomagnetically-induced currents and ionospheric and radiation storms, and (4) on developing a comprehensive specification of space climate, including the characterization of extreme space storms to guide resilient and robust engineering of technological infrastructures. The roadmap clusters its implementation recommendations by formulating three action pathways, and outlines needed instrumentation and research programs and infrastructure for each of these. An executive summary provides an overview of all recommendations.

The lower-hybrid drift instability is simulated in an ion-scale current sheet using a fully kinetic approach with values of the ion to electron mass ratio up to m i =m e 1836. Although the instability is localized on the edge of the layer, the nonlinear development increases the electron flow velocity in the central region resulting in a strong bifurcation of the current density and significant anisotropic heating of the electrons. This dramatically enhances the collisionless tearing mode and may lead to the rapid onset of magnetic reconnection for current sheets near the critical scale. DOI: 10.1103/PhysRevLett.93.105004 PACS numbers: 52.35.Vd, 52.35.Kt, 94.30.Ej, 94.30.Gm Current sheets with characteristic thickness of the order of a thermal ion gyroradius i are routinely observed in Earth's magnetosphere [1,2] and within laboratory experiments designed to examine the physics of magnetic reconnection [3], a topic with widespread application to space, astrophysical, and laboratory plasmas. Although current sheets are unstable to a variety of plasma instabilities including collisionless tearing [4] and the lower-hybrid drift instability [5], the relative importance of these instabilities to the onset and development of large scale magnetic reconnection remains controversial.The lower-hybrid drift instability (LHDI) is driven by the diamagnetic current in the presence of inhomogeneities in the density and magnetic field [6]. The LHDI has been considered extensively as a possible candidate to modify the reconnection physics through anomalous resistivity generated by wave particle interactions [5,7,8]. Unfortunately, theory predicts the fastest growing modes with a wavelength on the electron gyroscale k y e 1 are localized on the edge of the layer [5], while enhanced fluctuations are required in the central region to produce significant anomalous resistivity. This conclusion is supported by observations at the magnetopause [9], in the magnetotail [10], and by laboratory experiments [11].Based on this evidence, some researchers have concluded the LHDI does not play an important role in current sheet dynamics. However, new results from both theory and simulation are beginning to challenge this conclusion. In a number of simulations, a strong enhancement of the central current density associated with the LHDI is observed [12 -16] and it has been suggested this effect gives rise to the rapid onset of reconnection [14,15]. Most of these simulations were performed with artificial ion to electron mass ratios m i =m e 100-400 and very thin layers i =L 1:7-2:2, where L is the half thickness of the layer. Although the simulations in Ref.[13] considered thicker layers at realistic mass ratio, the focus was on long wavelength effects, and the spatial resolution was insufficient to resolve the full LHDI spectrum.The very thin layers considered in most of the simulations are comparable in thickness to laboratory reconnection experiments [3,17] but are considerably thinner than observed in the magnetotail prior to onset. In ...

The results of kinetic simulations of magnetic reconnection in Harris current sheets are analyzed. A range of guide fields is considered to study reconnection in plasmas characterized by different ␤ values, ␤Ͼm e /m i . Both an implicit particle-in-cell ͑PIC͒ simulation method and a parallel explicit PIC code are used. Simulations with mass ratios up to the physical value are performed. The simulations show that the reconnection rate decreases with the guide field and depends weakly on the mass ratio. The off-diagonal components of the electron pressure tensor break the frozen-in condition, even in low ␤ plasmas. In high ␤ plasmas, evidence is presented that whistler waves play a key role in the fast reconnection physics, while in low ␤ plasmas the kinetic Alfvén waves are important. The in-plane and the out-of-plane ion and electron motion are also considered, showing that they are influenced by the mass ratio and the plasma ␤.

Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: (i) it is much faster, developing on scales of the order of the Alfvén time, and (ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed-circulation patterns where the jets going out of the reconnection regions turn around and force their way back in, carrying along copious amounts of magnetic flux.

In this study, we apply a new method—the first‐order Taylor expansion (FOTE)—to find magnetic nulls and reconstruct magnetic field topology, in order to use it with the data from the forthcoming MMS mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the FOTE method can find a null both inside and outside the tetrahedron and also deduce its drift velocity. In addition, the FOTE method can (1) avoid limitations of the PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3‐D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2‐D (X and O); (3) reconstruct the magnetic field topology. We quantitatively test the accuracy of FOTE in positioning magnetic nulls and reconstructing field topology by using the data from 3‐D kinetic simulations. The influences of SC separation (0.05~1 di) and null‐SC distance (0~1 di) on the accuracy are both considered. We find that (1) for an isolated null, the method is accurate when the SC separation is smaller than 1 di, and the null‐SC distance is smaller than 0.25~0.5 di; (2) for a null pair, the accuracy is same as in the isolated‐null situation, except at the separator line, where the field is nonlinear. We define a parameter ξ ≡ |( λ1 + λ2 + λ3 )|/|λ|max in terms of the eigenvalues (λi) of the null to quantify the quality of our method—the smaller this parameter the better the results. Comparing to the previously used parameter (η≡|∇ ⋅ B|/|∇ × B|), ξ is more relevant for null identification. Using the new method, we reconstruct the magnetic field topology around a radial‐type null and a spiral‐type null, and find that the topologies are well consistent with those predicted in theory. We therefore suggest using this method to find magnetic nulls and reconstruct field topology with four‐point measurements, particularly from Cluster and the forthcoming MMS mission. For the MMS mission, this null‐finding algorithm can be used to trigger its burst‐mode measurements.

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We suggest a new approach that could be used for modeling both the large-scale behavior of astrophysical jets and the magnetically dominated explosions in astrophysics. We describe a method for modeling the injection of magnetic fields and their subsequent evolution in a regime where the free energy is magnetically dominated. The injected magnetic fields, along with their associated currents, have both poloidal and toroidal components, and they are not force free. The dynamic expansion driven by the Lorentz force of the injected fields is studied using threedimensional ideal magnetohydrodynamic simulations. The generic behavior of magnetic field expansion, the interactions with the background medium, and the dependence on various parameters are investigated.