A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

Magnetic reconnection is commonly considered as a mechanism of solar (eruptive) flares. A deeper study of this scenario reveals, however, a number of open issues. Among them is the fundamental question, how the magnetic energy is transferred from large, accumulation scales to plasma scales where its actual dissipation takes place. In order to investigate this transfer over a broad range of scales we address this question by means of a high-resolution MHD simulation. The simulation results indicate that the magnetic-energy transfer to small scales is realized via a cascade of consecutive smaller and smaller flux-ropes (plasmoids), in analogy with the vortex-tube cascade in (incompressible) fluid dynamics. Both tearing and (driven) "fragmenting coalescence" processes are equally important for the consecutive fragmentation of the magnetic field (and associated current density) to smaller elements. At the later stages a dynamic balance between tearing and coalescence processes reveals a steady (power-law) scaling typical for cascading processes. It is shown that cascading reconnection also addresses other open issues in solar flare research such as the duality between the regular largescale picture of (eruptive) flares and the observed signatures of fragmented (chaotic) energy release, as well as the huge number of accelerated particles. Indeed, spontaneous current-layer fragmentation and formation of multiple channelised dissipative/acceleration regions embedded in the current layer appears to be intrinsic to the cascading process. The multiple small-scale current sheets may also facilitate the acceleration of a large number of particles. The structure, distribution and dynamics of the embedded potential acceleration regions in a current layer fragmented by cascading reconnection are studied and discussed.

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere -a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases.

Context. Moving blob-like features observed in the soft X-ray and EUV range above flare-loops are often interpreted as signatures of plasmoids formed by the current sheet tearing in the flare-associated reconnection process. Aims. We investigate the evolution of the flare-associated current sheet numerically in order to analyse the kinematics and dynamics of plasmoids. The goal is to explain the broad diversity of kinematical properties of the plasmoid signatures recorded by various observational techniques. Methods. We performed a 2-dimensional resistive-MHD numerical simulation of the reconnection starting from the Harris-type current sheet. After identifying the plasmoids, we followed their motion to determine basic kinematical parameters (velocity and acceleration), and we analysed the associated magnetic field topology. Results. The simulation reveals a broad variety of the kinematical/dynamical properties of plasmoids -after formation, a plasmoid can move upward, downward, or can even change its direction of propagation. The highest velocities, in the range of the ambient Alfvén speed, are found in the case of upward propagating plasmoids. The acceleration is determined by the net magnetic field tension of the reconnected field lines. Downwardly propagating plasmoids achieve only a fraction of the ambient Alfvén speed. They strongly decelerate during the coalescence with low-lying flare-loops, when distinct energy-release peaks occur and loop system oscillations are excited. Conclusions. The presented results explain, qualitatively and quantitatively, the broad spectrum of kinematical properties of various observational features attributed to the current-sheet plasmoids.