Although high-resolution single-particle cryo-electron microscopy (cryo-EM) is now producing a rapid stream of breakthroughs in structural biology, it nevertheless remains the case that the preparation of suitable frozen-hydrated samples on electron microscopy grids is often quite challenging. Purified samples that are intact and structurally homogeneouswhile still in the test tube -may not necessarily survive the standard methods of making extremely thin, aqueous films on grids. As a result, it is often necessary to try a variety of experimental conditions before finally finding an approach that is optimal for the specimen at hand. Here, we summarize some of our collective experiences to date in optimizing sample preparation, in the hope that doing so will be useful to others, especially those new to the field. We also hope that an open discussion of these common challenges will encourage the development of more generally applicable methodology. Our collective experiences span a diverse range of biochemical samples and most of the commonly used variations in how grids are currently prepared. Unfortunately, none of the currently used optimization methods can be said, in advance, to be the one that ultimately will work when a project first begins. Nevertheless, there are some preferred first steps to explore when facing specific problems that can be more generally recommended, based on our experience and that of many others in the cryo-EM field.Correspondence to R.M. Glaeser

Magnetic reconnection is a fundamental process with explosive energy conversion from magnetic fields to plasmas and rapid reconfiguration of magnetic field lines. In general, the reconnecting magnetic fields do not have to be antiparallel, and an additional magnetic component known as the guide field (B g ) can appear in the direction perpendicular to the reconnecting plane. With the increase of B g , the guide field can gradually magnetize electrons in the central diffusion region (e.g., Le et al., 2013), resulting into the transition from antiparallel to guide-field reconnection (Swisdak et al., 2005). In guide-field reconnection, B g can cause the diamagnetic drift of the X-line, which may eventually suppress reconnection (e.g., Phan et al., 2013;Swisdak et al., 2003). The reconnection current sheet is deflected by the J L × B g force (J L is the current along the reconnecting direction, Goldman et al., 2011;Tang et al., 2022), and the Hall field structure is accordingly distorted (Eastwood et al., 2010). The reconnection electric field becomes parallel to the guide field in the vicinity of the X-line, which leads to the shift of the local energy conversion location to the magnetic field reversal points (Genestreti et al., 2017) and the significance of parallel energy dissipation (Wilder et al., 2018). In addition, this parallel electric field (E ‖ ) can accelerate electrons from one direction, and decelerate electrons from the other direction, forming a density cavity on one edge of the exhaust (Eastwood et al., 2018) and electron beams that are unstable for electron beamtype instabilities (e.g.,