Abstract. The Geospace Environmental Modeling (GEM) Reconnection Challengeproject is presented and the important results, which are presented in a series of companion papers, are summarized. Magnetic reconnection is studied in a simple Harris sheet configuration with a specified set of initial conditions, including a finite amplitude, magnetic island perturbation to trigger the dynamics. The evolution of the system is explored with a broad variety of codes, ranging from fully electromagnetic particle in cell (PIC) codes to conventional resistive magnetohydrodynamic (MHD) codes, and the results are compared. The goal is to identify the essential physics which is required to model collisionless magnetic reconnection. All models that include the Hall effect in the generalized Ohm's law produce essentially indistinguishable rates of reconnection, corresponding to nearly Alfv6nic inflow velocities. Thus the rate of reconnection is insensitive to the specific mechanism which breaks the frozen-in condition, whether resistivity, electron inertia, or electron thermal motion. The reconnection rate in the conventional resistive MHD model, in contrast, is dramatically smaller unless a large localized or current dependent resistivity is used. The Hall term brings the dynamics of whistler waves into the system. The quadratic dispersion property of whistlers (higher phase speed at smaller spatial scales) is the key to understanding these results. The implications of these results for trying to model the global dynamics of the magnetosphere are discussed.

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

How are biological structures maintained in a cellular environment that constantly threatens protein integrity? Here we elucidate proteostasis mechanisms affecting the Z disk, a protein assembly essential for actin anchoring in striated muscles, which is subjected to mechanical, thermal, and oxidative stress during contraction [1]. Based on the characterization of the Drosophila melanogaster cochaperone Starvin (Stv), we define a conserved chaperone machinery required for Z disk maintenance. Instead of keeping Z disk proteins in a folded conformation, this machinery facilitates the degradation of damaged components, such as filamin, through chaperone-assisted selective autophagy (CASA). Stv and its mammalian ortholog BAG-3 coordinate the activity of Hsc70 and the small heat shock protein HspB8 during disposal that is initiated by the chaperone-associated ubiquitin ligase CHIP and the autophagic ubiquitin adaptor p62. CASA is thus distinct from chaperone-mediated autophagy, previously shown to facilitate the ubiquitin-independent, direct translocation of a client across the lysosomal membrane [2]. Impaired CASA results in Z disk disintegration and progressive muscle weakness in flies, mice, and men. Our findings reveal the importance of chaperone-assisted degradation for the preservation of cellular structures and identify muscle as a tissue that highly relies on an intact proteostasis network, thereby shedding light on diverse myopathies and aging.

[1] Examination of Geotail measurements in the near-tail (X > À30 R E ) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of $1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hV x i = 431 km/s, and 38 moving tailward, hV x i = À451 km/s. We refer to these two populations as ''BBF-type'' and ''plasmoid-type'' flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal ÁB z perturbations, i.e., Ç for the ''BBF'' events and ± for the ''plasmoid'' events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within $1-2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 R E and 20 nT and 4.4 R E and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X-Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events.