ALBERT

Description: Abstract Magnetometers deployed on the largest satellite constellation to date are leveraged as a space‐based sensor network to study space‐time variability in auroral field‐aligned currents (FACs). The cubesat constellation of Planet Labs Inc. consists of nearly 200 satellites in two polar Sun‐synchronous orbits, with median spacecraft separations on the order of 375 km, and some occasions of opportunity providing much closer spacing. Each spacecraft contains a magnetoinductive magnetometer, able to sample the ambient magnetic field at 0.1 to 10 Hz with 〈200‐nT sensitivity. In this study, seven satellites from the Planet constellation were used to investigate space‐time variations in FACs over an active auroral display during a 10‐min interval. The aurora occurred during the early recovery phase of a geomagnetic storm and was characterized by large‐scale vortical motions and embedded rayed structure. Clear signatures of the large‐scale auroral current system were detected by the orbital magnetometers. Estimation of FAC patterns was carried out using three different methods. The results suggest a high degree of spatial and temporal variability during the 10‐min interval. The location of upward and downward current channels relative to the aurora was consistent with theoretical expectations, but current densities were not well correlated with visible features in the available imagery, suggesting unresolved small‐scale structure not captured by the collaborative observations. Advantages, limitations, and caveats in using opportunistic networks of low‐quality space‐based magnetometers to study dynamic auroral phenomena are discussed.

Description: Energy & Fuels DOI: 10.1021/acs.energyfuels.6b02510

Description: Context. The thermal Sunyaev-Zeldovich (SZ) effect presents a relatively new tool for characterizing galaxy cluster merger shocks, traditionally studied through X-ray observations. Widely regarded as the “textbook example” of a cluster merger bow shock, the western, most-prominent shock front in the Bullet Cluster (1E0657-56) represents the ideal test case for such an SZ study. Aims. We aim to characterize the shock properties using deep, high-resolution interferometric SZ effect observations in combination with priors from an independent X-ray analysis. Methods. Our analysis technique relies on the reconstruction of a parametric model for the SZ signal by directly and jointly fitting data from the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Compact Array (ACA) in Fourier space. Results. The ALMA+ACA data are primarily sensitive to the electron pressure difference across the shock front. To estimate the shock Mach number ℳ, this difference can be combined with the value for the upstream electron pressure derived from an independent Chandra X-ray analysis. In the case of instantaneous electron-ion temperature equilibration, we find ℳ = 2.08−0.12+0.12, in   ≈  2.4σ tension with the independent constraint from Chandra, MX = 2.74 ± 0.25. The assumption of purely adiabatic electron temperature change across the shock leads to ℳ = 2.53−0.25+0.33, in better agreement with the X-ray estimate ℳX = 2.57 ± 0.23 derived for the same heating scenario. Conclusion. We have demonstrated that interferometric observations of the thermal SZ effect provide constraints on the properties of the shock in the Bullet Cluster that are highly complementary to X-ray observations. The combination of X-ray and SZ data yields a powerful probe of the shock properties, capable of measuring ℳ and addressing the question of electron-ion equilibration in cluster shocks. Our analysis is however limited by systematics related to the overall cluster geometry and the complexity of the post-shock gas distribution. To overcome these limitations, a simultaneous, joint-likelihood analysis of SZ and X-ray data is needed.