Cosmic-ray hydrodynamics: Alfv\'en-wave regulated transport of cosmic rays

Star formation in galaxies appears to be self-regulated by energetic feedback processes. Among the most promising agents of feedback are cosmic rays (CRs), the relativistic ion population of interstellar and intergalactic plasmas. In these environments, energetic CRs are virtually collisionless and interact via collective phenomena mediated by kinetic-scale plasma waves and large-scale magnetic fields. The enormous separation of kinetic and global astrophysical scales requires a hydrodynamic description. Here, we develop a new macroscopic theory for CR transport in the self-confinement picture, which includes CR diffusion and streaming. The interaction between CRs and electromagnetic fields of Alfv\'enic turbulence provides the main source of CR scattering, and causes CRs to stream along the magnetic field with the Alfv\'en velocity if resonant waves are sufficiently energetic. However, numerical simulations struggle to capture this effect with current transport formalisms and adopt regularization schemes to ensure numerical stability. We extent the theory by deriving an equation for the CR momentum density along the mean magnetic field and include a transport equation for the Alfv\'en-wave energy. We account for energy exchange of CRs and Alfv\'en waves via the gyroresonant instability and include other wave damping mechanisms. Using numerical simulations we demonstrate that our new theory enables stable, self-regulated CR transport. The theory is coupled to magneto-hydrodynamics, conserves the total energy and momentum, and correctly recovers previous macroscopic CR transport formalisms in the steady-state flux limit. Because it is free of tunable parameters, it holds the promise to provide predictable simulations of CR feedback in galaxy formation.Comment: 34 pages, 6 figures, minor revision to match the accepted version to be published in MNRA

Diverging Regional Climate Preferences and the Assessment of Solar Geoengineering

Solar Geoengineering (SG) is a set of potential technologies to counteract climate change. While SG can only imperfectly compensate for temperature changes at the regional level, studies assessing regional SG impacts indicated so far that regional temperature disparities from SG may not be as severe as previously thought. A shortcoming of that literature is its assumption that regions’ temperature preferences correspond to some historic baseline climate. I extend the main framework for examining regional SG impacts by allowing for regions to have temperature preferences diverging from the baseline climate, showing that the impact of these diverging preferences can be split into two components. The first component changes the optimal SG level, but does not affect regional disagreement over SG. The second component leaves the optimal SG level unaffected, but changes regional disagreement over SG. I identify three aspects of SG performance in the presence of diverging preferences. A numerical implementation of the extended model shows that the presence of diverging preferences may change SG performance in either direction and that the direction generally depends on which of the three aspects of SG performance is considered

Astronomical seeing and ground-layer turbulence in the Canadian High Arctic

We report results of a two-year campaign of measurements, during arctic winter darkness, of optical turbulence in the atmospheric boundary-layer above the Polar Environment Atmospheric Laboratory in northern Ellesmere Island (latitude +80 deg N). The data reveal that the ground-layer turbulence in the Arctic is often quite weak, even at the comparatively-low 610 m altitude of this site. The median and 25th percentile ground-layer seeing, at a height of 20 m, are found to be 0.57 and 0.25 arcsec, respectively. When combined with a free-atmosphere component of 0.30 arcsec, the median and 25th percentile total seeing for this height is 0.68 and 0.42 arcsec respectively. The median total seeing from a height of 7 m is estimated to be 0.81 arcsec. These values are comparable to those found at the best high-altitude astronomical sites