The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

The Physical Processes of CME/ICME Evolution

As observed in Thomson-scattered white light, coronal mass ejections (CMEs) are manifest as large-scale expulsions of plasma magnetically driven from the corona in the most energetic eruptions from the Sun. It remains a tantalizing mystery as to how these erupting magnetic fields evolve to form the complex structures we observe in the solar wind at Earth. Here, we strive to provide a fresh perspective on the post-eruption and interplanetary evolution of CMEs, focusing on the physical processes that define the many complex interactions of the ejected plasma with its surroundings as it departs the corona and propagates through the heliosphere. We summarize the ways CMEs and their interplanetary CMEs (ICMEs) are rotated, reconfigured, deformed, deflected, decelerated and disguised during their journey through the solar wind. This study then leads to consideration of how structures originating in coronal eruptions can be connected to their far removed interplanetary counterparts. Given that ICMEs are the drivers of most geomagnetic storms (and the sole driver of extreme storms), this work provides a guide to the processes that must be considered in making space weather forecasts from remote observations of the corona.Peer reviewe

In vitro exposure of seal peripheral blood leukocytes to different metals reveal a sex-dependent effect of zinc on phagocytic activity

Although the immunotoxicity of heavy metals is well established, evaluation of their potential immunotoxicity in wildlife species is complicated by variables that could modulate the immune response to xenobiotics under field conditions. Phagocytosis plays a key role in the mammalian immune response. The objectives of our study were to develop a method for measuring the phagocytic activity of seal peripheral blood granulocytes, and to determine the effects of both zinc chloride (ZnCl2), cadmium chloride (CdCl2) and mercuric chloride (HgCl2) on this immune function of peripheral blood granulocytes in vitro. Peripheral blood leukocytes (PBLs) were isolated from the peripheral blood of either harbour seals (Phoca vitulina) or grey seals (Halichoerus grypus) captured in the St Lawrence Estuary. Cells exposed for 4 h to 10-4 M and 10-3 M HgCl2 displayed lower phagocytic activity but this was related to a general cytotoxic effect of HgCl2 as opposed to a specific effect on phagocytosis. Exposure of PBLs from either male or female PBLs to CdCl2 had no effect on phagocytic activity at the concentrations tested except a significant decrease in cells from male harbour seals exposed to 10-4 M. Exposure to ZnCl2 at physiologically relevant concentrations enhanced the phagocytic activity of PBLs from mature females of both species, whereas no effect was observed in cells from either males or immature females. In conclusion, we have developed an in vitro assay to test the effects of environmental contaminants on the phagocytic activity of seal PBLs. Our data indicate that exposure of PBLs to ZnCl2 results in a sex-dependent response and that PBLs from mature female seals are more sensitive to ZnCl2 than either male or immature seals. This sex-dependent response to ZnCl2 could lead to a differential sensitivity to heavy metal exposures for males and females seals in the field

Proliferation of adult Sertoli cells following conditional knockout of the gap junctional protein GJA1 (connexin 43) in mice

International audienc

Proliferation of adult sertoli cells in a conditional knockout mouse lacking the gap junctional protein connexin 43

International audienc