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 phase curve of cometary dust: Observations of comet 96P/Machholz 1 at large phase angle with the SOHO LASCO C3 coronagraph

We have analyzed brightness and polarization data of comet 96P/Machholz, obtained with the SOHO-LASCO C3 coronagraph at phase angles up to 167° and 157°, respectively. The polarization data are characteristic of a typical dusty comet. Within error limits the corresponding trigonometric fit describes the new data measured at larger phase angles as well as those of the previously known range. In the phase angle range from 110° to 167° the brightness increases almost linearly by about two orders of magnitude. The gradient is independent of wavelength. From the absence of a diffraction spike we conclude that the grains contributing significantly to the scattered light must have a size parameter $x = 2\pi r/\lambda \ge 20$, i.e. have a radius larger than 1 μm. Fits of the data with Mie calculations of particles having a power law distribution of power index ≈ 2.5 provide a best fit refractive index \emph{m} = 1.2 + \emph{i}0.004. In the framework of effective medium theory and on the assumption of a particle porosity $P= 0.5$ this leads to a complex refractive index of the porous medium \emph{m} = 1.43 + \emph{i}0.009. A higher refractive index is possible for more porous grains with very low absorption. The large particle sizes are in qualitative agreement with findings derived from the analysis of the motion of cometary dust under solar radiation pressure (Fulle and coworkers, see [CITE]; [CITE] 1997) and with the in-situ measurements of the dust of Halley's comet

Hyperpolarized 129Xe imaging of the brain : achievements and future challenges

Hyperpolarized (HP) xenon-129 (129Xe) brain MRI is a promising imaging modality currently under extensive development. HP 129Xe is nontoxic, capable of dissolving in pulmonary blood, and is extremely sensitive to the local environment. After dissolution in the pulmonary blood, HP 129Xe travels with the blood flow to the brain and can be used for functional imaging such as perfusion imaging, hemodynamic response detection, and blood–brain barrier permeability assessment. HP 129Xe MRI imaging of the brain has been performed in animals, healthy human subjects, and in patients with Alzheimer's disease and stroke. In this review, the overall progress in the field of HP 129Xe brain imaging is discussed, along with various imaging approaches and pulse sequences used to optimize HP 129Xe brain MRI. In addition, current challenges and limitations of HP 129Xe brain imaging are discussed, as well as possible methods for their mitigation. Finally, potential pathways for further development are also discussed. HP 129Xe MRI of the brain has the potential to become a valuable novel perfusion imaging technique and has the potential to be used in the clinical setting in the future