PERPENDICULAR TRANSPORT OF ENERGETIC CHARGED PARTICLES IN NONAXISYMMETRIC TWO-COMPONENT MAGNETIC TURBULENCE

ABSTRACT Q1,Q2 We examine energetic charged particle diffusion perpendicular to a mean magnetic field B 0 due to turbulent fluctuations in a plasma, relaxing the common assumption of axisymmetry around B 0 and varying the ratio of two fluctuation components, a slab component with parallel wavenumbers and a two-dimensional (2D) component with perpendicular wavenumbers. We perform computer simulations mostly for 80% 2D and 20% slab energy and a fluctuation amplitude on the order of B 0 . The nonlinear guiding center (NLGC) theory provides a reasonable description of asymptotic perpendicular diffusion as a function of the nonaxisymmetry and particle energy. These values are roughly proportional to the particle speed times the field line diffusion coefficient, with a prefactor that is much lower than in the classical field line random walk model of particle diffusion. NLGC predicts a prefactor in closer agreement with simulations. Next we consider extreme fluctuation anisotropy and the approach to reduced dimensionality. For 99% slab fluctuation energy, field line trajectories are diffusive, but the particle motion is subdiffusive. For 99% 2D fluctuation energy, both field lines and particle motions are initially subdiffusive and then diffusive, but NLGC gives unreliable results. The time dependence of the running particle diffusion coefficient shows that in all cases asymptotic diffusion is preceded by free streaming and subdiffusion, but the latter differs from standard compound subdiffusion. We can model the time profiles in terms of a decaying negative correlation of the perpendicular velocity due to the possibility of backtracking along magnetic field lines

Using under-ice hyperspectral transmittance to determine land-fast sea-ice algal biomass in Saroma-ko Lagoon, Hokkaido, Japan

Sea ice, which forms in polar and nonpolar areas, transmits light to ice-associated (sympagic) algal communities. To noninvasively study the distribution of sea-ice algae, empirical relations to estimate its biomass from under-ice hyperspectral irradiance have been developed in the Arctic and Antarctica but lack for nonpolar regions. This study examines relationships between normalised difference indices (NDI) calculated from hyperspectral transmittance and sympagic algal biomass in the nonpolar Saroma-ko Lagoon. We analysed physico-biogeochemical properties of snow and land-fast sea ice supporting 27 paired bio-optical measurements along three transects covering an area of over 250 m × 250 m in February 2019. Snow depth (0.08 ± 0.01 m) and ice-bottom brine volume fraction (0.21 ± 0.02) showed low (0.06) and high (0.58) correlations with sea-ice core bottom section chlorophyll a (Chl. a), respectively. Spatial analyses unveiled the patch size of sea-ice Chl. a to be ~65 m, which is in the same range reported from previous studies. A selected NDI (669, 596 nm) explained 63% of algal biomass variability. This reflects the bio-optical properties and environmental conditions of the lagoon that favour the wavelength pair in the orange/red part of the spectrum and suggests the necessity of a specific bio-optical relationship for Saroma-ko Lagoon

PERPENDICULAR TRANSPORT OF ENERGETIC CHARGED PARTICLES IN NONAXISYMMETRIC TWO-COMPONENT MAGNETIC TURBULENCE

ABSTRACT We examine energetic charged particle diffusion perpendicular to a mean magnetic field B 0 due to turbulent fluctuations in a plasma, relaxing the common assumption of axisymmetry around B 0 and varying the ratio of two fluctuation components, a slab component with parallel wavenumbers and a two-dimensional (2D) component with perpendicular wavenumbers. We perform computer simulations mostly for 80% 2D and 20% slab energy and a fluctuation amplitude on the order of B 0 . The nonlinear guiding center (NLGC) theory provides a reasonable description of asymptotic perpendicular diffusion as a function of the nonaxisymmetry and particle energy. These values are roughly proportional to the particle speed times the field line diffusion coefficient, with a prefactor that is much lower than in the classical field line random walk model of particle diffusion. NLGC predicts a prefactor in closer agreement with simulations. Next we consider extreme fluctuation anisotropy and the approach to reduced dimensionality. For 99% slab fluctuation energy, field line trajectories are diffusive, but the particle motion is subdiffusive. For 99% 2D fluctuation energy, both field lines and particle motions are initially subdiffusive and then diffusive, but NLGC gives unreliable results. The time dependence of the running particle diffusion coefficient shows that in all cases asymptotic diffusion is preceded by free streaming and subdiffusion, but the latter differs from standard compound subdiffusion. We can model the time profiles in terms of a decaying negative correlation of the perpendicular velocity due to the possibility of backtracking along magnetic field lines

Sub‐Ice Platelet Layer Physics: Insights From a Mushy‐Layer Sea Ice Model

International audienc

Saroma-ko Lagoon Observations for sea ice Physico-chemistry and Ecosystems 2019 (SLOPE2019)

Saroma-ko Lagoon, located on the Okhotsk Sea coast of Hokkaido, is seasonally covered by flat, homogeneous, easily accessible and safe sea ice. As such, it proves a very useful experimental site for the study of sea ice processes, the inter-comparison of methods, the testing of equipment, and the training of researchers new to the Polar regions. In this contribution, we describe a physical, chemical, and ecosystem survey at Saroma-ko Lagoon, conducted over February 23-28, 2019 under the auspices of the SLOPE2019 (Saroma-ko Lagoon Observations for sea ice Physico-chemistry and Ecosystems 2019) program. Sea ice cores were collected to examine temperature, salinity, oxygen isotopic ratio, thin sections, and chemical and biological parameters such as carbonate chemistry, CH4, nutrients, chlorophyll a concentrations, and ice algae community assemblage. Broadband and spectral irradiance measurements were carried out above/under the sea ice, and different sensors were inter-compared at close positions and environments. Equipment such as spectrometers, air-sea ice CO2/CH4 flux chamber, and under-ice turbulent heat flux systems were tested for future Arctic and Antarctic expeditions. Finally, an artificial pool was dug into the sea ice to understand the effect of snow particles on ice growth and to compare the gas exchange process over sea ice with an ice-free water surface. Our SLOPE2019 field campaign activities provided useful information for inter-comparison work and future sea ice research in the polar oceans

Antarctic Landfast Sea Ice: A Review of Its Physics, Biogeochemistry and Ecology

Antarctic landfast sea ice (fast ice) is stationary sea ice that is attached to the coast, grounded icebergs, ice shelves, or other protrusions on the continental shelf. Fast ice forms in narrow (generally up to 200 km wide) bands, and ranges in thickness from centimeters to tens of meters. In most regions, it forms in autumn, persists through the winter and melts in spring/summer, but can remain throughout the summer in particular locations, becoming multi-year ice. Despite its relatively limited extent (comprising between about 4% and 13% of overall sea ice), its presence, variability and seasonality are drivers of a wide range of physical, biological and biogeochemical processes, with both local and far-ranging ramifications for the Earth system. Antarctic fast ice has, until quite recently, been overlooked in studies, likely due to insufficient knowledge of its distribution, leading to its reputation as a “missing piece of the Antarctic puzzle.” This review presents a synthesis of current knowledge of the physical, biogeochemical and biological aspects of fast ice, based on the sub-domains of: fast ice growth, properties and seasonality; remote-sensing and distribution; interactions with the atmosphere and the ocean; biogeochemical interactions; its role in primary production; and fast ice as a habitat for grazers. Finally, we consider the potential state of Antarctic fast ice at the end of the 21st Century, underpinned by Coupled Model Intercomparison Project model projections. This review also gives recommendations for targeted future work to increase our understanding of this critically-important element of the global cryosphere.A. D. Fraser ... A. R. Klekociuk ... et al