High-frequency hopping conductivity in the quantum Hall effect regime: Acoustical studies

The high-frequency conductivity of Si delta-doped GaAs/AlGaAs heterostructures is studied in the integer quantum Hall effect (QHE) regime, using acoustic methods. Both the real and the imaginary parts of the complex conductivity are determined from the experimentally observed magnetic field and temperature dependences of the velocity and the attenuation of a surface acoustic wave. It is demonstrated that in the structures studied the mechanism of low-temperature conductance near the QHE plateau centers is hopping. It is also shown that at magnetic fields corresponding to filling factors 2 and 4, the doped Si delta- layer efficiently shunts the conductance in the two-dimensional electron gas (2DEG) channel. A method to separate the two contributions to the real part of the conductivity is developed, and the localization length in the 2DEG channel is estimated.Comment: 8pages, 9 figure

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

(abridged) The heating mechanism at high densities during M dwarf flares is poorly understood. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. The simulation produces bright continuum emission from a dense, hot chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015): updated to include comments by Guest Editor. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-

Spring primary production in relation to environmental drivers in central Hudson Bay

The environmental factors influencing the microalgal bloom during sea-ice breakup in Hudson Bay were investigated during June 2018, producing the first results ever on the seasonal development of the marine ecosystem in the offshore waters of this vast inland sea. As is typical in the Arctic, primary production was found to commence at the onset of ice melt, with surface nutrient depletion leading to the formation of a subsurface chlorophyll maximum in the open waters of western Hudson Bay. Simultaneously, the melting mobile ice cover in central Hudson Bay created favorable conditions for a diatom-dominated under-ice bloom, with the results of irradiance-photosynthesis curves confirming that phytoplankton cells were acclimated to increasing light levels in the surface water. The high production rates measured in ice-covered and ice-free waters highlight the considerable plasticity of phytoplankton in terms of photosynthetic performance in this highly variable environment. Interestingly, the maximum values of primary production and phytoplankton biomass observed under the sea ice (343 mg C m-2 d-1 and 35.10 mg TChl a m-2) were lower than those observed in open waters during the late-bloom stage in the western region (486 mg C m-2 d-1 and 57.12 mg TChl a m-2), which is attributed to a confined euphotic zone (reduced light availability? Since the euphotic zone in clear waters under the ice can be as thick as elsewhere, but simply receive less irradiance overall) under the ice and low surface concentrations of inorganic nitrogen (<2 mmol L-1) in central Hudson Bay. However, the highly abundant sub-ice diatom Melosira arctica contributed an estimated additional 287 mg C m-2 d-1 to under-ice production in this region, which implies that this filamentous diatom has a similar role in the seasonally ice-covered sub-Arctic as in the central Arctic Ocean where it significantly contributes to local production. Refining the historical total production estimates of Hudson Bay with our spring observations, we recalculated annual production to be ca. 69 g C m-2, which equates to mean value for interior Arctic shelves