title: Dynamical finite-amplitude magnetic reconnection at an X-type neutral point

authors: McClymont, A.N.

abstract: The linear theory of magnetic reconnection demonstrates that the rate of energy release at an X-type neutral point is "fast" - only logarithmically dependent on the plasma resistivity - if the field is strictly two-dimensional, gas pressure is absent, and perturbations are small. The present paper explores the response of the X-point to finite amplitude disturbances under the more realistic conditions of limited compressibility and a finite nonplanar magnetic field component. We show that fast reconnection is not inhibited by large amplitudes of the perturbation - in fact, both the reconnection rate and the ohmic dissipation rate increase with decreasing plasma resistivity. This "super fast" scaling can be understood by a simple, one-dimensional dynamic collapse model. However, the presence of finite gas pressure or an axial magnetic field component stalls the collapse by providing backpressure which retards the imploding magnetic wave: the current sheet is prevented from thinning and reconnection drops toward the static diffusion rate. Thus we overrule the initial implosion as a means of rapidly liberating magnetic energy when gas pressure or an axial magnetic field are present, as they are in the solar corona. But in this case the initial collapse does not provide the complete picture. Gas is subsequently squeezed out of the current sheet, allowing a higher current density to be attained. Thus the possibility of fast reconnection remains open. The dynamics of the evolution are complicated; it is not yet clear under exactly what conditions fast reconnection may be attainable. © 1996. The American Astronomical Society. All rights reserved.

text: Dynamical finite-amplitude magnetic reconnection at an X-type neutral point The linear theory of magnetic reconnection demonstrates that the rate of energy release at an X-type neutral point is "fast" - only logarithmically dependent on the plasma resistivity - if the field is strictly two-dimensional, gas pressure is absent, and perturbations are small. The present paper explores the response of the X-point to finite amplitude disturbances under the more realistic conditions of limited compressibility and a finite nonplanar magnetic field component. We show that fast reconnection is not inhibited by large amplitudes of the perturbation - in fact, both the reconnection rate and the ohmic dissipation rate increase with decreasing plasma resistivity. This "super fast" scaling can be understood by a simple, one-dimensional dynamic collapse model. However, the presence of finite gas pressure or an axial magnetic field component stalls the collapse by providing backpressure which retards the imploding magnetic wave: the current sheet is prevented from thinning and reconnection drops toward the static diffusion rate. Thus we overrule the initial implosion as a means of rapidly liberating magnetic energy when gas pressure or an axial magnetic field are present, as they are in the solar corona. But in this case the initial collapse does not provide the complete picture. Gas is subsequently squeezed out of the current sheet, allowing a higher current density to be attained. Thus the possibility of fast reconnection remains open. The dynamics of the evolution are complicated; it is not yet clear under exactly what conditions fast reconnection may be attainable. © 1996. The American Astronomical Society. All rights reserved.

year: 1996

title: Biomass estimation of invasive fish

authors: Hicks, Brendan J.

subjects: Invasive Fish

abstract: Invasive fish have a variety of effects on indigenous fish communities and freshwater ecosystems generally, and the magnitude of these effects is partly dependent on invasive fish biomass. For example, a koi carp biomass of 120–130 kg/ha was sufficient to depress macroinvertebrate and plant biomass, and to elevate chlorophyll a concentrations (Haas et al. 2007; Bajer et al. 2009; Hicks et al. 2011). The purpose of this section is to provide (i) estimates of the relative biomasses of invasive fish that have been established by boat electrofishing in a range of lake and riverine habitats in the North Island; and (ii) some estimates of absolute biomass derived from mark-recapture studies in shallow lakes. Collectively, these data provide a basis for future comparisons of invasive fish monitoring information in a region where coarse fish have proliferated.

text: Biomass estimation of invasive fish Invasive fish have a variety of effects on indigenous fish communities and freshwater ecosystems generally, and the magnitude of these effects is partly dependent on invasive fish biomass. For example, a koi carp biomass of 120–130 kg/ha was sufficient to depress macroinvertebrate and plant biomass, and to elevate chlorophyll a concentrations (Haas et al. 2007; Bajer et al. 2009; Hicks et al. 2011). The purpose of this section is to provide (i) estimates of the relative biomasses of invasive fish that have been established by boat electrofishing in a range of lake and riverine habitats in the North Island; and (ii) some estimates of absolute biomass derived from mark-recapture studies in shallow lakes. Collectively, these data provide a basis for future comparisons of invasive fish monitoring information in a region where coarse fish have proliferated.

year: 2015

title: A late-Holocene and prehistoric record of environmental change from Lake Waikaremoana, New Zealand

authors: Newnham, Rewi M.

subjects: Palynology

abstract: Further evidence in support of a late pre-European (Polynesian) settlement of New Zealand is provided by an 1850-year-long tephropalynological record from a remote region in New Zealand's North Island. The earliest unequivocally anthropogenic forest clearance is estimated from sedimentation rates to have occurred c. 375 14C years BP (c. ad 1523–1631), although the radiocarbon chronology, shown by tephrochron ology to be erroneous due to hard-water effects, suggested this occurred c. 900 years earlier. Delineation of the anthropogenic era, and the distinction between human activity and other agents of environmental change in the pollen/spore diagram, are supported by cluster analysis and detrended correspondence analysis. Two distinct phases of forest clearance are evident during the pre-European era, reflecting local changes either in population pressure or settlement patterns. We note that the lull between the two phases of forest clearance coincides with the maximum of the ‘Little Ice Age’ within the period c. late ad 1600s to early 1800s.

text: A late-Holocene and prehistoric record of environmental change from Lake Waikaremoana, New Zealand Further evidence in support of a late pre-European (Polynesian) settlement of New Zealand is provided by an 1850-year-long tephropalynological record from a remote region in New Zealand's North Island. The earliest unequivocally anthropogenic forest clearance is estimated from sedimentation rates to have occurred c. 375 14C years BP (c. ad 1523–1631), although the radiocarbon chronology, shown by tephrochron ology to be erroneous due to hard-water effects, suggested this occurred c. 900 years earlier. Delineation of the anthropogenic era, and the distinction between human activity and other agents of environmental change in the pollen/spore diagram, are supported by cluster analysis and detrended correspondence analysis. Two distinct phases of forest clearance are evident during the pre-European era, reflecting local changes either in population pressure or settlement patterns. We note that the lull between the two phases of forest clearance coincides with the maximum of the ‘Little Ice Age’ within the period c. late ad 1600s to early 1800s.

year: 1998