title: Current sheet formation in uniformly twisted magnetic flux tubes

authors: Ali, Farhad

abstract: A viable mechanism for coronal heating is conversion of magnetic field energy into heat through magnetic reconnection in current sheets. Two processes capable of generating current sheets in the solar corona are studied here. The corkscrew kink instability of an axisymmetric magnetic flux tube, as a result of uniform twisting, is addressed first. Linear growth rates of the instability due to a helical perturbation are calculated for a variety of tubes. It is shown that plasma pressure and tube radius reduction are stabilising.

The helically symmetric geometry allows us to use an effectively two-dimensional formulation. A magneto-frictional code is developed to simulate relaxation to a second helical equilibrium of the tubes. Lower energy equilibria resulting from the kink are studied. Only one tube, whose twist decays with radius, is shown to result in a true field singularity or a current sheet. The Gold-Hoyle tube relaxes to a smooth kinked equilibrium. Tubes having reversal of a field component result in current concentrations as they kink and relax to a neighboring equilibrium. Plasma pressure reduces the strength of the kink in all cases.

The second process resulting in current sheet formation is the interaction of multiple flux tubes. The same frictional code is used to show that repelling tubes move away from each other as the superposed field relaxes to equilibrium. Twisting them together results in current concentrations. Attracting tubes with no neutral point also give rise to current concentrations when they are twisted together and relax to equilibrium. Current sheets are formed when attracting flux tubes, whose initial combined field have a neutral point, coalesce in the absence of plasma pressure. Introducing plasma pressure prevents current sheet formation when the tubes are not twisted together. In case the tubes are twisted together we again see current sheets, the lack of symmetry permitting plasma to flow out from between approaching flux surfaces, allowing them to come into contact. Interaction of unequal attracting tubes results in curved current sheets.

text: Current sheet formation in uniformly twisted magnetic flux tubes A viable mechanism for coronal heating is conversion of magnetic field energy into heat through magnetic reconnection in current sheets. Two processes capable of generating current sheets in the solar corona are studied here. The corkscrew kink instability of an axisymmetric magnetic flux tube, as a result of uniform twisting, is addressed first. Linear growth rates of the instability due to a helical perturbation are calculated for a variety of tubes. It is shown that plasma pressure and tube radius reduction are stabilising.

The helically symmetric geometry allows us to use an effectively two-dimensional formulation. A magneto-frictional code is developed to simulate relaxation to a second helical equilibrium of the tubes. Lower energy equilibria resulting from the kink are studied. Only one tube, whose twist decays with radius, is shown to result in a true field singularity or a current sheet. The Gold-Hoyle tube relaxes to a smooth kinked equilibrium. Tubes having reversal of a field component result in current concentrations as they kink and relax to a neighboring equilibrium. Plasma pressure reduces the strength of the kink in all cases.

The second process resulting in current sheet formation is the interaction of multiple flux tubes. The same frictional code is used to show that repelling tubes move away from each other as the superposed field relaxes to equilibrium. Twisting them together results in current concentrations. Attracting tubes with no neutral point also give rise to current concentrations when they are twisted together and relax to equilibrium. Current sheets are formed when attracting flux tubes, whose initial combined field have a neutral point, coalesce in the absence of plasma pressure. Introducing plasma pressure prevents current sheet formation when the tubes are not twisted together. In case the tubes are twisted together we again see current sheets, the lack of symmetry permitting plasma to flow out from between approaching flux surfaces, allowing them to come into contact. Interaction of unequal attracting tubes results in curved current sheets.

year: 2001

title: A fluid flow perspective on the diagenesis of Te Aute limestones

authors: Ricketts, Brian D.

subjects: diagenesis

abstract: Pliocene cool-water, bioclastic Te Aute limestones in East Coast Basin, New Zealand, accumulated either in shelfal shoal areas or about structurally shallow growth fold structures in the tectonically active accretionary forearc prism. Up to five stages of carbonate cementation are recognised, based on cement sequence-stratigraphic concepts, that formed on the seafloor during exposure of the limestones before burial, during burial, uplift, and deformation. Two principal fluid types are identified--topography-driven meteoric fluids and compaction-driven fluids. We have developed conceptual and quantitative models that attempt to relate the physical characteristics of fluid flow to the cement paragenesis. In particular, we have simulated the effects of uplift of the axial ranges bordering East Coast Basin in terms of the degree of penetration of a meteoric wedge into the basin. The dynamics of meteoric flow changed dramatically during uplift over the last 2 m.y. such that the modelled extent of the meteoric wedge is at least 40 km across the basin, and the penetration depth 1500 m or more corresponding with measured freshwater intersections in some oil wells. Cement-fluid relationships include: (1) true marine cements that precipitated in areas remote from shallow freshwater lenses; (2) pre-compaction cements that formed in shallow freshwater lenses beneath limestone "islands"; (3) post-compaction cements derived from compaction-driven flow during burial; (4) early uplift-related fracture-fill cements formed during deformation of the accretionary prism and uplift of the axial ranges; and (5) late uplift-related cements associated with uplift into a shallow meteoric regime.

text: A fluid flow perspective on the diagenesis of Te Aute limestones Pliocene cool-water, bioclastic Te Aute limestones in East Coast Basin, New Zealand, accumulated either in shelfal shoal areas or about structurally shallow growth fold structures in the tectonically active accretionary forearc prism. Up to five stages of carbonate cementation are recognised, based on cement sequence-stratigraphic concepts, that formed on the seafloor during exposure of the limestones before burial, during burial, uplift, and deformation. Two principal fluid types are identified--topography-driven meteoric fluids and compaction-driven fluids. We have developed conceptual and quantitative models that attempt to relate the physical characteristics of fluid flow to the cement paragenesis. In particular, we have simulated the effects of uplift of the axial ranges bordering East Coast Basin in terms of the degree of penetration of a meteoric wedge into the basin. The dynamics of meteoric flow changed dramatically during uplift over the last 2 m.y. such that the modelled extent of the meteoric wedge is at least 40 km across the basin, and the penetration depth 1500 m or more corresponding with measured freshwater intersections in some oil wells. Cement-fluid relationships include: (1) true marine cements that precipitated in areas remote from shallow freshwater lenses; (2) pre-compaction cements that formed in shallow freshwater lenses beneath limestone "islands"; (3) post-compaction cements derived from compaction-driven flow during burial; (4) early uplift-related fracture-fill cements formed during deformation of the accretionary prism and uplift of the axial ranges; and (5) late uplift-related cements associated with uplift into a shallow meteoric regime.

year: 2004-12-01

title: Ngā waka o tawhiti

authors: Tuaupiki, Jackie Whetumarama

abstract: Mai anō ngā terenga waka a te Māori, ko ōna kōrero tuku iho i whakatauira tuatahingia e ngā atua tonu, taka mai ki te iwi tāngata o Te Moana-nui-a-Kiwa. Tā tēnei tuhinga, he wānanga i te mana o te kōrero tuku iho e haupū nei ngā pūrākau terenga waka onamata i takoto ai te tūāpapa mātauranga whakatere mō ngā waka tūpuna i heke iho i Hawaiki ki Aotearoa. Nō reira, ka riariakina ngā pūtoi whakatauiratanga waka a te Māori i ōna atua me ōna tūpuna me te ahunga mai o ana terenga mātāmua, ka aromātai hoki i te takiwā o ngā tau i tauria ai e ōna takere Te Ika-a-Māui e takoto nei.

text: Ngā waka o tawhiti Mai anō ngā terenga waka a te Māori, ko ōna kōrero tuku iho i whakatauira tuatahingia e ngā atua tonu, taka mai ki te iwi tāngata o Te Moana-nui-a-Kiwa. Tā tēnei tuhinga, he wānanga i te mana o te kōrero tuku iho e haupū nei ngā pūrākau terenga waka onamata i takoto ai te tūāpapa mātauranga whakatere mō ngā waka tūpuna i heke iho i Hawaiki ki Aotearoa. Nō reira, ka riariakina ngā pūtoi whakatauiratanga waka a te Māori i ōna atua me ōna tūpuna me te ahunga mai o ana terenga mātāmua, ka aromātai hoki i te takiwā o ngā tau i tauria ai e ōna takere Te Ika-a-Māui e takoto nei.

year: 2017