title: Anisotropic scaling of magnetohydrodynamic turbulence

authors: Horbury, Timothy S.

subjects: mathematics

abstract: We present a quantitative estimate of the anisotropic power and scaling of magnetic field fluctuations in inertial range magnetohydrodynamic turbulence, using a novel wavelet technique applied to spacecraft measurements in the solar wind. We show for the first time that, when the local magnetic field direction is parallel to the flow, the spacecraft-frame spectrum has a spectral index near 2. This can be interpreted as the signature of a population of fluctuations in field-parallel wavenumbers with a k║-2 spectrum but is also consistent with the presence of a "critical balance" style turbulent cascade. We also find, in common with previous studies, that most of the power is contained in wavevectors at large angles to the local magnetic field and that this component of the turbulence has a spectral index of 5/3.

text: Anisotropic scaling of magnetohydrodynamic turbulence We present a quantitative estimate of the anisotropic power and scaling of magnetic field fluctuations in inertial range magnetohydrodynamic turbulence, using a novel wavelet technique applied to spacecraft measurements in the solar wind. We show for the first time that, when the local magnetic field direction is parallel to the flow, the spacecraft-frame spectrum has a spectral index near 2. This can be interpreted as the signature of a population of fluctuations in field-parallel wavenumbers with a k║-2 spectrum but is also consistent with the presence of a "critical balance" style turbulent cascade. We also find, in common with previous studies, that most of the power is contained in wavevectors at large angles to the local magnetic field and that this component of the turbulence has a spectral index of 5/3.

year: 2008

title: Linear theory of fast reconnection at an X-type neutral point

authors: Craig, Ian J.D.

abstract: A linear theory is developed for the dissipation of the free magnetic energy in small disturbances imposed on the potential field of an X-type neutral point. An eigenmode analysis, using cylindrical coordinates centered on the neutral point, extends the work of Craig & McClymont (1991) to include non-azimuthally symmetric perturbations on the flux function. We demonstrate that all physically significant disturbances, both reconnective and nonreconnective, decay resistively on a "fast" time scale ∼|ln η|, where η is the nondimensional resistivity. Thus resistive diffusion is remarkably effective as a dissipation mechanism for all classes of perturbation not only those modes which induce changes in the field topology. The fundamental reason for the fast dissipation is the focusing of wave energy onto the neutral point by the gradient in Alfvén speed. This occurs for all types of disturbances launched from the outer boundary, but reconnective modes propagate purely radially while nonreconnective modes correspond to waves with an azimuthal motion and a smaller wave speed in the radial direction. Dissipation of reconnective disturbances takes place in a diffusion region of diameter ≈η1/2 around the neutral point, on a time scale ∝ |ln η|2, but nonreconnective perturbations decay before penetrating to the neutral point, because of the increased path lengths due to their azimuthal motion. These modes release their energy in an annulus of radius ≈[(m3/ 4k)η]1/2, where m and k are azimuthal and radial wavenumbers: because of their longer transit time, nonreconnective waves decay on a |ln η|3 time scale. We conclude by discussing the significance of the linear theory within the general context of steady state and dynamic reconnection studies. It is pointed out that, dynamically, disturbances can be expected to focus explosively in the vicinity of the neutral point. This suggests the formation of a "flux pile-up" current layer in which the bulk of the magnetic energy is released as heat rather than kinetic energy of mass motion.

text: Linear theory of fast reconnection at an X-type neutral point A linear theory is developed for the dissipation of the free magnetic energy in small disturbances imposed on the potential field of an X-type neutral point. An eigenmode analysis, using cylindrical coordinates centered on the neutral point, extends the work of Craig & McClymont (1991) to include non-azimuthally symmetric perturbations on the flux function. We demonstrate that all physically significant disturbances, both reconnective and nonreconnective, decay resistively on a "fast" time scale ∼|ln η|, where η is the nondimensional resistivity. Thus resistive diffusion is remarkably effective as a dissipation mechanism for all classes of perturbation not only those modes which induce changes in the field topology. The fundamental reason for the fast dissipation is the focusing of wave energy onto the neutral point by the gradient in Alfvén speed. This occurs for all types of disturbances launched from the outer boundary, but reconnective modes propagate purely radially while nonreconnective modes correspond to waves with an azimuthal motion and a smaller wave speed in the radial direction. Dissipation of reconnective disturbances takes place in a diffusion region of diameter ≈η1/2 around the neutral point, on a time scale ∝ |ln η|2, but nonreconnective perturbations decay before penetrating to the neutral point, because of the increased path lengths due to their azimuthal motion. These modes release their energy in an annulus of radius ≈[(m3/ 4k)η]1/2, where m and k are azimuthal and radial wavenumbers: because of their longer transit time, nonreconnective waves decay on a |ln η|3 time scale. We conclude by discussing the significance of the linear theory within the general context of steady state and dynamic reconnection studies. It is pointed out that, dynamically, disturbances can be expected to focus explosively in the vicinity of the neutral point. This suggests the formation of a "flux pile-up" current layer in which the bulk of the magnetic energy is released as heat rather than kinetic energy of mass motion.

year: 1993

title: 2021 NSCBI Paakehaa/Tauiwi Caucus Conference Session and Future Directions

authors: Graham, Rebekah Sarah

abstract: The National Standing Committee on Bicultural Issues (NSCBI) celebrated its 30th anniversary at the Annual NZPsS Conference, held virtually due to Covid-19 restrictions. The Paakehaa/Tauiwi caucus discussed the NSCBI’s core values, focusing on how to uphold and connect with each value, such as protecting values passed down by tūpuna and promoting a vision of the future. Anti-racism education was presented, emphasizing the journey from the “Fear Zone” to the “Learning Zone” and into the “Growth Zone” as a tool for self-assessment. Reflections on supporting and challenging each other to make psychological practice actively anti-racist concluded the session. Key takeaways included the discomfort of staying silent, the importance of learning and trying, and being a bridge for others on their anti-racist journey. The participants agreed on the usefulness of the discussion and expressed a commitment to organizing more regular online events focusing on anti-racism and anti-racist practice. The NSCBI Paakehaa/Tauiwi aims for 2021-2022 include strategically highlighting contributions to hostile working environments, developing relationships with anti-racism groups, promoting Māori psychologies, and facilitating online conversations, webinars, and discussions on Te Tiriti o Waitangi and biculturalism. The group also plans to develop training material and professional development for practitioners. The NSCBI acknowledges the ongoing progress in understanding racism and fostering openness among Paakehaa/Tauiwi in engaging with bicultural practices. Those interested in NSCBI events or contributing to their organization and promotion can visit the NZPsS Events page.

text: 2021 NSCBI Paakehaa/Tauiwi Caucus Conference Session and Future Directions The National Standing Committee on Bicultural Issues (NSCBI) celebrated its 30th anniversary at the Annual NZPsS Conference, held virtually due to Covid-19 restrictions. The Paakehaa/Tauiwi caucus discussed the NSCBI’s core values, focusing on how to uphold and connect with each value, such as protecting values passed down by tūpuna and promoting a vision of the future. Anti-racism education was presented, emphasizing the journey from the “Fear Zone” to the “Learning Zone” and into the “Growth Zone” as a tool for self-assessment. Reflections on supporting and challenging each other to make psychological practice actively anti-racist concluded the session. Key takeaways included the discomfort of staying silent, the importance of learning and trying, and being a bridge for others on their anti-racist journey. The participants agreed on the usefulness of the discussion and expressed a commitment to organizing more regular online events focusing on anti-racism and anti-racist practice. The NSCBI Paakehaa/Tauiwi aims for 2021-2022 include strategically highlighting contributions to hostile working environments, developing relationships with anti-racism groups, promoting Māori psychologies, and facilitating online conversations, webinars, and discussions on Te Tiriti o Waitangi and biculturalism. The group also plans to develop training material and professional development for practitioners. The NSCBI acknowledges the ongoing progress in understanding racism and fostering openness among Paakehaa/Tauiwi in engaging with bicultural practices. Those interested in NSCBI events or contributing to their organization and promotion can visit the NZPsS Events page.

year: 2021-11