Nonlinear Alfvén wave dynamics at a 2D magnetic null point: ponderomotive force

Context: In the linear, β = 0 MHD regime, the transient properties of magnetohydrodynamic (MHD) waves in the vicinity of 2D null points are well known. The waves are decoupled and accumulate at predictable parts of the magnetic topology: fast waves accumulate at the null point; whereas Alfvén waves cannot cross the separatricies. However, in nonlinear MHD mode conversion can occur at regions of inhomogeneous Alfvén speed, suggesting that the decoupled nature of waves may not extend to the nonlinear regime. Aims: We investigate the behaviour of low-amplitude Alfvén waves about a 2D magnetic null point in nonlinear, β = 0 MHD. Methods: We numerically simulate the introduction of low-amplitude Alfvén waves into the vicinity of a magnetic null point using the nonlinear LARE2D code. Results: Unlike in the linear regime, we find that the Alfvén wave sustains cospatial daughter disturbances, manifest in the transverse and longitudinal fluid velocity, owing to the action of nonlinear magnetic pressure gradients (viz. the ponderomotive force). These disturbances are dependent on the Alfvén wave and do not interact with the medium to excite magnetoacoustic waves, although the transverse daughter becomes focused at the null point. Additionally, an independently propagating fast magnetoacoustic wave is generated during the early stages, which transports some of the initial Alfvén wave energy towards the null point. Subsequently, despite undergoing dispersion and phase-mixing due to gradients in the Alfvén-speed profile (∇c_A ≠ 0) there is no further nonlinear generation of fast waves. Conclusions: We find that Alfvén waves at 2D cold null points behave largely as in the linear regime, however they sustain transverse and longitudinal disturbances - effects absent in the linear regime - due to nonlinear magnetic pressure gradients

Are nominal wage changes skewed away from wage cuts?

Wages

Genetic requirement for Ras in the transformation of fibroblasts and hematopoietic cells by the Bcr-Abl oncogene.

To determine the functional importance of Ras in transformation by Abl oncogenes, we used a genetic approach to measure the effect of impaired Ras activity on the ability of Bcr-Abl or v-Abl to transform cells. Expression of the catalytic domain of the GTPase activating protein for Ras (Gap C terminus) impaired soft agar colony formation by fibroblasts expressing v-Abl or Bcr-Abl by 70-80%. To test Ras function in a model that more closely resembles clinical diseases involving Bcr-Abl, double gene retroviruses expressing Bcr-Abl paired with the Gap C terminus or dominant negative Ras were introduced into naive mouse bone marrow cells. Transformation by Bcr-Abl was completely blocked in both situations. Coexpression of normal c-H-Ras accelerated the transforming activity of Bcr-Abl. These findings show that Ras activation is essential for the leukemogenic activity of Abl oncogenes in two distinct model systems. The results genetically define a connection between the Bcr-Abl cytoplasmic tyrosine kinase and Ras and add to the accumulating evidence that deregulation of Ras is a central event in the genesis of a number of molecularly distinct forms of human myeloid leukemia

Meeting update: faecal microbiota transplantation––bench, bedside, courtroom?

A group of stakeholders met, under the aegis of the British Society of Gastroenterology, to discuss the current landscape of faecal microbiota transplant- ation (FMT) within the UK and beyond. The meeting covered a wide range of topics, ranging from the practical aspects of establishing an FMT service and regu- latory issues relating to its delivery, to research implications and likely future directions