On magnetohydrodynamic waves in inhomogeneous magnetic fields

Abstract

The theoretical analysis of the propagation of magnetohydrodynamic waves in a cylindrical plasma immersed in a steady inhomogeneous magnetic field is here undertaken by assuming a model field of two constant field regions joined by the field B⃗ = A∇(l/r). We thus consider ‘beach’ and ‘cliff’ magnetic field configurations. The general solution of the coldplasma equations in the central (inhomogeneous field) region must be found numerically, but in the low-frequency case analytic solutions are obtained. The wave reflection and transmission found by applying boundary conditions at the constant field.Solutions are so obtained for a variety of cases, including simple reflection of the compressional and torsional modes, reflection of the compressional mode when the cutoff condition is encountered, absorption of the torsional mode at the ioncyclotron resonance, and conversion of one mode into the other after propagation through both of the above conditions. Finally, comparison is made with several recent experiments, which shows that the above theory is sufficient in the case that the wave frequency is not too close to the resonance frequency.The case of reflection of compressional waves from the cutoff shows good agreement between theory and experiment, except in the high field region, where apparently the waves are not confined by the magnetic field lines. Reflection of torsional waves close to the resonance condition shows disagreement in the transition region, where the experimental wave energy is as great as 4 times as large as that predicted both by the above theory and the WKB method.

Sites of methylation of purified transfer ribonucleic acid preparations by enzymes from normal tissues and from tumours induced by dimethylnitrosamine and 1,2-dimethylhydrazine

“…In the presence of Mg2+ alone the methylation of tRNA by crude extracts from mammalian tissues produces mainly methylated guanosine derivatives, but addition of polyamines or NH4+ allows the formation of 1-methyladenosine and 5-methylcytidine (Leboy, 1971;Pegg, 1972a,b;Leboy & Piester, 1973). These latter products are formed in the 'T#C loop' and the 'extra arm' region, which may be oriented in the tertiary structure of the tRNA (Levitt, 1969;Cramer, 1971;Kim etal., 1973) such that methylation occurs only when the tRNA structure is altered.…”

Effects of TψCG on the enzymatic binding of eukaryotic and prokaryotic initiator tRNAs to rat liver ribosomes

“…In 1964 Rosset et al discovered ribosomal5 S RNA [ 11, which was subsequently sequenced from a number of prokaryotic [2-41 and eukaryotic organisms [S-7]. Comparison of the sequence of KB cell 5 S RNA [5] with that of E. coli [2] lead Forget and Weissman [ 51 to propose that the common 5 S RNA sequence CGAAC would interact with the common GT\kCPu (loop IV) sequence of tRNA [8,9]. More recent data has shown that all 5 S RNAs contain the oligonucleotide or one similar to CGAAC [2-7, lo] .…”

Propagation of waves generated in tandem-mirror end cells

“…For z > 130 cm, fast waves typically have parallel wavelengths a few times the scale length, so the eikonal approximation must be examined more carefully. Cramer [10] studied propagation of fast and slow waves along a model inhomogeneous magnetic field. Cramer's results on fast-wave propagation show that little reflection occurs until the scale length is less than about 50% of the parallel wavelength.…”

“…To assess the validity of the eikonal approximation for the parallel variation of slow waves in TMX, we turn again to the work of Cramer [10]. He found little ( < 20%) reflection of slow waves unless the scale length was less than about 20% of the wavelength.…”

“…Significant conversion of slow to fast waves results from rapid change of k* due to rapidly changing magnetic-field strength B. This process was illustrated by Cramer [10]. By choosing a special model for the magnetic field and restricting himself to axisymmetric waves, Cramer was able to replace partial by ordinary differential equations.…”