On the inadmissibility of non-evolutionary shocks

In recent years, numerical solutions of the equations of compressible magnetohydrodynamic (MHD) flows have been found to contain intermediate shocks for certain kinds of problems. Since these results would seem to be in conflict with the classical theory of MHD shocks, they have stimulated attempts to reexamine various aspects of this theory, in particular the role of dissipation. In this paper, we study the general relationship between the evolutionary conditions for discontinuous solutions of the dissipation-free system and the existence and uniqueness of steady dissipative shock structures for systems of quasilinear conservation laws with a concave entropy function. Our results confirm the classical theory. We also show that the appearance of intermediate shocks in numerical simulations can be understood in terms of the properties of the equations of planar MHD, for which some of these shocks turn out to be evolutionary. Finally, we discuss ways in which numerical schemes can be modified in order to avoid the appearance of intermediate shocks in simulations with such symmetry

Rarefaction Shocks, Shock Errors and Low Order of Accuracy in ZEUS

We show that there are simple one dimensional problems for which the MHD code, ZEUS, generates significant errors, whereas upwind conservative schemes perform very well on these problems.Comment: 11 pages, accepted in ApJ Letter

Generation of density inhomogeneities by magnetohydrodynamic waves in two dimensions

Using two dimensional simulations, we study the formation of structures with a high-density contrast by magnetohydrodynamic waves in regions in which the ratio of thermal to magnetic pressure is small. The initial state is a uniform background perturbed by fast-mode wave. Our most significant result is that dense structures persist for far longer in a two-dimensional simulation than in the one-dimensional case. Once formed, these structures persist as long as the fast-mode amplitude remains high.Comment: 6 pages, 7 figures, accepted by MNRA

Sistem Gona pada Suku Bangsa Imian ( Studi Deskriptif Tentang Pertukaran Barang di Desa Tofot Distrik Seremuk Kabupaten Sorong Selatan )

Sistem gona pada suku Imian merupakan kegiatan untuk memenuhi kebutuhan ekonomi keluarga, yang dalam bahasa Tehit- Imian di sebut Frihene yang berarti perjumpaan antara kelompok atau individu- individu yang satu dengan yang lain dalam melakukan barter. Sistem frihene terdiri dari dua bagian yakni : sistem gona antara suku-suku Imian dengan suku-suku yang ada di Kabupaten Sorong Selatan, tak lain adalah untuk memenuhi kebutuhan hidup mereka sehari- hari. Sedangkan sistem gona antara waskan untuk membantu membayar maskawin. Pada dasarnya masyarakat suku Imian di desa Tofot mereka melakukan gona dengan suku – suku lain yang ada di kabupaten Sorong Selatan agar ikatan kekeluargaan mereka dapat terjaga dengan baik dari waktu ke waktu. Jenis penelitian yang dianggap tepat adalah penelitian kualitatif yang bersifat deskritif, dimana menurut Whitney ( 1960 ) metode deskritif adalah fakta dengan interprestasi yang tepat. Penelitian deskritif mempelajari masalah- masalah dalam masyarakat, serta tata cara yang berlaku dalam masyarakat, serta situasi-situasi tertentu, termasuk tentang hubungan, kegiatan-kegiatan, sikap-sikap, pandangan-pandangan serta proses-proses yang sedang berlangsung dan pengaruh-pengaruh dari suatu fenomena

Cosmic ray moderation of the thermal instability

We apply the Hermite-Bieler theorem in the analysis of the effect of cosmic rays on the thermal stability of an initially uniform, static background. The cosmic rays were treated in a fluid approximation and the diffusion coefficient was assumed to be constant in time and space. The inclusion of cosmic rays does not alter the criterion for the thermal stability of a medium subjected to isobaric perturbations. It does alter the criteria for the stability of a medium perturbed by small amplitude sound waves. In the limit of a high background cosmic ray pressure to thermal pressure ratio, the instability in response to high frequency sound waves is suppressed

The evolution of mass loaded supernova remnants: II. Temperature dependent mass injection rates

We investigate the evolution of spherically symmetric supernova remnants in which mass loading takes place due to conductively driven evaporation of embedded clouds. Numerical simulations reveal significant differences between the evolution of conductively mass loaded and the ablatively mass loaded remnants studied in Paper I. A main difference is the way in which conductive mass loading is extinguished at fairly early times, once the interior temperature of the remnant falls below ~ 107 K. Thus, at late times remnants that ablatively mass load are dominated by loaded mass and thermal energy, while those that conductively mass load are dominated by swept-up mass and kinetic energy. Simple approximations to the remnant evolution, complementary to those in Paper I, are given

Modeling Helical Structures in Relativistic Jets

Many jets exhibit twisted helical structures. Where superluminal motions are detected, jet orientation and pattern/flow speed are considerably constrained. In this case modeling efforts can place strong limits on conditions in the jet and in the external environment. This can be done by modeling the spatial development of helical structures which are sensitively dependent on these conditions. Along an expanding jet this sensitivity manifests itself in predictable changes in pattern speed and observed wavelength. In general, twists of low frequency relative to the local resonant frequency are advected along the expanding jet into a region in which the twist frequency is high relative to the local resonant frequency. The wave speed can be very different in these two frequency regimes. Potential effects include helical twists with a nearly constant apparent wavelength, an apparent wavelength scaling approximately with the jet radius for up to two orders of magnitude of jet expansion, or multiple twist wavelengths with vastly different intrinsic scale and vastly different wave speeds that give rise to similar observed twist wavelengths but with very different observed motion. In this paper I illustrate the basic intrinsic and observed behavior of these structures and show how to place constraints on jet conditions in superluminal jets using the apparent structures and motions in the inner 3C 120 jet.Comment: 18 pages, 7 figure