title: Properties of mass-loading shocks, 2. Magnetohydrodynamics

authors: Zank, Gary P.

subjects: MHD

abstract: The one-dimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets Giacobini-Zinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the Rankine-Hugoniot condition for the mass flux possess a source term. In a formal sense, this renders mass-loading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus mass-loading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, mass-loading shocks can behave like switch-on shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in mass-loading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of mass-loading fronts for which no classical MHD analogue exists, these being the fast and slow compound mass-loading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the Giotto-Halley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of mass-loading shock is the “slow-intermediate” shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of mass-loading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD.

text: Properties of mass-loading shocks, 2. Magnetohydrodynamics The one-dimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets Giacobini-Zinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the Rankine-Hugoniot condition for the mass flux possess a source term. In a formal sense, this renders mass-loading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus mass-loading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, mass-loading shocks can behave like switch-on shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in mass-loading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of mass-loading fronts for which no classical MHD analogue exists, these being the fast and slow compound mass-loading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the Giotto-Halley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of mass-loading shock is the “slow-intermediate” shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of mass-loading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD.

year: 1992

title: Weka: practical machine learning tools and techniques with Java Implementations

authors: Witten, Ian H.

abstract: The Waikato Environment for Knowledge Analysis(Weka) is a comprehensive suite of Java class libraries that implement many state-of-the-art machine learning and data mining algorithms.

Weka is freely available on the World-Wide Weband accompanies a new text on data mining which documents and fully explains all the algorithms it contains. Applications written using the Weka class libraries can be run on anycomputer with a Web browsing capability.

text: Weka: practical machine learning tools and techniques with Java Implementations The Waikato Environment for Knowledge Analysis(Weka) is a comprehensive suite of Java class libraries that implement many state-of-the-art machine learning and data mining algorithms.

Weka is freely available on the World-Wide Weband accompanies a new text on data mining which documents and fully explains all the algorithms it contains. Applications written using the Weka class libraries can be run on anycomputer with a Web browsing capability.

year: 1999-11-22

title: An ashy septingentenarian: the Kaharoa tephra turns 700 (with notes on its volcanological, archaeological, and historical importance)

authors: Lowe, David J.

abstract: Most of us are aware of the basaltic Tarawera eruption on 10th June 1886: the high toll on life (~120 people), landscape devastation, and loss of the Pink and White Terraces. But this was not the first time that Mt Tarawera produced an eruption of importance both to volcanology and human history. This edition of the GSNZ Newsletter marks the 700th anniversary of the Kaharoa eruption – its septingentenary to be precise – which occurred at Mt Tarawera in the winter of 1314 AD (± 12 years) (Hogg et al. 2003) (Fig. 1). The importance of the Kaharoa eruption is at least threefold. (1) It is the most recent rhyolite eruption in New Zealand, and the largest New Zealand eruption volumetrically of the last millennium. (2) The Kaharoa tephra is an important marker horizon in late Holocene stratigraphy and geoarchaeology (Lowe et al. 1998, 2000), and in particular helps to constrain the timing of settlement of early Polynesians in North Island (Newnham et al. 1998; Hogg et al. 2003; Lowe 2011). (3) There is a link between the soils that developed on the Kaharoa tephra, the animal ‘wasting’ disease known as ‘bush sickness’, and the birth of a government soil survey group as an independent organisation (Tonkin 2012).

text: An ashy septingentenarian: the Kaharoa tephra turns 700 (with notes on its volcanological, archaeological, and historical importance) Most of us are aware of the basaltic Tarawera eruption on 10th June 1886: the high toll on life (~120 people), landscape devastation, and loss of the Pink and White Terraces. But this was not the first time that Mt Tarawera produced an eruption of importance both to volcanology and human history. This edition of the GSNZ Newsletter marks the 700th anniversary of the Kaharoa eruption – its septingentenary to be precise – which occurred at Mt Tarawera in the winter of 1314 AD (± 12 years) (Hogg et al. 2003) (Fig. 1). The importance of the Kaharoa eruption is at least threefold. (1) It is the most recent rhyolite eruption in New Zealand, and the largest New Zealand eruption volumetrically of the last millennium. (2) The Kaharoa tephra is an important marker horizon in late Holocene stratigraphy and geoarchaeology (Lowe et al. 1998, 2000), and in particular helps to constrain the timing of settlement of early Polynesians in North Island (Newnham et al. 1998; Hogg et al. 2003; Lowe 2011). (3) There is a link between the soils that developed on the Kaharoa tephra, the animal ‘wasting’ disease known as ‘bush sickness’, and the birth of a government soil survey group as an independent organisation (Tonkin 2012).

year: 2014