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Title: Molecular Shock Structure in Multifluid MagnetohydrodynamicS
Author: Ahmed, Syed Muzaffor
ISNI:       0000 0001 3402 7429
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2007
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We present an investigation on the effects of magnetic dissipation and cooling due to spontaneous radiative emission in multifluid magnetohydrodynamic (MHD) shocks. Ideal MHD allows n small amplitude waves and therefore we can associate a shock with each. But, only non-linear fast and slow shocks are evolutionary. On smaller scales the structure ofshocks is determined by the non-ideal MHD equations and from neutral cooling. Therefore in a dense weakly ionised medium there exist three generic types of shock; C-type, J-type and C*-type. The shooting method can be used to calculate simple steady solutions, with constant ambipolar resistivity and radiative cooling. In this approach only coplanar transverse fields can vary i.e., these shocks are coplanar. But, this method is restricted to C-type non-reacting fluid· shocks, since J-type and C*-type contain a point of singularity in the transonic phase. For time dependent equations an upwind conservative scheme (Godunov's scheme) in one dimension is used. This method is less restricted; we have shown that it is extremely accurate in second order and that we call also capture all three generic interstellar shocks successfully. For completeness we : give expressions for the sources of mass, momentum and energy in a five fluid reacting model. We show that studies in zero dimensions can be used to reveal important shock structure parameter.s. Five fluid MHD shocks show that ionisation, recombination and . grain dynamics can have profound effects on the structure. Firstly we show that slow shock length scales are significantly enhanced and that cooling from molecular rotational and atomic fine structure lines contributes significantly in fast shocks. Thus the structure of the weakest and strongest shocks are characteristically adiabatic and characteristically finite cooling respectively. Conditions are such that both ambipolar resistivity and Hall resistivity can dominate, hence the waves are characteristically dissipative and dispersive, but, only in the fast regime a significant non-coplanar transverse field is induced. In slow shocks grain fluids are decoupled from the field, but in the fast regime they can reconnect· with the field and this is also dependent on their dimensions. We predict that slow shocks are generally C*-type, and that such shocks are more likely to be responsible for the: condensation of dense cores and therefore the formation of protostellar objects and stars.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available