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Ann. Geophys., 23, 545-552, 2005
www.ann-geophys.net/23/545/2005/
© European Geosciences Union 2005
Shock parameter calculations at weak interplanetary shock waves
J. M. Gloag and A. Balogh
Imperial College Space and Atmospheric Group Department of Physics, Prince Consort Road, London SW7 2BW, UK
Abstract. A large set of interplanetary shock waves observed using the Ulysses
spacecraft is analysed in order to determine their local parameters. For the
first time a detailed analysis is extended to the thermodynamic properties of
a large number of events. The intention is to relate the shock parameters to
the requirements set by MHD shock theory. A uniform approach is adopted in
the selection of up and downstream regions for this analysis and applied to
all the shock waves. Initially, the general case of a 3 component adiabatic
plasma is considered. However, the calculation of magnetosonic and Alfvénic
Mach numbers and the ratio of downstream to upstream entropy produce some
unexpected results. In some cases there is no clear increase in entropy
across the shock and also the magnetosonic Mach number can be less than 1. It
is found that a more discerning use of data along with an empirical value for
the polytropic index can raise the distribution of downstream to upstream
entropy ratios to a more acceptable level. However, it is also realised that
many of these shocks are at the very weakest end of the spectrum and
associated phenomena may also contribute to the explanation of these results.
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