The breakdown of magnetic flux-freezing in molecular clouds and
protostellar discs is usually approximated by ambipolar diffusion at low
densities or by resistive diffusion at high densities. However, there is
an intermediate regime in which the Hall term in the conductivity tensor
is significant, and the vector evolution of the magnetic field -- and
therefore the evolution of the system under consideration -- is dramatically
altered.
I begin by outlining how MHD differs in the Hall regime, and then present
calculations of charged particle abundances in dense molecular gas in
different environments that demonstrate that Hall diffusion is important
over a surprisingly broad range of conditions. The implications for the
dynamics of shock waves, dense cores, and protostellar discs are discussed.
Thursday, April 17th at 4:00 p.m.
**SPECIAL DATE**
Seminar is to be held at 4:00 PM in the conference room
on the second floor of Dearborn Observatory
Refreshments will be served at 3:30
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