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SUMMARY:Fluctuation dynamo in collisionless and weakly collisional\, magne
 tized plasmas - Dr. Denis St-Onge (Oxford)
DTSTART:20200210T140000Z
DTEND:20200210T150000Z
UID:TALK133576@talks.cam.ac.uk
CONTACT:Chris Hamilton
DESCRIPTION:The amplification of cosmic magnetic fields by chaotic fluid m
 otions is hampered by the adiabatic production of magnetic-field-aligned p
 ressure anisotropy. This anisotropy drives a viscous stress parallel to th
 e field that inhibits the plasma's ability to stretch magnetic-field lines
 . However\, in high-𝛽 plasmas\, kinetic ion-Larmor scale instabilities-
 --namely\, firehose and mirror---sever the adiabatic link between the ther
 mal and magnetic pressures\, reducing this viscous stress and thereby allo
 wing the dynamo to operate. We identify two distinct regimes of the fluctu
 ation dynamo in a magnetized plasma: one in which these instabilities effi
 ciently regulate the pressure anisotropy so that it does not venture much 
 beyond the firehose and mirror instability thresholds\, and one in which t
 his regulation is imperfect. Using kinetic and Braginskii-MHD simulations 
 and analytic theory\, we elucidate the role of these kinetic instabilities
  and determine how the fields and flows self-organize to allow the dynamo 
 to operate in the face of parallel viscous stresses. In the case of effici
 ent pressure-anisotropy regulation\, the plasma dynamo closely resembles i
 ts more traditional Pm ~ 1 MHD counterpart. When the regulation is imperfe
 ct\, the dynamo exhibits characteristics remarkably similar to those found
  in the saturated state of the MHD dynamo. An analytical model for the lat
 ter regime is developed that exploits this similarity. The model predicts 
 that the plasma dynamo ceases to operate if the ratio of field-aligned to 
 field-perpendicular viscosities is too large\, a behavior confirmed by num
 erical simulation. Leveraging these results\, we construct a novel set of 
 microphysical closures for fluid simulations that bridges these two regime
 s---one that exhibits explosive magnetic-field growth caused by a field-st
 rength-dependent viscosity set by the firehose and mirror instabilities.
LOCATION:MR14\,  Centre for Mathematical Sciences\, Wilberforce Road\, Cam
 bridge
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