In a space plasma there are few neutral particles. The pressure is close to a vacuum. So in general a plasma will be colllisionless. Heat will flow from the hot surface such as the sun’s surface to a cold surface, further out in space. The magnetic fields are generated by current flows but the magnitude of the magnetic fields will be small. To study the heat flow in such weakly magnetized collisionless plasmas in the presence of an imposed temperature gradient along an ambient magnetic field particle-in-cell simulations are very effective. The particle in cell codes tract particles in fields self-consistently generated by the average of particles in many cells. The codes can be one dimensional, two dimensional and even three dimensional. However the cost of running the code increase rapidly with the increase in dimensions.
In this paper a two dimensional particle in cell code is used to study two thermal reservoirs at different temperatures driving an electron heat flux that destabilizes off-angle whistler-type modes. The whistlers grow to large amplitude and resonantly scatter the electrons, significantly reducing the heat flux. A surprise is that the resulting steady state heat flux is largely independent of the thermal gradient. The rate of thermal conduction is instead controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir. The results are relevant to thermal transport in high beta astrophysical plasmas such as the Intracluster Medium and low-rate accretion flows.
This two dimensional study also shows that whistlers saturate at small amplitude in the low beta limit and are unable to effectively suppress the heat flux. Electrostatic double layers suppress the heat flux to a mostly constant factor of the free streaming value once this transition happens. The double layer physics is an example of ion-electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of beta explored. The range of plasma-beta studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including the intracluster medium of galaxy clusters, hot accretion flows, stellar and accretion disk coronae, and the solar wind.
Mike B Hopkins
Mike B Hopkins 