Four fluid model and numerical simulations of magnetic structures in the heliosheath

Details Four fluid model and numerical simulations of magnetic structures in the heliosheath Four fluid model and numerical simulations of magnetic structures in the heliosheath

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh23a1137a&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH23A-1137

Other

7524 Magnetic Fields, 7839 Nonlinear Phenomena (4400, 6944), 7852 Solitons And Solitary Waves (4455)

Scientific paper

A magnetic hole/hump is a stable structure with small scale minima/maxima of the mean magnetic field in the centre. Such structure has been observed in inter-planetary magnetic field, planetary magneto sheath, cometary's plasma, and very recently in the heliosheath, as revealed by Voyager I observations. Recently, we have proposed a realistic three fluid model that comprises of three fluids in the model are electrons, heliosheath ions, and neutrals. Stationary, time independent solutions of this model consisting of holes, humps, trains of holes and humps etc. were found to be consistent with Voyager observations e.g. a few tens of ion gyro-radii width, large magnetic maxima/minima, oblique angles of propagation and well approximated by Gaussians. In the first part of the present work, we extend the three fluid model to a four fluid model consisting of electrons, pick up ions (PUI), solar wind ions (SWI), and neutrals. The PUIs are generated by neutrals via charge exchange with SWI. The kinetic pressure of PUI is nearly three to four times the pressure of SWI. Hence these are more suited to mediate small scale structures in heliosheath like shocks, magnetic holes/humps etc. We show that the constant energy exchange between these two fluid drives them non adiabatic. The modified adiabatic index, is calculated by solving the corresponding enthalpy equation. The PUI are found to be isothermal ( = 1) while SWI have 1.25. In the four fluid model, these effects are captured by including a modified equation of state for PUIs and SWIs. The phase space of time independent solutions in terms of the Mach numbers of PUI and SWI is constructed to delineate the parameter space which allows structure formation. In the second part of the present work, we examine the stability of our time independent solution by evolving them via a full set of modified Hall-MHD equations. The evolutions are examined using two codes e.g. a pseudo spectral code and a code based on finite difference scheme and the results are cross checked. Our results show that (a) single structures, a hole or a hump, are robustly stable. They propagate without change of shape in homogenous plasma for long time (b) in collision of two of these structures (hole-hole, hump-hump, hole-hump), the structures are not quite stable. There seems to be a significant energy exchange between them, consequently structures which go in are different from the ones which come out of collision event. Besides a portion of energy is also transferred to high frequency noise and other type oscillations.

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