ISSI Team on modeling cometary environments in the context of the heritage of the Giotto mission to comet Halley and of forthcoming new observations at Comet 67P/Churyumov-Gerasimenko First Workshop, Bern, November 2012 Prediction / interpretation of the boundaries in the inner ionized coma by single fluid model Monio Kartalev, Murray Dryer, Valentina Keremidarska

Outline  Closer look at the inner ionized coma model and experimental Giotto results  Inner shock, contact discontinuity, inner “shocked” region between them  Possible mechanism of magnetic field diffusion due to the mass- loading process  Possible reinterpretations of inner ionized coma structures Attempt to address some of the Target Project’s QUESTIONS  Attempt to address some of the Target Project’s QUESTIONS  Attempt to formulate possible contribution for solving these questions questions  Possible coordination / cooperation with other Project Groups

Questions  Q2: Adequacy in reflecting important physical phenomena  During the Giotto passing through the inner Halley coma, this unique natural plasma physics laboratory posed some difficult for understanding and explanation questions.  Approaching the final stage of the Rosetta mission makes these questions again very actual

Diamagnetic cavity is one of the challenging experimental findings in the inner Coma of Comet Halley From Gombosi et al. 1996: Diamagnetic cavity found by Giotto at about 4800 km from the Halley nucleus Neubauer et al In the framework of the “conventional” ideal MHD approach, of course, the magnetic cavity boundary could be only the contact surface Inner shock “is in the magnetic cavity?”

Another challenging experimental finding in the inner Halley Coma: Ion pileup boundary, found by Giotto at approximately 8000 km from the nucleus From Gombosi et al. 1996: Comparison of modeled (their model) plasma density with measured ion mass density along the Giotto inbound pass as a function of cometocentric distance. HIS data (Altwegg et al.,1993), HERS data (Neugebauer et al., 1991). Dashed line: model by Schmidt et al, 1988 A coincidence in the frame of the MHD model was reached by these authors, but “paying” for this by introduction of some “ prescribed” outside to the model conditions Somewhere In the region B In this interpretation

Outside prescribed conditions, needed to explain the Ion pileup boundary in the frame of the ideal MHD approach From Gombosi et al. 1996: The radial electron temperature profile used in their calculation From Gombosi et al. 1996: The radial recombination rate profile used in their calculation Taken independently “from the experimental data”

In our single fluid model:  All parameters, participating in the reaction coefficients are determined self- consistently  The electron temperature, needed for the impact ionization and dissociative recombination is accepted to be equal to the temperature of the considered single fluid gas We studied thoroughly: HOW THE INCLUSION OF EACH OF THE POSSIBLE REACTIONS IN THE REGION C AFFECTS THE SOLUTION S p - photoionization S C - charge transfer L C - charge transfer L r - dissociative recombination I fr - frictional force With dissociative recombination (1-4) ; Without dissociative recombination (5-8)

The obtained positions for all the discontinuities are similar for all the cases: Another important test is the variation of the parameters along the Giotto trajectory

Dependence of the density distribution along the computational grid radiuses (108 o computational radius is a proxy of the Giotto trajectory) Case 1: All reactions included Only this model case predicts correctly the observed density variation along the trajectory Closest to the Giotto orbit Case 8: Only photoionization included

Projection of the cometary ion bulk flow vectors into the Halley–Sun– Ecliptic (HSE) plane Giotto IMS-HIS (blue) Rubin et al., 2009 Model velocity component, perpendicular to the radial direction (Region C) along the radius at 108 o from Comet-Sun line. Solid line – with all reaction included

When the dissociative recombination is included in the Region C, the obtained by the model temperature distribution along a Giotto trajectory also corresponds to the really measured one  The solid line is the single-fluid temperature distribution along Giotto trajectory, obtained by our model in the region inside the contact surface (between the INNER SHOCK and the contact surface)  The dashed line is the real, measured by Giotto electron temperature.  This measured distribution was introduced by Gombosi et al. (1996) in order to achieve a satisfactory density distribution but supposing that this happens outside the contact surface

The message from these results could be read as : It seems that the diamagnetic cavity boundary is the inner shock wave ??? Question:  Is the magnetic cavity boundary the inner shock wave? Despite of the shown results, it is not possible to answer “YES” without explaining the behavior of the interplanetary magnetic field  The “frozen in” IMF can reach the contact surface, but it is “not permitted” to enter the Region C and to reach the inner shock!  We can answer “YES” only if we are able to explain the magnetic field penetration till the inner shock Some kind of magnetic field diffusion is “needed” for such an explanation  Some kind of magnetic field diffusion is “needed” for such an explanation

Cravens, 1989 (JGR), was probably the first, who utilized artificially introduced magnetic diffusion, aiming better description of the inner comet region, including diamagnetic cavity problem.  One dimensional Navie-Stokes eqns  kinematic and magnetic viscosity  mass loading, frictional force,  Dissociative recombination  Boundary conditions: B=0 at 2000 km (from nucleus) V=0 at km

An attempt for introducing such a diffusion in a single - fluid MHD approach for the mass-loaded plasma was done in the paper: M.D. Kartalev (1998)”On the single-fluid modeling of mass-loaded plasma”, Geophys. Astrophs. Fluid Dynamics, vol.88, pp The picking up of new ions is non-instantaneous process Three basic stages passed by the new ions’ distribution function (Gaffey et al., 1988):  Ring-beam distribution  Shell – distribution  Relaxation to a supra-thermal distribution

Idealization (simplification) of this non-instantaneous process: Idealized model: there is a first (“fresh ions”) stage in the randomization process where this ions have still the velocity w of the comet neutrals the lifetime in this stage is much shorter than the gas dynamic infinitesimal time scale i.e. the “fresh ions” do not affect the source and loss terms- they remain “instantaneous” The “fresh ions” however do affect the electromagnetic field: they cause a kind of shielding electric field, which is external to the fluid and, correspondingly, effective external charges and currents As a result, a new term appears in the magnetic induction equation, causing a quasi-diffusion of the magnetic field into direction, opposite to the velocity of the cometary neutrals.  It is shown qualitatively that this quasi-diffusion is effective in the region C, ensuring the IMF penetration till the inner shock  It is also shown that this mechanism does not work in the region D  This is a possible way to explain all the mentioned results.

 ds is the initial (at the time moment t ) position of both elements  In the moment t + dt : the convected fluid material element moves to the positions ds’ the field line element moves to the positions ds’’ Schematic picture of the field line and fluid line elements kinematics: Generalization  of the consideration of the vortex line in fluid dynamics or  of the magnetic field line in MHD

Magnetic field lines supplemental convection  The analysis of the kinematics of the magnetic and fluid lines infers that: The magnetic field line is frozen in a certain virtual fluid with a virtual bulk velocity U: where u is the velocity of the ambient plasma with density n, w is the (radial) velocity of the background collisionless neutral particles flow and N f is the number density of the substance of the fresh new ions Two components of the obtained magnetic field diffusion:  Diffusion directed against the flow of the neutral particles  Pseudo-diffusion along the magnetic field line

 Kartalev and Nikolova, 1998 repeated the Cravens, 1989 solution for inner coma structure, replacing his artificial magnetic diffusion by the above described “suplemental” diffusion Sofer boundary conditions: dB/dr = 0 at 2000 km (from nucleus) dV/dr = 0 at km

 Numerically obtained density distribution along a segment of the Giotto trajectory in the inner Halley coma  Dotted line: measured density (Cravens, 1986). Segment AB- probably wide CD structure  Solid line: our computed distribution (all reactions included)  Vertical lines: our Inner Shock and Contact Discontinuity  Dashed line: Baranov and Lebedev, 1988 predictions  Point C: magnetic cavity boundary  Our model direct predictions:  The inner shock falls into the magnetic cavity boundary  The contact suface falls into the inner edge of the Pileup boundary  Density variation within the inner shocked region (CA) fits the measured one  Controversy remains: Why there is magnetic field in the inner shocked region?

Questions  Q2: Adequacy in reflecting important physical phenomena

Possible activity of our group in Sofia till the next team meeting and beyond:  We may contribute – together with experimentalists and other modeling groups – to revisiting some interesting or unresolved problems from past missions  We may participate in the interpretation of data from past missions, that have not been considered by now.  We may contribute to preparation of software / modeling tools for future missions

Thank you!