Geoscience Reference
In-Depth Information
The numerical simulation of the interaction between the cometary atmo-
sphere and the solar wind is a complex problem due to the difference
between the two plasmas involved in shaping this coma. The solar wind
is a hypersonic hot plasma composed of light ions (mainly protons) and
electrons embedded in an interplanetary magnetic field. The plasma in the
region of the inner coma formed from a cometary gas released from the
comet nucleus is much denser than the solar wind and is composed of ion
and neutral species with a range of masses. On release from the nucleus a
complex network of chemical reactions produces new chemical species. The
cometary gas expands and cools down rapidly once it is released from the
nucleus and is often considered to be completely cold (
T
n
∼
0K near
the nucleus surface) relative to the warmer unperturbed solar wind plasma
(
T
sw
∼
10
5
K). Our model includes the major charged particule species for
a water-rich nucleus.
In this paper we present early results of the three-dimensional (3-D)
version of the multi-fluid MHD simulator (CASIM3D: Comet Atmosphere
Simulator 3-D). This code was progressively assembled during the last
2 years to advance cometary modeling techniques by combining adaptive
mesh techniques and multi-fluid solvers. The basic multi-dimensional equa-
tions of this code and along with its two-dimensional (2-D) application
have been presented by Benna
et al.
4
Section 2.1 describes the physics
and chemistry, included in this model, while Sec. 2.2 details the adap-
tive mesh refinement technique that we implement on a 64-node paral-
lel processing high performance computer. Section 3 presents the output
of the MHD model for a comet of the activity level of Halley, using the
solar wind conditions relevant for the dates of the Giotto spacecraft flyby.
The plasma boundaries we compute are compared to the Giotto derived
values.
2. The Cometary Atmosphere Simulator
2.1.
The model
In the CASIM3D model, we assumed that water vapor molecules dominate
the cometary neutrals. When the neutral gas is released from the nucleus
by sublimation, it quickly expands, cools down and interacts with the solar
UV-EUV radiation and the solar wind plasma. The rich cometary atmo-
sphere is subject to a complex network of chemical reactions and physical
interactions that involve neutrals, ions, and electrons. In CASIM3D, we
