Geoscience Reference
In-Depth Information
Table 2.
Parameters used for the modeling of a Halley-type comet.
Parameter
Value
Parameter
Value
375 km s
−
1
Nucleus radius
6 km
Solar wind speed
7
×
10
29
mol s
−
1
Gas production rate
Solar wind mach number
10
Neutral gas surface
200 K
Solar wind mean
1 amu
temperature
molecular mass
7 ions cm
−
3
Mean neutrals
18 amu
Solar wind number
molecular mass
density
0.6 km s
−
1
10
5
K
Initial neutral gas
Solar wind ion
speed
temperature
10
6
s
2
.
5
×
10
5
K
Ionization life time
Solar wind electron
temperature
45
◦
IMF magnitude
5.6 nT
IMF angle
the boundary conditions and describe the major physical interactions. The
values of these variables are summarized in Table 2.
In CASIM3D the MHD equations are integrated in 3-D using the total
variation diminishing Lax-Friedrichs (TVDLF) solver. The TVDLF solver
is one of the high-resolution schemes designed to solve hyperbolic conser-
vation laws and capture complex hydrodynamic shock structures without
oscillations. Yee
17
provides a good review of this class of schemes. We have
described in detail in Ref. 4 these MHD equations that govern the behavior
of a cometary atmosphere composed of three fluids: ions, electrons, and
neutrals.
Figures 2-4 show the final steady state solution obtained through this
process. The solution was computed over 5,836,800 physical cells spread
over 25 levels of refinements. The largest computational cell is about
4,200,000 km and the smallest cell is about 250 m. These figures exhibit,
respectively, the ion density, temperature, and velocity profiles in the inner
coma (
R
c
≤
20,000 km) and in the outer part of the coma (
R
c
>
20,000 km).
This validates the capabilities of the CASIM3D code to model the cometary
atmosphere from the surface to large cometocentric distances.
We present in Fig. 5 a cross section along the the Sun-Comet line of
the ion Mach number, pressure and density along with the magnetic field
magnitude. These profiles show clearly the formation of the bow shock at
R
b
=5
.
5
10
5
km and the formation of the inner shock at
R
i
= 2,700 km
along the Sun-Comet line. The bow shock is formed as a result of the fast
deceleration of the hypersonic solar wind stream to subsonic speeds while
ions of cometary origin are imbedded in it. The inner shock is a result of
the deceleration of the hypersonic radially expanding cometary ions and its
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