Drift shell tracing code
Chris Paranicas and Andrew Cheng
JHU/Applied Physics Laboratory, 8/96
For Galileo ephemeris, we appreciate the help of Nat Bachman,
Robin Gary, Tom Garrard, and Neil Murphy.

I.  Output.
Output file contains time, position of Galileo (r, lat, east long),
L shell calculated, K used to calculate L, B of s/c calculated with 
model field, and B min on field line calculated with model field.
We scale lengths to 1 RJ = 71492 km.

II.  Calculated Parameters.
1)  The L value of the s/c in the particular field model for 
charged particles with constant K.

L is calculated following the method outlined in JGR, 99, 19433, 1994
by Paranicas and Cheng, where, 
 
L=2*pi*DM/abs(phi).

For any field model, this formula requires a scaling, DM, which
we take to be a nominal dipole moment.  Phi is the third adiabatic 
invariant equal to the magnetic flux enclosed by the particle drift shell.

We do not include a current sheet at the moment.

2)  The value of the second adiabatic invariant, K,
in the units sqrt(G)-RJ.  This is the K that was used to trace
the drift shell and is assumed to be constant on the drift shell.

3) B at the location of the s/c in Gauss as calculated by the
particular magnetic field model.

4) The minimum-B (in G) on the field line the
spacecraft occupies in the particular magnetic field model.

III.  Testing drift shell tracing code.
To test the code, we calculate the following:

LDC = L(OTD model) = L value using tracing code and an offset, tilted dipole
model for the field.

LDF = L(Dipole formula) = L value using a formula relating position to
L shell in an offset, tilted dipole field.

For 80 equally-spaced-in-time locations of Galileo between  
Dec 7, 1995 14:00 - Dec, 8, 1995 05:00, and Bm=0.25 G, we found,

Max (LDC/LDF) = 1.0015

and for these 80 points, 

(1/80) * sum (LDC - LDF)^2 = 3.4e-04 
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