Web Vectorial Model: Description of Parameters

Comet Name
Any string of characters to identify the comet (used for plot lables and header in tables). The only disallowed character is the single quote ('), which will be replaced by a backquote (`)

Heliocentric Distance (Rh)
Distance of the comet from the Sun (in astronomical units) at the time of observation.
Observer Distance (Delta)
Distance of the comet from the observer (in astronomical units) at the time of observation.
Slit Sizes
Two slit sizes are needed: the size of the slit (in arcseconds) in the direction perpendicular to the sun-comet line, and the size along the sun-comet line. In general, the geometry of the observation may not be aligned in this way, but arbitrary cangles cannot be calculated so the closest orthogonal orientation must be determined.
Production Rate Time Steps
In most cases, one assumes a constant gas production rate together with a time step long enough to allow the coma to establlish a permanent regimen. (The code calculates this optimal time length.) In that case, enter a single Q value and one time step, the time step being the start time in number of days before the time of observation (T=0). However, it is possible to model a time-variable gas production rate. In this case, up to 20 time steps can be pre-defined, and the user must enter the estimated relative Q values (in molecules per second) and the corresponding start time (in days before T=0) of each step. For example, entering:
Q = 1.0e28 6.3e28 2.5e29
TQ = 20.8 10.0 5.5
means that the modeled production rate will be:
Q=1.0e28 mol/sec for days T = -20.8 to -10.0 before the observation (duration = 10.8 days)
Q=6.3e28 mol/sec for days T = -10.0 to -5.5 before the observation (duration = 4.5 days)
Q=2.5e28 mol/sec for days T = -5.5 to 0 before the observation (duration = 5.5 days)
Parent velocity (Vp)
This is the velocity of the parent molecule at 1 AU. The program will internally adjust for the factor of sqrt(1/Rh).
The total lifetime of the parent molecules (Tpt)

The dissociative lifetime of the parent molecules (Tpd)

The destruction level of the parent molecules (Pp)
The code needs to know how far the coma should be modelled. If the destruction level is defined as 99%, the parent coma will have a diameter such that 99% of the parent species are destroyed inside of it. That number of 99% corresponds to approximately 3 parent scalelengths. This is enough in most applications. This choice does not imply that the density that is calculated is 99% of the total density. As a matter of fact, the parent species density near the center of coma willbe very accurate. However, at the edges of the coma, the parent density will be slightly underestimated. This will have not adverse consequence on the final calculation, that of the daugther species density.
Name of daughter radical
Any string of characters to identify the daughter species (used for plot lables and header in tables).

Excitation Rate g-factor of daughter radical (g)

Velocity of daughter radical (Vd)
It is assumed that the radicals are created from photolysis, and that collisions are not a significant source of parent disassociation. Therefore, Vd is independent of the heliocentric distance and no scaling for distance is made by the program (unlike the case for the parent velocity).
Total lifetime of the daughter radical (Tdt)

The destruction level of the daughter radical (Pd)
This parameter defines the overall extent of the calculated coma. The meaning of the parameter is exactly the same as that of the destruction level of the parent molecules. One must proceed by trial and errors to determine wich value of (Pd) best fits the user's needs. The spatial resolution depends on the choice of the two destruction parameters (Pp) and Pd. The user should thus have a look on this resolution and make sure it is not too large relative to the slit size. If it is too large, this parameter (Pd) should be decreased, and Pp should be kept constant. Then, a few runs with various values of Pd will allow the user to compare the results and find out what the best value is. For a molecule like OH, Pp=99% goes combined to Pd=95% provides good results over most of the coma.

Michel Festou and Joel Parker (joel@boulder.swri.edu)   [SwRI Homepage]