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 (R_h)

Distance of the comet from the Sun (in astronomical units) at the time of observation.

• Valid range: >0 and within of ±1 of the geocentric distance

Observer Distance (Delta)

Distance of the comet from the observer (in astronomical units) at the time of observation.

• Valid range: >0 and within of ±1 of the heliocentric distance unless the button is selected to indicate that the observer is not geocentric (geocentric typically also includes observatories in Earth orbit).
  The only other consideraton for this parameter is that the spatial resolution will be reduced if this parameter is large (this is due to the fact that a fixed numbers of points are used to simulate the coma structure). The user is advised to compare the spatial resolution at the center of coma (given in the output after the calculation) to the instrument spatial resolution.

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.

• Valid range: Although there are no explicit limits for the valid range, the practical limit the projected slit width and length should not be smaller than the spatial resolution.

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. 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 end time (in days) of each step. For example, entering:
Q = 1.0e28 6.3e28 2.5e29
T_Q = 5.5 10.0 20.8
means that the modeled production rate will be:
Q=1.0e28 mol/sec for days 0 to 5.5 (duration = 5.5 days)
Q=6.3e28 mol/sec for days 5.5 to 10.0 (duration = 4.5 days)
Q=2.5e28 mol/sec for days 10 to 20.8 (duration = 10.8 days)

• Valid range: There technically isn't a particular valid range of valid values, but for the case of a single, established production rate, the rule of thumb for the time step is at least 3 to 5 times the combined lifetimes of the parent and daugther.

Parent velocity (V_p)

This is the velocity of the parent molecule at 1 AU. The program will internally adjust for the factor of sqrt(1/R_h).

• Valid range: Any

The total lifetime of the parent molecules (T_pt)

• Valid range: < or = the dissociative lifetime of the parent molecules

The dissociative lifetime of the parent molecules (T_pd)

• Valid range: > or = the dissociative lifetime of the parent molecules

The destruction level of the parent molecules (P_p)

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.

• Valid range: 1-99

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)

• Valid range: Any

Velocity of daughter radical (V_d)

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

• Valid range: Any

Total lifetime of the daughter radical (T_dt)

• Valid range: Any

The destruction level of the daughter radical (P_d)

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 (P_d) best fits the user's needs. The spatial resolution depends on the choice of the two destruction parameters (P_p) and P_d. 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 (P_d) should be decreased, and P_p should be kept constant. Then, a few runs with various values of P_d will allow the user to compare the results and find out what the best value is. For a molecule like OH, P_p=99% goes combined to P_d=95% provides good results over most of the coma.

• Valid range: 1-99

Michel Festou and Joel Parker (joel@boulder.swri.edu)