3.1.34. Var: various settings for the plasma models

3.1.34.1. Overview

For the plasma models, there are several quantities that have useful default values but can be adjusted manually for specific purposes. We list them below.

3.1.34.1.1. Free-bound emission

Usually the freebound emission takes most of the computing time for the plasma models. This is because it needs to be evaluated for all energies, ions and atomic sublevels that are relevant. In order to reduce the computational burden, there is a parameter gfbacc in SPEX that is approximately the accuracy level for the free-bound calculation. The default value is . If this quantity is higher, less shells are taken into account and the computation proceeds faster (but less accurate). The users are advised not to change this parameter unless they are knowing what they do!

3.1.34.1.2. Line emission contributions

By default, all possible line emission processes are taken into account in the plasma models. For testing purposes, it is possible to include or exclude specific contributions. These are listed below in the table below.

Possible line emission processes

Abbrevation	Process
ex	electron excitation
rr	radiative recombination
dr	dielectronic recombination
ds	dielectronic recombination satellites
ii	inner shell ionisation

3.1.34.1.3. Doppler broadening

By default, thermal Doppler broadening is taken into account. However, the user can put this off for testing purposes. The options for the broadening are:

1. No broadening at all

2. Only Doppler broadening (default)

3. Only natural broadening (works only for var calc new)

4. Doppler and natural broadening, Voigt profiles (best physical representation, works only for var calc new)

3.1.34.1.4. Atomic data

The user can choose between the “old” Mekal code (the current default, also referred to as SPEXACT v2), the updated calculations with the command var calc new (referred to as SPEXACT v3), and a “quick-cie” mode with the command var calc qc.

The quick CIE mode is designed only for CIE-family models, including cie, dem, pdem, wdem, cf, and clus. It is not compatible with non-equilibrium scenarios like nei or any cases involving photon fields, like pion, tpho, and xabs.

3.1.34.1.5. Calculation level occupations

In the new line calculations, the occupation of the different atomic energy levels is determined self-consistently. Consider an ion for which we want to determine these occupations. If the occupation of the different levels of all neighbouring ions is known, then given our atomic database we know all the rates we need to calculate in one step the level occupations of our ion. However, our initial guess of the occupation of the neighbours may have been not correct, therefore the rates are not correct, and we need to iterate a few times before all ions of an element are converged. This usually takes between 2 and about 25 iterations, depending on the precise physical conditions.

As starting values we assume that all ions are in the ground state, and then we iterate (this is the default mode). By setting var occstart boltzmann, we can change this to an initial Boltzmann distribution for the occupation. This may be more appropriate for very high density plasmas. To reduce the number of iterations, one may also start with var occstart last, in which case the results from the last calculation are used as starting values. This can be useful for spectral fitting and/or error searches, where for each call the parameters of the model are close to those for the previous call.

Warning

The var occstart last option will have limited advantage if the model during a fit is far off from the final values (large parameter changes during fitting), or with multiple components (in which case the last parameters stored in the newlin subroutine may be associated to a different spectral component compared to the one studied).

3.1.34.1.6. Mekal code

Over the years, we have made several minor improvements to the original Mekal plasma model. These improvements are included by default. However, it is possible to discard some of these improvements by going back to the old code. The improvements are classified as follows (with the appropriate syntax word added here).

Wav: wavelength corrections according to the work of Phillips et al. (1999), based upon Solar flare spectra; in addition, the 1s-np wavelengths of Ni XXVII and Ni XXVIII have been improved.

Fe17: The strongest Fe XVII lines are corrected in order to agree with the rates as calculated by Doron & Behar (2002).

Update: several minor corrections: the Si IX C7 line has been deleted; the Si VIII N6 line now only has the 319.83 Å line strength instead of the total triplet strength; the Ni XVIII and Fe XVI Na1A and NA1B lines plus their satellites have been deleted.

3.1.34.1.7. Cooling by collisional excitation

In the oldest version of the pion model, for the cooling by collisional excitation the Mekal code was used, with all processes except for collisonal excitation on. This has now improved by using a mixture of more modern data for collisional excitation taken from our new SPEX calculations, or for ions for which no new data are yet available, from the Chianti database, and in a few rare cases we still use the old Mekal data (4 ions only). For a full description see Stofanova et al. (2020, subm.). This new calculation is now the default, but the user can switch between both by selecting the var newcoolexc #l option.

3.1.34.1.8. Cooling by dielectronic recombination

In the oldest version of the pion model, cooling by di-electronic recombination was not included. This has now been corrected. Wherever available, cooling rates are calculated directly from the di-electronic recombination model from the latest model, for those ions for which Auger rates and associated energies are available. For all other ions, we use the old calculations from the Mekal code (with only the dr and ds flags turned on).

The user has the option, however, to use only Mekal data (for comparison) by setting the command var newcooldr false.

3.1.34.1.9. Charge exchange recombination and ionization

The charge exchange recombination and ionization rates can be set either from Arnaud and Rothenflug (1985) or Kingdon and Ferland (1996) using the var cxcon command.

1. Use Arnaud and Rothenflug rates for charge exchange recombination and ionization (var cxcon 1).

2. Use Kingdon and Ferland rates for charge exchange recombination and ionization (var cxcon 2).

Default rates are Kingdon and Ferland.

3.1.34.1.10. Photoionization cross sections

The photoionization cross section in the balance calculation, and the relevant photoabsorption continuum and the free-bound emission, can be set either from the OPEN-ADAS database or by the classical recipe which involves Verner et al. (1996) for the photoionization/photoabsorption and Mao et al. (2016) for the free-bound emission. This option is available with the var pixsec command.

1. Use Verner for photoionization/photoabsorption and Mao for free-bound (var pixsec 1).

2. Use OPEN-ADAS for all (var pixsec 2).

Default rates are OPEN-ADAS. With this option the photoexcitation-autoionization resonances are evaluated in the balance calculation.

3.1.34.2. Syntax

The following syntax rules apply:

var gacc #r : Set the accuracy gfbacc for free-bound emission. Default is , maximum value 1 and minimum value 0. Do not change if you do not know what it does.

var gacc reset : Reset gfbacc to its default value.

var line #a #l : For process #a (where #a is one of the abbreviations in the Table above) the process is allowed (if #l is true) or disabled (if #l is false). By default, all processes are allowed.

var line reset : Enable all line emission processes

var line show : Show the status of the line emission processses

var doppler #i : Line broadening, see the four allowed values in the above description

var calc old : Use the old Mekal code

var calc new : Use the new updated atomic data (for SPEX version 3.0 and higher)

var calc qc : Use the quick-cie line calculation (for SPEX version 3.08 and higher)

var occstart ground : Start new line calculation iteration with initial guess that all ions are in the ground state. This is the default

var occstart boltzmann : Start new line calculation iteration with initial guess that all levels have a Boltzmann distribution.

var occstart boltzmann : Start new line calculation iteration with initial guess that all levels have same occupation as in last call to this routine.

var newmekal wav #l : if true (the default), use the updated wavelengths for the Mekal code

var newmekal fe17 #l : if true (the default), use the updated Fe XVII calculations for the Mekal code

var newmekal update #l : if true (the default), use the updates for some lines for the Mekal code

var newmekal all #l : if true (default), use all the above three corrections for the Mekal code

var ibalmaxw #l : if true use multi-Maxwellians (if relevant) for both the ionisation balance and the spectrum (default); if false, only use it for the spectrum.

var newcoolexc #l : if true (default), use the latest cooling by collisional excitation calculations

var newcooldr #l : if true (default), use the latest cooling by dielectronic recombination calculations

var cxcon #i : If 1, Arnaud and Rothenflug charge exchange rates are used. If 2 (default), Kingdon and Ferland rates are used.

3.1.34.3. Examples

var gacc 0.01 : Set the accuracy gfbacc for free-bound emission.to 0.01

var gacc reset : Reset the accuracy gfbacc for free-bound emission.to its default value of 0.001

var line ex f : Exclude electron excitation

var line ds t : Include dielectronic satellites

var line reset : Include all line emission processes

var line show : Show status of all line emission proceses

var doppler f : Do not use thermal Doppler bvroadening

var calc new : Use the new atomic data

var calc qc : Use the quick-cie line calculation (experimental)

var calc boltzmann : Start level occupation calculations with Boltzmann distribution

var newmekal wav f : Use the original Mekal wavelengths instead

var newmekal fe17 t : Use the updated Fe XVII calculations

var newmekal all f : Go back to the full old Mekal code

var newmekal all t : Take the full updated Mekal code

var ibalmaxw f : Do not use Multi-Maxwellians for the ionisation balance

var newcoolexc f : Change to the old collisional excitation cooling calculations

var newcooldr f : Change to the old dielectronic recombination cooling calculations

var cxcon 1 : Change to Arnaud and Rothenflug (1985) charge exchange rates.

var pixsec 1 : Change to Verner and Mao rates for photoionization.