Intensity-based Emission with pure absorption
------------------------------------------------------

Uses ArtPureEmis class
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To perform an ibase (intensity-based) calculation using ``ArtEmisPure`` in ExoJAX, 
one simply needs to change the ``rtsolver`` option to either "ibased" or "ibased_linsap". 

The difference between **ibased** and **ibased_linsap** (ibased w/ linear source approximation) in ExoJAX lies in the assumptions 
regarding the distribution of the source function within the layer. 
The former assumes a uniform source function, while the latter assumes a linearly approximated source function.
The latter can also be described as a linear version of Olson and Kunasz's method.

In the case of ibased, the number of streams can be specified. This is done using the ``nstream`` option (which needs to be an even number).

.. code:: ipython
    
    from exojax.spec.atmrt import ArtEmisPure

    art = ArtEmisPure(pressure_top=1.e-8,
                      pressure_btm=1.e2,
                      nlayer=100,
                      nu_grid=nu_grid, 
                      rtsolver="ibased", # "ibased" or "ibased_linsap"
                      nstream=8)         # specify the number of the streams
    
    art.change_temperature_range(400.0, 1500.0) #sets temperature range
    Tarr = art.powerlaw_temperature(1300.0, 0.1) # sets a powerlaw T-P profile
    mmr_arr = art.constant_mmr_profile(0.1) # sets a constant mass mixing ratio
    gravity = art.constant_gravity_profile(2478.57) #sets a constant gravity 

    # we here call OpaPremodit as opa just to compute xsmatrix 
    opa = OpaPremodit(mdb=mdb,
                      nu_grid=nu_grid,
                      diffmode=diffmode,
                      auto_trange=[art.Tlow, art.Thigh]) 
    xsmatrix = opa.xsmatrix(Tarr, art.pressure)

    dtau = art.opacity_profile_xs(xsmatrix, mmr_arr, opa.mdb.molmass, gravity) # computes optical depth profile  
    F0 = art.run(dtau, Tarr) # computes spectrum