change name of A_sum and add note in fiasco.io about its origin

Author: Jeffrey Reep

fiasco/io/sources/continuum_sources.py

@@ -252,15 +252,21 @@ def to_hdf5(self, hf, df, **kwargs):
 
 
 class HSeqParser(GenericParser):
-    """
+    r"""
     Parameters for calculating two-photon continuum for hydrogen-like ions
+
+    Notes
+    -----
+    * The parameter :math: `\psi_{\text{norm}}` (called :math: `A_{\text{sum}}` in CHIANTI) is a normalization factor of
+    the integral of the spectral distribution function :math:`\psi(y)` from 0 to 1, such
+    that :math: `\frac{1}{\psi_{\text{norm}}} \int_{0}^{1} \psi(y) dy = 2`.
     """
     filetype = 'hseq_2photon'
     dtypes = [int, float, int, float, float, float]
     units = [None, u.dimensionless_unscaled, None, 1/u.s, u.dimensionless_unscaled, u.dimensionless_unscaled]
-    headings = ['Z', 'y', 'Z_0', 'A', 'A_sum', 'psi']
+    headings = ['Z', 'y', 'Z_0', 'A', 'psi_norm', 'psi']
     descriptions = ['atomic number', 'fraction of energy carried by one of the two photons',
-                    'nominal atomic number', 'radiative decay rate', 'summed radiative decay rate',
+                    'nominal atomic number', 'radiative decay rate', 'normalization of the integral of psi from 0 to 1',
                     'spectral distribution function']
 
     def __init__(self, filename, **kwargs):

fiasco/ions.py

@@ -1490,11 +1490,11 @@ def two_photon(self, wavelength: u.angstrom, electron_density: u.cm**(-3)) -> u.
                               'erg cm^3 s^-1 Angstrom^-1')
         if self.hydrogenic:
             A_ji = self._hseq['A']
-            A_sum = self._hseq['A_sum']
+            psi_norm = self._hseq['psi_norm']
             cubic_spline = CubicSpline(self._hseq['y'], self._hseq['psi'])
         if self.helium_like:
             A_ji = self._heseq['A']
-            A_sum = 1.0 * u.dimensionless_unscaled
+            psi_norm = 1.0 * u.dimensionless_unscaled
             cubic_spline = CubicSpline(self._heseq['y'], self._heseq['psi'])
 
         # store the rest wavelength:
@@ -1509,7 +1509,7 @@ def two_photon(self, wavelength: u.angstrom, electron_density: u.cm**(-3)) -> u.
         indices = np.where(wavelength < rest_wavelength)
         psi_interp[indices] = 0.0  # intensity below rest_wavelength is 0
 
-        energy_dist = (A_ji * rest_wavelength * psi_interp) / (A_sum * wavelength**3)
+        energy_dist = (A_ji * rest_wavelength * psi_interp) / (psi_norm * wavelength**3)
 
         pe_ratio = proton_electron_ratio(self.temperature)
         level_population = self.level_populations(electron_density)[:,:,1]