Add free-free f2 small-scale template generation notebook

[zonca]

Free-free f2 small-scale template

This PR adds a notebook that generates a high-resolution (nside=2048) free-free intensity template (f2) by injecting statistically consistent small-scale structure into the Hutschenreuter et al. (2024) Emission Measure map.

Method summary

1. Convert the Emission Measure map to brightness temperature at 30 GHz using the Gaunt-factor formalism (Dickinson+ 2003, Draine 2011)
2. Transform to log-space with a data-driven epsilon floor
3. Fit a power-law to the angular power spectrum in the ℓ = 30–150 range
4. Extrapolate small-scale power with a sigmoid high-pass filter
5. Build a spatially-varying modulation map from patch-based spectral analysis
6. Combine large and small scales using power-conserving harmonic cross-fade filters (f_LS² + f_SS² = 1)
7. Transform back to linear space

Output files

Three FITS maps are produced:

• ff_highres_map_nside2048_f2_30GHz.fits — the final free-free template at 30 GHz (μK_RJ)
• ff_modulation_map_nside128_f2_30GHz.fits — spatially-varying modulation map
• ff_HE_map_nside256_30GHz.fits — input Hutschenreuter & Enßlin map converted to brightness temperature

The output files are available at NERSC:

• Path: /global/cfs/cdirs/cmb/www/pysm-dev/f2/
• Public URL: https://portal.nersc.gov/project/cmb/pysm-dev/f2/

Executed notebook

The fully executed notebook (with all outputs and plots at nside=2048) is available here:
https://gist.github.com/zonca/22ca2c4e94e6756a6c87f7bde489c294

The notebook in this PR has outputs stripped to keep the file size small in git.

Looking for reviewers

We are looking for a reviewer who can help assess whether this new f2 template is suitable for use in PySM simulations — in particular, whether the small-scale power injection, the large/small-scale transition, and the spatial modulation produce physically reasonable results compared to the existing f1 template.

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