Rheinhardt, M., & Brandenburg, A.: 2010, ``Test-field method for mean-field coefficients with MHD background,'' Astron. Astrophys. 520, A28 (arXiv:1004.0689, ADS, DOI, PDF)
Sur, S., Subramanian, K., & Brandenburg, A.: 2007, ``Kinetic and magnetic alpha effects in nonlinear dynamo theory,'' Monthly Notices Roy. Astron. Soc. 376, 1238-1250 (astro-ph/0701001, ADS, PDF)
Figure 2 of Sur et al. (2007), but with the -4 slope added.