Spruit, H. C. High Energy Astrophysics, MPI for Astrophysics, Max Planck Society;
Spruit, H. C., Scharmer, G. B., & Löfdahl, M. G. (2010). Striation and convection in penumbral filaments. Astronomy & Astrophysics, 521: A72, pp. 1-8. doi:10.1051/0004-6361/200912519.