Four regions in the color–period diagram. The red dashed lines denote the critical periods or Rossby number that separate the gap and I region in each temperature bin (the ZAIS line). The blue dashed lines denote the critical periods or Rossby number that separate the C and gap region in each temperature bin. The brown dashed line denotes the critical periods that separate the I region and the W region. The Sun is marked with an ⊙ symbol.