(a) S ₄ time profile over San Miguel de Tucumán during March 2011. (b) Relative energy of each intrinsic mode component of the S ₄ index as a function of the period (in days); colors indicate three different intervals of periods: low periods (yellow), middle periods (red) and high periods (green). (c–e) Report the time profiles of the sum of the S ₄ intrinsic mode components of the low (S _4L)_, middle (S _4M) and high (S _4H) periods, respectively.