Fig. 5

The total magnetic field B _t and the three magnetic field components B _x , B _y , B _z in the GSM coordinate and the amplitude–frequency–time spectrogram of the decomposed intrinsic mode functions (IMFs) obtained by the HHT method for the δB _x , δB _y , and δB _z perturbations observed by (a) Th D and (b) Th E spacecraft, respectively. The bottom panels show the ion beta, which is much larger than unity prior to the substorm onset. Note that the Pi2 disturbances are excited prior to both the substorm onset time (~0404:30 UT) and the higher frequency disturbances. The lowest frequency IMF components are in the Pi2 frequency range