Analysing Turbulent Energy Cascade in a Coronal Mass Ejection using Empirical Mode Decomposition
Coronal mass ejections (CMEs) are large-scale expulsions of plasma and magnetic flux from the Sun's corona into the heliosphere. In interplanetary space they are referred to as interplanetary CMEs (ICMEs), often characterised by a shock, a sheath, and in some cases a magnetic cloud, and are capable of triggering geomagnetic storms. We apply empirical mode decomposition (EMD) in conjunction with Hilbert spectral analysis (HSA) to investigate turbulence characteristics at different stages of an ICME event observed on 27 June 2013 by the MAG instrument onboard NASA's ACE spacecraft. The event is divided into four regions: (i) preceding solar wind, (ii) sheath, (iii) magnetic cloud, and (iv) trailing solar wind. The magnetic field components (Bx, By, Bz) are decomposed into intrinsic mode functions using EMD, and instantaneous frequencies and amplitudes are derived via HSA. Spectral slopes in the inertial range are calculated from the second-order marginal Hilbert spectra. The preceding solar wind shows a slope near the Kolmogorov value (α_HHT \approx -1.68), indicating fully developed turbulence at 1 AU. Clear steepening is observed in the sheath and trailing solar wind (α_HHT \approx -1.78 and -1.79), consistent with enhanced intermittency and non-linear activity from shock compression and solar wind-ICME interactions. Within the magnetic cloud the exponent is slightly less steep (α_HHT \approx -1.71), suggesting the effects driving steepening are less prevalent inside the flux rope. ICME passage thus modifies the turbulent energy distribution across scales, and the EMD-HSA method provides smoother and more stable spectral estimates than conventional Fourier approach.