Speaker
Description
Coronal mass ejections (CMEs) are large-scale eruptions of plasma and magnetic flux from the Sun’s corona that propagate through the heliosphere. They play a crucial role in driving space weather phenomena and are responsible for some of the most intense geomagnetic storms. Upon entering the interplanetary space, they are termed interplanetary coronal mass ejections (ICMEs). In-situ observations have shown ICMEs to exhibit a forward shock followed by a region of enhanced magnetic field known as the CME sheath, formed by the combined effects of CME propagation and expansion. In certain cases, ICME-driven shocks are followed by a magnetic cloud (MC)—a structure characterized by smooth magnetic field rotation, enhanced magnetic strength, and low plasma beta (β < 1). The smoothly rotating field signature within the MCs can be attributed towards the presence of coherent structures of twisted magnetic field lines, known as flux ropes.
While turbulence in the solar wind and CMEs has been extensively studied using Fourier and wavelet-based methods, we adopt a novel approach based on Empirical Mode Decomposition (EMD) (Huang et al., 1998) to investigate the turbulence characteristics during a CME event. EMD adaptively decomposes the original magnetic field signal into intrinsic mode functions (IMFs), each representing a dominant oscillatory component. This is followed by Hilbert Spectral Analysis (HSA) to extract instantaneous frequency and amplitude information. While the Fourier and wavelet-based methods assume local stationarity and linearity in the signal data, the EMD-HSA method is well-suited for analyzing nonlinear and non-stationary signals, making it ideal for studying real-time data.
In this study, we examine the turbulence properties during different phases of the ICME event observed on 27 June 2013 at 13:51 UTC by NASA’s ACE spacecraft. The event is divided into three intervals, characterized by the arrival of the CME shock, the sheath region, and the trailing magnetic cloud. The magnetic field components (Bx, By, and Bz) in each interval were processed using EMD followed by HSA to generate Hilbert spectra, representing the time-frequency distribution of amplitude. To evaluate the energy density across frequencies, we computed the second-order marginal Hilbert spectra and performed linear fits within the inertial range to determine spectral slopes.
Our results indicate the presence of fully developed turbulence in the solar wind preceding the CME shock, with a spectral slope of -1.664 ± 0.015—consistent with the Kolmogorov scaling. The sheath region exhibited a steeper slope of -1.700 ± 0.015, suggesting enhanced turbulence due to CME-driven compression. In the trailing magnetic cloud region, a slope of -1.751 ± 0.015 was observed, indicating continued strong magnetic activity. These findings highlight the evolving nature of turbulence across different CME phases and underscore the effectiveness of EMD-HSA in capturing subtle changes in CME dynamics.
| Do you plan to attend in-person or online? | In-person |
|---|
