A Multi-Diagnostic Observational Framework for Magnetosonic Solitary Waves During Geomagnetic Storms in Solar Cycles 24 and 25 using Cluster II Mission

Abstract

Solitary structures, commonly known as solitons, are a class of nonlinear plasma waves that are abundantly found in near-Earth plasmas and planetary magnetospheres. They are nonlinear, localized plasma waves that maintain their shape and velocity over time and distance. While their occurrence in various space plasma environments has been extensively reported, their observation during geomagnetic storms, large-scale disturbances driven by interactions between the solar wind and Earth's magnetosphere, remains limited. In this study, we present a comparative investigation of magnetosonic soliton signatures during geomagnetic storms associated with Solar Cycles 24 and 25. Using high-resolution in-situ magnetic field measurements from the Cluster II mission, we systematically examine the plasma conditions favorable for soliton generation and their evolution during storm-time dynamics. A comprehensive multi-diagnostic observational framework, incorporating several state-of-the-art analytical techniques, is developed to reliably detect and characterize magnetosonic solitons. The results demonstrate that solitary structures in both storms predominantly occur during the early storm intervals, prior to the main phase, suggesting that they may serve as potential precursor signatures of enhanced geomagnetic activity.

Figures (16)

• Figure 1: Temporal evolution of the geomagnetic indices Kp (top panel) and Dst (bottom panel) during the March 2015 geomagnetic storm associated with SC 24. The storm onset is marked by a rapid enhancement of Kp beginning on 17 March, reaching peak values close to 8, indicative of severe geomagnetic activity. This intensification is followed by a sharp decrease in Dst, which reaches a minimum of approximately $-223$ nT on 17-18 March, corresponding to the main phase of the well-known St. Patrick’s Day storm. The subsequent gradual recovery of Dst over the following days reflects the decay of the storm-time ring current and the transition into the recovery phase, accompanied by a progressive reduction of Kp to moderate levels.
• Figure 2: Time series of the magnetic field magnitude $|B|$ (black) and the three magnetic field components $B_x$ (red), $B_y$ (green), and $B_z$ (purple) measured by the FGM instrument onboard Cluster C1, in GSE coordinates during 15-16 March 2015, corresponding to the St. Patrick’s Day geomagnetic storm (SC 24). The interval captures the storm onset and early main phase, characterized by a strong magnetic field enhancement associated with magnetospheric compression, followed by pronounced, non-sinusoidal fluctuations in the field components. The large-amplitude, asymmetric variations and sharp gradients observed in both $|B|$ and the individual components indicate highly disturbed, nonlinear magnetospheric conditions, providing a favorable environment for the generation of nonlinear plasma structures.
• Figure 3: Magnetic field observations from the C1 FGM during the early main phase of the 16 March 2015 geomagnetic storm (Solar Cycle 24). The upper panel shows the magnetic field magnitude, $\lvert B \rvert$, while the lower panel displays the GSE components $B_x$, $B_y$, and $B_z$ over the interval 10:50--11:00 UT in the magnetopause region.
• Figure 4: Minimum Variance Analysis (MVA) of the magnetic field data from C1 spacecraft during the SC24 interval over a short, localized time window (10:50:30--10:51:00 UT). Panels (a)--(c) show hodograms in the MVA frame corresponding to projections in the $L$--$M$, $L$--$N$ and $M$--$N$ planes, where $L$, $M$, and $N$ denote the maximum, intermediate and minimum variance directions, respectively. The start and end points of each hodogram are marked to illustrate the temporal evolution across the structure. Panel (d) shows the magnetic field components resolved in the MVA frame as a function of time, highlighting the dominance of the maximum variance component and comparatively weaker fluctuations along the minimum variance direction. The corresponding eigenvalues ($\lambda_L$, $\lambda_M$ & $\lambda_N$), their ratios ($\lambda_L$/$\lambda_M$ & $\lambda_M$/$\lambda_N$) and eigenvectors (L, M & N) are indicated, confirming a well-defined variance hierarchy over the selected interval. The short analysis window is chosen to isolate the intrinsic geometry of the soliton train and to minimize contamination from background turbulence.
• Figure 5: Detection of magnetic field solitary structures during SC25 observed by Cluster C1 FGM during a geomagnetic storm interval on 16 March 2015 between 10:50 and 11:00 UT. The top panel shows the magnetic field magnitude $|B|$, where sharp, localized magnetic enhancements marked by red arrows indicate individual solitary pulses. Vertical dashed lines distinguishes successive groups of pulses, suggesting the occurrence of multiple distinct solitary wave trains with irregular spacing and varying amplitudes. The middle panel presents the continuous wavelet transform (CWT) power spectrum of $|B|$, revealing intermittent and temporally localized energy enhancements primarily within characteristic periods of approximately $150$--$250$ s. The color scale represents wavelet power in units of $\mathrm{nT}^2$, with higher intensities (yellow to red; up to $\sim 2.7 \times 10^{3}\ \mathrm{nT}^2$) corresponding to strong magnetic fluctuations associated with the detected structures while the bottom panel displays the logarithmic wavelet power spectrum.