Multiscale Features of the Near-Hermean Environment as Derived Through the Hilbert-Huang Transform

Tommaso Alberti,Christina Plainaki,Elisabetta De Angelis,Stavro L Ivanovski,Stefano Orsini,Alessandro Ippolito,Alessandro Mura,Rosanna Rispoli,Valeria Mangano,Monica Laurenza,Stefano Massetti,Anna Milillo

doi:10.3389/fphy.2021.668098

Abstract

The interaction between the interplanetary medium and planetary environments gives rise to different phenomena on several temporal and spatial scales. Here, we propose for the first time, the application of the Hilbert-Huang Transform (HHT) to characterize both the local and global properties of Mercury's environment as seen during two Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) flybys. In particular, we compute the energy-time-frequency distribution of the observed magnetic field components and the reconstruction of these signals at large, magnetohydrodynamics (MHD) and kinetic scales through the empirical mode decomposition. We show that the HHT analysis allows to capture and reproduce some interesting features of the Hermean environment such as flux transfer events (FTEs), Kelvin-Helmholtz vortices, and ultralow frequency (ULF) wave activity. Moreover, our findings support the ion kinetic nature of the Hermean plasma structures, the characterization of the magnetosheath by anisotropic ion-kinetic intermittent fluctuations, superimposed to both MHD fluctuations and large-scale field structure. Our approach has proven to be very promising for characterizing the structure and dynamics of planetary magnetic field at different scales, for identifying the boundaries, and for discriminating the different scale-dependent features of global and local source processes that can be used for modeling purposes.

Highlights

The interplanetary medium significantly affects the dynamics of planetary environments by means of energy and mass transfer processes [1]
Applying the Empirical Mode Decomposition (EMD) we obtained a set of N1 = {20, 23, 24} and N2 = {23, 25, 26} empirical modes for each magnetic field component measured during the first and the second Mercury flybys, respectively
It is important to note that the EMD acts as a sort of dyadic filter [28], and it is expected to filter out a number of empirical modes of the order of log2(Nt), where Nt is the number of points of the considered time series

Summary

Introduction

The interplanetary medium significantly affects the dynamics of planetary environments by means of energy and mass transfer processes [1]. This interaction involves several types of phenomena such as magnetic reconnection, plasma instabilities, magnetic flux transport, particle precipitation, turbulence, and waves [2]. The basic structure of the magnetosphere of the planets is quite similar and can be understood by scaling the Earth’s case, the dynamical features peculiar to the specific body are completely dependent on its intrinsic/induced nature [8], on the different particle population and plasma composition (especially the presence of heavy ions) [9,10,11], and on the effects of solar transient phenomena impacting their boundaries [12, 13] and affecting planetary exospheres [14] and environments [15, 16]. These systems are characterized by a wide variety of phenomena occurring on different temporal and spatial scales, as well as in different surrounding regions, which need to be properly identified and investigated [1, 17]

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Results

Conclusion