Kinetic-scale magnetic turbulence and finite Larmor radius effects at Mercury

TL;DR

This study reveals that kinetic-scale magnetic turbulence and finite Larmor radius effects significantly influence Mercury's magnetosphere, highlighting the importance of non-MHD effects in planetary space environments.

Contribution

First to analyze kinetic-scale magnetic turbulence at Mercury using a nonstationary structure function approach, emphasizing finite Larmor radius effects.

Findings

Kinetic-scale fluctuations are significant up to ~20 s timescale.

Presence of a highly turbulent foreshock with ULF oscillations.

Detection of ion-kinetic turbulence and finite Larmor radius effects.

Abstract

We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near Mercury's space environment, with the emphasis on key boundary regions participating in the solar wind -- magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable time scale ~20 s imposed by the signal nonstationarity, suggesting that turbulence at this planet is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.