Low-frequency magnetic field fluctuations in Earth's plasma environment observed by THEMIS

Abstract

Abstract. Low-frequency magnetic wave activity in Earth's plasma environment was determined based on a statistical analysis of THEMIS magnetic field data. We observe that the spatial distribution of low-frequency magnetic field fluctuations reveals highest values in the magnetosheath, but the observations differ qualitatively from observations at Venus presented in a previous study since significant wave activity at Earth is also observed in the nightside magnetosheath. Outside the magnetosheath the low-frequency wave activity level is generally very low. By means of an analytical streamline model for the magnetosheath plasma flow, we are able to investigate the spatial and temporal evolution of wave intensity along particular streamlines in order to characterise possible wave generation mechanisms. We observe a decay of wave intensity along the streamlines, but contrary to the situation at Venus, we obtain good qualitative agreement with the theoretical concept of freely evolving/decaying turbulence. Differences between the dawn region and the dusk region can be observed only further away from the magnetopause. We conclude that wave generation mechanisms may be primarily attributed to processes at or in the vicinity of the bow shock. The difference with the observations of the Venusian magnetosheath we interpret to be the result of the different types of solar wind interaction processes since the Earth possesses a global magnetic field while Venus does not, and therefore the observed magnetic wave activities may be caused by diverse magnetic field controlled characteristics of wave generation processes.

Kinetic‐Scale Ion Flux Fluctuations Behind the Quasi‐Parallel and Quasi‐Perpendicular Bow Shock

“…Similar statistical result was obtained by Guicking et al () for lower frequencies (up to 0.167 Hz). The authors showed that processes driving electromagnetic waves in the MSH are more likely to occur close to the BS rather than deeper in the MSH.…”

A statistical study of magnetic field fluctuations in the dayside magnetosheath and their dependence on upstream solar wind conditions

“…They also suggested that the amplitude of these fluctuations increased with decreased proximity to the MP, suggesting the MP as a significant driving mechanism. Guicking et al [2012] used Time History of Events and Macroscale Interactions (THEMIS) data to study wave activity in the Earth's MS for frequencies between 30 and 167 mHz. The authors reported that instabilities and other mechanisms driving electromagnetic waves in the MS are more likely to occur within close vicinity to the BS rather than deeper in the MS.…”

Evolution of the Earth’s Magnetosheath Turbulence: A Statistical Study Based on MMS Observations

“…Based on a previous case study, the intermittency of plasma turbulence increases in amplitude and anisotropy away from the bow shock (Yordanova et al 2008), and the break frequency of ionflux spectra evolves to higher frequencies approaching the magnetopause (Rakhmanova et al 2017). From a statistical perspective, Guicking et al (2012) find a decay of wave intensity of low-frequency magnetic-field fluctuations along the streamlines in the Earth's magnetosheath, which quantitatively agrees with the theoretical concept of freely evolving/decaying turbulence; Huang et al (2017) find that the ∼f −1 spectral scaling of magnetic-field spectra at MHD scales is more likely located in the vicinity of the bow shock, while the Kolmogorov-like scaling at MHD scales located away from the bow shock toward the flank and magnetopause regions. Moreover, the spectral scaling at sub-ion scales flattens from the bow shock to the flank and the magnetopause.…”

“…First, the bow shock and the magnetopause influence the magnetosheath turbulence properties (Gurnett et al 1979;Rezeau & Belmont 2001;Sahraoui et al 2006;Rakhmanova et al 2018b). Second, strong temperature anisotropy generally observed in the magnetosheath can generate various instabilities under different conditions, likely the Alfvén/ ion-cyclotron instability, the mirror-mode instability, the fast magnetosonic/whistler instability, and the fire-hose instability (e.g., Southwood & Kivelson 1993;Quest & Shapiro 1996;Gary et al 1998;Hellinger & Matsumoto 2000;Guicking et al 2012;Kunz et al 2014;Verscharen et al 2016), which is verified by many studies (e.g., Anderson & Fuselier 1993;Anderson et al 1994;Schwartz et al 1996;Czaykowska et al 2001;Lucek et al 2001;Sahraoui et al 2006;Chen & Boldyrev 2017;Teh & Zenitani 2019;Vörös et al 2019;Zhao et al 2019aZhao et al , 2019b. Third, other turbulent fluctuations related to local structures, e.g., current sheets, magnetic islands, and vortices, can further complicate the magnetosheath turbulence picture (e.g., Alexandrova 2008;Karimabadi et al 2014;Huang et al 2018).…”