The spectral scalings of magnetic fluctuations upstream and downstream of the Venusian bow shock

S D Xiao,G Q Wang,M Y Wu,T L Zhang,Y Q Chen

doi:10.1186/s40623-020-01343-7

Abstract

We statistically investigate the spectral scalings of magnetic fluctuations at the upstream and downstream regions near the Venusian bow shock and perform a differentiation by shock geometry. Based on the Venus Express data, 115 quasi-parallel (Q_{parallel }) bow shock crossings and 303 quasi-perpendicular (Q_{ bot }) bow shock crossings are selected. The statistical results suggest that the bow shock tends to modify the upstream spectra flatter to 1/f noise in the magnetohydrodynamics (MHD) regime and steeper to turbulence in the kinetic regime after the magnetic fluctuations crossing the bow shock, and this modification for the Q_{parallel } and Q_{ bot } bow shocks is basically consistent. However, the upstream spectral scalings are associated with the shock geometry. The changes of the spectral scalings of magnetic fluctuations near the Q_{parallel } bow shocks are not as significant as near the Q_{ bot } bow shock crossings. That might result from the fluctuations generated by the backstreaming ions which can escape across the Q_{parallel } bow shock into the foreshock. Our results suggest that the energy cascade and dissipation near Venus can be modified by the Venusian bow shock, and the Q_{parallel } bow shock plays an important role on the energy injection and dissipation in the solar wind interaction with Venus. The large dispersion of spectral scalings indicates that this fluctuation environment is complicated, and the shock geometry is not the only key factor in the fluctuations across the Venusian bow shock. Other possible factors in the shock modification to the upstream fluctuations will be explored in future.

Highlights

Statistical observations To statistically differentiate the spectral scaling indices for the upstream and downstream regions near the Venusian bow shock and emphasize the shock geometry effects, we examine the Venus Express magnetic field data for ~ 7 years (2006.05–2012.08) and identify the bow shock crossings
We find the Venusian bow shock tends to flatten the spectra of upstream MHD fluctuations and steepen the kinetic spectra
The MHD magnetic fluctuations and turbulence tend to be modified to 1/f noise and the kinetic turbulence can be fully developed behind the shock, which is consistent with the previous studies (e.g., Vörös et al 2008a; Xiao et al 2020b)

Summary

Introduction

As a typical unmagnetized planet, Venus has no global intrinsic magnetic field. An induced magnetosphere is created by the solar wind (SW) interaction with Venus (e.g., Zhang et al 2008a), consisting of the magnetic barrier (e.g., Zhang et al 1991, 2008b; Xiao and Zhang 2018) and the magnetotail (e.g., Rong et al 2014; Xiao et al 2016). In the near-Venusian space, magnetic field fluctuations play an important role in the transformation of momentum and energy, and their properties are widely reported (e.g., Luhmann et al 1983; Guicking et al 2010; Du et al 2010; Xiao et al 2017). The power of these fluctuations generally exhibits a frequency function as P ∝ 1/f α (where P is the power spectral density (PSD), f is the frequency, and α is the spectral scaling index). In the kinetic frequency range, the turbulence has a larger value of α ~ 2.8, in which regime

Objectives

Conclusion