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

Abstract

Properties of kinetic‐scale plasma turbulence are examined observationally with the help of Spektr‐R measurements in the Earth's magnetosheath. The statistics covers 6 years of ion flux measurements during years 2011–2017 and includes a few dozens of the magnetosheath crossings with total duration of 250 hr. Several types of Fourier spectra are distinguished in the magnetosheath according to a spectral shape at the scales of transition from magnetohydrodynamic to kinetic regimes. Spectral indices and occurrence rate of different spectral shapes are examined at different locations with respect to the magnetosheath boundaries—the magnetopause and the bow shock. Magnetosheath plasma behind the quasi‐parallel and quasi‐perpendicular bow shock is explored separately. Clear dynamics of spectral shapes across the magnetosheath is shown: spectra with broad bump at the transition frequencies occur more frequently in the vicinity of the bow shock, while spectra with plateau at the same frequencies are usually observed closer to the magnetopause. Spectral indices such as spectral slope at the kinetic scales and break frequency are shown to be weakly affected by the boundaries behind the quasi‐perpendicular bow shock. The steepest spectra are observed close to the quasi‐parallel bow shock.

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

“…Moreover, the spectral scaling at sub-ion scales flattens from the bow shock to the flank and the magnetopause. Similar results are obtained for the ion-flux spectra in the dayside magnetosheath (Rakhmanova et al 2018b(Rakhmanova et al , 2018a.…”

“…Various wave modes/ instabilities introduced immediately after the bow shock (e.g., Schwartz et al 1996) may be responsible for these phenomena. Rakhmanova et al (2018a) show that "bump"-type ion-flux spectra (i.e., peaked at 0.25-0.35 Hz) are more likely observed near the bow shock rather than near the magnetopause, which leads to spectral flattening at MHD scales and spectral steepening at sub-ion scales. Sahraoui et al (2006) argue that mirror-mode structures are more likely responsible for the "bumps" in magnetic field and plasma spectra with the peak at ∼0.1 Hz.…”

“…For simplicity, we automatically determine the break frequency, f b , by minimizing the chi-square value of a two-stage power-law fitting procedure on the PSDs. Several spectral shapes at the scales of transition from MHD to kinetic regimes have been proposed, such as bumps or plateaus (Riazantseva et al 2017;Rakhmanova et al 2018a). To reduce interferences, we obtain the spectral indices at MHD scales (a 1 ) and at sub-ion scales (a 2 ) from the linear least-squares fitting of PSDs over two separated frequency bands.…”

“…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). In addition, the turbulence properties are also different in the quasi-parallel and quasi-perpendicular magnetosheath (e.g., Czaykowska et al 2001;Shevyrev et al 2007;Macek et al 2015;Breuillard et al 2018;Rakhmanova et al 2018a). Here, the quasi-parallel magnetosheath is defined as the magnetosheath behind a bow shock with quasi-parallel field geometry and likewise the quasi-perpendicular magnetosheath.…”

Characteristics of Solar Wind Fluctuations at and below Ion Scales

“…This range is usually defined by achange of the PSD steepness (relative to the inertial range). For PSDs of bulk V sw and thermal velocity, V th , the spectral index steepens to values between −2 and −4 (Šafránková et al 2016), whereas for N the slope first flattens (Unti et al 1973;Chen et al 2013b;Šafránková et al 2015) and then decreases to avalue of −2.8 (Alexandrova et al 2009;Chen et al 2012b;Šafránková et al 2015;Riazantseva et al 2016;Rakhmanova et al 2018). In the range between the ion and electron scales, magnetic field fluctuations typically exhibit aspectral scaling similar to that of the density…”