[Paper Review] Average observed properties of the Earth's quasi-perpendicular and quasi-parallel bow shock

A. Czaykowska, Thomas Bauer+2 more|ArXiv.org|Sep 13, 2000

Solar and Space Plasma Dynamics49 references3 citations

TL;DR

This study analyzes 132 dayside bow shock crossings by the AMPTE/IRM spacecraft using superposed epoch analysis to examine plasma and magnetic field properties in quasi-perpendicular and quasi-parallel shocks. It finds that proton temperature anisotropy drives left-hand polarized ion cyclotron waves in low-β quasi-perpendicular shocks, while mirror waves appear in high-β, high-θBn conditions, with strong correlation between anisotropy and wave intensity but no simple link to compressive fluctuations.

ABSTRACT

We present a statistical analysis of 132 dayside (LT 0700-1700) bow shock crossings of the AMPTE/IRM spacecraft. We perform a superposed epoch analysis of plasma and magnetic field parameters as well as of low frequency magnetic power spectra some minutes upstream and downstream of the bow shock by dividing the events into categories depending on the angle between bow shock normal and interplanetary magnetic field and on the plasma-beta, i.e., the ratio of plasma to magnetic pressure. Downstream of the quasi-perpendicular low-beta (beta < 0.5) bow shock we find a dominance of the left-hand polarized component at frequencies just below the ion cyclotron frequency with amplitudes of about 3 nT. These waves are identified as ion cyclotron waves which grow in a low-beta regime due to the proton temperature anisotropy. We find a strong correlation of this anisotropy with the intensity of the left-hand polarized component. Downstream of some nearly perpendicular high-beta (beta > 1.0) crossings mirror waves are identified. However, there are also cases where the conditions for mirror modes are met downstream of the nearly perpendicular shock, but no mirror waves are observed.

Motivation & Objective

• To characterize average plasma and magnetic field properties downstream of Earth's bow shock under varying shock geometry and plasma conditions.
• To investigate the role of proton temperature anisotropy and plasma beta in driving low-frequency magnetic fluctuations upstream and downstream of the shock.
• To determine the conditions under which ion cyclotron and mirror mode waves are generated and sustained in the magnetosheath.
• To assess the correlation between wave intensity and plasma parameters such as β and T⊥/T∥ in different shock regimes.
• To resolve discrepancies in bow shock position estimates by analyzing data under solar minimum conditions.

Proposed method

• Superposed epoch analysis of 132 dayside bow shock crossings from AMPTE/IRM spacecraft data.
• Categorization of events based on θBn (angle between shock normal and IMF) and plasma β (ratio of plasma to magnetic pressure).
• Averaging of plasma parameters, magnetic field, and low-frequency magnetic power spectra upstream and downstream of the shock.
• Analysis of magnetic fluctuation power spectra in the 0.01–0.04 Hz range to identify wave polarization and growth mechanisms.
• Use of wave polarization analysis to distinguish left-hand and right-hand circularly polarized components, identifying ion cyclotron waves.
• Comparison of observed wave amplitudes and anisotropy with theoretical models of mirror and ion cyclotron instabilities.

Experimental results

Research questions

• RQ1 How does the proton temperature anisotropy (T⊥/T∥) vary downstream of quasi-perpendicular versus quasi-parallel bow shocks?
• RQ2 What is the relationship between plasma β and the generation of ion cyclotron or mirror mode waves downstream of the shock?
• RQ3 Why is magnetic wave activity significantly higher upstream of quasi-parallel shocks compared to quasi-perpendicular shocks?
• RQ4 Is there a consistent correlation between the intensity of left-hand polarized magnetic fluctuations and proton temperature anisotropy?
• RQ5 Why are mirror waves not observed even when all instability criteria are satisfied in high-β, high-θBn conditions?

Key findings

• Downstream of quasi-perpendicular shocks (θBn > 45°), proton temperature anisotropy T⊥/T∥ ≈ 1.5 is observed, while it remains nearly isotropic downstream of quasi-parallel shocks (θBn < 45°).
• Upstream of quasi-parallel shocks, magnetic fluctuation power is ~10 times higher (δB ≈ 4 nT at 0.01–0.04 Hz) than upstream of quasi-perpendicular shocks.
• At the quasi-parallel shock itself, magnetic wave power is enhanced by a factor of 4, likely due to wave amplification or generation at the interface.
• In quasi-perpendicular low-β shocks, left-hand polarized ion cyclotron waves with amplitudes ~3 nT dominate just below the proton gyrofrequency, strongly correlated with T⊥/T∥.
• Mirror waves are observed downstream of nearly perpendicular (θBn ≈ 90°) high-β shocks, but their presence is not guaranteed even when all instability conditions are met.
• The simulated wave amplitudes (δB/B ≈ 0.2) and anisotropy reduction timescales (τ ≈ 2.5/fcp) from McKean et al. (1994) are consistent with observed data, supporting the role of pitch-angle scattering in saturation.