Abstract
Abstract. Observations of Earth's bow shock during high-β (ratio of thermal to magnetic pressure) solar wind streams are rare. However, such shocks are ubiquitous in astrophysical plasmas. Typical solar wind parameters related to high β (here β>10) are as follows: low speed, high density, and a very low interplanetary magnetic field of 1–2 nT. These conditions are usually quite transient and need to be verified immediately upstream of the observed shock crossings. In this report, three characteristic crossings by the Cluster project (from the 22 found) are studied using multipoint analysis, allowing us to determine spatial scales. The main magnetic field and density spatial scale of about a couple of hundred of kilometers generally corresponds to the increased proton convective gyroradius. Observed magnetic variations are different from those for supercritical shocks, with β∼1. Dominant magnetic variations in the shock transition have amplitudes much larger than the background field and have a frequency of ∼ 0.3–0.5 Hz (in some events – 1–2 Hz). The wave polarization has no stable phase and is closer to linear, which complicates the determination of the wave propagation direction. Spatial scales (wavelengths) of variations are within several tens to a couple of hundred of kilometers.
Highlights
Shocks are the primary dissipation mechanism in space plasmas with supersonic flows (Sagdeev, 1966; Kennel et al, 1985; Krasnoselskikh et al, 2013)
Interplanetary shocks develop inside the heliosphere after solar eruptions, when large-scale transient structures propagate relative to the regular solar wind flow
When a spacecraft is found to be in the right place, the plots of solar wind, interplanetary magnetic field (IMF), local magnetic field, and plasma parameters are analyzed visually in the 5 h window around the selected hour
Summary
Shocks are the primary dissipation mechanism in space plasmas with supersonic flows (Sagdeev, 1966; Kennel et al, 1985; Krasnoselskikh et al, 2013). Oblique shocks (angles around 45◦) are, in a sense, intermediate with respect to their properties, and ions are partially capable of escaping upstream but generally have a rather spatially localized transition, similar to quasi-perpendicular shocks. Besides this large-scale magnetic field structure, relatively low-frequency magnetic variations (from one tenth to a few Hertz) with visually maximal amplitudes, which form the primary shock front structure and dissipate ions, are Published by Copernicus Publications on behalf of the European Geosciences Union. To the best of our knowledge, the first extended experimental study of high-β bow shocks with multipoint analysis of dominating low-frequency magnetic variations at high-β shock transition using observations from the Cluster project. All vectors in this paper are in the GSE frame of reference
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