Wave activity in front of high-β Earth bow shocks

Abstract

Earth's bow shock in high β (ratio of thermal to magnetic pressure) solar wind environment is relatively rare phenomenon. However such a plasma object may be of interest for astrophysics. We survey statistics of high-β (β > 10) shock observations by near-Earth spacecraft since 1995. Typical solar wind parameters related with high β are: low speed, high density and very low IMF 1-2 nT. These conditions are usually quite transient and need to be verified immediately upstream of the observed shock crossings. About a hundred crossings were initially identified mostly with quazi-perpendicular geometry 5 and high Mach number. In this report 22 Cluster project crossings are studied with spacecraft separation within 30-200 km.Observed shock front structure is different from that for quaziperpendicular supercritical shocks with β ∼1. There is no well defined ramp. Dominating magnetic waves have frequency 0.1-0.5 Hz (in some events 1-2 Hz). Polarization has no stable phase and is closer to linear. In some cases it is possible to determine wavelength at 0.1-0.5 Hz of the order of 200-900 km.10 Copyright statement. IntroductionShocks are the primary dissipation mechanism in space plasmas with supersonic flows (Sagdeev, 1966;Kennel et al., 1985;Krasnoselskikh et al., 2013). A brand new branch of plasma science, theory of collisionless shocks, appeared in the sixties, in response to new observational data on solar flares and solar wind interaction with Earth magnetic field. In the solar system solar wind forms the bow shocks at planets and comets, the termination shock at the heliospheric interface. 15Interplanetary shocks develop, when large-scale transient structures propagate in solar wind after solar eruptions. In the distant space, shocks are associated with supernova explosions, stellar winds, collisions of galaxy clusters. Astrophysical shocks are believed to have a leading role in the acceleration process of cosmic rays (Krymskii, 1977;Axford et al., 1977). The review of space shock physics can be found in AGU Geophysical Monographs 34 and 35 (1985). The Earth bow shock has been most thoroughly studied since the launch of the first spacecraft and is the main source of our in-situ knowledge of collisionless shock 20 structure and dynamics.Of particular interest to astrophysical applications are shocks in weak magnetic field environment (high-β shocks) (e.g., Markevitch and Vikhlinin, 2007). β is a dimensionless parameter, a ratio of plasma thermal to magnetic energy density. Unfortunately, observations of high β shocks near the Earth are quite rare, since the solar wind plasma usually has β ∼1. Very 1 Ann. Geophys. Discuss., https://doi.