Abstract
Abstract. Mirror modes are among the most intense low frequency plasma wave phenomena observed in the magnetosheaths of magnetized planets. They appear as large amplitude non-propagating fluctuations in the magnetic field magnitude and plasma density. These structures are widely accepted to represent a non-linear stage of the mirror instability, dominant in plasmas with large ion beta and a significant ion temperature anisotropy T⊥/T∥>1. It has long been recognized that the mirror instability both in the linear and non-linear stage is a kinetic process and that the behavior of resonant particles at small parallel velocities is crucial for its development and saturation. While the dynamics of the instability and the effect of trapped particles have been studied extensively in theoretical models and numerical simulations, only spurious observations of the trapped ions were published to date. In this work we used data from the Cluster spacecraft to perform the first detailed experimental study of ion velocity distribution associated with mirror mode oscillations. We show a conclusive evidence for the predicted cooling of resonant ions at small parallel velocities and heating of trapped ions at intermediate pitch angles.
Highlights
Mirror modes are a well known phenomenon occurring in a variety of space plasmas
If plasma β is sufficiently high, mirror structures are generated behind the bow shock, where the heated plasma often satisfies the mirror instability threshold condition β⊥(T⊥/T − 1) > 1 (Hasegawa, 1969; Hellinger, 2007)
The mirror structures are very important for the global magnetosheath dynamics under high β conditions: they are responsible for dissipation of the temperature anisotropy excess generated at the bow shock, keeping the magnetosheath plasma in a marginally stable state (Fuselier et al, 1994; Hellinger et al, 2003)
Summary
They are typically observed as large amplitude nonpropagating variations in magnetic field magnitude accompanied by an anti-correlated variation plasma pressure. For more details see Soucek et al (2008); Genot et al (2009) and references therein These structures are generally believed to be generated by mirror instability (Vedenov and Sagdeev, 1958; Hasegawa, 1969) in plasmas with sufficient ion temperature anisotropy. In this work we use ion and magnetic data from the Cluster spacecraft (Escoubet et al, 2001) to investigate the details of the ion distribution observed within large amplitude mirror structures. The more complicated problem of fully non-linear mirror structures has been approached by phenomenological models of particle behavior (Kivelson and Southwood, 1996) and numeric simulations (Califano et al, 2008) which demonstrated the importance of trapped particles for instability saturation at large amplitudes.
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