Cluster observations of trapped ions interacting with magnetosheath mirror modes

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.

Vlasiator: First global hybrid-Vlasov simulations of Earth's foreshock and magnetosheath

“…Kis et al, 2007) and in the magnetosheath (e.g. Souček and Escoubet, 2011). We will show that the ion foreshock includes the kinetic features present in Omidi et al (2005), while mirror mode structures in the magnetosheath display characteristics present in Southwood and Kivelson (1993), Souček et al (2008), promising fruitful avenues for further global simulations.…”

“…In the magnetosheath region the simulations presented here as well as in Pokhotelov et al (2013) reproduce highly anisotropic biMaxwellian distributions and associated mirror mode structures. Such anisotropic distributions and large scale mirror mode structures are routinely observed deep in the magnetosheath (Souček and Escoubet, 2011;Souček et al, 2008). Other instabilities that are responsible for the generation of smaller scale ion cyclotron waves observed in the magnetosheath are not resolved in the current simulations due to insufficient spatial resolution.…”

Ion distributions upstream and downstream of the Earth's bow shock: first results from Vlasiator

“…Such mirror mode structures with spatial scales of a few ion inertial lengths are routinely observed across the Earth's magnetosheath (Soucek et al, 2008). The loss cone seen in the magnetosheath ion distribution is also a characteristic feature of the mirror modes and is due to ion trapping between magnetic mirrors (Soucek and Escoubet, 2011).…”

Cluster and TC-1 observation of magnetic holes in the plasma sheet

“…The electrons could affect the threshold of mirror instability and also the scale size of magnetic holes, but not very significantly. Recent observations have also shown that the trapped ions could be heated at intermediate pitch angles and cooled at small parallel velocities in the trough of nonlinear mirror mode structures (Soucek and Escoubet, 2011).…”