Mirror mode physics: Amplitude limit

Abstract

The mirror mode evolving in collisionless magnetised high-temperature thermally anisotropic plasmas is shown to resemble a macro-quantum state. Starting as a classical zero frequency ion fluid instability it saturates quasi-linearly at very low magnetic level, while forming extended magnetic bubbles.It traps the electron component into an adiabatic bounce motion along the magnetic field which causes a bulk electron anisotropy. This can drive an electron mirror mode (see Treumann & Baumjohann, 2018b, who identified it in old spacecraft data). More important, however, we show that trapped electrons play 5 the dominant role of further evolution towards a stationary state. Interaction of the trapped bouncing electrons with the thermal level of ion sound waves causes attractive potentials between electrons and forms electron pairs in the lowest-energy singlet state of two combined electrons. This happens preferentially near the electron mirror points resulting in a diamagnetic current effect which ultimately drives evolution of the magnetic field into large amplitude mirror bubbles causing diamagnetism and expelling a larger fraction of magnetic flux from the interior of the initial quasi-linearly stable mirror mode bottle. Estimates 10 given in view of mirror modes in the magnetosheath are in reasonable numerical agreement with observation. We derive the self-consistent final state of the mirror bubbles. This analysis demonstrates that the observed mirror mode in high temperature space plasmas (solar wind, magnetosheath, magnetotail) is not a simple magnetohydrodynamic instability. It resembles a classical super-conducting, super-fluid state in high temperature plasma under conditions when electron pairs form. This is a most interesting observation which suggests that pair formation can become relevant in space and astrophysics. 15 but finite real frequency (Hasegawa, 1969). Various formal properties of the instability were added under different plasma conditions and in different wavenumber ranges like finite gyroradius effects, dependencies on electron temperature and electron anisotropies, as well as on streaming plasma conditions. It was moreover shown that the instability saturates quasilinearly at a rather low level by exhausting the bulk thermal anisotropy (cf., e.g., Treumann & Baumjohann, 1997;Noreen et al., 2017, for the quasilinear numerics). Finally it was shown that the trapped particle components give rise to the excitation of ion cyclotron 30 and electron whistler waves if only a thermal anisotropy of resonant particles evolves. Identification in real plasmas became possible when measuring the pressure balance between the magnetic field and plasma. It was shown that in observations the two pressures in ion-mirror modes are indeed anti-correlated, a condition which generally is considered the key identifier of mirror modes: mere pressure balance. Thus it seemed that the physics of mirror modes was completely understood though it remained unclear how in an ideally conducting plasma at high temperature the...

Electron Mirror-mode Structure: Magnetospheric Multiscale Observations

“…Our observation shows that the mirror mode excited by electrons can be intensively excited, indicating their important roles in kinetic-scale plasma physics. Recent studies also show the importance of electrons in the evolution even in the ion mirror mode (e.g., Shaaban et al 2018;Treumann & Baumjohann 2019). The electron mirror-mode structures were observed in the foreshock and its downstream regions, which are highly turbulent, and rich in particle acceleration, energy conversion, and mass and momentum transport.…”

Observation of Whistler Mode Waves Inside Mirror Mode Structures in the Earth's Outer Magnetosphere

“…Mirror mode structures are generated by anisotropic ions with T i⊥ /T i∥ > 1 in high β plasmas (Hasegawa, 1969;Treumann & Baumjohann, 2019). In the statistical result, the MM-Ss are mainly observed in the dusk sector, suggesting that substorm injected protons are the main source of free energy.…”