Mirror and ion cyclotron anisotropy instabilities in the magnetosheath

Abstract

Magnetosheath plasmas are typically characterized by a proton/• > 1 and a proton temperature anisotropy V.l_/T]l > 1. Such a plasma is unstable to both the ion cyclotron anisotropy and mirror instabilities, and evidence for both of these growing modes has been found in the magnetosheath. In this paper, we use one-dimensional hybrid simulations to investi•[ate the kinetic properties of these two instabilities. We find that, for moderate values of ion/• and temperature anisotropy, the two instabilities produce similar levels of turbulence, but that the cyclotron instability apparently produces higher fluctuation levels than the mirror instability for high /• or anisotropy. Although both instabilities saturate by reducing the ion temperature anisotropy, mirror waves accomplish this through the use of the wave magnetic fields, whereas cyclotron waves do so by using the wave electric fields in resonant interactions with the ions. Consequently, mirror waves preferentially affect those ions with large magnetic moment, while cyclotron waves affect a broader range of ions and thereby dominate the isotropization process even when the mirror mode is somewhat stronger. We also investigate the utility of transport ratios as mode identifiers, and find that the Alfv6n ratio in particular can be useful in distinguishing between the two modes. than the ion temperature 2ql parallel to Bo [Gary et al., 1976, 1992]. These conditions are achieved by the heating and reflection of ions at the bow shock and persist downstream in the magnetosheath [Sckopke et al., 1983, 1990; Thomsen et al., 1985], and are also achieved by the draping and compression of the magnetic field in the magnetosheath [Zwan and Wolf, 1976; Crooker and Siscoe, 1977].The characteristics and properties of both the ion cyclotron anisotropy and mirror instabilities have been studied in detail. The mirror instability can be described by fluid theory in the long wavelength limit, has zero real frequency in the plasma rest frame, and has a maximum growth rate at a direction of the wave vector k that is at a_. large angle with respect to the background magnetic field Bo. Consequently, the magnetic perturbations caused by the mirror waves are approximately parMid to Bo. The ion cyclotron anisotropy instability can only be described by kinetic theory and has a maximum growth rate at a nonzero frequency below the ion gyrofrequency with k parallel to Bo. Hence, the magnetic perturbations associated with the ion cyclotron waves are oriented approximately perpendicular to Bo. In terms of polarization, linear theory and numerical simulations show that parallel-propagating ion cyclotron waves are left-hand circularly polarized [Tajima et al., 1977; Ambrosiano and Brecht, 1987; Brinca et al., 1990]. Linear anMyses of the two instabilities in electron-proton plasmas have found that, for •3ill < 6.0 (where •3ill is the proton beta associated with Tll), the cyclotron instability should grow more quickly, and be excited at a lower anisotropy threshold, than the mirror instability [Gar...

Mirror‐mode storms inside stream interaction regions and in the ambient solar wind: A kinetic study

“…A possible explanation is that because ion-cyclotron waves have phase velocities much greater than the mirror-mode waves, we are not able to observe them in the same region where MMSs are present because they might have left the region of observation. That both waves are cogenerated even when their growth rates are very different was shown observationally by Russell et al [2006] for the low beta conditions in the Saturnian magnetosphere and has also been observed in simulations [McKean et al, 1992[McKean et al, , 1994.…”

Low-frequency waves in the Earth's magnetosheath: present status

“…Recent attempts to illustrate the role of the Alfve´n/ioncyclotron modes in the magnetosheath have stressed the mode's ability to pitch-angle scatter particles, as revealed by one-and two-dimensional hybrid simulations (McKean et al, 1992b;Gary and Winske, 1993;Gary et al, 1993c;McKean et al, 1994). This scattering thereby results in a more-thermalized equilibrium state Denton et al, 1994b;Gary and Lee, 1994;Gary et al, 1993a;Gary et al, 1994a;Gary et al, 1994b;Gary et al, 1994c).…”

Shaping the solar wind temperature anisotropy by the interplay of electron and proton instabilities

“…Resonant interactions, i.e., cyclotron resonance with plasma particles, are therefore expected to be effective in the relaxation of temperature anisotropy in this case. The interest to explain these effects and their consequences in the solar wind has been boosted after the simulations suggested that the mirror instability is not an effective pitch angle scatterer of protons (McKean et al 1992Gary et al 1993.a) and the cyclotron anisotropy instability may be assumed the primary mechanism to constrain the proton anisotropy (Gary and Lee 1994.a). Further confirmation of this constraint can also be obtained in a straightforward way, namely, by fitting the instability thresholds predicted by the linear theory with the anisotropy limits reported by the observations (see explanatory arguments and references in Gary et al (1994.b)).…”