The proton cyclotron instability and the anisotropy/β inverse correlation

Abstract

Spacecraft observations in the strongly compressed subsolar magnetosheath show an inverse correlation between the proton temperature anisotropy (Wlp/•l p • 1 where _L and II denote directions perpendicular and parallel to the background magnetic field) and the parallel proton/• (/•llp)' This manuscript uses one-dimensional hybrid simulations of the proton cyclotron anisotropy instability in homogeneous electron-proton plasmas to study this correlation which may represent a limited closure relation for fluid theories of anisotropic space plasmas. The emphasis is on driven simulations which increase the temperature anisotropy by periodically reducing the magnetic-field-aligned velocities of the protons. The late-time states from ensembles of both initial value and driven simulations yield very similar expressions for the proton anisotropy//•ll p inverse correlation, and provide a basis for explaining differences between sheath observations from different spacecraft. The driven simulations also yield expressions for the maximum instability growth rate and the fluctuating field energy as functions of ]•llp and a parameter characterizing the anisotropy driver. Paper number 93JA03583. 0148-0227/94/93JA-03583505.00 number. These closure conditions often correspond to assumptions such as isotropicity and adiabaticity, conditions which may be useful for collision-dominated fluids but which are not necessarily appropriate for the collisionless plasmas of space. Spacecraft observations often demonstrate distribution functions that are dearly non-Maxwellian, implying that Vlasov theory may be an appropriate mechanism for determining closure under some circumstances [e.g., Feldman el al., 1973, 1976]. Tllp ]•llp with a linear correlation coefficient of R -0.996, a result very similar to equation (4) from the late-time initial 59•o GARY ET AL.. PROTON CYCLOTRON INSTABILITY _Lp ß . -1 T ß lip ß ß ß te ß ß 0.2

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

“…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). Moreover, this work established a significant correlation between the proton temperature anisotropy and the proton in the magnetosheath, and one which has the same slope as that required to keep the Alfve´n/ion-cyclotron mode near to marginal stability.…”

“…This paradigm suggests that these waves are being driven within the magnetosheath and in turn react back on the particle distributions to reduce the mode growth rate from large values to that corresponding to further macroscopic pumping rates. Artificially driven simulations (Gary et al, 1994c;Denton et al, 1993) have been used to support this paradigm, although they do not completely represent the pumping processes in the magnetosheath. The Alfve´n/ ion-cyclotron mode should be particularly effective at low .…”

Solar wind vs magnetosheath turbulence and Alfvén vortices

“…at frequencies below f ci . For the magnetosheath conditions (T ⊥ >T ) the linear Vlasov theory for homogeneous plasmas predicts that if the ion plasma beta β=8πp/B 2 0 , the ratio between the pressure of the ions p and the magnetic pressure, is small (β≤1) Alfvén Ion Cyclotron (AIC) waves will grow with wave vectors mainly parallel to the background field k k ⊥ , while for a high beta (β≥5) mirror waves with k ⊥ k will grow (Schwartz et al, 1996;Gary et al, 1994b). These waves then scatter and diffuse the particles, reducing the anisotropy.…”

Electromagnetic proton/proton instabilities in the solar wind: Simulations

“…Our previous research with single proton bi-Maxwellian distributions [Gary et al, 1994[Gary et al, , 1995[Gary et al, , 1997] has shown that there exist constraints on both the proton anisotropy and the saturation value of the fluctuating magnetic field energy density which may be expressed as con- Further study is necessary to more completely characterize (10).…”