Plasma sheet pressure anisotropies

Empirical modeling of plasma pressure and magnetic field for the quiet time nightside magnetosphere is investigated. Two models are constructed for this study. One model, referred to here as T89R, is basically the magnetic field model of Tsyganenko [1989] but is modified by the addition of an inner eastward ring current at a radial distance of ~3 RE as suggested by observation. The other is a combination of the T89R model and the long version of the magnetic field model of Tsyganenko [1987] such that the former dominates the magnetic field in the inner magnetosphere while the latter prevails in the distant tail. The distribution of plasma pressure which is required to balance the magnetic force for each of these two field models is computed along the tail axis in the midnight meridian. The occurrence of pressure anisotropy in the inner magnetospheric region is also taken into account by determining an empirical fit to the observed plasma pressure anisotropy. This represents the first effort to obtain the plasma pressure distribution in force equilibrium with magnetic stresses from an empirical field model with the inclusion of pressure anisotropy. The inclusion of pressure anisotropy alters the plasma pressure by as much as a factor of -3 in the inner magnetosphere. The deduced plasma pressure profile along the tail axis is found to be in good agreement with the observed quiet time plasma pressure for geocentric distances between ~2 and ~35 RE-

Section: The Modified Magnetic Field Modelsmentioning

confidence: 99%

Empirical modeling of the quiet time nightside magnetosphere

Lui¹,

Spence²,

Stern³

1994

“…The RCM implicitly assumes that the particles are subject to a pitch angle scattering process that randomizes pitch angles in a time small compared to a drift period without changing particle energy. This is the simplest reasonable assumption for the plasma sheet, which is observed to have a basically isotropic distribution [Stiles et al, 1978] In the ionosphere the electric field in (2) is normally assumed to be a potential field, for timescales more than a few minutes. In the magnetosphere, where the magnetic field changes significantly with time, E has both potential and inductive terms.…”

Section: Distributionsmentioning

confidence: 99%

Comprehensive computational model of Earth's ring current

Fok¹,

Wolf²,

Spiro³

et al. 2001

254

303

Abstract. A comprehensive ring current model (CRCM) has been developed that couples the Rice Convection Model (RCM) and the kinetic model of Fok and coworkers. The coupled model is able to simulate, for the first time using a self-consistently calculated electric field, the evolution of an inner magnetosphere plasma distribution that conserves the first two adiabatic invariants. The traditional RCM calculates the ionospheric electric fields and currents consistent with a magnetospheric ion distribution that is assumed to be isotropic in pitch angle. The Fok model calculates the plasma distribution by solving the Boltzmann equation with specified electric and magnetic fields. To combine the RCM and the Fok model, the RCM Birkeland current algorithm has been generalized to arbitrary pitch angle distributions. Given a specification of height-integrated ionospheric conductance, the RCM component of the CRCM computes the ionospheric electric field and currents. The Fok model then advances the ring current plasma distribution using the electric field computed by the RCM and at the same time calculates losses along particle drift paths. We present the logic of CRCM and the first validation results following the H + distribution during the previously studied magnetic storm of May 2, 1986. The H + fluxes calculated by the coupled model agree very well with observations by AMPTE/CCE. In particular, the coupled model is able to reproduce the high H + flux seen on the dayside at L -2.3 that the previous simulation, which employed a Stern-Volland convection model with shielding factor 2, failed to produce. Though the Stern-Volland and CRCM electric fields differ in several respects, the most notable difference is that the CRCM predicts strong electric fields near Earth in the storm main phase, particularly in the dusk-midnight quadrant. Thus the CRCM injects particles more deeply and more quickly.

“…If this particle is in an isotropic particle distribution, which is a good approximation for the plasma sheet [Stiles et al, 1978;Nakamura et al, 1991], its bounce-averaged drift at position x and time t is independent of its pitch angle and can be described as [Wolf, 1983] B(x, t) x V q•(x, t) B(x, t) x VE k (x, t) V D (X, t) = I g(x, t)12 + 12 .…”

Section: Magnetospheric Specification Modelmentioning

confidence: 99%

Modeling the quiet time inner plasma sheet protons

Wang¹,

Lyons²,

Chen³

et al. 2001

Abstract. In order to understand the characteristics of the quiet time inner plasma sheet protons, we use a modified version of the Magnetospheric Specification Model to simulate the bounce averaged electric and magnetic drift of isotropic plasma sheet protons in an approximately selfconsistent magnetic field. Proton differential fluxes are assigned to the model boundary to mimic a mixed tail source consisting of hot plasma from the distant tail and cooler plasma from the low latitude boundary layer (LLBL). The source is local time dependent and is based on Geotail observations and the results of the finite tail width convection model. For the purpose of selfconsistently simulating plasma motion and a magnetic field, the Tsyganenko 96 magnetic field model is incorporated with additional adjustable ring-current shaped current loops. We obtain equatorial proton flow and midnight and equatorial profiles of proton pressure, number density, and temperature. We find that our results agree well with observations. This indicates that the drift motion dominates the plasma transport in the quiet time inner plasma sheet. Our simulations show that cold plasma from the LLBL enhances the number density and the proton pressure in the inner plasma sheet and decreases the dawn-dusk asymmetry of the equatorial proton pressure. From our approximately force-balanced simulations the magnetic field responds to the increase of pressure gradient force in the inner plasma sheet by changing its configuration to give a stronger magnetic force. At the same time, the plasma dynamics is affected by the changing field configuration and its associated pressure gradient force becomes smaller. Our model predicts a quiet time magnetic field configuration with a local depression in the equatorial magnetic field strength at the inner edge of the plasma sheet and a cross-tail current separated from the ring current, results that are supported by observations. A scale analysis of our results shows that in the inner plasma sheet the magnitude of the Hall term in the generalized Ohm's law is not small compared with the quiet time electric field. This suggests that the frozen-in condition E =-vxB is not valid in the inner plasma sheet and that the Hall term needs to be included to obtain an appropriate approximation of the generalized Ohm's law in that region.