SM13A-1113
Tests and consequences of ballooning in the RCM-E
The Rice Convection Model – Equilibrium (RCM-E) combines the drift physics and magnetosphere-ionosphere coupling computational machinery of the RCM with a model of equilibrium magnetic field that computes the magnetic field that is in static force balance with the RCM-computed pressures. RCM-E simulations of a growth phase frequently exhibit growing ripples associated with the highly stretched magnetic-field configurations. We have applied the Fast-MHD stability analysis algorithm [Crabtree et al., 2003] to RCM-E computed configurations with the result that suggests the system is MHD ballooning unstable. If one assumes that in the magnetosphere, the occurrence of ballooning would result in the modification of the specific entropy of the plasma this would result not only in the injection of plasma into the inner magnetosphere, but also the reconfiguration of the system to a more stable state. We will use the RCM-E to model the consequences on the overall stability of the system when a depleted channel is put in the model and the code was then allowed to evolve in a similar fashion similar to that described by Lemon et al [2004].
SM13A-1114
Geotail observations of temperature anisotropies in the cold plasma sheet on the duskside
To further our understanding of the solar wind entry across the magnetopause under northward IMF, we have studied temperature anisotropies of the two-component protons and electrons in the cold plasma sheet on the duskside. The two-component protons result from mixing of the cold component from solar wind and the hot component of magnetospheric origin, and may be the most eloquent evidence for the transport process across the magnetopause. The cold component occasionally has a strong anisotropy in the dusk flank, and the sense of the anisotropy depends on the observed locations; the parallel temperature is enhanced in the tail flank while the perpendicular temperature is enhanced on the dayside. The parallel anisotropy of electrons is stronger than that of the cold proton component, which is attributed to selective heating of electrons. We further find that strengths of the parallel anisotropies in the tail flank depend on the latitudinal angle of the IMF; strong parallel anisotropies occur under strongly northward IMF. Next we have performed a case study of a duskside Kelvin-Helmholtz (KH) vortices event on 24 March 1995 to investigate the origin of the anisotropies in more detail. The cold proton component occasionally consisted of counter-streaming beams near the current layer in the KH vortical structure. Low-energy bidirectional electron beams or flat-topped electron distribution functions in the direction along the local magnetic field are apparent on the magnetosphere side of the current layer. We discuss that the bidirectionality of electrons and the cold proton component implies magnetic reconnection inside the KH vortical structure. In addition, we suggest selective heating of electrons inside the vortical structure. Comparing temperatures in the magnetosphere-like region inside the vortical structure with those in the cold plasma sheet, we show that further heating is taking place in the cold plasma sheet or on the way from the vortices to the cold plasma sheet. We discuss that both adiabatic heating and wave-particle interactions may be taking place in the vortical structures and the cold plasma sheet.
SM13A-1115
The Role of Low-Entropy Flux Tubes During Geomagnetic Activity
This investigation uses plasma and magnetic field observations taken onboard the Geotail satellite near perigee (-9 RE > XGSE > -12 RE) in the midnight sector of the plasma sheet during geomagnetic bays (driven activity) and near times of substorm onsets and pseudobreakups (unloading activity). Injections are comprised of low-entropy flux tubes ("bubbles") that have lower pV5/3 than their pre-onset predecessors. Interchange with their surroundings result in their injection earthward. Injected flux tubes have pV5/3 ≤ 0.08 nPa(RE/nT)5/3, a value observed at geosynchronous altitude during injections. In pseudobreakups, observed pV5/3 returns to its larger, pre-onset values within a few minutes, whereas for substorm expansions, pV5/3 remains at depressed levels through the expansion phase. Geomagnetic bays are episodes of driven activity evidenced by enhanced eastward and westward electrojets, enhanced convection and a stable auroral band across the nightside. Steady magnetospheric convection (SMC) intervals are geomagnetic bays that do not show unloading activity. During geomagnetic bays, pV5/3 < 0.08 nPa(RE/nT)5/3 in the near-Earth plasma sheet, typical of low-entropy, expansion-phase flux tubes rather than high-content, growth-phase flux tubes. While we cannot say how such low-entropy flux tubes were formed during geomagnetic bays and SMCs, their presence allows Earthward convection to proceed into the inner plasma sheet without encountering the pressure-balance catastrophe.
SM13A-1116
Interchange Instability Criterion in the Plasma Sheet
The interchange instability analysis of Xing and Wolf [2007] has been generalized to the case of arbitrary plasma β, where β is the ratio of plasma thermal pressure to magnetic pressure, averaged within a magnetic flux tube. The theoretical model focuses on a boundary layer between two plasma regions with uniform entropy parameters PV5/3 on each side (where P is the plasma thermal pressure and V=\int ds/B is the flux tube volume); the gradients of PV5/3 and V are at an angle α to each other in the boundary layer. The development of a small perturbation was investigated numerically and the results suggest that shorter wavelength (wave length λ compared with boundary layer thickness 2Δ) perturbations have a wider rage of stability in the plasma boundary layer than the longer wavelength perturbations do. For example when λ=100Δ, the plasma with β<<1 is unstable when α>π/2, but when λ=10Δ, the unstable angles are α>1.73. When λ=Δ, the unstable range goes to α>2.38, and the threshold angle keep getting closer to π as the wavelength decreases. The results for higher magnetic β values indicate more instability. The test calculation for λ=100Δ with β=10 shows that the plasma is unstable when α>1.03; for λ=10Δ the unstable range is α>1.45; and when λ=Δ, the unstable range becomes α>2.20. These critical angles are all reduced compare to the β<<1 cases. The overall conclusion is that increasing either β or wavelength increases the region of instability. Combined with the characteristic of the Earth's plasma sheet calculated from empirical models, our criterion shows that the statistical-average plasma sheet is interchange stable.
SM13A-1117
Entropy Constraints on Particle Entry and Tail Transport from DMSP Measurements
Entropy maps are constructed by mapping DMSP observations of diffuse auroral precipiation to the plasma sheet for quiescent IMF northward condition. These maps show a significant increase in entropy from the flank boundaries to the central plasma sheet. We further examine the entropy profiles for distinguishable hot and cold populations. Differences in entropy of hot particles seem to imply a heat flux from dawn to dusk, while differences in the cold population are related to transport and heating of the sheath population. We discuss these observations in the context of diffusive entry due to kinetic Alfven waves, Kelvin Helmholtz instability, and cusp reconnection. For active conditions, entropy maps are also constructed from a statistical ensemble of substorm events which are binned according to substorm growth, expansion, early-recovery, and late-recovery phases. We investigate the global profiles of total and specific entropy. We find a significant loss of total entropy in the near-earth plasma sheet in the transition between growth and expansion while changes in the specific entropy are less noticeable. Finally, we discuss changes in the global plasma profiles and entropy during substorms in the context of existing substorm models and consider what underlying physical processes could be responsible for the observed morphology. http://w3.pppl.gov/~jrj/magnetopause.html
SM13A-1118
Plasma Equilibrium With Entropy Input and Applications
During slow, quasi-static evolution in the collisionless magnetotail the entropy (S) as a function of magnetic flux (A) is a much better conserved quantity than the plasma pressure function P(A). The entropy profile S(A) has been found to be quasi-conserved even in scenarios involving topology change through magnetic reconnection, as long as the dissipation is very localized. Even though entropy conservation is a better constraint for space plasmas, most available plasma equilibrium solvers take the pressure profile P(A) as an input. This is mostly due to the difficulty of computing equilibria with S(A) input: as opposed to prescribing the current or pressure profiles for plasma equilibrium (which leads to the conventional Grad-Shafranov elliptic partial differential equation), prescribing the entropy results in a nonstandard differential equation whose solution by usual iterative techniques is not guaranteed. To solve this nonstandard equation we have developed a numerical code based on the 'alternating dimensions method' (ADM) suggested by Grad et al., [1975]; the code alternates between solving the 2D Grad-Shafranov equation to obtain the field configuration (flux function A) from the pressure profile P(A) and a 1D ordinary differential equation that uses the entropy profile to derive the pressure function P(A) from a flux surface average. We apply this method to 2 cases relevant to the magnetotail, both involving topological change in the transition from the initial to the final state: first, the code is applied to a forced magnetic reconnection problem (the 'Newton Challenge'), and the resulting equilibria obtained with various boundary deformations are compared with snapshots from dynamic MHD simulations. The second application starts from a more realistic magnetotail geometry that includes magnetic field line flaring. Assuming entropy conservation, we compute possible final equilibria with reconnected topology, and contrast them to equilibria obtained with unchanged P(A) profile. We also discuss how much magnetic energy is converted into thermal energy through topological change in both scenarios.
SM13A-1119
Stationary diffusion process at the Earth magnetopause
Using hybrid simulations, we examine how particles can diffuse across the Earth's magnetopause because of finite Larmor radius effects. We focus on tangential discontinuities and consider a reversal of the magnetic field that closely models the magnetopause under southward interplanetary magnetic field. When the Larmor radius is on the order of the field reversal thickness, we show that particles can cross the discontinuity. We also show that with a realistic initial shear flow, a Kelvin-Helmholtz instability develops that increases the efficiency of the crossing process. We investigate the distribution functions of the transmitted ions and demonstrate that they are structured according to a D-shape. It accordingly appears that magnetic reconnection at the magnetopause is not the only process that leads to such specific distribution functions. A simple analytical model that describes the built-up of these functions is proposed.
SM13A-1120
Self Consistent Calculation of Fokker-Planck Coefficients for Transport in Space Plasmas
Most space plasmas present velocity distributions with superthermal tails, fitted with kappa distributions. Some investigators have demonstrated a kappa distribution solution to the Fokker-Planck equation containing a longitudinal diffusion coefficient with an added (velocity)-1 dependence. These calculations rely on transport coefficients evaluated with an underlying equilibrium (Maxwellian) distribution, although the steady state solution remains kappa and never attains full equilibrium. This investigation presents the functional form for transport coefficients calculated self-consistently with an underlying kappa distribution rather than a Maxwellian distribution. These transport coefficients, retain the same velocity dependence in the high velocity (asymptotic) limit, so that (velocity)-1 dependence to the diffusion coefficient, that these earlier investigators seek is revalidated. However, below this limit, the functional form of the transport coefficients calculated self-consistently differs from these coefficients used in earlier solutions to the steady state Fokker-Planck equation. Relaxation times for plasma transport based on these revised functional forms will be calculated and compared with earlier calculated relaxation times. Implication to space weather modeling will be presented.