---------------------------------------------------------------- REPORT ON 1994 SNOWMASS WORKSHOP - MAGNETOTAIL/SUBSTORM CAMPAIGN ---------------------------------------------------------------- Tail/Substorm Working Group 3: Quantitative Tail and Substorm Models Co-chairs: M. Hesse, W. Lotko The working group pursued topics in three general areas: 1. Magnetotail Equilibria and Structure 2. Scale-Interactive Processes 3. Connectivity and Dynamics The discussion was structured into 1 hour segments. During each segment a prescheduled discussion leader pursued a particular subtopic, usually by presenting concepts, models, and/or results to inform and stimulate the discussion. This format seemed to work well. The discussions were very lively. Strengths and weaknesses of various models were identified, and, in at least one critical area described below, no adequate models were found to exist. Although the theoretical discussion sometimes proceeded without conscience, occasional interjections by observationalists provided an essential reality check. Two action items will be pursued when the working group meets again at Snowmass 95. 1. Magnetotail Equilibria and Structure * Equilibrium Models Discussion leader: J. Birn Philosophy: Determine influence of boundary conditions on tail structure. Approach: EQUATIONS HERE (tail approximation). Findings: Nightside region 1 currents are found primarily in the region surrounding the boundary between open and closed field lines where the shear in EQN is large. It is likely (although not included in the equilibrium model) that these currents originate along the tail flanks. Question: How does the magnetostatic tail field compare with (1) global MHD and (2) with the IMP statistical tail? To be resolved: (1) As the boundary conditions are continuously varied, does a loss of equilibrium lead to the formation of a thin current sheet and substorm onset? (2) Although it is not clear at this time how to incorporate equilibrium models into a ``substorm module,'' a simplified equilibrium pressure distribution characterizing the tail may allow a quick determination of substorm onset for given solar wind conditions. * Inner P. S. / Outer R. C. Region Discussion leader: L. Lyons Importance: The inner plasma sheet/outer ring current region (1) maps into the auroral region, and (2) is the likely site of substorm onset. Difficulty: Overlap region where neither the simplifying tail approximation given above nor the low beta drift physics approximation can be used. Conjecture: EQN in the tail may require a plasma source; the mantle may be the most important source (Lyons). Serious deficiency: No existing models treat this region adequately! 2. Scale-Interactive Processes * Including Non-MHD Physics in MHD Discussion leader: J. Lyon Irrelevant physics: ``Physics that does not couple into MHD'' (only for purposes of this discussion!). Run global or regional MHD models with various non-MHD effects to determine irrelevance. Traditional approach: Anomalous transport coefficients (ATCs) have met with mixed success. Action: G. Burkhart will provide an inventory of ATCs and their utility at Snowmass 95. Flux coupling: Try to convey non-MHD effects to MHD-scale processes via fluxes in the primitive equations. The equations take the form EQUATION HERE where EQN is a vector representing the 8 primitive MHD variables (velocity, B-field, density, and pressure) and EQN contains the usual MHD terms as well as non-MHD effects arising from non-ideal terms in Ohm's law, the mass continuity equation (e.g., a mass source), and the pressure and heat flux tensors. Self-consistency may also require appropriate flux coupling terms in the equations defining the non-MHD effects. Question: Is the form of the (non-MHD) dissipation mechanism important? Answers: Steady-state - probably not. Transients - YES! Operational models: Implement engineering fixes now; pursue scientific resolution over the long haul (M. Heinemann). The detailed nature of localized dissipation is probably irrelevant as long as approximately correct electric fields are produced (M. Hesse). Data assimilation: Not being done systematically in tail/substorm models at this point. Action: G. Siscoe to review data assimilation techniques in dynamic meteorology at Snowmass 95. Limitation: ``We should not ask too much of global MHD'' (A. Bhattacharjee). * Transport Model for Ion Weibel Mode Discussion leader: M. Hesse cum T. Lui Premise: Current disruption/reduction is part of substorm cycle (consensus); but no agreement on whether it is a cause or effect. CFCI requirements: The cross-field current instability (CFCI) requires a relatively large (1) EQN, and (2) cross-field ion flows with EQN. Dipolarization scenario: A localized perturbation electric field (e.g. resistive) exists at onset. Assume, for simplicity, that initially in the induction equation, so that the motional electric field term is negligible compared with the resistive term. The assumption is reasonable if the bulk velocity is initially zero, but must be violated as time proceeds. The situation is illustrated schematically in the figure below. Dipolarization occurs in the region where EQN. Does the magnetic field reduction in the region where EQN lead to neutral line formation? FIGURE HERE Exercise: It may be worth imbedding Lui's model in a regional or global MHD model to understand its implications in a global context. 3. Connectivity and Dynamics A last minute cancellation by the scheduled discussion leader J. Kan prompted an ad hoc and very impromptu discussion of Auroral Arc - Tail Coupling . The following points and questions were raised by J. Borovsky: -Auroral arc structures contain mulitple scale sizes from 100 m up to 100 km. -Some of this structure maps into the plasma sheet. -Auroral arcs affect plasma sheet properties (cooling, composition, ...) -What determines auroral arc geometry (long and narrow) and location? -What is the relationship between substorm onset, auroral arc formation, and causitive mechanism(s) in the plasma sheet? R. Lysak, W. Lotko, and J. Samson addressed most of the points raised by Borovsky, largely with emphasis on the common properties of auroral arcs and resonant Alfv en waves, (The latter were detailed in several posters presented earlier in the week.) Salient points, proceeding from larger to smaller scales, included: -Substorm onset arc maps to the inner edge of the plasma sheet (EQN) on closed field lines. -Field line resonances (Alfv en waves standing on closed field lines between northern and southern ionospheres) are naturally conjugate and are sometimes (usually?) correlated with 10-100 km scale auroral precipitation structures. -Locations (L shells) of stimulation are related to natural frequencies of the geomagnetic cavity (so-called global modes). -Dispersive properties of kilometer scale Alfv en waves further regulate long and narrow geometry as well as location. Substantial energy accumulates in resonances that form in steep radial Alfv en speed gradients (inner edge of plasma sheet?) and that are unstructured, or weakly structured, in the azimuthal direction (east-west). -Parallel electric fields, due to kinetic and electron inertial effects, accompany dispersive Alfv en waves and accelerate electrons and ions, especially at low altitudes. -Field amplitudes of dispersive Alfv en waves tend to increase substantially at low altitudes equatorial magnetospheric signatures may be difficult to recognize. -Small-scale ( 100 m) substructure is a likely result of 1-10 sec time-scale, dispersive resonances excited in the more localized, low altitude auroral resonator formed by the ionosphere at one end and the relatively steep and highly refractive, field-aligned Alfv en speed gradient that extends up to about 1 altitude. -Seemingly turbulent structure arising during breakup may be associated with inertial tearing, Kelvin-Helmholtz, and/or fast ionospheric feedback instabilities of auroral arcs. T/S WG 3 has no plans to meet again until Snowmass 95.